Some components of the genetic loads in irradiated experimental populations of Drosophila melanogaster.

OPULATIONS of sexual, diploid, and outbreeding organisms contain con‘cealed genetic loads, which can be greatly increased by radiation exposures. The influence of radiation-induced genetic loads on fitness has been studied in Drosophila (WALLACE 1956,1958; STONE, WHEELER and WILSON 1962; CARSON 1964), swine (Cox 1964), mice (GOWEN and STADLER 1964; SEARLE 1964; SPALDING. BROOKS and MCWILLIAMS 1964), in Tribolium (CRENSHAW 1965) and other organisms. SANKARANARAYANAN (1964, 1965) has examined the effects of X-irradiation on the egg-adult viability in experimental populations of Drosophila melanogaster maintained under conditions designed to minimize the natural selection due to larval competition and crowding. Populations in which the males were irradiated generation after generation showed a decline in the egg-adult viability for the first four or five generations, after which equilibria were reached, the levels of which depended on the amount and the history of the radiation exposure. Subpopulations derived from such “chronically irradiated” populations, and then propagated without further irradiation recovered rapidly. Most of them reached 90 to 95% of the egg-adult viability of the Control population in three to seven generations, while others recovered less fully. The present report summarizes the data on the egg-adult viability in three populations which showed incomplete recovery until the time of when the experiments were concluded in July 1965.