The “Far-West” of Anopheles gambiae Molecular Forms

Abstract

The main Afrotropical malaria vector, Anopheles gambiae sensu stricto, is undergoing a process of sympatric ecological diversification leading to at least two incipient species (the M and S molecular forms) showing heterogeneous levels of divergence across the genome. The physically unlinked centromeric regions on all three chromosomes of these closely related taxa contain fixed nucleotide differences which have been found in nearly complete linkage disequilibrium in geographic areas of no or low M-S hybridization. Assays diagnostic for SNP and structural differences between M and S forms in the three centromeric regions were applied in samples from the western extreme of their range of sympatry, the only area where high frequencies of putative M/S hybrids have been reported. The results reveal a level of admixture not observed in the rest of the range. In particular, we found: i) heterozygous genotypes at each marker, although at frequencies lower than expected under panmixia; ii) virtually all possible genotypic combinations between markers on different chromosomes, although genetic association was nevertheless detected; iii) discordant M and S genotypes at two X-linked markers near the centromere, suggestive of introgression and inter-locus recombination. These results could be indicative either of a secondary contact zone between M and S, or of the maintenance of ancestral polymorphisms. This issue and the perspectives opened by these results in the study of the M and S incipient speciation process are discussed.

Spatiotemporal dynamics of gene flow and hybrid fitness between the M and S forms of the malaria mosquito, Anopheles gambiae

“…This idea is central to the incidental islands model described above. The notion that association is maintained by strong selection against mixed genotypes is weakened by reports of linkage equilibrium in coastal populations where hybridization rates are high (20). We included two coastal populations in this study, the villages of Abu and Prabis in Guinea-Bissau, and we likewise found low LD among the three divergence islands (r 2 < 0.17).…”

“…These were all observed in populations in coastal West Africa, an area now thought to represent a zone of secondary contact (20,21). These reports resulted in the emergence of this species as the focus of research aimed at exploring the evolution and maintenance of genetic divergence with gene flow (11, 12, 22, 23).…”

Swarming behaviour in natural populations of Anopheles gambiae and An. coluzzii: Review of 4 years survey in rural areas of sympatry, Burkina Faso (West Africa)

“…Although these molecular forms are able to hybridise in the laboratory, M/S hybrids are very rare in nature (della Torre et al 2001Torre et al , 2005. The issue of reproductive isolation between these molecular forms has created much debate della Torre et al 2002;Caputo et al 2008Lawniczak et al 2010 as the assessment of reproductive isolation varies considerably across their geographical distribution (della Torre et al 2001;Tripet et al 2001;Oliveira et al 2008;Diabaté et al 2009;Caputo et al 2011), and we do not yet know how reproductive barriers between these molecular forms function. In fact a cryptic group of An.…”

Characterisation of Species and Diversity of Anopheles gambiae Keele Colony

“…Instead, spatial segregation of the two forms in mating swarms [10–12], or assortative mating behaviour [13] probably contributes to the usually very low rates of hybridisation seen in natural An . gambiae populations where the two forms are sympatric [12,14], although hybridisation can reach up to 20% in some sympatric populations [15–17], particularly at the extremes of the geographical distribution of sympatry [18]. Genetic divergence between the two forms in nature has been extensively studied, and was found to be widely distributed across the M- and S-form genomes, supporting the separation of the two forms into species [19–23].…”