Life Cycle and the Expression of Heterosis in Inversion Heterozygotes in Drosophila funebris and Drosophila pavani

In three stocks of D. funebris and in two stocks of D. pavani it has been demonstrated that, in general, there is a higher frequency of inversion heterozygotes among adult flies than among larvae. Nevertheless, there exist some differences according to the species, the stock, or the chromosome concerned. In D. funebris, 100 day old flies contain a greater frequency of heterozygotes than "young" ten day old flies. In D. pavani, the incidence of second chromosome inversion heterozygotes is about the same in "young" and in "old" flies, but is significantly higher in these than in larvae. In this same species, the frequency of fourth chromosome inversion heterozygotes is about the same in larvae and in young adults, but, at least in one stock, it is significantly higher in 100 day old flies. The general conclusion which may be drawn is that there is differential mortality favoring the inversion heterozygotes. The selective pressures which confer a higher fitness on these heterozygotes seem to act both at the preadult and adult stages of life.