Cold temperature resistance, chromosomal polymorphism and interpopulation heterosis in Drosophila pseudoobscura.

Descendants of two Colorado, U.S.A., populations of D. pseudoobscura, Boulder and La Foret, were exposed to +2 degrees and -2 degrees . In third instar larvae from Boulder AR and PP third chromosome gene arrangements survived better than TL and others, while the reverse situation occurred for La Foret. Deleterious dominant effects were observed for AR from La Foret. In adults from Boulder there was a trend towards greater survival for AR and PP than for other gene arrangements, while AR from La Foret showed relatively poor cold resistance. Survival of Boulder and La Foret flies, and their interpopulation hybrid, was determined after exposure to -2 degrees at two humidities. Order of survival of developmental stages was: adults >> third instar larvae > mixture of first and second instar larvae. Adults survived better at low humidity, while larvae survived better at high humidity. Boulder adults and larvae survived better than those from La Foret. Advantage in survival of hybrids over the midparent ranged from 23% to 138%. Hybrid advantage over the higher parent ranged from 5% to 111%. Order of expression of heterosis was: mixture of first and second instar larvae > third instar larvae > adults. Relation of all results to the chromosomal polymorphisms at Boulder (seasonally constant) and La Foret (seasonally cyclic) is discussed.