PATTERNS OF MITOCHONDRIAL VARIATION WITHIN AND BETWEEN AFRICAN MALARIA VECTORS, ANOPHELES-GAMBIAE AND ANOPHELES-ARABIENSIS, SUGGEST EXTENSIVE GENE FLOW

Anopheles gambiae and An. arabiensis are mosquito species responsible for most malaria transmission in sub-Saharan Africa. They are also clo sely related sibling species that share chromosomal and molecular poly morphisms as a consequence of incomplete lineage sorting or introgress ive hybridization. To help resolve these processes, this study examine d the partitioning of mtDNA sequence variation within and between spec ies across Africa, from both population genetic and phylogeographic pe rspectives. Based on partial gene sequences from the cytochrome b, ND1 and ND5 genes, haplotype diversity was high but sequences were very c losely related. Within species, little or no population subdivision wa s detected, and there was no evidence for isolation by distance. Betwe en species, there were no fixed nucleotide differences, a high proport ion of shared polymorphisms, and eight haplotypes in common over dista nces as great as 6000 km. Only one of 16 shared polymorphisms led to a n amino acid difference, and there was no compelling evidence for nonn eutral variation. Parsimony networks constructed of haplotypes from bo th species revealed no correspondence of haplotype with either geograp hy or taxonomy. This trend of low intraspecific genetic divergence is consistent with evidence from allozyme and microsatellite data and is interpreted in terms of both extensive gene flow and recent range expa nsion from relatively large, stable populations. We argue that retenti on of ancestral polymorphisms is a plausible but insufficient explanat ion for low interspecific genetic divergence, and that extensive hybri dization is a contributing factor.