Living at the edge: biogeographic patterns of habitat segregation conform to speciation by niche expansion in Anopheles gambiae

Field-evolved resistance to neonicotinoids in the mosquito, Anopheles gambiae, is associated with mutations of nicotinic acetylcholine receptor subunits combined with cytochrome P450-mediated detoxification

New insecticides prequalified for malaria control interventions include modulators of nicotinic acetylcholine receptors that act selectively on different subunits leading to variable sensitivity among arthropods. This study aimed to investigate the molecular mechanisms underlying contrasting susceptibility to neonicotinoids observed in wild populations of two mosquito sibling species. Bioassays and a synergist test with piperonyl butoxide revealed that the sister taxa, Anopheles gambiae and An. coluzzii, from Yaounde, Cameroon, both have the potential to develop resistance to acetamiprid through cytochrome P450-mediated detoxification. However, contrary to An. coluzzii, An. gambiae populations are evolving cross-resistance to several active ingredients facilitated by mutations of nicotinic acetylcholine receptors (nAChRs). We sequenced coding regions on the β1 and α6 nAChR subunits where variants associated with resistance to neonicotinoids or to spinosyns have been found in agricultural pests and detected no mutation in An. coluzzii. By contrast, six nucleotide substitutions including an amino acid change in one of the loops that modulate ligand binding and affect sensitivity were present in the resistant species, An. gambiae. Allele frequency distributions were consistent with the spread of beneficial mutations that likely reduce the affinity of An. gambiae nAChRs for synthetic modulators. Our findings provide critical information for the application and resistance management of nAChR modulators in malaria prevention.

Natural variation in timing of egg hatching, response to water agitation, and bidirectional selection of early and late hatching strains of the malaria mosquito Anopheles gambiae sensu lato

BACKGROUND

Eggs of anopheline mosquitoes hatch within a few days of laying and require high levels of humidity to survive. Assessing natural variation in egg hatching and its environmental and genetic determinants in sibling species of the malaria vector Anopheles gambiae s.l. is important for understanding their adaptation to variable aquatic habitats. Crucially, it can also inform insectary rearing practices toward the optimization of mosquito production for genetic vector control strategies.

METHODS

Hatching rates and timing of egg hatching in long-established and recently colonized strains of An. gambiae s.s, Anopheles arabiensis, and Anopheles coluzzii, were compared under still water conditions (26 ℃) and with cold (4 ℃) and (15 ℃) water agitation regimes. Next, early and late hatching strains of the recently colonized An. coluzzii VK colony were generated through bidirectional selection for 18-23 generations to detect a genetic component for this trait.

RESULTS

Hatching rates differed significantly between species and treatments. The older An. arabiensis Senn and An. gambiae s.s. Kisumu strains had the highest proportion of hatching and preferred the nonagitation treatment at 26 °C. In contrast, the more recently colonized An. coluzzii VK and An. arabiensis Rufisque strains had lower overall hatching success but responded strongly to agitation at 4 °C, while the An. coluzzii Mopti strain did not significantly respond to water agitation. In all strains, eggs hatching started at day 2 and continued till day 5 in the older strains, whilst it was more staggered and extended up to day 6 in the younger strains. Bidirectional selection for early and late hatching over many generations resulted in early hatching selected strains with eggs hatching 2-3 days earlier than in late hatching ones indicating a significant heritable component for these traits.

CONCLUSIONS

Water agitation and temperature and age of colonization are likely important determinants of egg hatching in natural An. gambiae s.l.

POPULATIONS

Current rearing protocols systematically select for fast hatching and result in the progressive loss of staggered egg hatching in older laboratory strains. The selection of novel slow-hatching strains may prove instrumental to enable the mass production, shipping, and release of Anopheles mosquitoes across Africa as part of genetic vector control programs.

Whole-genome sequencing of major malaria vectors reveals the evolution of new insecticide resistance variants in a longitudinal study in Burkina Faso

BACKGROUND

Intensive deployment of insecticide based malaria vector control tools resulted in the rapid evolution of phenotypes resistant to these chemicals. Understanding this process at the genomic level is important for the deployment of successful vector control interventions. Therefore, longitudinal sampling followed by whole genome sequencing (WGS) is necessary to understand how these evolutionary processes evolve over time. This study investigated the change in genetic structure and the evolution of the insecticide resistance variants in natural populations of Anopheles gambiae over time and space from 2012 to 2017 in Burkina Faso.

METHODS

New genomic data have been generated from An. gambiae mosquitoes collected from three villages in the western part of Burkina Faso between 2012 and 2017. The samples were whole-genome sequenced and the data used in the An. gambiae 1000 genomes (Ag1000G) project as part of the Vector Observatory. Genomic data were analysed using the analysis pipeline previously designed by the Ag1000G project.

RESULTS

The results showed similar and consistent nucleotide diversity and negative Tajima's D between An. gambiae sensu stricto (s.s.) and Anopheles coluzzii. Principal component analysis (PCA) and the fixation index (FST) showed a clear genetic structure in the An. gambiae sensu lato (s.l.) species. Genome-wide FST and H12 scans identified genomic regions under divergent selection that may have implications in the adaptation to ecological changes. Novel voltage-gated sodium channel pyrethroid resistance target-site alleles (V402L, I1527T) were identified at increasing frequencies alongside the established alleles (Vgsc-L995F, Vgsc-L995S and N1570Y) within the An. gambiae s.l.

POPULATIONS

Organophosphate metabolic resistance markers were also identified, at increasing frequencies, within the An. gambiae s.s. populations from 2012 to 2017, including the SNP Ace1-G280S and its associated duplication. Variants simultaneously identified in the same vector populations raise concerns about the long-term efficacy of new generation bed nets and the recently organophosphate pirimiphos-methyl indoor residual spraying in Burkina Faso.

CONCLUSION

These findings highlighted the benefit of genomic surveillance of malaria vectors for the detection of new insecticide resistance variants, the monitoring of the existing resistance variants, and also to get insights into the evolutionary processes driving insecticide resistance.

Mitochondrial Variation in Anopheles gambiae and Anopheles coluzzii: Phylogeographic Legacy and Mitonuclear Associations With Metabolic Resistance to Pathogens and Insecticides

Mitochondrial DNA has been a popular marker in phylogeography, phylogeny, and molecular ecology, but its complex evolution is increasingly recognized. Here, we investigated mitochondrial DNA variation in Anopheles gambiae and Anopheles coluzzii, in relation to other species in the Anopheles gambiae complex, by assembling the mitogenomes of 1,219 mosquitoes across Africa. The mitochondrial DNA phylogeny of the Anopheles gambiae complex was consistent with previously reported highly reticulated evolutionary history, revealing important discordances with the species tree. The three most widespread species (An. gambiae, An. coluzzii, and Anopheles arabiensis), known for extensive historical introgression, could not be discriminated based on mitogenomes. Furthermore, a monophyletic clustering of the three saltwater-tolerant species (Anopheles merus, Anopheles melas, and Anopheles bwambae) in the Anopheles gambiae complex also suggested that introgression and possibly selection shaped mitochondrial DNA evolution. Mitochondrial DNA variation in An. gambiae and An. coluzzii across Africa revealed significant partitioning among populations and species. A peculiar mitochondrial DNA lineage found predominantly in An. coluzzii and in the hybrid taxon of the African "far-west" exhibited divergence comparable to the interspecies divergence in the Anopheles gambiae complex, with a geographic distribution matching closely An. coluzzii's geographic range. This phylogeographic relict of the An. coluzzii and An. gambiae split was associated with population and species structure, but not with the rare Wolbachia occurrence. The lineage was significantly associated with single nucleotide polymorphisms in the nuclear genome, particularly in genes associated with pathogen and insecticide resistance. These findings underline potential mitonuclear coevolution history and the role played by mitochondria in shaping metabolic responses to pathogens and insecticides in Anopheles.

Field-evolved resistance to neonicotinoids in the mosquito, Anopheles gambiae, is associated with downregulation and mutations of nicotinic acetylcholine receptor subunits combined with cytochrome P450-mediated detoxification

Neonicotinoid insecticides act selectively on their nicotinic receptor targets leading to variable sensitivity among arthropods. This study aimed to investigate the molecular mechanisms underlying contrasting susceptibility to neonicotinoids observed in wild populations of two mosquito sibling species. Bioassays and a synergism test revealed that the sister taxa, Anopheles gambiae and An. coluzzii, from Yaounde, Cameroon, rely on cytochrome P450s to detoxify neonicotinoids and develop resistance. However, contrary to An. coluzzii, An. gambiae populations are evolving stronger resistance to several active ingredients facilitated by mutations and reduced expression of nicotinic acetylcholine receptors. Six mutations were detected in coding sequences of the β1 and α6 subunits, including two substitutions in one of the loops that modulate ligand binding and sensitivity. Allele frequencies were strongly correlated with a susceptibility gradient between An. coluzzii and An. gambiae suggesting that the mutations may play a key role in sensitivity. Messenger RNA expression levels of the β1, α3, and α7 subunits decreased dramatically, on average by 23.27, 17.50, 15.80-fold, respectively, in wild An. gambiae populations compared to a susceptible insectary colony. By contrast, only the β2 and α9-1 subunits were moderately downregulated (5.28 and 2.67-fold change, respectively) in field-collected An. coluzzii adults relative to susceptible colonized mosquitoes. Our findings provide critical information for the application and resistance management of neonicotinoids in malaria prevention.

