Abstract
The random load, A, and the inbreeding load, B, proposed by Morton et al. (1956), in four experimental populations, which were derived from a successively irradiated population of D. melanogaster with a dose of 5, 000r of X-rays, were examined. The experimental populations A, B, C, and D had been subjected to the accumulative radiation exposures of 10, 000, 25, 000, 50, 000, and 75, 000r, respectively.
With the cessation of the irradiation, both the A and B values decreased exponentially in successive generation. Furthermore, the random load appeared to reach the original level more rapidly than the inbreeding load. That is, the random load appeared to arrive close to the original level within 11 to 16 generations after the cessation of the irradiation, while it took longer time for the inbreeding load to drop to the original level. Especially, the amount of the inbreeding load in Population D was still significantly greater than that in the control population.
The analysis of the data indicated that most of the radiation-induced mutant genes, which had dominant or semidominant effect reducing the Darwinian fitness, were relatively rapidly elimianted from the populations in the early generations after the cessation of the irradiation, and after that the slightly detrimental mutants were left in the populations.
