Population genetic structure of the malaria mosquito Anopheles arabiensis across Nigeria suggests range expansion

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.

Isolation and Characterization of Polymorphic Microsatellite Markers from the Malaria Vector Anopheles fluviatilis Species T (Diptera: Culicidae)

Anopheles fluviatilis James is an important malaria vector in India, Pakistan, Nepal, and Iran. It has now been recognized as a complex of at least four sibling species-S, T, U, and V, among which species T is the most widely distributed species throughout India. The taxonomic status of these species is confusing owing to controversies prevailing in the literature. In addition, chromosomal inversion genotypes, which were considered species-diagnostic for An. fluviatilis species T, are unreliable due to the existence of polymorphism in some populations. To study the genetic diversity at population level, we isolated and characterized 20 microsatellite markers from microsatellite-enriched genomic DNA library of An. fluviatilis T, of which 18 were polymorphic while two were monomorphic. The number of alleles per locus among polymorphic markers ranged from 4 to 19, and values for observed and expected heterozygosities varied from 0.352 to 0.857 and from 0.575 to 0.933, respectively. Thirteen markers had cross-cryptic species transferability to species S and U of the Fluviatilis Complex. This study provides a promising genetic tool for the population genetic analyses of An. fluviatilis.

A genotypically distinct, melanic variant of Anopheles arabiensis in Sudan is associated with arid environments

BACKGROUND

Anopheles arabiensis, an important malaria vector in Sudan and other countries in sub-Saharan Africa, exhibits considerable ecological and behavioural plasticity allowing it to survive in the harsh conditions of arid regions. It has been shown that adult populations of An. arabiensis in the semi-desert habitat of western Khartoum State survive through the long dry season in a state of partial aestivation, characterized by limited feeding activity and a degree of arrested ovarian development. Anopheles arabiensis in these sites occurs in two phenotypic forms. One is large and heavily melanized, the other has the typical characteristics of An. arabiensis as found elsewhere in Africa. The extent of genetic variation in these forms was examined in widely separated locations in Sudan, including Kassala, Gedaref and the Northern States between 1998 and 1999 and 2004 and 2006.

METHODS

Each mosquito specimen was identified using standard morphological keys and a species-specific PCR test. Sequence variation in a 660 bp fragment of the mtDNA ND5 coding region was examined and the extent of genetic divergence between the forms was estimated from FST values using DNASP version 4.9. TCS 1.13 software was used to determine the genealogical relationships and to reflect clustering among mtDNA haplotypes.

RESULTS

The melanic and normal forms were found in sympatry in Kassala, Gedaref and Khartoum states, with the melanic form commonest in the hottest and most arid areas. Both forms were encountered in the periods of study: 1998-1999, and 2004-2006. Only ten specimens of An. arabiensis were collected from the Northern State in February 2006, all of which were of the normal form.Based on the ND5 analysis, there was a marked subdivision between the normal and melanic forms (FST = 0.59). Furthermore, the melanic form showed more genetic variability, as measured by haplotype diversity (0.95) compared with the normal form (0.57), suggesting larger effective population.

CONCLUSIONS

This is the first demonstration of correspondent phenotypic and genetic structuring in An. arabiensis. The high level of genetic differentiation shown by the mtDNA ND5 locus suggests that the two forms may represent separate species. It is hypothesized that the melanic form is better adapted to hot and arid environments.

Transmission patterns of Plasmodium falciparum by Anopheles gambiae in Benin

BACKGROUND

To better control malaria, the clear and urgent need is for improved data to inform decision makers, but in several African countries, there is a lack of baseline data on vectors and variation in the intensity of malaria transmission. This has resulted in the implementation of vector control efforts that ignore variation in vector behaviour and intensity of transmission, an approach that is most often not cost-effective. This study presents a detailed entomological description of mosquito distribution and variation in potentially transmissible contacts of Plasmodium falciparum following a south to north transect in Benin.

