Parallel evolution in mosquito vectors - a duplicated esterase locus is associated with resistance to pirimiphos-methyl in An. gambiae

The primary control methods for the African malaria mosquito, Anopheles gambiae, are based on insecticidal interventions. Emerging resistance to these compounds is therefore of major concern to malaria control programmes. The organophosphate, pirimiphos-methyl, is a relatively new chemical in the vector control armoury but is now widely used in indoor residual spray campaigns. Whilst generally effective, phenotypic resistance has developed in some areas in malaria vectors. Here, we used a population genomic approach to identify novel mechanisms of resistance to pirimiphos-methyl in Anopheles gambiae s.l mosquitoes. In multiple populations, we found large and repeated signals of selection at a locus containing a cluster of detoxification enzymes, some of whose orthologs are known to confer resistance to organophosphates in Culex pipiens. Close examination revealed a pair of alpha-esterases, Coeae1f and Coeae2f, and a complex and diverse pattern of haplotypes under selection in An. gambiae, An. coluzzii and An. arabiensis. As in Cx. pipiens, copy number variation seems to play a role in the evolution of insecticide resistance at this locus. We used diplotype clustering to examine whether these signals arise from parallel evolution or adaptive introgression. Using whole-genome sequenced phenotyped samples, we found that in West Africa, a copy number variant in Anopheles gambiae is associated with resistance to pirimiphos-methyl. Overall, we demonstrate a striking example of contemporary parallel evolution which has important implications for malaria control programmes.

Vectorial competence, insecticide resistance in Anopheles funestus and operational implications for malaria vector control strategies in Benin Republic

The primary reason for the failure of malaria vector control across endemic regions is the widespread insecticide resistance observed in Anopheles vectors. The most dominant African vectors of malaria parasites are Anopheles gambiae and Anopheles funestus mosquitoes. These species often exhibit divergent behaviours and adaptive changes underscoring the importance of deploying active and effective measures in their control. Unlike An. gambiae, An. funestus mosquitoes are poorly studied in Benin Republic. However, recent reports indicated that An. funestus can adapt and colonize various ecological niches owing to its resistance against insecticides and adaptation to changing breeding habitats. Unfortunately, scientific investigations on the contribution of An. funestus to malaria transmission, their susceptibility to insecticide and resistance mechanism developed are currently insufficient for the design of better control strategies. In an attempt to gather valuable information on An. funestus, the present review examines the progress made on this malaria vector species in Benin Republic and highlights future research perspectives on insecticide resistance profiles and related mechanisms, as well as new potential control strategies against An. funestus. Literature analysis revealed that An. funestus is distributed all over the country, although present in low density compared to other dominant malaria vectors. Interestingly, An. funestus is being found in abundance during the dry seasons, suggesting an adaptation to desiccation. Among the An. funestus group, only An. funestus sensu stricto (s.s.) and Anopheles leesoni were found in the country with An. funestus s.s. being the most abundant species. Furthermore, An. funestus s.s. is the only one species in the group contributing to malaria transmission and have adapted biting times that allow them to bite at dawn. In addition, across the country, An. funestus were found resistant to pyrethroid insecticides used for bed nets impregnation and also resistant to bendiocarb which is currently being introduced in indoor residual spraying formulation in malaria endemic regions. All these findings highlight the challenges faced in controlling this malaria vector. Therefore, advancing the knowledge of vectorial competence of An. funestus, understanding the dynamics of insecticide resistance in this malaria vector, and exploring alternative vector control measures, are critical for sustainable malaria control efforts in Benin Republic.

Genome-wide association studies reveal novel loci associated with pyrethroid and organophosphate resistance in Anopheles gambiae and Anopheles coluzzii

Resistance to insecticides in Anopheles mosquitoes threatens the effectiveness of malaria control, but the genetics of resistance are only partially understood. We performed a large scale multi-country genome-wide association study of resistance to two widely used insecticides: deltamethrin and pirimiphos-methyl, using sequencing data from An. gambiae and An. coluzzii from ten locations in West Africa. Resistance was highly multi-genic, multi-allelic and variable between populations. While the strongest and most consistent association with deltamethrin resistance came from Cyp6aa1, this was based on several independent copy number variants (CNVs) in An. coluzzii, and on a non-CNV haplotype in An. gambiae. For pirimiphos-methyl, signals included Ace1, cytochrome P450s, glutathione S-transferases and the nAChR target site of neonicotinoid insecticides. The regions around Cyp9k1 and the Tep family of immune genes showed evidence of cross-resistance to both insecticides. These locally-varying, multi-allelic patterns highlight the challenges involved in genomic monitoring of resistance, and may form the basis for improved surveillance methods.

