Population structure and population history of Anopheles dirus mosquitoes in southeast Asia

Separating the confounding effects of long-term population history from gen e flow can he difficult. Here, we address the question of what inferences a bout gene flow can be made from mitochondrial sequence data in three closel y related species of mosquitoes, Anopheles dirus species A, C, and D, from southeast Asia. A total of 84 sequences of 923 bp of the mitochondrial cyto chrome oxidase I gene were obtained from 14 populations in Thailand, Myanma r, and Bangladesh. The genealogy of sequences obtained from two populations of Ail. dirus C indicates no contemporary gene flow between them. The F-ST value of 0.421 therefore probably represents a recent common history, perh aps involving colonization events. Anopheles dirus A and D are parapatric. yet no differentiation was seen either within or between species. The starl ike genealogy of their haplotypes, smooth unimodal mismatch distributions, and excess of low frequency mutations indicate population expansion in Ail. dirus A and D. This, rather than widespread gene flow, explains their low within-species F-ST values (0.018 and 0.022). The greater genetic diversity of An. dirus D suggests that expansion occurred first in species D and sub sequently in species A. The current geographical separation and low hybrid fitness of these species also argue against ongoing interspecific gene how. They suggest instead either historical introgression of mtDNA from An. dir us D into species A followed by independent range expansions, or a selectiv e sweep of mtDNA that originated in An. dirus D. While not excluding contem porary gene flow, historical population processes are sufficient to explain the data in An. dirus A and D. The genealogical relationships between hapl otypes could not be used to make inferences of gene flow because of extensi ve homoplasy due to hypervariable sites and possibly also recombination. Ho wever, it is concluded that this approach, rather than the use of fixation indices, is required in the future to understand contemporary gene flow in these mosquitoes. The implications of these results Tor understanding gene flow in another important and comparable group of malaria vector mosquitoes in Africa, the An. gambiae complex, are also discussed.