Population Structure of Anopheles arabiensis and its Implications for SIT

An. arabiensis is the most widely distributed malaria vector in Africa and is also found in the Arabian peninsula and there are distinct differences in genetic variation throughout its range. There is less chromosomal polymorphism and greater zoophily and exophily east of the Rift valley with distinctive populations in Namibia and Madagascar. There are now powerful tools to analyse DNA and they are being used to try to understand gene flow and genetic drift in field populations of An. arabiensis. Microsatellite analysis was used to analyse populations of An. arabiensis in Tanzania and Mozambique, a distance of some 2000 km. There were significant differences in allele frequencies between the two populations but high rates of gene flow. This suggests recent population separation and not large-scale present day migrations. A second study was made of populations from Sudan (2), Ethiopia, Tanzania, Malawi (2) and Mozambique (3) with 8 microsatellite loci. The two populations studied in both Malawi and Ethiopia were not different from each other but populations separated by more than 2000 km showed highly significant differences with extensive barriers to gene flow. These data again suggest that the present distribution of An. arabiensis is due to a recent range expansion rather than contemporary gene flow. This recent range expansion is probably closely related to human activities. The information on population genetics is important for SIT as it indicates that there are probably no pre-mating isolation barriers between the different populations of An. arabiensis.