High amounts of genetic differentiation between populations of the malariavector Anopheles arabiensis from west Africa and eastern outer islands

Polymorphism at nine microsatellite loci was examined to assess the level o f genetic differentiation between four Anopheles arabiensis populations fro m Senegal, the high plateau of Madagascar, and Reunion and Mauritius island s. Eight of nine loci showed great polymorphism (2-16 alleles/locus) and si gnificant genetic differentiation was revealed between all four populations by F- and R-statistics, with Fst estimates ranging from 0.080 to 0.215 and equivalent Rst values ranging between 0.022 and 0.300. These high amounts of genetic differentiation are discussed in relation to geographic distance including large bodies of water, and history of mosquito settlement, and i nsecticide use on the islands. The results suggest that historical events o f drift rather than mutation are probably the forces generating genetic div ergence between these populations, with homogenization of the gene pool by migration being drastically restricted across the ocean.