ENVIRONMENTAL MODIFICATION OF HETEROSIS IN DROSOPHILA PSEUDOOBSCURA

Abstract

F1 hybrids were made between 12 strains homozygous for ST (Standard gene arrangements) and 12 strains homozygous for CH (Chiricahua gene arrangements). These hybrids which at the start contained 50% of ST and 50% of CH chromosomes, were raised in population cages either in incubators at 25[degree] and 21[degree]C or in a constant-temp. room at 16[degree]C. The two spp. of yeast used were Zygosaccharomyces dobzhanskii Shehata and Kloeckera apiculata (Lindner) Dvornik. Samples of 300 chromosomes (150 larvae) were taken in 6 subsamples on as many successive days at intervals somewhat longer than a generation in the early stages and at intervals of 2 or 3 generations later on. No statistically significant changes in the frequences of ST and CH chromosomes were observed in the populations kept at 16[degree]C for 100 days and fed on the two yeast spp. When these populations were transferred to 21[degree]C after living for 115 days at 16[degree]C, the frequencies of ST chromosomes rose from approx. 50% to approx. 70% in 135 days after the transfer. In the populations fed on Zygosaccharomyces the ST chromosomes continued to increase to a level of approx. 95% at the end of the expt. (535 days), whereas in the populations fed on Kloeckera a genetic equilibrium of about 75% ST chromosomes and 25% CH chromosomes was reached at 215 days. When 4 populations (2 fed on Zygosaccharomyces and 2 on Kloeckera) were kept at 25 C, the frequency of ST chromosomes rose rapidly for the first three generations from the initial 50%, but later the increase was very slow and an equilibrium was established at 6 to 8 generations, with no effect of the different yeasts. At 21[degree]C, when fed on Zygosaccharomyces, the adaptive value of ST/ST homozygotes was equal to or greater than that of the ST/CH heterozygotes, but when fed on Kloeckera, the heterozygotes were superior to both homozygotes. At 25[degree]C the heterozygotes were superior to the homozygotes on the two yeasts and also on Saccharomyces cerevisiae (Fleischmann''s yeast). Chi-square tests for replicates within treatments indicated good reproducibility of results.