Effects of temperature on the viabilities of homozygotes and heterozygotes for second chromosomes of Drosophila melanogaster.

B A N D (1963) has reported that in Drosophila melanoguster significant viability changes among random heterozygotes for lethal, semilethal and normal chromosomes could be induced when the same chromosome combinations were tested in wide and narrow temperature range conditions having a mean temperature of 25°C. In a study utilizing only lethal and normal chromosomes, TOBARI (1966) found that viabilities of lethal heterozygotes were higher than those of normal heterozygotes at an above standard temperature, analogous to the results of BAND obtained in the narrow range conditions; however, viabilities of the lethal heterozygotes were lower than those of normal heterozygotes both in the standard laboratory temperature conditions and at the lower temperature of 17°C. In neither study by BAND (1963) nor TOBARI (1966) were the viability effects of the homozygous chromosomes scored at temperatures other than the standard laboratory temperature. On the other hand, BAND and IVES (1963) studied the correlation between viabilities of homozygotes and heterozygotes at the standard laboratory temperature of 25°C and at a lower temperature of 17"C, using the second chromosomes isolated from a natural population of D. melanogaster. They obtained negative but nonsignificant values at both temperatures. This and other evidence led them to conclude that the heterozygous effects of second chromosomes on viability are largely independent of the homozygous effects of these same chromosomes when experiments are carried out at the standard laboratory temperature and at a lower temperature, and that the genetic load is primarily of balanced type. The present investigation was undertaken to study the homozygous and heterozygous effects of second chromosomes isolated from a laboratory population of D. melanogaster at the standard laboratory temperature and the higher temperature of 29°C.