PATTERNS OF SPONTANEOUS AND RADIATION INDUCED MUTATION RATES DURING SPERMATOGENESIS IN DROSOPHILA MELANOGASTER

The daily pattern of mutational response to gamma rays was determined for three types of young adult, heterozygous, Oregon-R males which were mated exhaustively during days 1 to 12 after irradiation. Doses of 250r, 500r, and 1kr were tested for the production of dominant visible and sex-linked hemizygous mutations, and 1kr for Y and autosomal translocations. Translocations were also scored in tests of irradiated mature sperm, and of sperm maturing five to six days later, after doses of 250r, 500r, 2kr and 4kr. The observed pattern was that of an even rate for the visibles, chiefly Minutes, and was U-shaped for sex- linked mutations, chiefiy lethals, with the lower rate appearing in sperm from days 2 to 8. A linear relationship with dose is indicated for the rates of all types of induced point mutations in sperin from days 1 to 8. In premeiotic stages of spermatogenesis the relationship varies between dosage levels and between the types of heterozygous males tested. Sperin derived from gonial cells which did not mutate at the ttme of exposure to doses of 1kr to 5kr showed that the same rate of spontaneous mutations occurred in subsequent stages of spermatogenesis as occurred in the spermmore » of similarly aged Control males. After 1kr, day-5 sperm showed 3.7 times more sex-linked mutations than appeared in sperm from days 1 to 3, and day-8 sperm showed 11.8 times more translocations. This favors the view that changes in mutation frequency during days 1 to 6 reflect changes in the rate at which both point mutations and chromosome rearrangements occur during the various stages of spermatogenesis from ineiosis to maturity. The rate of translocations increases at approximately the 1.8 power of the increase in dose in both mature sperm and sperre maturing five to six days after irradiation, and the rate of point mutations increases linearly at both of those times. This supports the view that the mutation process does not change during the meiosis-to- maturation period of spermatogenesis. At the time of peak response in meiosisspermiogenesis, as well as in mature sperm, after 250r to 1kr, dominant visibles appear to be a mixture of point and position-effect mutations. Increasing the time required for development by lowering the temperature increases proportionately the number of days to the peak mutational response in irradiated sperm. This favors the interpretation that peak mutation rates are associated with specific stages in spermatogenesis. (auth)« less