Variation in vectorial capacity for human malaria among Anopheles mosquito species is determined by many factors, including behavior, immunity, and life history. To investigate the genomic basis of vectorial capacity and explore new avenues for vector control, we sequenced the genomes of 16 anopheline mosquito species from diverse locations spanning ~100 million years of evolution. Comparative analyses show faster rates of gene gain and loss, elevated gene shuffling on the X chromosome, and more intron losses, relative to Drosophila. Some determinants of vectorial capacity, such as chemosensory genes, do not show elevated turnover, but instead diversify through protein-sequence changes. This dynamism of anopheline genes and genomes may contribute to their flexible capacity to take advantage of new ecological niches, including adapting to humans as primary hosts.

BackgroundOngoing lineage splitting within the African malaria mosquito Anopheles gambiae is compatible with ecological speciation, the evolution of reproductive isolation by divergent natural selection acting on two populations exploiting alternative resources. Divergence between two molecular forms (M and S) identified by fixed differences in rDNA, and characterized by marked, although incomplete, reproductive isolation is occurring in West and Central Africa. To elucidate the role that ecology and geography play in speciation, we carried out a countrywide analysis of An. gambiae M and S habitat requirements, and that of their chromosomal variants, across Burkina Faso.ResultsMaps of relative abundance by geostatistical interpolators produced a distinct pattern of distribution: the M-form dominated in the northernmost arid zones, the S-form in the more humid southern regions. Maps of habitat suitability, quantified by Ecological Niche Factor Analysis based on 15 eco-geographical variables revealed less contrast among forms. M was peculiar as it occurred proportionally more in habitat of marginal quality. Measures of ecological niche breadth and overlap confirmed the mismatch between the fundamental and realized patterns of habitat occupation: forms segregated more than expected from the extent of divergence of their environmental envelope – a signature of niche expansion. Classification of chromosomal arm 2R karyotypes by multilocus genetic clustering identified two clusters loosely corresponding to molecular forms, with 'mismatches' representing admixed individuals due to shared ancestral polymorphism and/or residual hybridization. In multivariate ordination space, these karyotypes plotted in habitat of more marginal quality compared to non-admixed, 'typical', karyotypes. The distribution of 'typical' karyotypes along the main eco-climatic gradient followed a consistent pattern within and between forms, indicating an adaptive role of inversions at this geographical scale.ConclusionEcological segregation between M and S is consistent with niche expansion into marginal habitats by chromosomal inversion variants during early lineage divergence; presumably, this process is promoted by inter-karyotype competition in the higher-quality core habitat. We propose that the appearance of favourable allelic combinations in other regions of suppressed recombination (e.g. pericentromeric portions defining speciation islands in An. gambiae) fosters development of reproductive isolation to protect linkage between separate chromosomal regions.

BackgroundInsecticide resistance in malaria vectors is a growing concern in many countries which requires immediate attention because of the limited chemical arsenal available for vector control. The current extent and distribution of this resistance in many parts of the continent is unknown and yet such information is essential for the planning of effective malaria control interventions.MethodsIn 2008, a network was established, with financial support from WHO/TDR, to investigate the extent of insecticide resistance in malaria vectors in five African countries. Here, the results of bioassays on Anopheles gambiae sensu lato from two rounds of monitoring from 12 sentinel sites in three of the partner countries are reported.ResultsResistance is very heterogeneous even over relatively small distances. Furthermore, in some sites, large differences in mortality rates were observed during the course of the malaria transmission season. Using WHO diagnostic doses, all populations from Burkina Faso and Chad and two of the four populations from Sudan were classified as resistant to permethrin and/or deltamethrin. Very high frequencies of DDT resistance were found in urban areas in Burkina Faso and Sudan and in a cotton-growing district in Chad. In areas where both An. gambiae s.s. and Anopheles arabiensis were present, resistance was found in both species, although generally at a higher frequency in An gambiae s.s. Anopheles gambiae s.l. remains largely susceptible to the organophosphate fenitrothion and the carbamate bendiocarb in the majority of the sentinel sites with the exception of two sites in Burkina Faso. In the cotton-growing region of Soumousso in Burkina Faso, the vector population is resistant to all four classes of insecticide available for malaria control.ConclusionsPossible factors influencing the frequency of resistant individuals observed in the sentinel sites are discussed. The results of this study highlight the importance of standardized longitudinal insecticide resistance monitoring and the urgent need for studies to monitor the impact of this resistance on malaria vector control activities.

Population subgroups of the African malaria vector Anopheles gambiae have not been comprehensively characterized owing to the lack of unbiased sampling methods. In the arid savanna zone of West Africa, where potential oviposition sites are scarce, widespread collection from larval pools in the peridomestic human habitat yielded a comprehensive genetic survey of local A. gambiae population subgroups, independent of adult resting behavior and ecological preference. A previously unknown subgroup of exophilic A. gambiae is sympatric with the known endophilic A. gambiae in this region. The exophilic subgroup is abundant, lacks differentiation into M and S molecular forms, and is highly susceptible to infection with wild Plasmodium falciparum. These findings might have implications for the epidemiology of malaria transmission and control.Much of the current genetic knowledge of African malaria vector populations is based on collection and analysis of indoor house-resting (endophilic) mosquitoes. Indoor capture of mosquitoes is often regarded as an unbiased collection method for epidemiologically important vectors of human malaria, including A. gambiae sensu stricto (ss), because they are thought to be 'naturally endophilic' (1). Outdoor-resting (exophilic) mosquitoes can contribute to malaria transmission but are underrepresented or absent from indoor collections, particularly if they also bite outdoors (2-7). Collection methods for exophilic mosquitoes are much less efficient than for indoor-resting mosquitoes, involving variants of i) artificial resting sites such as pits, boxes or pots, ii) manual aspiration from vegetation and holes, or iii) capture of mosquitoes landing on animal or human bait, now largely proscribed due to the risk of infection of human collectors (2). Further highlighting the challenge in sampling exophilic mosquitoes, resting sites thought to harbor large A. gambiae populations during the dry season have resisted detection for decades (8)(9)(10) Genetic division of A. gambiae populations into subgroups allows fine ecological partitioning by the species, mediating the expansion of malaria transmission spatially and temporally. Chromosome inversion polymorphisms define certain subgroups (10,12). Frequency of the 2La inversion follows a geographic cline from the humid Central African forest, where the wild-type 2La+ allele is fixed, north to the arid West African savanna, where the inverted 2La allele is fixed (12). One likely phenotype of the 2La inversion is adaptation to aridity. Another example of niche expansion of A. gambiae by genetic subdivision is represented by two genetically diverged molecular forms, termed M and S (13). These molecular forms are detected by assays for fixed nucleotide differences on the X chromosome (Molecular Form Diagnostic SNPs, MFDS), and display other fixed SNPs in genomic 'speciation islands ' (14). The M form dominates in marginal and disturbed habitats where S is less competitive (15).We collected mosquito larvae from natural breeding pools in an arid...

Our study reveals that the infectious reservoir peaks at the start of the wet season, with prominent roles for infections in children and submicroscopic infections. These findings have important consequences for strategies and the timing of interventions, which need to include submicroscopic infections and be implemented in the dry season.