Genetic variation and genetic load due to the male reproductive component of fitness in Drosophila.

The genetic variation and genetic load due to virility, the male reproductive component of fitness, was measured in Drosophila melanogaster and D. pseudoobscura using males homozygous and heterozygous for the second chromosome of each species. Virility was determined in a female-choice, male mating competition experiment where both mating propensity and fertility were taken into account.--The mean virility of the homozygous D. melanogaster males relative to the heterozygous males was 0.50; the relative mean virility of the quasinormal homozygotes was 0.56. The mean virility of the homozygous D. pseudoobscura males relative to the heterozygous males was 0.70; the relative mean virility of the nonsterile homozygotes was 0.72, and of the quasinormal homozygotes, 0.68.--Depending on the species and chromosome sampled, fertile homozygous males had a mean virility 15 to 50% lower than the mean viability of individuals homozygous for a chromosome with quasinormal viability. The genetic load due to virility was also greater than that due to the female reproductive component. This higher level of hidden genetic variation (or genetic load) indicates that the results of Prout (1971a, b) and Bundgaard and Christian (1972), where the virility component of fitness dominated the dynamics of an artificial polymorphism, may be more general and that virility may dominate the dynamics of natural polymorphisms as well.