Effects of temperature on the viability of heterozygotes of lethal chromosomes in Drosophila melanogaster.

INCE the pioneering work of CHETVERIKOV ( 1927), many studies have shown that a large number of phenotypically normal individuals in natural populations of outbred diploid species, such as Drosophila, carry numerous recessive lethal genes concealed in heterozygous condition. The experiments of STERN, CARSON, KINST, NOVITSKY and UPHOFF (1952), MULLER and CAMPBELL (see MORTON, CROW and MULLER 1956), HIRAIZUMI and CROW (1960), and OSHIMA and KITAGAWA (1961 ) showed that the lethal genes were not completely recessive but reduced the viability of heterozygotes by 2 to 4% on the average. On the other hand, DOBZHANSKY, RIMBAS and KRIMBAS (1960), and WALLACE and DOBZHANSKY ( 1962) found no evidence of reduced viability of heterozygotes for lethal chromosomes. The data of MUKAI and BURDICK (1959, 1960) on experimental laboratory populations showed that the superior fitness of the lethal heterozygotes could be. attributed to single-gene heterosis. In all of the above studies, the experiments were conducted under constant environmental conditions. In natural populations, however, the environment in which natural selection takes place is not constant, and the degree of dominance of lethal genes does not necessarily remain constant. It is possible that the degree of dominance can be altered by such factors as temperature, nutritional conditions, population density and genetic background in which lethal genes occur. Thus, it is important to know the effects of these factors on the fitness of lethalgene carriers. In a review article, LANGRIDGE (1962) noted that the heterosis caused by nonlethal genes for several component traits of fitness is dependent on the temperature at which the organisms are raised. Namely, at the optimum or below-optimum temperature heterosis is usually poorly expressed or absent, while at high temperature, heterosis is pronounced. It is, therefore, of interest to know whether this also holds true for lethal genes. This investigation was conducted to examine this point by using lethal genes on the second chromosome of Drosophila melanogaster.