Population structure in the malaria vector, Anopheles arabiensis Patton, in East Africa

The population structure of the malaria vector Anopheles arabiensis was inv estigated using data from six microsatellite loci in samples from localitie s in Mozambique and Tanzania. Genotype frequencies were neither significant ly different between houses in a village in Tanzania nor between villages w ithin a 20-km radius in Mozambique. Thus a deme has an area greater than 20 km in radius. At five of the six loci the heterozygosity of the population from Mozambique was lower than that from Tanzania, implying a lower effect ive population size (N-e) at this southern edge of the species range. There were significant differences in genotype frequencies between the Tanzanian and Mozambique populations at five of the six loci (P < 0.05). Values for both F-ST (mean = 0.069) and R-ST (mean = 0.025) were significantly differe nt from zero (P < 0.05) at four and three out of five loci, respectively, b ut there was no significant correlation between the two statistics. The wid e variation in values of F-ST and R-ST across loci suggests that care shoul d be taken in interpreting values derived from averaging across loci. Wheth er the variation results from sampling effects or selectional constraints o n some loci is unclear. Although there is evidence for significant differen tiation between these populations, estimates of gene flow (Nm) calculated f rom mean F-ST and R-ST statistics were relatively high, 3.4 and 4.9, respec tively. We argue that this is more likely to reflect recent separation of t hese populations and/or large effective population size rather than large-s cale present day migrations.