Studies of non-disjunction of the major autosomes in Drosophila melanogaster. II. Effects of dose-fractionation, low radiation doses, EMS, and ageing.

To test whether the induction of two-hit events plays a role in the induction of disomic gametes by exposure to X-rays of the oocytes of females carrying compound second chromosomes, exposures of 1000 R, 2000 R or 3000 R were given as an acute dose or as two equal fractions separated by a 3-h interval. The dose-effect relationship for acute exposure is linear over this exposure range (1000–3000 R) and the yields obtained are remarkebly similar to those recorded in an earlier study by Clark and Sobels. At 3000 R a significant reduction in yield was observed after exposure fractionation. At exposure levels of 2000 and 1000 R, however exposure fractionation tends to enhance the yield od disomics.The dose-effect relationship study was then extended to 125, 250 and 500 R. After an exposure of 125 R the induced frequency of disomics was significantly higher than control, but not different from the frequencies induced by 250, 500 and 1000 R. There is no obvious explanation for this plateau which, together with the linear increase from 1000–3000 R, provide an indication that there may be several mechanisms involved in the induction of disomic gemetes.Treatment with ethyl methanesulfonate (EMS) and ageing of the females for a week did not affect the yield of disomic progeny.