METHOD

The study was conducted in five locations where environmental parameters were different and malaria prevalence ranged between 14 and 51%. The locations represent the main eco-epidemiological malaria areas in Benin. Mosquitoes were collected using human landing catches, pyrethrum spray catches and windows traps. They were taxonomically and molecularly identified. Head-thoraces of Anopheles gambiae s.l. were tested by enzyme-linked immunosorbent assay. Entomological indicators were estimated following WHO guidelines.

RESULTS

The results showed variation between location and period in distribution of Anopheles coluzzii, An. gambiae, and Anopheles arabiensis (p < 0.05). An extension of the reported range of An. arabiensis was also observed. Densities of malaria vectors varied significantly between rural and urban sites, however, indoor/outdoor biting ratios remained constant. Proportions of malaria vectors with circumsporozoite protein of P. falciparum were similar between locations. The entomological inoculation rates ranged between zero and eight bites/man/night with significant variations between areas.Four profiles of human exposure to infectious malaria vector bites were observed and included location with one season of high transmission (June - August), two seasons of lower transmission (March-August; October-November), moderate continuous transmission season, and high continuous transmission season of P. falciparum.

CONCLUSION

The study revealed several entomological patterns in transmission of P. falciparum in Benin. The data could be used for purposes of planning a more cost-effective vector control strategy, by stratifying the country into higher and lower transmission zones. The information could also be used to guide extension of indoor residual spray based on a targeted use of IRS at sites where the duration of insecticidal effect following spraying coincides with the peak transmission period.

Islands and stepping-stones: comparative population structure of Anopheles gambiae sensu stricto and Anopheles arabiensis in Tanzania and implications for the spread of insecticide resistance

Population genetic structures of the two major malaria vectors Anopheles gambiae s.s. and An. arabiensis, differ markedly across Sub-Saharan Africa, which could reflect differences in historical demographies or in contemporary gene flow. Elucidation of the degree and cause of population structure is important for predicting the spread of genetic traits such as insecticide resistance genes or artificially engineered genes. Here the population genetics of An. gambiae s.s. and An. arabiensis in the central, eastern and island regions of Tanzania were compared. Microsatellite markers were screened in 33 collections of female An. gambiae s.l., originating from 22 geographical locations, four of which were sampled in two or three years between 2008 and 2010. An. gambiae were sampled from six sites, An. arabiensis from 14 sites, and both species from two sites, with an additional colonised insectary sample of each species. Frequencies of the knock-down resistance (kdr) alleles 1014S and 1014F were also determined. An. gambiae exhibited relatively high genetic differentiation (average pairwise FST = 0.131), significant even between nearby samples, but without clear geographical patterning. In contrast, An. arabiensis exhibited limited differentiation (average FST = 0.015), but strong isolation-by-distance (Mantel test r = 0.46, p = 0.0008). Most time-series samples of An. arabiensis were homogeneous, suggesting general temporal stability of the genetic structure. An. gambiae populations from Dar es Salaam and Bagamoyo were found to have high frequencies of kdr 1014S (around 70%), with almost 50% homozygote but was at much lower frequency on Unguja Island, with no. An. gambiae population genetic differentiation was consistent with an island model of genetic structuring with highly restricted gene flow, contrary to An. arabiensis which was consistent with a stepping-stone model of extensive, but geographically-restricted gene flow.

Limited usefulness of microsatellite markers from the malaria vector Anopheles gambiae when applied to the closely related species Anopheles melas

Anopheles melas is a brackish water mosquito found in coastal West Africa where it is a dominant malaria vector locally. In order to facilitate genetic studies of this species, 45 microsatellite loci originally developed for Anopheles gambiae were sequenced in An. melas. Those that were suitable based on repeat number and flanking regions were examined in 2 natural populations from Equatorial Guinea. Only 15 loci were eventually deemed suitable as polymorphic markers in An. melas populations. These loci were screened in 4 populations from a wider geographic range. Heterozygosity estimates ranged from 0.18 to 0.79, and 2.5-15 average alleles were observed per locus, yielding 13 highly polymorphic markers and 2 loci with lower variability. To examine the usefulness of microsatellite markers when applied in a sibling species, the original An. gambiae specific markers were used to amplify 5 loci in An. melas. Null alleles were found for 1 An. gambiae marker. We discuss the pitfalls of using microsatellite loci across closely related species and conclude that in addition to the problem of null alleles associated with this practice, many loci may prove to be of very limited use as polymorphic markers even when used in a sibling species.

