Estimates of heterozygosity and patterns of geographic differentiation in natural populations of the medfly (Ceratitis capitata)

Abstract

Allozyme variation at 25 gene loci was studied in fifteen populations of Ceratitis capitata (Wiedemann) (medfly,) originating from different countries. The study of genetic variation in natural populations of medfly was undertaken to identify different groups of polymorphic loci whose variation patterns might suggest different kinds of selection forces. The findings of this study are as follows: (1) the African populations have mean heterozygosity 17.8 % and the introduced 5.7 %. (2) An average population is polymorphic for 22.3 % of the gene loci and an average individual is heterozygous for 8.7 % of its gene loci. (3) The within population heterozygosity (Hs) is divided into two groups. The first had an Hs varying from 0 to 20 % and the second from 35 to 43 %. This abrupt transition between the two groups provides initial clues about selection on at least some loci. (4) The FST (fixation index) values have a wide range among loci from 40.7 % (Pep3) to 0 (Adh). The differentiation could be partly due to drift and partly by some sort of selection that affects particular genes. (5) Total heterozygosity (HT) could be separated into two groups with a gap between 20 % and 40 %. This distribution can sort out loci under purifying selection (low HT) and balancing selection (high HT). In conclusion, different evolutionary forces could have contributed to genetic changes during colonization by the medfly. In addition to the bottleneck effect, natural selection may have played an important role in the observed differences in the patterns of variation seen for different groups of loci in natural populations of C. capitata.