Considerations for first field trials of low-threshold gene drive for malaria vector control

Sustainable reductions in African malaria transmission require innovative tools for mosquito control. One proposal involves the use of low-threshold gene drive in Anopheles vector species, where a 'causal pathway' would be initiated by (i) the release of a gene drive system in target mosquito vector species, leading to (ii) its transmission to subsequent generations, (iii) its increase in frequency and spread in target mosquito populations, (iv) its simultaneous propagation of a linked genetic trait aimed at reducing vectorial capacity for Plasmodium, and (v) reduced vectorial capacity for parasites in target mosquito populations as the gene drive system reaches fixation in target mosquito populations, causing (vi) decreased malaria incidence and prevalence. Here the scope, objectives, trial design elements, and approaches to monitoring for initial field releases of such gene dive systems are considered, informed by the successful implementation of field trials of biological control agents, as well as other vector control tools, including insecticides, Wolbachia, larvicides, and attractive-toxic sugar bait systems. Specific research questions to be addressed in initial gene drive field trials are identified, and adaptive trial design is explored as a potentially constructive and flexible approach to facilitate testing of the causal pathway. A fundamental question for decision-makers for the first field trials will be whether there should be a selective focus on earlier points of the pathway, such as genetic efficacy via measurement of the increase in frequency and spread of the gene drive system in target populations, or on wider interrogation of the entire pathway including entomological and epidemiological efficacy. How and when epidemiological efficacy will eventually be assessed will be an essential consideration before decisions on any field trial protocols are finalized and implemented, regardless of whether initial field trials focus exclusively on the measurement of genetic efficacy, or on broader aspects of the causal pathway. Statistical and modelling tools are currently under active development and will inform such decisions on initial trial design, locations, and endpoints. Collectively, the considerations here advance the realization of developer ambitions for the first field trials of low-threshold gene drive for malaria vector control within the next 5 years.

The Anopheles coluzzii range extends into Kenya: detection, insecticide resistance profiles and population genetic structure in relation to conspecific populations in West and Central Africa

BACKGROUND

Anopheles coluzzii is a primary vector of malaria found in West and Central Africa, but its presence has hitherto never been documented in Kenya. A thorough understanding of vector bionomics is important as it enables the implementation of targeted and effective vector control interventions. Malaria vector surveillance efforts in the country have tended to focus on historically known primary vectors. The current study sought to determine the taxonomic status of samples collected from five different malaria epidemiological zones in Kenya as well as describe the population genetic structure and insecticide resistance profiles in relation to other An. coluzzii populations.

METHODS

Mosquitoes were sampled as larvae from Busia, Kwale, Turkana, Kirinyaga and Kiambu counties, representing the range of malaria endemicities in Kenya, in 2019 and 2021 and emergent adults analysed using Whole Genome Sequencing (WGS) data processed in accordance with the Anopheles gambiae 1000 Genomes Project phase 3. Where available, historical samples from the same sites were included for WGS. Comparisons were made with An. coluzzii cohorts from West and Central Africa.

RESULTS

This study reports the detection of An. coluzzii for the first time in Kenya. The species was detected in Turkana County across all three time points from which samples were analyzed and its presence confirmed through taxonomic analysis. Additionally, there was a lack of strong population genetic differentiation between An. coluzzii from Kenya and those from the more northerly regions of West and Central Africa, suggesting they represent a connected extension to the known species range. Mutations associated with target-site resistance to DDT and pyrethroids and metabolic resistance to DDT were found at high frequencies up to 64%. The profile and frequencies of the variants observed were similar to An. coluzzii from West and Central Africa but the ace-1 mutation linked to organophosphate and carbamate resistance present in An. coluzzii from coastal West Africa was absent in Kenya.

CONCLUSIONS

These findings emphasize the need for the incorporation of genomics in comprehensive and routine vector surveillance to inform on the range of malaria vector species, and their insecticide resistance status to inform the choice of effective vector control approaches.

Adult mosquitoes of the sibling species Anopheles gambiae and Anopheles coluzzii exhibit contrasting patterns of susceptibility to four neonicotinoid insecticides along an urban-to-rural gradient in Yaoundé, Cameroon

BACKGROUND

Neonicotinoids are potential alternatives for controlling pyrethroid-resistant mosquitoes, but their efficacy against malaria vector populations of sub-Saharan Africa has yet to be investigated. The aim of the present study was to test the efficacy of four neonicotinoids against adult populations of the sibling species Anopheles gambiae and Anopheles coluzzii sampled along an urban-to-rural gradient.

METHODS

The lethal toxicity of three active ingredients for adults of two susceptible Anopheles strains was assessed using concentration-response assays, and their discriminating concentrations were calculated. The discriminating concentrations were then used to test the susceptibility of An. gambiae and An. coluzzii mosquitoes collected from urban, suburban and rural areas of Yaoundé, Cameroon, to acetamiprid, imidacloprid, clothianidin and thiamethoxam.

RESULTS

Lethal concentrations of neonicotinoids were relatively high suggesting that this class of insecticides has low toxicity against Anopheles mosquitoes. Reduced susceptibility to the four neonicotinoids tested was detected in An. gambiae populations collected from rural and suburban areas. By contrast, adults of An. coluzzii that occurred in urbanized settings were susceptible to neonicotinoids except acetamiprid for which 80% mortality was obtained within 72 h of insecticide exposure. The cytochrome inhibitor, piperonyl butoxide (PBO), significantly enhanced the activity of clothianidin and acetamiprid against An. gambiae mosquitoes.

CONCLUSIONS

These findings corroborate susceptibility profiles observed in larvae and highlight a significant variation in tolerance to neonicotinoids between An. gambiae and An. coluzzii populations from Yaoundé. Further studies are needed to disentangle the role of exposure to agricultural pesticides and of cross-resistance mechanisms in the development of neonicotinoid resistance in some Anopheles species.

The Anopheles coluzzii range extends into Kenya: Detection, insecticide resistance profiles and population genetic structure in relation to conspecific populations in West and Central Africa

Background

Anopheles coluzzii is a primary vector of malaria found in West and Central Africa, but its presence has hitherto never been documented in Kenya. A thorough understanding of vector bionomics is important as it enables the implementation of targeted and effective vector control interventions. Malaria vector surveillance efforts in the country have tended to focus on historically known primary vectors. In the current study, we sought to determine the taxonomic status of samples collected from five different malaria epidemiological zones in Kenya as well asdescribe the population genetic structure and insecticide resistance profiles in relation to other An. coluzzi populations.

Methods

Mosquitoes were sampled as larvae from Busia, Kwale, Turkana, Kirinyaga and Kiambu counties, representing the range of malaria endemicities in Kenya, in 2019 and 2021 and emergent adults analysed using Whole Genome Sequencing data processed in accordance with the Anopheles gambiae 1000 Genomes Project phase 3. Where available, historical samples from the same sites were included for WGS.

Results

This study reports the detection of Anopheles coluzzii for the first time in Kenya. The species was detected in Turkana County across all three time points sampled and its presence confirmed through taxonomic analysis. Additionally, we found a lack of strong population genetic differentiation between An. coluzzii from Kenya and those from the more northerly regions of West and Central Africa, suggesting they represent a connected extension to the known species range. Mutations associated with target-site resistance to DDT and pyrethroids and metabolic resistance to DDT were found at high frequencies of ~60%. The profile and frequencies of the variants observed were similar to An. coluzzii from West and Central Africa but the ace-1 mutation linked to organophosphate and carbamate resistance present in An. coluzzii from coastal West Africa was absent in Kenya.

Conclusions

These findings emphasise the need for the incorporation of genomics in comprehensive and routine vector surveillance to inform on the range of malaria vector species, and their insecticide resistance status to inform the choice of effective vector control approaches.

Species composition and distribution of the Anopheles gambiae complex circulating in Kinshasa

Understanding the distribution of Anopheles species is essential for planning and implementing malaria control programmes. This study assessed the composition and distribution of cryptic species of the main malaria vector, the Anopheles gambiae complex, in different districts of Kinshasa. Anopheles were sampled using CDC light traps in the four Kinshasa districts between July 2021 and June 2022, and then morphologically identified. Equal proportions of Anopheles gambiae s.l. per site were subjected to polymerase chain reaction to identify the cryptic species of the Anopheles gambiae complex. Anopheles gambiae complex specimens were identified throughout Kinshasa. The average density significantly differed inside and outside households. Two species of this complex circulate in Kinshasa: Anopheles gambiae and Anopheles coluzzii. In all the study sites, Anopheles gambiae was the most widespread species. Our results provide an important basis for future studies on the ecology and dynamics of cryptic species of the Anopheles gambiae complex in Kinshasa.

A novel tetra-primer ARMS-PCR approach for the molecular karyotyping of chromosomal inversion 2Ru in the main malaria vectors Anopheles gambiae and Anopheles coluzzii

BACKGROUND

Chromosomal inversion polymorphisms have been associated with adaptive behavioral, physiological, morphological and life history traits in the two main Afrotropical malaria vectors, Anopheles coluzzii and Anopheles gambiae. The understanding of the adaptive value of chromosomal inversion systems is constrained by the feasibility of cytological karyotyping. In recent years in silico and molecular approaches have been developed for the genotyping of most widespread inversions (2La, 2Rb and 2Rc). The 2Ru inversion, spanning roughly 8% of chromosome 2R, is commonly polymorphic in West African populations of An. coluzzii and An. gambiae and shows clear increases in frequency with increasing rainfall seasonally and geographically. The aim of this work was to overcome the constraints of currently available cytological and high-throughput molecular assays by developing a simple PCR assay for genotyping the 2Ru inversion in individual specimens of both mosquito species.

METHODS

We designed tetra-primer amplification refractory mutation system (ARMS)-PCR assays based on five tag single-nucleotide polymorphisms (SNPs) previously shown to be strongly correlated with 2Ru inversion orientation. The most promising assay was validated against laboratory and field samples of An. coluzzii and An. gambiae karyotyped either cytogenetically or molecularly using a genotyping-in-thousands by sequencing (GT-seq) high-throughput approach that employs targeted sequencing of multiplexed PCR amplicons.