Single-nucleotide polymorphisms for high-throughput genotyping of Anopheles arabiensis in East and southern Africa

Anopheles arabiensis Patton is one of the principal vectors of malaria in sub-Saharan Africa, occupying a wide variety of ecological zones. This species is increasingly responsible for malaria transmission in Africa and is becoming the dominant vector species in some localities. Despite its growing importance, little is known about genetic polymorphisms in this species. Multiple sequences of various gene fragments from An. arabiensis isolates from Cameroon were obtained from GenBank. In total, 20 gene fragments containing single-nucleotide polymorphisms (SNPs) at moderate density were selected for direct sequencing from field collected specimens from Tanzania and Zambia. We obtained 301 SNPs in total from the 20 gene fragments, 60 of which were suitable for Illumina GoldenGate SNP genotyping. A greater number of SNPs (n = 185) was suitable for analysis using Sequenom iPLEX, an alternative high-throughput genotyping technology using mass spectrometry. An SNP was present every 59 (+/- 44.5) bases on average. Overall, An. arabiensis from Tanzania and Zambia are genetically closer (mean F(ST) = 0.075) than either is to populations in Cameroon (F(ST, TZ-CM) = 0.250, F(ST,ZA-CM) = 0.372). A fixed polymorphism between East/southern and Central Africa was identified on AGAP000574, a gene on the X chromosome. We have identified SNPs in natural populations of An. arabiensis. SNP densities in An. arabiensis were higher than Anopheles gambiae s.s., suggesting a greater challenge in the development of high-throughput SNP analysis for this species. The SNP markers provided in this study are suitable for a high-throughput genotyping analysis and can be used for population genetic studies and association mapping efforts.

Nigeria Anopheles vector database: an overview of 100 years' research

Anopheles mosquitoes are important vectors of malaria and lymphatic filariasis (LF), which are major public health diseases in Nigeria. Malaria is caused by infection with a protozoan parasite of the genus Plasmodium and LF by the parasitic worm Wuchereria bancrofti. Updating our knowledge of the Anopheles species is vital in planning and implementing evidence based vector control programs. To present a comprehensive report on the spatial distribution and composition of these vectors, all published data available were collated into a database. Details recorded for each source were the locality, latitude/longitude, time/period of study, species, abundance, sampling/collection methods, morphological and molecular species identification methods, insecticide resistance status, including evidence of the kdr allele, and P. falciparum sporozoite rate and W. bancrofti microfilaria prevalence. This collation resulted in a total of 110 publications, encompassing 484,747 Anopheles mosquitoes in 632 spatially unique descriptions at 142 georeferenced locations being identified across Nigeria from 1900 to 2010. Overall, the highest number of vector species reported included An. gambiae complex (65.2%), An. funestus complex (17.3%), An. gambiae s.s. (6.5%). An. arabiensis (5.0%) and An. funestus s.s. (2.5%), with the molecular forms An. gambiae M and S identified at 120 locations. A variety of sampling/collection and species identification methods were used with an increase in molecular techniques in recent decades. Insecticide resistance to pyrethroids and organochlorines was found in the main Anopheles species across 45 locations. Presence of P. falciparum and W. bancrofti varied between species with the highest sporozoite rates found in An. gambiae s.s, An. funestus s.s. and An. moucheti, and the highest microfilaria prevalence in An. gambiae s.l., An. arabiensis, and An. gambiae s.s. This comprehensive geo-referenced database provides an essential baseline on Anopheles vectors and will be an important resource for malaria and LF vector control programmes in Nigeria.

