Lack of experimental evidence for frequency-dependent selection at the alcohol dehydrogenase locus in Drosophila melanogaster.

Two experiments were conducted for testing frequency-dependent selection at the alcohol dehydrogenase locus (Adh; map position, 2--50.1), where two types of alleles, AdhF and AdhS, have been found to be polymorphic in natural populations. First, a competition experiment was carried out between inversion-free AdhF/AdhF and AdhS/AdhS genotypes at five gene frequency levels (those of AdhF = 0.83, 0.67, 0.5, 0.33, and 0.17). First-instar larvae of the two homozygous types in various ratios were placed in a vial, and the frequency of adult flies of each genotype was determined. However, the relative viabilities of these two types did not change with the change in the frequencies of genotypes or genes. The second test was performed by using the results of viability tests with the Cy method of Wallace, using the results of inversion-free crosses only. Although there was variation in the gene frequencies among the vials in testing the viability of individuals consistng of random combinations of two second chromosomes, no difference was found in average viabilities among the vials in which the gene frequencies were different. Thus, no evidence for frequency-dependent selection was obtained.