Evidence for the two-hit nature of X-ray induced crossing-over in the centromeric region of Drosophila males

Abstract

In order to study the relationship between dosage and frequency of X-ray induced crossing-over in males of Drosophila melanogaster, four samples of about 130 males each were irradiated with doses of 500, 1000, 2500 and 4000 R respectively. On the 10th–11th day after irradiation, the two classes of complementary crossovers were found to occur independently of each other in the progeny of the individual males, and the distribution of crossovers per irradiated male followed a Poisson distribution. In consequence, the clusters in this brood were considered as pseudo-clusters, originating from different crossover events in the latest gonial cells. It is this stage that is characterized by the highest sensitivity to the induction of crossing-over. The number of crossovers found on these days has therefore been used as a direct estimate of the number of crossing-overs produced, taking into account the probable induction of crossing-over at the four-strand stage. After logarithmic transformation, a linear relationship was observed between the dose and the percentage of crossovers produced in the late gonial cells in the centromeric region of the second chromosome (regression coefficient = 1.768 ± 0.030). Crossing-over induced in this chromosome region therefore is most probably a “two-hit” phenomenon, to be explained in terms of two distinct and independent breaks. This conclusion is further supported by the observation that fractionation of the dose significantly reduced the frequency of induced crossing-over. When N 2 was given between the fractions the crossing-over frequency was restored to that of unfractionated radiation. It is thought that N 2 acts by preventing restitution of the breaks required for the pseudo-crossing-over. Post-treatment with N 2 after unfractionated radiation resulted in a significant enhancement of the crossing-over frequency. Irradiation in N 2 reduced the frequency of induced crossing-over to about one fifth of that observed in air, whereas radiation exposure in oxygen approximately doubled the crossing-over frequency as compared to that induced in air. The ratio-sensitivity in O 2 and N 2 differed by a factor of about 10. This is considered to offer further support for the two-hit origin of induced centromeric crossing-over in this stage.