THE EFFECTIVE POPULATION-SIZE OF ANOPHELES-GAMBIAE IN KENYA - IMPLICATIONS FOR POPULATION-STRUCTURE

We estimated current and long-term effective population size (N-e) of two Anopheles gambiae (savanna cytotype) populations in Kenya. Tempora l variation at nine microsatellite loci in each population sampled 7 a nd 9 years apart and genetic diversity in each sample were analyzed to answer the following questions. (1) Do bottlenecks occur in Kenyan po pulations of A. gambiae? (2) How variable are different populations wi th respect to their current and long-term N-e values? (3) What are the implications of these results on population structure and history? Th e estimates of N-e of Asembo and Jego were 6,359 and 4,258, respective ly, and the lower 95% limits were 2,455 and 1,669, respectively. Thus, despite the typical observation of low density at the village level d uring the dry season, large populations are maintained annually. Large current N-e is consistent with previous studies showing low different iation across the continent, especially under Wright's isolation-by-di stance model. Current N-e in Asembo was 1.5-fold higher than in Jego, but this difference was not significant. Long-term N-e in Asembo (22,6 67) was 2.9-fold higher than that in Jego (7,855) based on the stepwis e mutation model. The difference between populations was significant a t both time points regardless of whether long-term N-e values were cal culated based on the stepwise mutation model or the infinite-alleles m odel. Heterozygosity in Jego declined significantly between 1987 (59%) and 1996 (54%), whereas heterozygosity in Asembo was stable (66%-65%) . Despite the relatively high and significant differentiation between Asembo and Jego (F-ST = 0.072-0.10, R-ST = 0.037-0.038), all alleles i n Jego were found in Asembo but not vice versa. All of these findings suggest that lower N-e in Jego magnifies differentiation between the t wo populations. The long-term N-e was biased downward, because its cal culation was based on an upper bound estimate of microsatellite mutati on rate. N-e values based on mtDNA and allozymes were an order of magn itude higher. Long-term N-e therefore, is probably measured in hundred s of thousands and hence does not support a recent expansion of this s pecies from a small population.