Selection for Egg Number with X-ray-induced Variation.

XPERIMENTS designed to utilize radiation-induced genetic variability E through artificial selection are relatively recent. Unlike studies with mutations of specific well known genes where mutation frequencies can be obtained by enumeration, the quantitative experiments rely on the measure of genetic variation and selection response as the measure of induced mutation effects. Neither the frequencies nor the magnitude of individual gene effects are known under those circumstances, so that much less precise statements can be made about genetic mechanisms underlying radiation response. Nevertheless, it has been shown for Drosophila and many species of plants that radiation-inkred polygenic variation can be of sufficient magnitude to be important. With relatively high doses of radiation (up to 100,000r) plant geneticists have been able to increase genetic variation up to sixfold of that observed in natural populations, but the effectiveness of such treatments clearly depends on whether the character considered shows an average decline from irradiation (BROCK and LATTER 1961). Thus. improvements from selection following irradiation appear more pronounced for traits such as plant height (RAWLINGS, HANWAY, and GARDNER 1958 for soya beans) o r early maturity (BROCK and LATTER 1961 for subterranean clover) than for yield, which is initially reduced by radiation (GREGORY 1955 for peanuts; RAWLINGS, HANWAY, and GARDNER 1958 for soya beans). In animals almost all published evidence on selection with radiation-induced variation has been obtained with Drosophila. Two early experiments by SEREBROVSKY (1935) and ROKIZKY (1936) in Drosophila melanoguster showed relatively little added selection response in abdominal bristles due to radiation. More striking responses were obtained by SCOSSIROLI (1954) who was able to almost double the number of sternopleural bristles by alternative selection and radiation in a population that had presumably reached a plateau for that trait under previous selection. He ( SCOSSIROLI and SCOSSIROLI 1959) later repeated essentially the same experiment with isogenic lines and a population based on crossing two isogenic lines with very similar results. In the latter case he was able to increase sternopleural bristle number from about 17 to 25 over ten generations of selection with 3000r of X radiation given to both males and females. Smaller but consistent effects of radiation on selection response and genetic variation were obtained by CLAYTON and ROBERTSON (1955) for abdominal bristles in highly inbred lines of D. melunoguster derived from their Kaduna population, and using a dose of only 1800r of X rays per generation. In contrast to these successful selection experiments with bristles there is little evidence that viability in Drosophila can be improved by radiation followed by selection (WALLACE 1956; CARSON 1964).