The relationship between radiation-induced and transposon-induced genetic damage during Drosophila oogenesis.

The combined effect of X-irradiation and transposon mobility on the frequencies of X-linked recessive lethals and dominant lethals was investigated in female hybrids in the P-M system of hybrid dysgenesis. X-linked lethals were measured in G2 hybrid dysgenic females whose X chromosome was derived from the M X P cross. To test for additivity or synergism, the mutation rate in irradiated dysgenic females was compared to that of unirradiated females as well as to irradiated nondysgenic hybrid females derived from M X M crosses. Eggs collected for 2 days after irradiation, were represented by the more radiation-sensitive A and B oocytes (about 75%) and the least sensitive C oocytes (about 25%). The production of X-linked lethal events in X-irradiated dysgenic females was 8.1%, as compared to 4.5% in dysgenic controls and 3.4% in irradiated, nondysgenic controls, demonstrating an additive effect of radiation and dysgenesis-induced genetic damage. The effect of irradiation on sterility of dysgenic hybrid females was a negative one, resulting in 20% less sterility than expected from an additive effect. The combined effect of radiation and dysgenesis on dominant lethality tested in A, B and C oocytes of the same hybrid females was synergistic. Egg broods collected for 3.5 days after irradiation showed that significantly more damage was induced in the presence of ionizing radiation in dysgenic females than in their nondysgenic counterparts. This effect was most obvious in B and C oocytes. The synergism observed may be related to the inability of cells to repair the increased number of chromosome breaks induced both by radiation and transposon mobility.