RESULTS

A successful assay was designed based on the tag SNP at position 2R, 31710303, which is highly predictive of the 2Ru genotype. The assay, which requires only one PCR, and no additional post-PCR processing other than electrophoresis, produced a clear banding pattern for 98.5% of the 454 specimens tested, which is a 96.7% agreement with established karyotyping methods. Sequences were obtained for nine of the An. coluzzii specimens manifesting 2Ru genotype discrepancies with GT-seq. Possible sources of these discordances are discussed.

CONCLUSIONS

The tetra-primer ARMS-PCR assay represents an accurate, streamlined and cost-effective method for the molecular karyotyping of the 2Ru inversion in An. coluzzii and An. gambiae. Together with approaches already available for the other common polymorphic inversions, 2La, 2Rb and 2Rc, this assay will allow investigations of the adaptive value of the complex set of inversion systems observed in the two major malaria vectors in the Afrotropical region.

The role of recombination landscape in species hybridisation and speciation

It is now well recognised that closely related species can hybridize and exchange genetic material, which may promote or oppose adaptation and speciation. In some cases, interspecific hybridisation is very common, making it surprising that species identity is preserved despite active gene exchange. The genomes of most eukaryotic species are highly heterogeneous with regard to gene density, abundance of repetitive DNA, chromatin compactisation etc, which can make certain genomic regions more prone or more resistant to introgression of genetic material from other species. Heterogeneity in local recombination rate underpins many of the observed patterns across the genome (e.g. actively recombining regions are typically gene rich and depleted for repetitive DNA) and it can strongly affect the permeability of genomic regions to interspecific introgression. The larger the region lacking recombination, the higher the chance for the presence of species incompatibility gene(s) in that region, making the entire non- or rarely recombining block impermeable to interspecific introgression. Large plant genomes tend to have highly heterogeneous recombination landscape, with recombination frequently occurring at the ends of the chromosomes and central regions lacking recombination. In this paper we review the relationship between recombination and introgression in plants and argue that large rarely recombining regions likely play a major role in preserving species identity in actively hybridising plant species.

Enhancing the efficacy of neonicotinoids against mosquitoes and overcoming resistance issues

Background

Neonicotinoids are potential alternatives for targeting pyrethroid-resistant mosquitoes, but their efficacy against malaria vector populations of Sub-Saharan Africa has yet to be investigated. Here we tested and compared the efficacy of four neonicotinoids alone or in combination with a synergist against two major vectors of Plasmodium .

Results

Using standard bioassays, we first assessed the lethal toxicity of three active ingredients against adults of two susceptible Anopheles strains and we determined discriminating doses for monitoring susceptibility in wild populations. We then tested the susceptibility of 5532 Anopheles mosquitoes collected from urban and rural areas of Yaoundé, Cameroon, to discriminating doses of acetamiprid, imidacloprid, clothianidin and thiamethoxam. We found that in comparison with some public health insecticides, neonicotinoids have high lethal concentration, LC ₉₉ , reflecting their low toxicity to Anopheles mosquitoes. In addition to this reduced toxicity, resistance to the four neonicotinoids tested was detected in An. gambiae populations collected from agricultural areas where larvae are intensively exposed to crop-protection neonicotinoids. However, adults of another major vector that occurred in urbanized settings, An. coluzzii , were fully susceptible to neonicotinoids except acetamiprid for which 80% mortality was obtained within 72 h of insecticide exposure. Importantly, the cytochrome inhibitor, piperonyl butoxide (PBO), was very effective in enhancing the activity of clothianidin and acetamiprid providing opportunities to create potent neonicotinoid formulations against Anopheles .

Conclusion

These findings suggest that to successfully repurpose agricultural neonicotinoids for malaria vector control, it is essential to use formulations containing synergists such as PBO or surfactants to ensure optimal efficacy.

Gene drive in species complexes: defining target organisms

Engineered gene drives, which bias their own inheritance to increase in frequency in target populations, are being developed to control mosquito malaria vectors. Such mosquitoes can belong to complexes of both vector and nonvector species that can produce fertile interspecific hybrids, making vertical gene drive transfer (VGDT) to sibling species biologically plausible. While VGDT to other vectors could positively impact human health protection goals, VGDT to nonvectors might challenge biodiversity ones. Therefore, environmental risk assessment of gene drive use in species complexes invites more nuanced considerations of target organisms and nontarget organisms than for transgenes not intended to increase in frequency in target populations. Incorporating the concept of target species complexes offers more flexibility when assessing potential impacts from VGDT.

Insecticide resistance intensity and efficacy of synergists with pyrethroids in Anopheles gambiae (Diptera: Culicidae) from Southern Togo

BACKGROUND

This study was designed to provide insecticide resistance data for decision-making in terms of resistance management plans in Togo.

METHODS

The susceptibility status of Anopheles gambiae sensu lato (s.l.) to insecticides used in public health was assessed using the WHO tube test protocol. Pyrethroid resistance intensity bioassays were performed following the CDC bottle test protocol. The activity of detoxification enzymes was tested using the synergists piperonyl butoxide, S.S.S-tributlyphosphorotrithioate and ethacrinic acid. Species-specific identification of An. gambiae s.l. and kdr mutation genotyping were performed using PCR techniques.

RESULTS

Local populations of An. gambiae s.l. showed full susceptibility to pirimiphos methyl at Lomé, Kovié, Anié, and Kpèlè Toutou. At Baguida, mortality was 90%, indicating possible resistance to pirimiphos methyl. Resistance was recorded to DDT, bendiocarb, and propoxur at all sites. A high intensity of pyrethroid resistance was recorded and the detoxification enzymes contributing to resistance were oxidases, esterases, and glutathione-s-transferases based on the synergist tests. Anopheles gambiae sensu stricto (s.s.) and Anopheles coluzzii were the main species identified. High kdr L1014F and low kdr L1014S allele frequencies were detected at all localities.

CONCLUSION

This study suggests the need to reinforce current insecticide-based malaria control interventions (IRS and LLINs) with complementary tools.

Temporal and biting dynamics of the chromosomal inversion 2La in the malaria vectors Anopheles gambiae and Anopheles coluzzii in Bangui, Central African Republic

The chromosomal rearrangement 2La has been directly involved in the ecological and deadly epidemiological success of the malaria mosquitoes Anopheles gambiae and Anopheles coluzzii in sub-Saharan Africa. However, little is known about the biological and ecological factors that drive the local and temporal dynamics of this inversion in both species. Here, we performed a year-round longitudinal survey in Bangui, Central African Republic. We monthly sampled A. gambiae and A. coluzzii mosquitoes indoor and outdoor using human landing catches (HLC) for 48 h non-stop. We molecularly karyotyped all specimens to study the 2La inversion frequency variations, and monitored the mosquito spatial and temporal biting behavior throughout the year. In total, we successfully karyotyped 5121 A. gambiae and 986 A. coluzzii specimens. The 2La inversion frequency was higher in A. coluzzii than in A. gambiae across the year. In A. gambiae and A. coluzzii, the inversion frequency or karyotypes did not influence the biting behavior, either location or time. Moreover, the inversion frequency variation in both species was also independent of local climatic changes. Overall, our results revealed that in Bangui, the 2La inversion segregates at different frequency in each species, but this is not influenced by their trophic behavior. Studying the impact of urban settings and the population genetic structure of these two A. gambiae complex members could bring insights into the intrinsic relationship between 2La inversion and local conditions. More studies are needed to understand the polymorphic equilibrium of this inversion in Bangui.

Systematic Review on Diversity and Distribution of Anopheles Species in Gabon: A Fresh Look at the Potential Malaria Vectors and Perspectives

Gabon is located in the malaria hyper-endemic zone, where data concerning malaria vector distribution remains fragmentary, making it difficult to implement an effective vector control strategy. Thus, it becomes crucial and urgent to undertake entomological surveys that will allow a better mapping of the Anopheles species present in Gabon. In this review, we examined different articles dealing with Anopheles in Gabon from ProQuest, Web of Science, PubMed, and Google scholar databases. After applying the eligibility criteria to 7543 articles collected from four databases, 42 studies were included that covered a 91-year period of study. The review revealed a wide diversity of Anopheles species in Gabon with a heterogeneous distribution. Indeed, our review revealed the presence of 41 Anopheles species, of which the most abundant were members of the Gambiae and Nili complexes and those of the Funestus and Moucheti groups. However, our review also revealed that the major and minor vectors of malaria in Gabon are present in both sylvatic, rural, and urban environments. The observation of human malaria vectors in sylvatic environments raises the question of the role that the sylvatic environment may play in maintaining malaria transmission in rural and urban areas. Ultimately, it appears that knowledge of biodiversity and spatial distribution of Anopheles mosquitoes is fragmentary in Gabon, suggesting that additional studies are necessary to complete and update these entomological data, which are useful for the implementation of vector control strategies.

Spatial modelling for population replacement of mosquito vectors at continental scale

Malaria is one of the deadliest vector-borne diseases in the world. Researchers are developing new genetic and conventional vector control strategies to attempt to limit its burden. Novel control strategies require detailed safety assessment to ensure responsible and successful deployments. Anopheles gambiae sensu stricto (s.s.) and Anopheles coluzzii, two closely related subspecies within the species complex Anopheles gambiae sensu lato (s.l.), are among the dominant malaria vectors in sub-Saharan Africa. These two subspecies readily hybridise and compete in the wild and are also known to have distinct niches, each with spatially and temporally varying carrying capacities driven by precipitation and land use factors.

We model the spread and persistence of a population-modifying gene drive system in these subspecies across sub-Saharan Africa by simulating introductions of genetically modified mosquitoes across the African mainland and its offshore islands. We explore transmission of the gene drive between the two subspecies that arise from different hybridisation mechanisms, the effects of both local dispersal and potential wind-aided migration to the spread, and the development of resistance to the gene drive. Given the best current available knowledge on the subspecies’ life histories, we find that an introduced gene drive system with typical characteristics can plausibly spread from even distant offshore islands to the African mainland with the aid of wind-driven migration, with resistance beginning to take over within a decade. Our model accounts for regional to continental scale mechanisms, and demonstrates a range of realistic dynamics including the effect of prevailing wind on spread and spatio-temporally varying carrying capacities for subspecies. As a result, it is well-placed to answer future questions relating to mosquito gene drives as important life history parameters become better understood.