Molecular characterization and evolution of a gene family encoding male-specific reproductive proteins in the African malaria vector Anopheles gambiae

Background

During copulation, the major Afro-tropical malaria vector Anopheles gambiae s.s. transfers male accessory gland (MAG) proteins to females as a solid mass (i.e. the "mating plug"). These proteins are postulated to function as important modulators of female post-mating responses. To understand the role of selective forces underlying the evolution of these proteins in the A. gambiae complex, we carried out an evolutionary analysis of gene sequence and expression divergence on a pair of paralog genes called AgAcp34A-1 and AgAcp34A-2. These encode MAG-specific proteins which, based on homology with Drosophila, have been hypothesized to play a role in sperm viability and function.

Results

Genetic analysis of 6 species of the A. gambiae complex revealed the existence of a third paralog (68-78% of identity), that we named AgAcp34A-3. FISH assays showed that this gene maps in the same division (34A) of chromosome-3R as the other two paralogs. In particular, immuno-fluorescence assays targeting the C-terminals of AgAcp34A-2 and AgAcp34A-3 revealed that these two proteins are localized in the posterior part of the MAG and concentrated at the apical portion of the mating plug. When transferred to females, this part of the plug lies in proximity to the duct connecting the spermatheca to the uterus, suggesting a potential role for these proteins in regulating sperm motility. AgAcp34A-3 is more polymorphic than the other two paralogs, possibly because of relaxation of purifying selection. Since both unequal crossing-over and gene conversion likely homogenized the members of this gene family, the interpretation of the evolutionary patterns is not straightforward. Although several haplotypes of the three paralogs are shared by most A. gambiae s.l. species, some fixed species-specific replacements (mainly placed in the N- and C-terminal portions of the secreted peptides) were also observed, suggesting some lineage-specific adaptation.

Conclusions

Progress in understanding the signaling cascade in the A. gambiae reproductive pathway will elucidate the interaction of this MAG-specific protein family with their female counterparts. This knowledge will allow a better evaluation of the relative importance of genes involved in the reproductive isolation and fertility of A. gambiae species and could help the interpretation of the observed evolutionary patterns.

Molecular evolution of a gene cluster of serine proteases expressed in the Anopheles gambiae female reproductive tract

Background

Genes involved in post-mating processes of multiple mating organisms are known to evolve rapidly due to coevolution driven by sexual conflict among male-female interacting proteins. In the malaria mosquito Anopheles gambiae - a monandrous species in which sexual conflict is expected to be absent or minimal - recent data strongly suggest that proteolytic enzymes specifically expressed in the female lower reproductive tissues are involved in the processing of male products transferred to females during mating. In order to better understand the role of selective forces underlying the evolution of proteins involved in post-mating responses, we analysed a cluster of genes encoding for three serine proteases that are down-regulated after mating, two of which specifically expressed in the atrium and one in the spermatheca of A. gambiae females.

Results

The analysis of polymorphisms and divergence of these female-expressed proteases in closely related species of the A. gambiae complex revealed a high level of replacement polymorphisms consistent with relaxed evolutionary constraints of duplicated genes, allowing to rapidly fix novel replacements to perform new or more specific functions. Adaptive evolution was detected in several codons of the 3 genes and hints of episodic selection were also found. In addition, the structural modelling of these proteases highlighted some important differences in their substrate specificity, and provided evidence that a number of sites evolving under selective pressures lie relatively close to the catalytic triad and/or on the edge of the specificity pocket, known to be involved in substrate recognition or binding. The observed patterns suggest that these proteases may interact with factors transferred by males during mating (e.g. substrates, inhibitors or pathogens) and that they may have differently evolved in independent A. gambiae lineages.

Conclusions

Our results - also examined in light of constraints in the application of selection-inference methods to the closely related species of the A. gambiae complex - reveal an unexpectedly intricate evolutionary scenario. Further experimental analyses are needed to investigate the biological functions of these genes in order to better interpret their molecular evolution and to assess whether they represent possible targets for limiting the fertility of Anopheles mosquitoes in malaria vector control strategies.

