COMPARISON OF FITNESS AMONG GENOTYPES IN DROSOPHILA VIRILIS WITH THE DIFFERENT LEVEL OF ESTERASE ACTIVITY

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As a part of the study in which the maintenance of esterase isozyme polymorphisms of D. virilis is experimentally analyzed, the effects of esterase activity on two major components of fitness, fecundity and larva-to-adult viability, were studied by using a null strain (00/00) having very low esterase activity in comparison with an ordinary 7B/7B strain. The following three experiments were performed: 1) Fecundity and larva-to-adult viability under near optimal conditions. 2) Larva-to-adult viability in mixed cultures. 3) Larva-to-adult viability under competitive conditions in which three genotypes coexist at various frequencies.
In Experiments 1 and 2, there were no significant differences in the component of fitness among three homozygous genotypes (00/00, 0B/0B and 7B/7B). In Experiment 3, in which larvae of three genotypes (00/00, 00/7B and 7B/7B) coexist in the Hardy-Weinberg ratio, the heterozygotes seem to be slightly superior to both homozygotes in the larva-to-adult viability but the viability did not differ between two homozygotes having quite different esterase activities. The heterozygote superiority could be ascribed to the heterotic effect of a gene block including two esterase loci on the second chromosome. No evidence of frequency-dependent selection was recognized in Experiment 3.
From the results summarized above it could be concluded that there may be little effect of esterase activity on two components of fitness, fecundity and viability, and that selective differences among genotypes at two esterase loci are very small, if any.