Coadaptation and Recessive Lethal Content in DDT Resistant Populations Ofdrosophila Melanogaster

The vast literature dealing with recessive lethals in Drosophila populations dates back to the report of Dubinin (1931) that a large number of wild flies carried recessive lethal genes concealed in the heterozygous condition. Subsequent work intended to define the genetic significance of recessive lethals often gave what appeared to be contradictory results (Dobzhansky, 1964). Much of the inconsistency in experimental findings has been ascribed to deficiencies in the control and evaluation of all the genetic and environmental parameters shown to influence lethal heterozygote expression (Oshima and Kitagawa, 1961; Tobari, 1966; Ytterborn, 1971). Alternative hypotheses regarding recessive lethals are that they contribute either to the balanced component of the genetic load or to the mutational component (Crow, 1958). A controversy exists over the propolrtion of lethal genes that belong to each category. Experimental work by Hiraizumi and Crow (1960) and an analysis by Crow and Temin (1964) of data from a number of studies have been interpreted to mean that most of the naturally occurring lethal load in Drosophila populations is dependent upon recurrent mutations for replacement of lethals lost through selection against homozygotes, and more importantly, their heterozygous combinations. However, Wallace and Dobzhansky (1962) have presented experimental evidence to support an argument that the balanced portion of the lethal load may be