Genome-wide association studies reveal novel loci associated with pyrethroid and organophosphate resistance in Anopheles gambiae s.l

Resistance to insecticides in Anopheles mosquitoes threatens the effectiveness of the most widespread tools currently used to control malaria. The genetic underpinnings of resistance are still only partially understood, with much of the variance in resistance phenotype left unexplained. We performed a multi-country large scale genome-wide association study of resistance to two insecticides widely used in malaria control: deltamethrin and pirimiphos-methyl. Using a bioassay methodology designed to maximise the phenotypic difference between resistant and susceptible samples, we sequenced 969 phenotyped female An. gambiae and An. coluzzii from ten locations across four countries in West Africa (Benin, Côte d'Ivoire, Ghana and Togo), identifying single nucleotide polymorphisms (SNPs) and copy number variants (CNVs) segregating in the populations. The patterns of resistance association were highly multiallelic and variable between populations, with different genomic regions contributing to resistance, as well as different mutations within a given region. While the strongest and most consistent association with deltamethrin resistance came from the region around Cyp6aa1 , this resistance was based on a combination of several independent CNVs in An. coluzzii , and on a non-CNV bearing haplotype in An. gambiae . Further signals involved a range of cytochrome P450, mitochondrial, and immunity genes. Similarly, for pirimiphos-methyl, while the strongest signal came from the region of Ace1 , more widespread signals included cytochrome P450s, glutathione S-transferases, and a subunit of the nAChR target site of neonicotinoid insecticides. The regions around Cyp9k1 and the Tep family of immune genes were associated with resistance to both insecticide classes, suggesting possible cross-resistance mechanisms. These locally-varying, multigenic and multiallelic patterns highlight the challenges involved in genomic monitoring and surveillance of resistance, and form the basis for improvement of methods used to detect and predict resistance. Based on simulations of resistance variants, we recommend that yet larger scale studies, exceeding 500 phenotyped samples per population, are required to better identify associated genomic regions.

Organophosphate Insecticide Exposure Impacts Reproductive Success in Insensitive Acetylcholinesterase Anopheles gambiae Mosquitoes

Extensive use of insecticides has led to the selection of resistance alleles in malaria vectors threatening the control programs. Even if mosquitoes are not killed directly in the contact of insecticide-treated bed nets, their capacity to transmit malaria parasite could be decreased because of the consequences on their life-history traits after repeated exposure. The current work investigated the effects of organochlorine, carbamate, organophosphate, and pyrethroid insecticide exposure on the reproductive success in Anopheles gambiae s.s. Two Anopheles gambiae strains, AcerKis, KisKdr, were used. According to WHO recommendations, female mosquitoes of these resistant strains were exposed to discriminant doses of DDT, chlorpyriphos-methyl, bendiocarb, and permethrin insecticides. Surviving mosquitoes were then fed and allowed to lay eggs. Fecundity was assessed by examining the number of eggs per mosquito, the number of larvae per egg batch and larval hatching rates were used to evaluate the fertility. The data showed that AcerKis females surviving chlorpyriphos-methyl exposure significantly laid few eggs. No significant difference in the hatching rate was noticed in AcerKis females exposed to bendiocarb compared to their control. No significant effect on the fecundity and fertility was observed in KisKdr females exposed to permethrin. Our finding showed that organophosphate insecticides represented here by chlorpyriphos-methyl could hamper egg-laying in insensitive acetylcholinesterase An. gambiae female mosquitoes. This knowledge could help design alternative vector control strategies targeting fecundity and fertility in resistant malaria vectors.

Malaria-Transmitting Vectors Microbiota: Overview and Interactions With Anopheles Mosquito Biology

Malaria remains a vector-borne infectious disease that is still a major public health concern worldwide, especially in tropical regions. Malaria is caused by a protozoan parasite of the genus Plasmodium and transmitted through the bite of infected female Anopheles mosquitoes. The control interventions targeting mosquito vectors have achieved significant success during the last two decades and rely mainly on the use of chemical insecticides through the insecticide-treated nets (ITNs) and indoor residual spraying (IRS). Unfortunately, resistance to conventional insecticides currently being used in public health is spreading in the natural mosquito populations, hampering the long-term success of the current vector control strategies. Thus, to achieve the goal of malaria elimination, it appears necessary to improve vector control approaches through the development of novel environment-friendly tools. Mosquito microbiota has by now given rise to the expansion of innovative control tools, such as the use of endosymbionts to target insect vectors, known as "symbiotic control." In this review, we will present the viral, fungal and bacterial diversity of Anopheles mosquitoes, including the bacteriophages. This review discusses the likely interactions between the vector microbiota and its fitness and resistance to insecticides.

Putative pleiotropic effects of the knockdown resistance (L1014F) allele on the life-history traits of Anopheles gambiae

Background

Existing mechanisms of insecticide resistance are known to help the survival of mosquitoes following contact with chemical compounds, even though they could negatively affect the life-history traits of resistant malaria vectors. In West Africa, the knockdown resistance mechanism kdr^R (L1014F) is the most common. However, little knowledge is available on its effects on mosquito life-history traits. The fitness effects associated with this knockdown resistance allele in Anopheles gambiae sensu stricto (s.s.) were investigated in an insecticide-free laboratory environment.