Conventional control methods have dramatically reduced malaria, but it still kills over 300,000 children in Africa each year, and this number could increase as their effectiveness wanes. Novel control methods using gene drives rapidly reduce or modify malaria vector populations in laboratory settings, and hence are now being considered for field applications. We use modelling to assess how a gene drive might spread and persist in the malaria-carrying subspecies Anopheles gambiae sensu stricto (s.s.) and Anopheles coluzzii. These two subspecies interbreed and compete, so we model how these interactions affect the spread of the drive at a continental scale. In scenarios that allow mosquitoes to travel on prevailing wind currents, we find that a gene drive can potentially spread across national borders—and jump from offshore islands to the African mainland—but spread is eventually arrested when the drive allele is ousted by a resistant allele. As we learn more about the population dynamics of both genetically modified and wild mosquitoes, and as gene drive systems are further developed to allow local containment and evade resistance, our model will be able to answer more detailed questions about how they can be applied in the field effectively and safely.

Local adaptation and colonization are potential factors affecting sexual competitiveness and mating choice in Anopheles coluzzii populations

The mating behaviour of the malaria vector Anopheles gambiae complex is an important aspect of its reproduction biology. The success of mosquito release programmes based on genetic control of malaria crucially depends on competitive mating between both laboratory-reared and wild individuals, and populations from different localities. It is known that intrinsic and extrinsic factors can influence the mating success. This study addressed some of the knowledge gaps about factors influcencing mosquito mating success. In semi-field conditions, the study compared the mating success of three laboratory-reared and wild allopatric An. coluzzii populations originating from ecologically different locations in Burkina Faso. Overall, it was found that colonization reduced the mating competitiveness of both males and females compared to that of wild type individuals. More importly, females were more likely to mate with males of their own population of origin, be it wild or colonised, suggesting that local adaptation affected mate choice. The observations of mating behaviour of colonized and local wild populations revealed that subtle differences in behaviour lead to significant levels of population-specific mating. This is the first study to highlight the importance of local adaptation in the mating success, thereby highlighting the importance of using local strains for mass-rearing and release of An. coluzzii in vector control programmes.

Transposable element variants and their potential adaptive impact in urban populations of the malaria vector Anopheles coluzzii

Anopheles coluzzii is one of the primary vectors of human malaria in sub-Saharan Africa. Recently, it has spread into the main cities of Central Africa threatening vector control programs. The adaptation of An. coluzzii to urban environments partly results from an increased tolerance to organic pollution and insecticides. Some of the molecular mechanisms for ecological adaptation are known, but the role of transposable elements (TEs) in the adaptive processes of this species has not been studied yet. As a first step toward assessing the role of TEs in rapid urban adaptation, we sequenced using long reads six An. coluzzii genomes from natural breeding sites in two major Central Africa cities. We de novo annotated TEs in these genomes and in an additional high-quality An. coluzzii genome, and we identified 64 new TE families. TEs were nonrandomly distributed throughout the genome with significant differences in the number of insertions of several superfamilies across the studied genomes. We identified seven putatively active families with insertions near genes with functions related to vectorial capacity, and several TEs that may provide promoter and transcription factor binding sites to insecticide resistance and immune-related genes. Overall, the analysis of multiple high-quality genomes allowed us to generate the most comprehensive TE annotation in this species to date and identify several TE insertions that could potentially impact both genome architecture and the regulation of functionally relevant genes. These results provide a basis for future studies of the impact of TEs on the biology of An. coluzzii.

Spatio-temporal analysis and prediction of malaria cases using remote sensing meteorological data in Diébougou health district, Burkina Faso, 2016-2017

Malaria control and prevention programs are more efficient and cost-effective when they target hotspots or select the best periods of year to implement interventions. This study aimed to identify the spatial distribution of malaria hotspots at the village level in Diébougou health district, Burkina Faso, and to model the temporal dynamics of malaria cases as a function of meteorological conditions and of the distance between villages and health centres (HCs). Case data for 27 villages were collected in 13 HCs. Meteorological data were obtained through remote sensing. Two synthetic meteorological indicators (SMIs) were created to summarize meteorological variables. Spatial hotspots were detected using the Kulldorf scanning method. A General Additive Model was used to determine the time lag between cases and SMIs and to evaluate the effect of SMIs and distance to HC on the temporal evolution of malaria cases. The multivariate model was fitted with data from the epidemic year to predict the number of cases in the following outbreak. Overall, the incidence rate in the area was 429.13 cases per 1000 person-year with important spatial and temporal heterogeneities. Four spatial hotspots, involving 7 of the 27 villages, were detected, for an incidence rate of 854.02 cases per 1000 person-year. The hotspot with the highest risk (relative risk = 4.06) consisted of a single village, with an incidence rate of 1750.75 cases per 1000 person-years. The multivariate analysis found greater variability in incidence between HCs than between villages linked to the same HC. The time lag that generated the better predictions of cases was 9 weeks for SMI1 (positively correlated with precipitation variables) and 16 weeks for SMI2 (positively correlated with temperature variables. The prediction followed the overall pattern of the time series of reported cases and predicted the onset of the following outbreak with a precision of less than 3 weeks. This analysis of malaria cases in Diébougou health district, Burkina Faso, provides a powerful prospective method for identifying and predicting high-risk areas and high-transmission periods that could be targeted in future malaria control and prevention campaigns.

Review of Anopheles Mosquito Species, Abundance, and Distribution in Ethiopia

Background

Malaria is a major mosquito-borne disease in Ethiopia, and it is one of the leading causes of morbidity and mortality. Plasmodium falciparum and P. vivax are the two malaria-causing parasitic species commonly known to cause human malaria in Ethiopia. To better manage and control vectors transmitting malaria parasites, the abundance, distribution, and updated annotated list of Anopheles species present in Ethiopia are very important.

Methods

In order to compile a list of the species recorded in Ethiopia, 33 original research articles were collected. This work gives an updated list of Anopheles mosquito species in Ethiopia and their abundance, distribution, and composition.

Results

According to this review, 110305 Anopheles mosquitoes were collected and 35 Anopheles species were recorded in different parts of Ethiopia. A. arabiensis was the most abundant when compared to other species, whereas A. maculipalpis and A. wilsonii were the least abundant species. The most abundant Anopheles species was recorded in central and the least abundant, from eastern Ethiopia. The second, third, and fourth abundant species were also collected from southern, northern, and western parts of Ethiopia.

Intraspecific Transcriptome Variation and Sex-Biased Expression in Anopheles arabiensis

The magnitude and functional patterns of intraspecific transcriptional variation in the anophelines, including those of sex-biased genes underlying sex-specific traits relevant for malaria transmission, remain understudied. As a result, how changes in expression levels drive adaptation in these species is poorly understood. We sequenced the female, male, and larval transcriptomes of three populations of Anopheles arabiensis from Burkina Faso. One-third of the genes were differentially expressed between populations, often involving insecticide resistance-related genes in a sample type-specific manner, and with the females showing the largest number of differentially expressed genes. At the genomic level, the X chromosome appears depleted of differentially expressed genes compared with the autosomes, chromosomes harboring inversions do not exhibit evidence for enrichment of such genes, and genes that are top contributors to functional enrichment patterns of population differentiation tend to be clustered in the genome. Further, the magnitude of variation for the sex expression ratio across populations did not substantially differ between male- and female-biased genes, except for some populations in which male-limited expressed genes showed more variation than their female counterparts. In fact, female-biased genes exhibited a larger level of interpopulation variation than male-biased genes, both when assayed in males and females. Beyond uncovering the extensive adaptive potential of transcriptional variation in An. Arabiensis, our findings suggest that the evolutionary rate of changes in expression levels on the X chromosome exceeds that on the autosomes, while pointing to female-biased genes as the most variable component of the An. Arabiensis transcriptome.

Entomological baseline data collection and power analyses in preparation of a mosquito swarm-killing intervention in south-western Burkina Faso

Background

Insecticides are currently the main tools used to reduce the transmission of malaria; therefore, the development of resistance to insecticides in malaria vectors is of major concern for malaria control. The resistance level to pyrethroids is particularly high in the Western region of Burkina Faso and may affect the efficacy of insecticidal bed nets and indoor residual spraying. Adult mosquito swarming and other nocturnal behaviours exhibit spatial and temporal patterns that suggest potential vulnerability to targeted space spraying with effective insecticides. Indeed, targeted space-spraying against adult mosquito swarms has been used to crash mosquito populations and disrupt malaria transmission.

Methods

Prior to impact assessment of swarm killing, a baseline data collection was conducted from June to November 2016 in 10 villages divided into two areas in western Burkina Faso. The data considered both ecological and demographic characteristics to monitor the key entomological parameters.

Results

The mean number of swarms observed was 35 per village, ranging from 25 to 70 swarms according to the village. Female density in both areas varied significantly as a function of the village and the period of collection. The human biting rate was significantly affected by the period of collection and depended upon whether the collection was carried out indoors or outdoors. Averages of parity rate were high in both areas for all periods of collection, ranging from 60 to 90%. These values ranged from 80 to 100% for inseminated females. Sporozoite rates ranged between 1.6 and 7.2% depending upon the village. The molecular identification of resting and swarming mosquitoes showed the presence of the three major malaria vectors in Burkina Faso, but in different proportions for each village.

Conclusions

The distribution of the potential swarm markers and swarms in villages suggested that swarms are clustered across space, making intervention easier. Power simulations showed that the direct sampling of swarms provides the highest statistical power, thereby reducing the number of villages needed for a trial.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03877-x.

