Evidence for genetic differentiation between the molecular forms M and S within the Forest chromosomal form of Anopheles gambiae in an area of sympatry.

We studied genetic variation at ten microsatellite DNA loci in Anopheles gambiae populations from the Forest chromosomal form collected in four villages in Cameroon (Central Africa). Both recently described M and S molecular forms occur in sympatry in this area. Geographic differentiation within form was low (Fst < 0.017) despite geographical distance between collection sites ranging from 35 to 350 km. However, higher (Fst > 0.035) and statistically significant levels of genetic differentiation were observed between forms, being the highest between sympatric M and S populations collected within the same village. Results were consistent across all loci spread throughout the genome, therefore reflecting a genome-wide pattern. Considering previous findings of strong assortative mating within forms and general lack of hybrids in areas of sympatry, we propose that there is now sufficient direct and indirect evidence to consider both M and S molecular forms of An. gambiae as distinct species that have probably speciated recently.