Malaria vectors in the Republic of Benin: distribution of species and molecular forms of the Anopheles gambiae complex

Members of the Anopheles gambiae complex are among the best malaria vectors in the world, but their vectorial capacities vary between species and populations. A large-scale sampling of An. gambiae sensu lato was carried out in 2006 and 2007 in various bioclimatic areas of Benin (West Africa). The objective of this study was to collate data on the relative frequencies of species and forms within the An. gambiae complex and to produce a map of their spatial distribution. Sampling took place at 30 sites and 2122 females were analyzed. Two species were identified through molecular methods. The overall collection showed a preponderance of An. gambiae s.s., but unexpectedly, An. arabiensis was reported in the coastal-Guinean bioclimatic area characterized by a mean annual rainfall of >1500 mm where only An. gambiae s.s. was reported previously. Our study of Benin indicates that An. arabiensis would be adapted not only to the urban areas but also to the rural humid regions. Among 1717 An. gambiae s.s., 26.5% were of the M form and 73.3% were S form. Few hybrid specimens between the M and S forms were observed (0.2%). Only the spatial distribution of the M form appears to be mainly a function of bioclimatic area. Factors that influence the distribution of these malaria vectors are discussed. This study underlines the need of further investigations of biological, ecological, and behavioral traits of these species and forms to better appreciate their vectorial capacities. Acquisition of entomological field data appears essential to better estimate the stratification of malaria risk and help improve malaria vector control interventions.

Population genetic structure of Anopheles gambiae and Anopheles arabiensis in Niger

The increasing usage of long-lasting insecticide-treated nets allows protection of millions of people from malaria infection. Monitoring studies should be planned during any wide-scale malaria control program integrating insecticide-treated materials, to evaluate their effects and effectiveness on epidemiologically relevant parameters. Such operational control interventions may be challenged by insecticide resistance spread within vector populations, as a result of wide insecticide pressure. A nationwide distribution of long-lasting insecticidal nets was implemented throughout Niger in 2005. We studied the population genetic structure of major malaria vectors across Nigerien Sahel, and investigated potential effects of this large malaria control intervention. Wild-caught Anopheles gambiae sensu lato females from seven villages and two wet seasons were genotyped at 12 microsatellite loci. The genetic diversity within both species appeared homogenous between villages and years. The estimated genetic differentiation among samples was very low within both species, indicating high gene flow across the area. An absence of differentiation was also found between 2005 and 2006 wet seasons, for all samples but one, showing that the net distribution did not impact significantly the genetic diversity and structure of vector populations in a single year. We provide valuable results participating to document effects of large malaria control programs, to maximize the efficiency of available tools in future interventions.

Evidence for late Pleistocene population expansion of the malarial mosquitoes, Anopheles arabiensis and Anopheles gambiae in Nigeria

Anopheles gambiae Giles s.s. and Anopheles arabiensis Patton (Diptera: Culicidae) are major vectors of malaria in Nigeria. We used 1115 bp of the mitochondrial COI gene to assess their population genetic structures based on samples from across Nigeria (n = 199). The mtDNA neighbour-joining tree, based on F(ST) estimates, separated An. gambiae M and S forms, except that samples of An. gambiae M from Calabar clustered with all the An. gambiae S form. Anopheles arabiensis and An. gambiae could be combined into a single star-shaped, parsimonious haplotype network, and shared three haplotypes. Haplotype diversity values were high in An. arabiensis and An. gambiae S, and intermediate in An. gambiae M; all nucleotide diversities were relatively low. Taken together, patterns of haplotype diversity, the star-like genealogy of haplotypes, five of seven significant neutrality tests, and the violation of the isolation-by-distance model indicate population expansion in An. arabiensis and An. gambiae S, but the signal was weak in An. gambiae M. Selection is supported as an important factor shaping genetic structure in An. gambiae in Nigeria. There were two geographical subdivisions in An. arabiensis: one included all southern localities and all but two central localities; the other included all northern and two central localities. Re-analysing an earlier microsatellite dataset of An. arabiensis using a Bayesian method determined that there were two distinctive clusters, northern and southern, that were fairly congruent with the mtDNA subdivisions. There was a trend towards decreasing genetic diversity in An. arabiensis from the northern savannah to the southern rainforest that corroborated previous data from microsatellites and polytene chromosomes.