Ecological plasticity to ions concentration determines genetic response and dominance of Anopheles coluzzii larvae in urban coastal habitats of Central Africa

In Central Africa, the malaria vector Anopheles coluzzii is predominant in urban and coastal habitats. However, little is known about the environmental factors that may be involved in this process. Here, we performed an analysis of 28 physicochemical characteristics of 59 breeding sites across 5 urban and rural sites in coastal areas of Central Africa. We then modelled the relative frequency of An. coluzzii larvae to these physicochemical parameters in order to investigate environmental patterns. Then, we assessed the expression variation of 10 candidate genes in An. coluzzii, previously incriminated with insecticide resistance and osmoregulation in urban settings. Our results confirmed the ecological plasticity of An. coluzzii larvae to breed in a large range of aquatic conditions and its predominance in breeding sites rich in ions. Gene expression patterns were comparable between urban and rural habitats, suggesting a broad response to ions concentrations of whatever origin. Altogether, An. coluzzii exhibits a plastic response to occupy both coastal and urban habitats. This entails important consequences for malaria control in the context of the rapid urban expansion in Africa in the coming years.

Data-driven and interpretable machine-learning modeling to explore the fine-scale environmental determinants of malaria vectors biting rates in rural Burkina Faso

Background

Improving the knowledge and understanding of the environmental determinants of malaria vector abundance at fine spatiotemporal scales is essential to design locally tailored vector control intervention. This work is aimed at exploring the environmental tenets of human-biting activity in the main malaria vectors (Anopheles gambiae s.s., Anopheles coluzzii and Anopheles funestus) in the health district of Diébougou, rural Burkina Faso.

Methods

Anopheles human-biting activity was monitored in 27 villages during 15 months (in 2017–2018), and environmental variables (meteorological and landscape) were extracted from high-resolution satellite imagery. A two-step data-driven modeling study was then carried out. Correlation coefficients between the biting rates of each vector species and the environmental variables taken at various temporal lags and spatial distances from the biting events were first calculated. Then, multivariate machine-learning models were generated and interpreted to (i) pinpoint primary and secondary environmental drivers of variation in the biting rates of each species and (ii) identify complex associations between the environmental conditions and the biting rates.

Results

Meteorological and landscape variables were often significantly correlated with the vectors’ biting rates. Many nonlinear associations and thresholds were unveiled by the multivariate models, for both meteorological and landscape variables. From these results, several aspects of the bio-ecology of the main malaria vectors were identified or hypothesized for the Diébougou area, including breeding site typologies, development and survival rates in relation to weather, flight ranges from breeding sites and dispersal related to landscape openness.

Conclusions

Using high-resolution data in an interpretable machine-learning modeling framework proved to be an efficient way to enhance the knowledge of the complex links between the environment and the malaria vectors at a local scale. More broadly, the emerging field of interpretable machine learning has significant potential to help improve our understanding of the complex processes leading to malaria transmission, and to aid in developing operational tools to support the fight against the disease (e.g. vector control intervention plans, seasonal maps of predicted biting rates, early warning systems).

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04851-x.

Mosquito sound communication: are male swarms loud enough to attract females?

Given the unsurpassed sound sensitivity of mosquitoes among arthropods and the sound source power required for long-range hearing, we investigated the distance over which female mosquitoes detect species-specific cues in the sound of station-keeping mating swarms. A common misunderstanding, that mosquitoes cannot hear at long range because their hearing organs are 'particle-velocity' receptors, has clouded the fact that particle velocity is an intrinsic component of sound whatever the distance to the sound source. We exposed free-flying Anopheles coluzzii females to pre-recorded sounds of male An. coluzzii and An. gambiae s.s. swarms over a range of natural sound levels. Sound levels tested were related to equivalent distances between the female and the swarm for a given number of males, enabling us to infer distances over which females might hear large male swarms. We show that females do not respond to swarm sound up to 48 dB sound pressure level (SPL) and that louder SPLs are not ecologically relevant for a swarm. Considering that swarms are the only mosquito sound source that would be loud enough to be heard at long range, we conclude that inter-mosquito acoustic communication is restricted to close-range pair interactions. We also showed that the sensitivity to sound in free-flying males is much enhanced compared to that of tethered ones.

Novel genotyping approaches to easily detect genomic admixture between the major Afrotropical malaria vector species, Anopheles coluzzii and An. gambiae

The two most efficient and most recently radiated Afrotropical vectors of human malaria - Anopheles coluzzii and An. gambiae - are identified by single-locus diagnostic PCR assays based on species-specific markers in a 4 Mb region on chromosome-X centromere. Inherently, these diagnostic assays cannot detect interspecific autosomal admixture shown to be extensive at the westernmost and easternmost extremes of the species range. The main aim of this study was to develop novel, easy-to-implement tools for genotyping An. coluzzii and An. gambiae-specific ancestral informative markers (AIMs) identified from the Anopheles gambiae 1000 genomes (Ag1000G) project. First, we took advantage of this large set of data in order to develop a multilocus approach to genotype 26 AIMs on all chromosome arms valid across the species range. Second, we tested the multilocus assay on samples from Guinea Bissau, The Gambia and Senegal, three countries spanning the westernmost hybridization zone, where conventional species diagnostic is problematic due to the putative presence of a novel "hybrid form". The multilocus assay was able to capture patterns of admixture reflecting those revealed by the whole set of AIMs and provided new original data on interspecific admixture in the region. Third, we developed an easy-to-use, cost-effective PCR approach for genotyping two AIMs on chromosome-3 among those included in the multilocus approach, opening the possibility for advanced identification of species and of admixed specimens during routine large scale entomological surveys, particularly, but not exclusively, at the extremes of the range, where WGS data highlighted unexpected autosomal admixture.

Systematic identification of plausible pathways to potential harm via problem formulation for investigational releases of a population suppression gene drive to control the human malaria vector Anopheles gambiae in West Africa

BACKGROUND

Population suppression gene drive has been proposed as a strategy for malaria vector control. A CRISPR-Cas9-based transgene homing at the doublesex locus (dsxFCRISPRh) has recently been shown to increase rapidly in frequency in, and suppress, caged laboratory populations of the malaria mosquito vector Anopheles gambiae. Here, problem formulation, an initial step in environmental risk assessment (ERA), was performed for simulated field releases of the dsxFCRISPRh transgene in West Africa.

METHODS

Building on consultative workshops in Africa that previously identified relevant environmental and health protection goals for ERA of gene drive in malaria vector control, 8 potentially harmful effects from these simulated releases were identified. These were stratified into 46 plausible pathways describing the causal chain of events that would be required for potential harms to occur. Risk hypotheses to interrogate critical steps in each pathway, and an analysis plan involving experiments, modelling and literature review to test each of those risk hypotheses, were developed.

RESULTS

Most potential harms involved increased human (n = 13) or animal (n = 13) disease transmission, emphasizing the importance to subsequent stages of ERA of data on vectorial capacity comparing transgenics to non-transgenics. Although some of the pathways (n = 14) were based on known anatomical alterations in dsxFCRISPRh homozygotes, many could also be applicable to field releases of a range of other transgenic strains of mosquito (n = 18). In addition to population suppression of target organisms being an accepted outcome for existing vector control programmes, these investigations also revealed that the efficacy of population suppression caused by the dsxFCRISPRh transgene should itself directly affect most pathways (n = 35).

CONCLUSIONS

Modelling will play an essential role in subsequent stages of ERA by clarifying the dynamics of this relationship between population suppression and reduction in exposure to specific potential harms. This analysis represents a comprehensive identification of plausible pathways to potential harm using problem formulation for a specific gene drive transgene and organism, and a transparent communication tool that could inform future regulatory studies, guide subsequent stages of ERA, and stimulate further, broader engagement on the use of population suppression gene drive to control malaria vectors in West Africa.

Speciation across the Tree of Life

Much of what we know about speciation comes from detailed studies of well-known model systems. Although there have been several important syntheses on speciation, few (if any) have explicitly compared speciation among major groups across the Tree of Life. Here, we synthesize and compare what is known about key aspects of speciation across taxa, including bacteria, protists, fungi, plants, and major animal groups. We focus on three main questions. Is allopatric speciation predominant across groups? How common is ecological divergence of sister species (a requirement for ecological speciation), and on what niche axes do species diverge in each group? What are the reproductive isolating barriers in each group? Our review suggests the following patterns. (i) Based on our survey and projected species numbers, the most frequent speciation process across the Tree of Life may be co-speciation between endosymbiotic bacteria and their insect hosts. (ii) Allopatric speciation appears to be present in all major groups, and may be the most common mode in both animals and plants, based on non-overlapping ranges of sister species. (iii) Full sympatry of sister species is also widespread, and may be more common in fungi than allopatry. (iv) Full sympatry of sister species is more common in some marine animals than in terrestrial and freshwater ones. (v) Ecological divergence of sister species is widespread in all groups, including ~70% of surveyed species pairs of plants and insects. (vi) Major axes of ecological divergence involve species interactions (e.g. host-switching) and habitat divergence. (vii) Prezygotic isolation appears to be generally more widespread and important than postzygotic isolation. (viii) Rates of diversification (and presumably speciation) are strikingly different across groups, with the fastest rates in plants, and successively slower rates in animals, fungi, and protists, with the slowest rates in prokaryotes. Overall, our study represents an initial step towards understanding general patterns in speciation across all organisms.

A PCR-RFLP method for genotyping of inversion 2Rc in Anopheles coluzzii

BACKGROUND

Genotyping of polymorphic chromosomal inversions in malaria vectors such as An. coluzzii Coetzee & Wilkerson is important, both because they cause cryptic population structure that can mislead vector analysis and control and because they influence epidemiologically relevant eco-phenotypes. The conventional cytogenetic method of genotyping is an impediment because it is labor intensive, requires specialized training, and can be applied only to one gender and developmental stage. Here, we circumvent these limitations by developing a simple and rapid molecular method of genotyping inversion 2Rc in An. coluzzii that is both economical and field-friendly. This inversion is strongly implicated in temporal and spatial adaptations to climatic and ecological variation, particularly aridity.

