Effect of Environment upon the Manifestation of Heterosis and Homeostasis in Drosophila Pseudoobscura.

A NUMBER of studies involving the comparison of the F, and F, generations from crosses between geographic populations of Drosophila with the populations themselves have been carried out. In general the findings were that Fl hybrids showed a heterotic increase in Darwinian fitness over the parental populations and the F, generation showed a breakdown of heterosis. Possible explanations for this kind of effect are of some interest and have been discussed by several workers, for example, by WALLACE and VETUKHIV (1 955). However, the present experiments were initiated with another end in view. A number of observations had shown that the relative performance of two or more genotypes may be different under different environmental conditions. It appeared interesting to test whether the heterosis and breakdown observed in crosses between geographic populations at 25°C was maintained over a range of environmental conditions. Experiments of BEARDMORE (unpublished data) had shown that the superiority in developmental stability of F, hybrids over their inbred parents was dependent upon the type of environment in which the experiments were carried out. In an environment embodying a regularly fluctuating temperature cycle, Drosophila F, hybrids were not significantly more stable or homeostatic than the inbred parental lines, although under constant temperature conditions the F, superiority was consistently manifested as a lower degree of asymmetry of the sternopleural chaetae. In the present study we tested whether F,s derived from crosses between geographic populations show heterosis and whether the F,s show breakdown over a range of environmental conditions. Comparable experiments testing flies homozygous and heterozygous for various second chromosomes of D. pseudoobscura have been described by DOBZHANSKY and WALLACE (1953). These experiments were concerned essentially with the between-homologues component of heterosis in Fls, whereas in the present study we have looked at the over-all interaction between haploid sets of chromosomes in the F, and, as well, at recombination between sets in the F, generation.