Molecular differentiation between chromosomally defined incipient species of Anopheles funestus

Anopheles funestus Giles is one of the most important vectors of malaria in sub-Saharan Africa. The population structure of this mosquito in Burkina Faso, West Africa based on chromosomal inversion data led to the description of two chromosomal forms, Kiribina and Folonzo. Because both forms co-occur in the same locales yet differ significantly, both in the frequency of inverted arrangements on chromosome arms 3R and 2R and in vectorial capacity, they were hypothesized to be emerging species with at least partial barriers to gene flow. This hypothesis would be strengthened by molecular evidence of differentiation between Kiribina and Folonzo at loci outside chromosomal inversions. We surveyed molecular variation in sympatric populations of the two forms using sequences from the mitochondrial ND5 gene and genotypes at sixteen microsatellite loci distributed across the genome. Both classes of marker revealed slight but significant differentiation between the two forms (mtDNA F(ST) = 0.023, P < 0.001; microsatellite F(ST) = 0.004, P < 0.001; R(st) = 0.009, P = 0.002). Locus-by-locus analysis of the microsatellite data showed that significant differentiation was not genome-wide, but could be attributed to five loci on chromosome 3R (F(ST) = 0.010, P < 0.001; R(st) = 0.016, P = 0.002). Importantly, three of these loci are outside of, and in linkage equilibrium with, chromosomal inversions, suggesting that differentiation between chromosomal forms extends beyond the inversions themselves. The slight overall degree of differentiation indicated by both marker classes is likely an underestimate because of recent population expansion inferred for both Folonzo and Kiribina. The molecular evidence from this study is consistent with the hypothesis of incipient speciation between Kiribina and Folonzo.