Female Fecundity of Drosophila Melanogaster Second Chromosome Recessive Lethal Heterozygotes in Homozygous and Heterozygous Genetic Background.

Received August 26. 1964 A large number of both spontaneous and induced second chromosome recessive lethals in Drosophila melanogaster have been reported. Almost of all these lethals are rather rapidly eliminated from populations under random mating conditions. IVES (1945) and WALLACE (1950) have estimated the number of second chromosome loci of lethal genes as 495 and 400, respectively. Some lethal genes have been reported to remain in random mating populations for a long time. For example, BURDICK (1961 ) has collected a particular lethal (Z(2) 55i) at the same collection site on two different occasions six years apart. OSHIMA and KITAGAWA (1961a) have reported on the second chromosome recessive lethals in Japanese populations of D. melanogaster. Two lethal genes showed a remarkably high persistence in random mating populations after 15 generations (80 percent and 50 percent lethal heterozygotes). Frequencies of lethal genes are different from population to population. A number of components affecting the degree of persistence of lethal heterozygotes must be considered in order to know the fitness of lethal heterozygotes in populations. If an autosomal lethal heterozygote shows higher than wild-type male mating ability, sperm competitive ability, zygotic viability and/or higher female fecundity, the lethal gene will be maintained in the population (MUKAI and BURDICK 1959). STERN and his co-workers (1952) estimated the larval viability of sex-linked recessive lethals in heterozygous condition and their results showed that 27 out of 75 lethals have some heterotic effect on the viability of zygotes. MUKAI and BURDICK (1 959) investigated laboratory populations segregating for a lethal (Z(2)55iI which showed a heterotic effect under random mating. Their results showed that the superior fitness of the lethal heterozygotes in both homozygous and heterozygous genetic background could be attributed to single gene heterosis. Almost all the studies of the heterozygous effect of deleterious genes in Drosophila have been an assessment of the zygotic viability of larvae. MUKAI and