AMYLASE POLYMORPHISM IN DROSOPHILA-MELANOGASTER - HAPLOTYPE FREQUENCIES IN TROPICAL AFRICAN AND AMERICAN POPULATIONS

The frequencies of phenotypic haplotypes at the Amylase loci of D melanogaster were determined in 10 samples from 7 different tropical origins, including the African mainland, Indian Ocean Islands and the French West Indies. Altogether, 2110 haplotypes were scored and 10 different electrophoretic alleles were identified. Allelic frequencies were calculated with the assumption that 2 functional loci occur on each second chromosome. The data of 3 temperate populations from Texas, Japan and France (1238 haplotypes) were also included for comparisons. Genetic diversity, measured either at the allelic or haplotypic levels, was extremely variable between populations, with expected heterozygosities ranging from 2 to almost 90%. The most diverse populations are found on the African mainland while temperate populations are characterized by the predominance of the Amy-1 allele; a very low diversity was also found in the Mascarene islands. Genetic distances were similarly close between populations from temperate regions, Guadeloupe islands and Mascarene islands, in spite of large geographic distances. On the other hand, African mainland populations, despite their high diversity and geographic proximity, could be very distantly related at the genetic level. With 10 different alleles, 55 different phenotypic haplotypes (ie not discriminating between the proximal and distal loci) may be produced, and 34 were identified. Among the 21 missing haplotypes, 20 had very low expectancy under the assumption of free recombination (total expected number 5.9). Only one (Amy 3-5) had a higher expectancy (8.9). Therefore, most of the possible haplotypes have been produced during the course of evolution in spite of the tight linkage between the 2 loci, and 3 possible mechanisms are discussed. All these observations seem better explained by stochastic processes than by selective pressures. Ancestral populations on the African mainland have accumulated a large number of alleles and haplotypes, but their genetic differentiation suggests restricted gene flows. In other parts of the world, the low diversity could be explained by demographic bottlenecks related to recent colonizations.