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

Abstract

Two experiments were conducted for testing frequency-dependent selection at the alcohol dehydrogenase locus (Adh; map position, 2-50.1), where 2 types of alleles, AdhF and AdhS, were polymorphic in natural populations. A competition experiment was carried out between inversion-free AdhF/AdhF and AdhS/AdhS genotypes at 5 gene frequency levels (those of AdhF = 0.83, 0.67, 0.5, 0.33 and 0.17). First-instar larvae of the 2 homozygous types in various ratios were placed in a vial and the frequency of adult flies of each genotype was determined. The relative viabilities of these 2 types did not change with the change in the frequencies of genotypes or genes. The 2nd 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 consisting of random combinations of 2 2nd chromosomes, no difference was found in average viabilities among the vials in which the gene frequencies were different. No evidence for frequency-dependent selection was obtained.