METHODS

Using a set of tag single-nucleotide polymorphisms (SNPs) strongly correlated with inversion orientation, we identified those that overlapped restriction enzyme recognition sites and developed four polymerase chain reaction (PCR) restriction fragment length polymorphism (RFLP) assays that distinguish alternative allelic states at the tag SNPs. We assessed the performance of these assays using mosquito population samples from Burkina Faso that had been cytogenetically karyotyped as well as genotyped, using two complementary high-throughput molecular methods based on tag SNPs. Further validation was performed using mosquito population samples from additional West African (Benin, Mali, Senegal) and Central African (Cameroon) countries.

RESULTS

Of four assays tested, two were concordant with the 2Rc cytogenetic karyotype > 90% of the time in all samples. We recommend that these two assays be employed in tandem for reliable genotyping. By accepting only those genotypic assignments where both assays agree, > 99% of assignments are expected to be accurate.

CONCLUSIONS

We have developed tandem PCR-RFLP assays for the accurate genotyping of inversion 2Rc in An. coluzzii. Because this approach is simple, inexpensive, and requires only basic molecular biology equipment, it is widely accessible. These provide a crucial tool for probing the molecular basis of eco-phenotypes relevant to malaria epidemiology and vector control.

High-Throughput Genotyping of Common Chromosomal Inversions in the Afrotropical Malaria Mosquito Anopheles Funestus

Polymorphic chromosomal inversions have been implicated in local adaptation. In anopheline mosquitoes, inversions also contribute to epidemiologically relevant phenotypes such as resting behavior. Progress in understanding these phenotypes and their mechanistic basis has been hindered because the only available method for inversion genotyping relies on traditional cytogenetic karyotyping, a rate-limiting and technically difficult approach that is possible only for the fraction of the adult female population at the correct gonotrophic stage. Here, we focus on an understudied malaria vector of major importance in sub-Saharan Africa, Anopheles funestus. We ascertain and validate tag single nucleotide polymorphisms (SNPs) using high throughput molecular assays that allow rapid inversion genotyping of the three most common An. funestus inversions at scale, overcoming the cytogenetic karyotyping barrier. These same inversions are the only available markers for distinguishing two An. funestus ecotypes that differ in indoor resting behavior, Folonzo and Kiribina. Our new inversion genotyping tools will facilitate studies of ecotypic differentiation in An. funestus and provide a means to improve our understanding of the roles of Folonzo and Kiribina in malaria transmission.

Inversion Genotyping in the Anopheles gambiae Complex Using High-Throughput Array and Sequencing Platforms

Chromosomal inversion polymorphisms have special importance in the Anopheles gambiae complex of malaria vector mosquitoes, due to their role in local adaptation and range expansion. The study of inversions in natural populations is reliant on polytene chromosome analysis by expert cytogeneticists, a process that is limited by the rarity of trained specialists, low throughput, and restrictive sampling requirements. To overcome this barrier, we ascertained tag single nucleotide polymorphisms (SNPs) that are highly correlated with inversion status (inverted or standard orientation). We compared the performance of the tag SNPs using two alternative high throughput molecular genotyping approaches vs. traditional cytogenetic karyotyping of the same 960 individual An. gambiae and An. coluzzii mosquitoes sampled from Burkina Faso, West Africa. We show that both molecular approaches yield comparable results, and that either one performs as well or better than cytogenetics in terms of genotyping accuracy. Given the ability of molecular genotyping approaches to be conducted at scale and at relatively low cost without restriction on mosquito sex or developmental stage, molecular genotyping via tag SNPs has the potential to revitalize research into the role of chromosomal inversions in the behavior and ongoing adaptation of An. gambiae and An. coluzzii to environmental heterogeneities.

Climate change could shift disease burden from malaria to arboviruses in Africa

Malaria is a long-standing public health problem in sub-Saharan Africa, whereas arthropod-borne viruses (arboviruses) such as dengue and chikungunya cause an under-recognised burden of disease. Many human and environmental drivers affect the dynamics of vector-borne diseases. In this Personal View, we argue that the direct effects of warming temperatures are likely to promote greater environmental suitability for dengue and other arbovirus transmission by Aedes aegypti and reduce suitability for malaria transmission by Anopheles gambiae. Environmentally driven changes in disease dynamics will be complex and multifaceted, but given that current public efforts are targeted to malaria control, we highlight Ae aegypti and dengue, chikungunya, and other arboviruses as potential emerging public health threats in sub-Saharan Africa.

Behavioural plasticity of Anopheles coluzzii and Anopheles arabiensis undermines LLIN community protective effect in a Sudanese-savannah village in Burkina Faso

BACKGROUND

Despite the overall major impact of long-lasting insecticide treated nets (LLINs) in eliciting individual and collective protection to malaria infections, some sub-Saharan countries, including Burkina Faso, still carry a disproportionately high share of the global malaria burden. This study aims to analyse the possible entomological bases of LLIN limited impact, focusing on a LLIN-protected village in the Plateau Central region of Burkina Faso.

METHODS

Human landing catches (HLCs) were carried out in 2015 for 12 nights both indoors and outdoors at different time windows during the highest biting activity phase for Anopheles gambiae (s.l.). Collected specimens were morphologically and molecularly identified and processed for Plasmodium detection and L1014F insecticide-resistance allele genotyping.

RESULTS

Almost 2000 unfed An. gambiae (s.l.) (54% Anopheles coluzzii and 44% Anopheles arabiensis) females landing on human volunteers were collected, corresponding to a median number of 23.5 females/person/hour. No significant differences were observed in median numbers of mosquitoes collected indoors and outdoors, nor between sporozoite rates in An. coluzzii (6.1%) and An. arabiensis (5.5%). The estimated median hourly entomological inoculation rate (EIR) on volunteers was 1.4 infective bites/person/hour. Results do not show evidence of the biting peak during night hours typical for An. gambiae (s.l.) in the absence of bednet protection. The frequency of the L1014F resistant allele (n = 285) was 66% in An. coluzzii and 38% in An. arabiensis.

CONCLUSIONS

The observed biting rate and sporozoite rates are in line with the literature data available for An. gambiae (s.l.) in the same geographical area before LLIN implementation and highlight high levels of malaria transmission in the study village. Homogeneous biting rate throughout the night and lack of preference for indoor-biting activity, suggest the capacity of both An. coluzzii and An. arabiensis to adjust their host-seeking behaviour to bite humans despite bednet protection, accounting for the maintenance of high rates of mosquito infectivity and malaria transmission. These results, despite being limited to a local situation in Burkina Faso, represent a paradigmatic example of how high densities and behavioural plasticity in the vector populations may contribute to explaining the limited impact of LLINs on malaria transmission in holo-endemic Sudanese savannah areas in West Africa.

Genetic analysis and population structure of the Anopheles gambiae complex from different ecological zones of Burkina Faso

The Anopheles gambiae complex (Diptera: Culicidae) is the most important vector for malaria in Sub-Saharan Africa, besides other vectors such as Anopheles funestus. Malaria vector control should encompass specific identification, genetic diversity and population structure of An. gambiae to design vector control strategies. The aim of this study was to determine the distribution of sibling species of the An. gambiae complex according to climatic regions related to cotton-growing or cotton-free areas by using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Then, variation in mitochondrial cytochrome c oxidase 1 (COI) was used to assess the genetic structure within and between populations from our selected ecological zones. At the sibling species level, the following proportions were found across all samples (n = 180): An. coluzzii 65.56%, An. gambiae stricto sensu (s.s). 21.11%, and An. arabiensis 3.33%. Hybrids between An. gambiae s.s. and An. coluzzii (7.78%) and hybrids between An. coluzzii and An. arabiensis (2.22%) were found. The phylogenetic tree and Integer Neighbour-Joining (IntNJ) haplotype network did not reveal any distinct genetic structure pattern related to climatic or agricultural conditions in Burkina Faso. The Fst (Wright's F-statistic) values close to zero showed a free gene flow and no differentiation in An. gambiae complex populations. Furthermore, neutrality indices calculated by Tajima's D, Fu and Li's D^⁎, Fu and Li's F^⁎, Fu's Fs tests suggested an excess of rare mutations in the populations. Overall, variation in the proportions of An. gambiae s.s., An. coluzzii and An. arabiensis was found according to climatic regions, but COI analysis did not evidence any population structuring of the An. gambiae complex. These scientific contributions can be used as a basis for further in-depth study of the genetic diversity of the An. gambiae complex for epidemiological risk assessment of malaria in Burkina Faso.

A survey of Anopheles species composition and insecticide resistance on the island of Bubaque, Bijagos Archipelago, Guinea-Bissau

Background

Bubaque is the most populous island of the Bijagos archipelago, a group of malaria-endemic islands situated off the coast of Guinea-Bissau, West Africa. Malaria vector control on Bubaque relies almost exclusively on the use of long-lasting insecticidal nets (LLINs). However, there is little information on local vector bionomics and insecticide resistance.

Methods

A survey of mosquito species composition was performed at the onset of the wet season (June/July) and the beginning of the dry season (November/December). Sampling was performed using indoor adult light-traps and larval dipping. Anopheles mosquitoes were identified to species level and assessed for kdr allele frequency by TaqMan PCR. Females were analysed for sporozoite positivity by CSP-ELISA. Resistance to permethrin and α-cypermethrin was measured using the CDC-bottle bioassay incorporating the synergist piperonyl-butoxide.

