Temperature sensitivity of chromosomal polymorphism inDrosophila pseudoobscura

Population cages ofDrosophila pseudoobscura with the gene arrangements Arrowhead and Chiricahua (CH) were studied at 15 1/2°, 17 1/2°, 19°, 22°C, and a fluctuating temperature (F°) ranging from 17° to 27° with mean of 22°. At the three lower temperatures there were only small but detectable changes in the frequency of CH, but at 22° and F° the earlier populations (V and K) showed the elimination of CH and the later ones (W and L) showed an equilibrium similar to that at 25°. Evidence is presented to show that an environmental change strongly affecting population size need have no effect on the inversion system. Several environmental factors that could have influenced the results are discussed.The temperature sensitivity of the inversion system is discussed in relation toWright's adaptive surface model of genotypes. It is proposed that at about 22° there are a number of narrowly separated steep-sided peaks, some of which represent polymorphic and others monomorphic genetic systems. At 25° the surface has a single peak favoring inversion polymorphism, while from 15° to 19° no well-defined peaks are discernible.Dedicated to Prof. L. C. Dunn in friendship and in recognition of his many contributions to the science of genetics.National Science Foundation (U.S.A.) Predoctoral Fellow, 1956–1959; Columbia University Fellow, 1959–1960; Boese Postdoctoral Fellow, Columbia University, 1961–1962.