Methods

The life-history traits of Kisumu (susceptible) and KisKdr (kdr resistant) strains of An. gambiae s.s. were compared. Larval survivorship and pupation rate were assessed as well as fecundity and fertility of adult females. Female mosquitoes of both strains were directly blood fed through artificial membrane assays and then the blood-feeding success, blood volume and adult survivorship post-blood meal were assessed.

Results

The An. gambiae mosquitoes carrying the kdr^R allele (KisKdr) laid a reduced number of eggs. The mean number of larvae in the susceptible strain Kisumu was three-fold overall higher than that seen in the KisKdr strain with a significant difference in hatching rates (81.89% in Kisumu vs 72.89% in KisKdr). The KisKdr larvae had a significant higher survivorship than that of Kisumu. The blood-feeding success was significantly higher in the resistant mosquitoes (84%) compared to the susceptible ones (34.75%). However, the mean blood volume was 1.36 µL/mg, 1.45 µL/mg and 1.68 µL/mg in Kisumu, homozygote and heterozygote KisKdr mosquitoes, respectively. After blood-feeding, the heterozygote KisKdr mosquitoes displayed highest survivorship when compared to that of Kisumu.

Conclusions

The presence of the knockdown resistance allele appears to impact the life-history traits, such as fecundity, fertility, larval survivorship, and blood-feeding behaviour in An. gambiae. These data could help to guide the implementation of more reliable strategies for the control of malaria vectors.

Interplay Between Oxytetracycline and the Homozygote kdr (L1014F) Resistance Genotype on Fecundity in Anopheles gambiae (Diptera: Culicidae) Mosquitoes

The insecticide resistance in Anopheles gambiae mosquitoes has remained the major threat for vector control programs but the fitness effects conferred by these mechanisms are poorly understood. To fill this knowledge gap, the present study aimed at testing the hypothesis that antibiotic oxytetracycline could have an interaction with insecticide resistance genotypes and consequently inhibit the fecundity in An. gambiae. Four strains of An. gambiae: Kisumu (susceptible), KisKdr (kdr (L1014F) resistant), AcerKis (ace-1 (G119S) resistant) and AcerKdrKis (both kdr (L1014F) and ace-1 (G119S) resistant) were used in this study. The different strains were allowed to bloodfeed on a rabbit previously treated with antibiotic oxytetracycline at a concentration of 39·10-5 M. Three days later, ovarian follicles were dissected from individual mosquito ovaries into physiological saline solution (0.9% NaCl) under a stereomicroscope and the eggs were counted. Fecundity was substantially lower in oxytetracycline-exposed KisKdr females when compared to that of the untreated individuals and oxytetracycline-exposed Kisumu females. The exposed AcerKis females displayed an increased fecundity compared to their nontreated counterparts whereas they had reduced fecundity compared to that of oxytetracycline-exposed Kisumu females. There was no substantial difference between the fecundity in the treated and untreated AcerKdrKis females. The oxytetracycline-exposed AcerKdrKis mosquitoes had an increased fecundity compared to that of the exposed Kisumu females. Our data indicate an indirect effect of oxytetracycline in reducing fecundity of An. gambiae mosquitoes carrying kdrR (L1014F) genotype. These findings could be useful for designing new integrated approaches for malaria vector control in endemic countries.

Phenotypic Insecticide Resistance in Anopheles gambiae (Diptera: Culicidae): Specific Characterization of Underlying Resistance Mechanisms Still Matters

An effective control of malaria vectors requires an extensive knowledge of mechanisms underlying the resistance-phenotypes developed by these vectors against insecticides. We investigated Anopheles gambiae mosquitoes from Benin and Togo for their intensity of insecticide resistance and we discussed the involvement of genotyped mechanisms in the resistance-phenotypes observed. Three- to five-day-old adult mosquitoes emerged from field and laboratory An. gambiae larvae were assayed using WHO tube intensity tests against various doses of deltamethrin: 1× (0.05%); 2× (0.1%); 5× (0.25%); 7.5× (0.375%) and those of pirimiphos-methyl: 0.5× (0.125%); 1× (0.25%). Members of An. gambiae complex were screened in field populations using polymerase chain reaction (PCR) assays. The presence of kdrR(1014F/1014S) and ace-1R(119S) mutations was also investigated using TaqMan and PCR-RFLP techniques, respectively. Anopheles gambiae from field were very resistant to deltamethrin, whereas KisKdr and AcerKdrKis strains displayed 100% mortality rates at 2× the diagnostic dose. In contrast, the field mosquitoes displayed a low resistance-intensity against 1× the diagnostic dose of pirimiphos-methyl, whereas AcerKis and AcerKdrKis strains showed susceptibility at 0.5× the diagnostic dose. Anopheles gambiae s.s., Anopheles coluzzii, and Anopheles arabiensis were identified. Allelic frequencies of kdrR (1014F) and ace-1R (119S) mutations in the field populations varied from 0.65 to 1 and 0 to 0.84, respectively. The field An. gambiae displayed high-resistance levels against deltamethrin and pirimiphos-methyl when compared with those of the laboratory An. gambiae-resistant strains. These results exhibit the complexity of underlying insecticide resistance mechanisms in these field malaria vectors.