Results

Several Anopheles species were found on the island, all belonging to the Anopheles gambiae sensu lato (s.l.) complex, including An. gambiae sensu stricto, Anopheles coluzzii, Anopheles melas, and An. gambiae/An. coluzzii hybrids. Endophagic Anopheles species composition and abundance showed strong seasonal variation, with a majority of An. gambiae (50% of adults collected) caught in June/July, while An. melas was dominant in November/December (83.9% of adults collected). Anopheles gambiae had the highest sporozoite rate in both seasons, with infection rates of 13.9% and 20% in June/July and November/December, respectively. Moderate frequencies of the West African kdr allele were found in An. gambiae (36%), An. coluzzii (35%), An. gambiae/An. coluzzii hybrids (42%). Bioassays suggest moderate resistance to α-cypermethrin, but full susceptibility to permethrin.

Conclusions

The island of Bubaque maintained an An. gambiae s.l. population in both June/July and November/December. Anopheles gambiae was the primary vector at the onset of the wet season, while An. melas is likely to be responsible for most dry season transmission. There was moderate kdr allele frequency and synergist assays suggest likely metabolic resistance, which could reduce the efficacy of LLINs. Future control of malaria on the islands should consider the seasonal shift in mosquito species, and should employ continuous monitoring for insecticide resistance.

Highly specific PCR-RFLP assays for karyotyping the widespread 2Rb inversion in malaria vectors of the Anopheles gambiae complex

Background

Chromosomal inversion polymorphisms play a role in adaptation to heterogeneous environments. Inversion polymorphisms are implicated in the very high ecological flexibility of the three main malaria vector species of the Afrotropical Anopheles gambiae complex, facilitating the exploitation of anthropogenic environmental modifications and promoting a strong association with humans. In addition to extending the species’ spatial and temporal distribution, inversions are associated with epidemiologically relevant mosquito behavior and physiology, underscoring their medical importance. We here present novel PCR-RFLP based assays strongly predictive of genotype for the cosmopolitan 2Rb inversion in An. coluzzii and An. gambiae, a development which overcomes the numerous constraints inherent to traditional cytological karyotyping.

Methods

We designed PCR-RFLP genotyping assays based on tag SNPs previously computationally identified as strongly predictive (> 95%) of 2Rb genotype. We targeted those tags whose alternative allelic states destroyed or created the recognition site of a commercially available restriction enzyme, and designed assays with distinctive cleavage profiles for each inversion genotype. The assays were validated on 251 An. coluzzii and 451 An. gambiae cytologically karyotyped specimens from nine countries across Africa and one An. coluzzii laboratory colony.

Results

For three tag SNPs, PCR-RFLP assays (denoted DraIII, MspAI, and TatI) reliably produced robust amplicons and clearly distinguishable electrophoretic profiles for all three inversion genotypes. Results obtained with the DraIII assay are ≥ 95% concordant with cytogenetic assignments in both species, while MspAI and TatI assays produce patterns highly concordant with cytogenetic assignments only in An. coluzzii or An. gambiae, respectively. Joint application of species-appropriate pairs of assays increased the concordance levels to > 99% in An. coluzzii and 98% in An. gambiae. Potential sources of discordance (e.g. imperfect association between tag and inversion, allelic dropout, additional polymorphisms in the restriction target site, incomplete or failed restriction digestion) are discussed.

Conclusions

The availability of highly specific, cost effective and accessible molecular assays for genotyping 2Rb in An. gambiae and An. coluzzii allows karyotyping of both sexes and all developmental stages. These novel tools will accelerate deeper investigations into the role of this ecologically and epidemiologically important chromosomal inversion in vector biology.

Sex aggregation and species segregation cues in swarming mosquitoes: role of ground visual markers

BACKGROUND

Mating swarm segregation in closely related insect species may contribute to reproductive isolation. Visual markers are used for swarm formation; however, it is unknown whether they play a key role in swarm location, species segregation and sex aggregation.

METHODS

Using two sympatric closely related species of the Anopheles gambiae complex, An. coluzzii and An. gambiae (s.s.), we investigated in both laboratory and semi-field conditions (i) whether males of the two species use visual markers (black cloths) to locate their swarm; and (ii) whether the presence/absence and size of the marker may differentially affect swarm characteristics. We also investigated whether conspecific virgin females use these markers to join male swarm sites.

RESULTS

We showed that males of the two species used visual markers but in different ways: An. coluzzii swarm right above the marker whereas An. gambiae (s.s.) locate their swarm at a constant distance of 76.4 ± 0.6 cm from a 20 × 20 cm marker in the laboratory setup and at 206 ± 6 cm from a 60 × 60 cm marker in the semi-field setup. Although increased marker size recruited more mosquitoes and consequently increased the swarm size in the two species, An. coluzzii swarms flew higher and were stretched both vertically and horizontally, while An. gambiae (s.s.) swarms were only stretched horizontally. Virgin females displayed a swarm-like behavior with similar characteristics to their conspecific males.

CONCLUSIONS

Our results provided experimental evidence that both An. coluzzii and An. gambiae (s.s.) males use ground visual markers to form and locate their swarm at species-specific locations. Moreover, the marker size differentially affected swarm characteristics in the two species. Our results also showed that virgin females displayed a swarm-like behavior. However, these "swarms" could be due to the absence of males in our experimental conditions. Nevertheless, the fact that females displayed these "swarms" with the same characteristics as their respective males provided evidence that visual markers are used by the two sexes to join mating spots. Altogether, this suggests that visual markers and the way species and sexes use them could be key cues in species segregation, swarm location and recognition.

In Silico Karyotyping of Chromosomally Polymorphic Malaria Mosquitoes in the Anopheles gambiae Complex

Chromosomal inversion polymorphisms play an important role in adaptation to environmental heterogeneities. For mosquito species in the Anopheles gambiae complex that are significant vectors of human malaria, paracentric inversion polymorphisms are abundant and are associated with ecologically and epidemiologically important phenotypes. Improved understanding of these traits relies on determining mosquito karyotype, which currently depends upon laborious cytogenetic methods whose application is limited both by the requirement for specialized expertise and for properly preserved adult females at specific gonotrophic stages. To overcome this limitation, we developed sets of tag single nucleotide polymorphisms (SNPs) inside inversions whose biallelic genotype is strongly correlated with inversion genotype. We leveraged 1,347 fully sequenced An. gambiae and Anopheles coluzzii genomes in the Ag1000G database of natural variation. Beginning with principal components analysis (PCA) of population samples, applied to windows of the genome containing individual chromosomal rearrangements, we classified samples into three inversion genotypes, distinguishing homozygous inverted and homozygous uninverted groups by inclusion of the small subset of specimens in Ag1000G that are associated with cytogenetic metadata. We then assessed the correlation between candidate tag SNP genotypes and PCA-based inversion genotypes in our training sets, selecting those candidates with >80% agreement. Our initial tests both in held-back validation samples from Ag1000G and in data independent of Ag1000G suggest that when used for in silico inversion genotyping of sequenced mosquitoes, these tags perform better than traditional cytogenetics, even for specimens where only a small subset of the tag SNPs can be successfully ascertained.

Anopheline species composition and the 1014F-genotype in different ecological settings of Burkina Faso in relation to malaria transmission

Background

A three-year longitudinal study was conducted in four sentinel sites from different ecological settings in Burkina Faso, between 2008 and 2010 to identify longitudinal changes in insecticide resistance within Anopheles gambiae complex species based on larval collection. During this study, adult mosquitoes were also collected indoor and outdoor using several methods of collection. The present study reports the diversity of malaria vectors and the 1014F-genotype from this adult collection and investigates the association between this 1014F-genotype and sporozoite rate.

Methods

Adult mosquitoes were collected from July to August (corresponding to the start of rainy season) and October to November (corresponding to the end of rainy season) over 3 years (2008–2010) at four sites across the country, using pyrethrum spray catches (PSC), exit traps and pit shelters. Anopheles gambiae complex mosquitoes were identified to species and genotyped for the L1014F kdr mutation by PCR using genomic DNA. The circumsporozoite antigen of Plasmodium falciparum was detected in mosquitoes using sandwich ELISA.

Results

Overall 9212 anopheline mosquitoes were collected during the study period. Of those, 6767 mosquitoes were identified as Anopheles gambiae sensu lato (s.l.). Anopheles arabiensis, Anopheles coluzzii, Anopheles gambiae and or Anopheles funestus were incriminated as vectors of P. falciparum in the study area with an average sporozoite rate of 5%, (95% CI 4.14–5.99%). The kdr1014F-genotype frequencies were 11.44% (95% CI 2.5–39.85%), 19.2% (95% CI 4.53–53.73%) and 89.9 (95% CI 63.14–97.45%), respectively for An. arabiensis, An. coluzzii and An. gambiae. The proportion of the 1014F-genotype varied between sporozoite-infected and uninfected An. gambiae s.l. group. There was no significant difference in the 1014F-genotype frequency between infected and uninfected mosquitoes.

Conclusion

The current study shows the diversity of malaria vectors and significant interaction between species composition and kdr1014F-genotype in An. gambiae complex mosquitoes from Burkina Faso. In this study, no associations were found between the 1014F-genotype and P. falciparum infection in the major malaria vector An. gambiae s.l.

Electronic supplementary material

The online version of this article (10.1186/s12936-019-2789-8) contains supplementary material, which is available to authorized users.

Forecasting Zoonotic Infectious Disease Response to Climate Change: Mosquito Vectors and a Changing Environment

Infectious diseases are changing due to the environment and altered interactions among hosts, reservoirs, vectors, and pathogens. This is particularly true for zoonotic diseases that infect humans, agricultural animals, and wildlife. Within the subset of zoonoses, vector-borne pathogens are changing more rapidly with climate change, and have a complex epidemiology, which may allow them to take advantage of a changing environment. Most mosquito-borne infectious diseases are transmitted by mosquitoes in three genera: Aedes, Anopheles, and Culex, and the expansion of these genera is well documented. There is an urgent need to study vector-borne diseases in response to climate change and to produce a generalizable approach capable of generating risk maps and forecasting outbreaks. Here, we provide a strategy for coupling climate and epidemiological models for zoonotic infectious diseases. We discuss the complexity and challenges of data and model fusion, baseline requirements for data, and animal and human population movement. Disease forecasting needs significant investment to build the infrastructure necessary to collect data about the environment, vectors, and hosts at all spatial and temporal resolutions. These investments can contribute to building a modeling community around the globe to support public health officials so as to reduce disease burden through forecasts with quantified uncertainty.

