THE GENETICS OF VIABILITY IN DROSOPHILA-MELANOGASTER - EFFECTS OF INBREEDING AND ARTIFICIAL SELECTION

Inbreeding and artificial selection experiments were conducted to inve stigate the genetic properties of egg-to-pupa viability in a populatio n of Drosophila melanogaster. The effect of different levels of inbree ding (F = 0, 0.25, 0.50, and 0.73) was studied. Up to F = 0.50, a line ar depression of the mean viability was observed, accompanied by a sig nificant increase of both within-line additive variance and between-li ne variance. At F = 0.73, no further changes were detected. This can b e attributed to natural selection opposing high levels of homozygosity . In parallel, artificial selection to increase viability was performe d for 27 generations in (1) a single undivided population (U) and (2) two populations with cycles of subdivision and between-line selection, followed by reconstitution of selected lines (S-0 and S-I). During th e first cycle (generations 0-4), most of the final total response was achieved under all selection regimes. An advantage of the S-0 and S-I strategies was observed after the completion of the first cycle. Howev er, the same limit was reached in all cases because of a delayed respo nse experienced by line U. Reverse selection for viability resulted in positive correlated responses for fecundity and mating success. Both inbreeding and selection results are compatible with the genetic varia nce of viability in the base population being generated by segregation at a few loci with substantial additive effects and several deleterio us recessives at low initial frequencies. Possible reasons for the mai ntenance of that variance in natural populations are discussed.