Patterns of selection in anti-malarial immune genes in malaria vectors: evidence for adaptive evolution in LRIM1 in Anopheles arabiensis

Background

Co-evolution between Plasmodium species and its vectors may result in adaptive changes in genes that are crucial components of the vector's defense against the pathogen. By analyzing which genes show evidence of positive selection in malaria vectors, but not in closely related non-vectors, we can identify genes that are crucial for the mosquito's resistance against Plasmodium.

Methodology/Principle Findings

We investigated genetic variation of three anti-malarial genes; CEC1, GNBP-B1 and LRIM1, in both vector and non-vector species of the Anopheles gambiae complex. Whereas little protein differentiation was observed between species in CEC1 and GNBP-B1, McDonald-Kreitman and maximum likelihood tests of positive selection show that LRIM1 underwent adaptive evolution in a primary malaria vector; An. arabiensis. In particular, two adjacent codons show clear signs of adaptation by having accumulated three out of four replacement substitutions. Furthermore, our data indicate that this LRIM1 allele has introgressed from An. arabiensis into the other main malaria vector An. gambiae.

Conclusions/Significance

Although no evidence exists to link the adaptation of LRIM1 to P. falciparum infection, an adaptive response of a known anti-malarial gene in a primary malaria vector is intriguing, and may suggest that this gene could play a role in Plasmodium resistance in An. arabiensis. If so, our data also predicts that LRIM1 alleles in An. gambiae vary in their level of resistance against P. falciparum.

Feeding and resting behaviour of malaria vector, Anopheles arabiensis with reference to zooprophylaxis

Background

The most important factor for effective zooprophylaxis in reducing malaria transmission is a predominant population of a strongly zoophilic mosquito, Anopheles arabiensis. The feeding preference behaviour of Anopheline mosquitoes was evaluated in odour-baited entry trap (OBET).

Methods

Mosquitoes were captured daily using odour-baited entry traps, light traps and hand catch both indoor and in pit traps. Experimental huts were used for release and recapture experiment. The mosquitoes collected were compared in species abundances.

Results

Anopheles arabiensis was found to account for over 99% of Anopheles species collected in the study area in Lower Moshi, Northern Tanzania. In experimental release/capture trials conducted at the Mabogini verandah huts, An. arabiensis was found to have higher exophilic tendency (80.7%) compared to Anopheles gambiae (59.7%) and Culex spp. (60.8%). OBET experiments conducted at Mabogini collected a total of 506 An. arabiensis in four different trials involving human, cattle, sheep, goat and pig. Odours from the cattle attracted 90.3% (243) compared to odours from human, which attracted 9.7% (26) with a significant difference at P = 0.005. Odours from sheep, goat and pig attracted 9.7%, 7.2% and 7.3%, respectively. Estimation of HBI in An. arabiensis collected from houses in three lower Moshi villages indicated lower ratios for mosquitoes collected from houses with cattle compared to those without cattles. HBI was also lower in mosquitoes collected outdoors (0.1–0.3) compared to indoor (0.4–0.9).

Conclusion

In discussing the results, reference has been made to observation of exophilic, zoophilic and feeding tendencies of An. arabiensis, which are conducive for zooprophylaxis. It is recommended that in areas with a predominant An. arabiensis population, cattle should be placed close to dwelling houses in order to maximize the effects of zooprophylaxis. Protective effects of human from malaria can further be enhanced by keeping cattle in surroundings of residences.