Modelling the potential of genetic control of malaria mosquitoes at national scale

BACKGROUND

The persistence of malaria in large parts of sub-Saharan Africa has motivated the development of novel tools to complement existing control programmes, including gene-drive technologies to modify mosquito vector populations. Here, we use a stochastic simulation model to explore the potential of using a driving-Y chromosome to suppress vector populations in a 10⁶ km² area of West Africa including all of Burkina Faso.

RESULTS

The consequence of driving-Y introductions is predicted to vary across the landscape, causing elimination of the target species in some regions and suppression in others. We explore how this variation is determined by environmental conditions, mosquito behaviour, and the properties of the gene-drive. Seasonality is particularly important, and we find population elimination is more likely in regions with mild dry seasons whereas suppression is more likely in regions with strong seasonality.

CONCLUSIONS

Despite the spatial heterogeneity, we suggest that repeated introductions of modified mosquitoes over a few years into a small fraction of human settlements may be sufficient to substantially reduce the overall number of mosquitoes across the entire geographic area.

Vector bionomics and malaria transmission in an area of sympatry of An. arabiensis, An. coluzzii and An. gambiae

Despite extensive genetic studies on their variability and differentiation, few is known about the specific and relative role of An. coluzzii, An. gambiae and An. arabiensis in areas of sympatry. Indeed, their behavioral dissimilarities and divergent population dynamics can impact on malaria transmission level and intensity. This study was undertaken in four sympatric sites belonging to two different ecosystems with differential insecticide pressure to study the bionomics of these species and their relative role in malaria transmission. Mosquitoes were collected monthly from July to December 2011 when landing on human volunteers and by pyrethrum spray catches. Specimens belonging to the An. gambiae complex were further identified using molecular tools. Plasmodium falciparum infection and blood-feeding preferences were studied using the ELISA techniques. Overall, the three species were in sympatry in each of the four sites with the predominance of An. gambiae. Mosquito populations' dynamics varied temporally depending on the rainy season for each zone. The anthropophilic rates varied between 45.7 and 78.1% for An. arabiensis, 81.8 and 100% for An. coluzzii and 80 and 96.7% for An. gambiae. Plasmodium infection rates were higher in An. gambiae (range: 2.17%-6.54%) while for An. arabiensis and An. coluzzii it varied respectively between 0-1.24% and 0-3.66%. Malaria transmission occured in each of the four sites both indoors and outdoors and was due mainly to An. gambiae. An. arabiensis and An. coluzzii played a limited role due both to a low anthropophilic rate and a lower biting rate for An. coluzzii in comparison with An. gambiae. This study showed that, while present in sympatric areas, species from the An. gambiae complex could exhibit differential involvement in malaria transmission. Even less involved in malaria transmission, the occurrence of ecological and environmental changes tending to a good adaptation of An. coluzzii could lead to a great risk for malaria transmission in time and space in human populations.

Unexpectedly high Plasmodium sporozoite rate associated with low human blood index in Anopheles coluzzii from a LLIN-protected village in Burkina Faso

Despite the effectiveness of mass distribution of long-lasting insecticidal nets (LLINs) in reducing malaria transmission in Africa, in hyperendemic areas such as Burkina Faso the burden of malaria remains high. We here report the results of a 4-month survey on the feeding habits and Plasmodium infection in malaria vectors from a village in Burkina Faso one year following a national LLIN distribution programme. Low values of human blood index (HBI) observed in the major malaria vectors in the area (Anopheles coluzzii: N = 263, 20.1%; An. arabiensis: 5.8%, N = 103) are consistent with the hypothesis that LLINs reduced the availability of human hosts to mosquitoes. A regression meta-analysis of data from a systematic review of published studies reporting HBI and sporozoite rates (SR) for An. gambiae complex revealed that the observed SR values (An. coluzzii: 7.6%, N = 503; An. arabiensis: 5.3%, N = 225) are out of the ranges expected based on the low HBI observed. We hypothesize that a small fraction of inhabitants unprotected by bednets acts as a "core group" repeatedly exposed to mosquito bites, representing the major Plasmodium reservoir for the vectors, able to maintain a high risk of transmission even in a village protected by LLINs.

Systems genetic analysis of inversion polymorphisms in the malaria mosquito Anopheles gambiae

Inversion polymorphisms in the African malaria vector Anopheles gambiae segregate along climatic gradients of aridity. Despite indirect evidence of their adaptive significance, little is known of the phenotypic targets of selection or the underlying genetic mechanisms. Here we adopt a systems genetics approach to explore the interaction of two inversions on opposite arms of chromosome 2 with gender, climatic conditions, and one another. We measure organismal traits and transcriptional profiles in 8-d-old adults of both sexes and four alternative homokaryotypic classes reared under two alternative climatic regimes. We show that karyotype strongly influences both organismal traits and transcriptional profiles but that the strength and direction of the effects depend upon complex interactions with gender and environmental conditions and between inversions on independent arms. Our data support the suppressed recombination model for the role of inversions in local adaptation, and-supported by transcriptional and physiological measurements following perturbation with the drug rapamycin-suggest that one mechanism underlying their adaptive role may be the maintenance of energy homeostasis.

Emergence of knock-down resistance in the Anopheles gambiae complex in the Upper River Region, The Gambia, and its relationship with malaria infection in children

BACKGROUND

Insecticide resistance threatens malaria control in sub-Saharan Africa. Knockdown resistance to pyrethroids and organochlorines in Anopheles gambiae sensu lato (s.l.) is commonly caused by mutations in the gene encoding a voltage-gated sodium channel which is the target site for the insecticide. The study aimed to examine risk factors for knockdown resistance in An. gambiae s.l. and its relationship with malaria infection in children in rural Gambia. Point mutations at the Vgsc-1014 locus, were measured in An. gambiae s.l. during a 2-year trial. Cross-sectional surveys were conducted at the end of the transmission season to measure malaria infection in children aged 6 months-14 years.

RESULTS

Whilst few Anopheles arabiensis and Anopheles coluzzii had Vgsc-1014 mutations, the proportion of An. gambiae sensu stricto (s.s.) mosquitoes homozygous for the Vgsc-1014F mutation increased from 64.8 to 90.9% during the study. The Vgsc-1014S or 1014F mutation was 80% higher in 2011 compared to 2010, and 27% higher in the villages with indoor residual spraying compared to those without. An increase in the proportion of An. gambiae s.l. mosquitoes with homozygous Vgsc-1014F mutations and an increase in the proportion of An. gambiae s.s. in a cluster were each associated with increased childhood malaria infection. Homozygous Vgsc-1014F mutations were, however, most common in An. gambiae s.s. and almost reached saturation during the study meaning that the two variables were colinear.

CONCLUSIONS

As a result of colinearity between homozygous Vgsc-1014F mutations and An. gambiae s.s., it was not possible to determine whether insecticide resistance or species composition increased the risk of childhood malaria infection.

Analysis of natural female post-mating responses of Anopheles gambiae and Anopheles coluzzii unravels similarities and differences in their reproductive ecology

Anopheles gambiae and An. coluzzii, the two most important malaria vectors in sub-Saharan Africa, are recently radiated sibling species that are reproductively isolated even in areas of sympatry. In females from these species, sexual transfer of male accessory gland products, including the steroid hormone 20-hydroxyecdysone (20E), induces vast behavioral, physiological, and transcriptional changes that profoundly shape their post-mating ecology, and that may have contributed to the insurgence of post-mating, prezygotic reproductive barriers. As these barriers can be detected by studying transcriptional changes induced by mating, we set out to analyze the post-mating response of An. gambiae and An. coluzzii females captured in natural mating swarms in Burkina Faso. While the molecular pathways shaping short- and long-term mating-induced changes are largely conserved in females from the two species, we unravel significant inter-specific differences that suggest divergent regulation of key reproductive processes such as egg development, processing of seminal secretion, and mating behavior, that may have played a role in reproductive isolation. Interestingly, a number of these changes occur in genes previously shown to be regulated by the sexual transfer of 20E and may be due to divergent utilization of this steroid hormone in the two species.

Modelling the persistence of mosquito vectors of malaria in Burkina Faso

Background

Populations of the Anopheles gambiae complex are found during the rainy season throughout West Africa, even in arid areas with long dry seasons during which mosquitoes appear to be absent. Several hypotheses have been proposed to explain this apparent paradox, including aestivation, dispersal between neighbouring settlements, and long distance migration using high-altitude wind currents.

Methods

An individual-based, spatially explicit model of mosquito populations was developed for a region of West Africa centred on, and including all of, Burkina Faso. Populations associated with human settlements were linked by dispersal and the model incorporated geospatial data on the distribution of settlements, water bodies and rainfall.

Results

Local dispersal (at rates consistent with experimental data) was necessary to explain observed patterns of rainy season populations across all of the simulation area, but by itself failed to account for the presence of populations in the arid North (the Sahel). The presence of rare dry-season larval sites could explain these northern populations, but seems inconsistent with field surveys. Aestivation by female mosquitoes explained rainy-season populations in all but the very sparsest and driest areas of human habitation, while long-distance migration based on annual wind patterns could account for all observed populations.

Conclusions

Modelling studies such as this can help assess the potential validity of different hypotheses and suggest priority areas for experimental study. In particular, the results highlight a shortage of empirical research on mosquito dispersal between neighbouring settlements, which may be critically important to the continued presence of many mosquito populations in West Africa. Further research that establishes the extent to which mosquitoes aestivate, and migrate using high altitude winds, is also much needed to understand Sahelian mosquito populations.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2288-3) contains supplementary material, which is available to authorized users.