Genetic Loads and Cold Temperature Resistance in Drosophila pseudoobscura

The genetic load associated with cold temperature resistance in Drosophila pseudoobscura was investigated by comparing the survival of heterozygotes and homozygotes for certain chromosomes, following exposure of adult flies to several cold temperature treatments. A large genetic load was found on chromosome III, where survival of homozygotes was 50% less than that of heterozygotes. A much smaller load was uncovered on chromosome II. This difference could be due to a concentration of genes affecting cold temperature resistance on chromosome III, or to the use of flies from a cold winter habitat for the third-chromosome analysis as opposed to flies from a warm winter habitat for the second-chromosome analysis. The pronounced heterozygote advantage associated with genes on chromosome III is another example of heterosis at suboptimal temperatures in poikilotherms. Among third-chromosome gene arrangements in the locality studied, AR and PP appeared to have superior cold temperature resistance, and there was no evidence of heterokaryotypic advantage. Since AR and PP are also the predominant gene arrangements in summer, it is suggested that their chromosomes carry a gene complex which provides selective superiority throughout the year. Evidence is presented which favors the view that separate groups of genes govern cold temperature resistance, egg-to-adult viability at 25⚬ C, and fecundity at 25⚬ C.