Northwestern song sparrow populations show genetic effects of sequential colonization

Two genetic consequences are often considered evidence of a founder effect: substantial loss in genetic diversity and rapid divergence between source and founder populations. Single-step founder events have been studied for these effects, but with mixed results, causing continued controversy over the role of founder events in divergence. Experiments of serial bottlenecks have shown losses of diversity, increased divergence, and rapid behavioural changes possibly leading to reproductive isolation between source and final populations. The few studies conducted on natural, sequentially founded systems show some evidence of these effects. We examined a natural vertebrate system of sequential colonization among northwestern song sparrows (Melospiza melodia). This system has an effectively linear distribution, it was probably colonized within the last 10,000 years, there are morphological and behavioural differences among populations, and the westernmost populations occur in atypical habitats for the species. Eight microsatellite loci from eight populations in Alaska and British Columbia (n = 205) showed stepwise loss of genetic diversity, genetic evidence for strong population bottlenecks, and increased population divergence. The endpoint population on Attu Island has extremely low diversity (H(E) = 0.18). Our study shows that sequential bottlenecks or founder events can have powerful genetic effects in reducing diversity, possibly leading to rapid evolutionary divergence.

Identification and expression of odorant-binding proteins of the malaria-carrying mosquitoes Anopheles gambiae and Anopheles arabiensis

Host preference and blood feeding are restricted to female mosquitoes. Olfaction plays a major role in host-seeking behaviour, which is likely to be associated with a subset of mosquito olfactory genes. Proteins involved in olfaction include the odorant receptors (ORs) and the odorant-binding proteins (OBPs). OBPs are thought to function as a carrier within insect antennae for transporting odours to the olfactory receptors. Here we report the annotation of 32 genes encoding putative OBPs in the malaria mosquito Anopheles gambiae and their tissue-specific expression in two mosquito species of the Anopheles complex; a highly anthropophilic species An. gambiae sensu stricto and an opportunistic, but more zoophilic species, An. arabiensis. RT-PCR shows that some of the genes are expressed mainly in head tissue and a subset of these show highest expression in female heads. One of the genes (agCP1588) which has not been identified as an OBP, has a high similarity (40%) to the Drosophila pheromone-binding protein 4 (PBPRP4) and is only expressed in heads of both An. gambiae and An. arabiensis, and at higher levels in female heads. Two genes (agCP3071 and agCP15554) are expressed only in female heads and agC15554 also shows higher expression levels in An. gambiae. The expression profiles of the genes in the two members of the Anopheles complex provides the first step towards further molecular analysis of the mosquito olfactory apparatus.

The role of environment in shaping the genetic diversity of the subalpine mosquito, Aedes rusticus (Diptera, Culicidae)

The relative involvement of larval dietary tolerance to the leaf-litter toxic polyphenols in shaping population genetic structure of the subalpine mosquito Aedes rusticus was examined. This was compared with other parameters such as geographical range, type of vegetation surrounding the breeding site, and occurrence of annual larvicidal treatments. Population genetic structure was analysed at 10 presumed neutral polymorphic isoenzyme loci. Toxicological comparisons involved standard bioassays performed on larvae fed on toxic decomposed leaf litter. Significant overall genetic differentiation was observed among the 22 studied populations and within the five defined geographical groups. Analysis of molecular variance revealed an absence of relation between genetic and environmental parameters, genetic variance being essentially found within populations. This suggested that the larval dietary tolerance to the toxic leaf litter and the other studied parameters poorly influence population genetic structure. The local adaptation of subalpine mosquito populations to the surrounding vegetation thus appears as a labile trait. Such a dynamic adaptation is also suggested by the correlation between geographical and toxicological distances and the correlation between dietary tolerance to the leaf-litter toxic polyphenols and annual larvicidal treatments.

Molecular systematics of Anopheles: from subgenera to subpopulations

The century-old discovery of the role of Anopheles in human malaria transmission precipitated intense study of this genus at the alpha taxonomy level, but until recently little attention was focused on the systematics of this group. The application of molecular approaches to systematic problems ranging from subgeneric relationships to relationships at and below the species level is helping to address questions such as anopheline phylogenetics and biogeography, the nature of species boundaries, and the forces that have structured genetic variation within species. Current knowledge in these areas is reviewed, with an emphasis on the Anopheles gambiae model. The recent publication of the genome of this anopheline mosquito will have a profound impact on inquiries at all taxonomic levels, supplying better tools for estimating phylogeny and population structure in the short term, and ultimately allowing the identification of genes and/or regulatory networks underlying ecological differentiation, speciation, and vectorial capacity.