Genetic population structure and introgression in Anopheles dirus mosquitoes in South-east Asia

Genetic structure and species relationships were studied in three closely r elated mosquito species, Anopheles dirus A, C and D in Thailand using 11 mi crosatellite loci and compared with previous mitochondrial DNA (mtDNA) data on the same populations. All three species were well differentiated from e ach other at the microsatellite loci. Given the almost complete absence of mtDNA differentiation between An. dirus A and D, this endorses the previous suggestion of mtDNA introgression between these species. The high degree o f differentiation between the northern and southern population of An. dirus C (R-ST = 0.401), in agreement with mtDNA data, is suggestive of incipient species. The lack of genetic structure indicated by microsatellites in fou r populations of An. dirus A across northern Thailand also concurs with mtD NA data. However, in An. dirus D a limited but significant level of structu re was detected by microsatellites over similar to 400 km in northern Thail and, whereas the mtDNA detected no population differentiation over a much l arger area (>1200 km). There is prior evidence for population expansion in the mtDNA. If this is due to a selective sweep originating in An. dirus D, the microsatellite data may indicate greater barriers to gene flow within A n. dirus D than in species A. Alternatively, there may have been historical introgression of mtDNA and subsequent demographic expansion which occurred first in An. dirus D so enabling it to accumulate some population differen tiation. In the latter case the lack of migration-drift equilibrium preclud es the inference of absolute or relative values of gene flow in An. dirus A and D.