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Malaria research in parasite biology
Minimal genomic differentiation between putative incipient species of Anopheles gambiae with extensive interbreeding at ecologically diverse sites in West Africa.
M and S molecular forms of the major African malaria vector Anopheles gambiae sensu stricto (s.s.) are widely considered as incipient species with strong reproductive isolation, with M/S hybrids rarely seen except in a few sites in the extreme west of Africa.
We undertook new surveys of 12 sites in four contiguous countries in this region, with a total of 3499 An. gambiae s.s. identified and genotyped. High frequencies of M/S hybrid forms were seen at each site, ranging from 5% to 42%, and there was a large spectrum of inbreeding coefficient values from 0.11 to 0.76 (spanning most of the range expected between the alternative extremes of panmixia and assortative mating). At a site in The Gambia sampled each month for 2 years, M/S hybrid forms were seen throughout, and a genome-wide scan with an Affymetrix 400,000 single nucleotide polymorphism (SNP) microarray showed marked differentiation between M and S forms only in the pericentromeric region of the X chromosome that contains the molecular form-specific marker locus. Such minimal inter-form genomic differentiation was also seen in such a scan of two other populations, in Senegal and Guinea Bissau. These results clearly indicate that M and S forms are not strongly isolated in this part of Africa.