Observations on Crossing over Induced by X-Rays in the Males of Drosophila.

HE induction of crossing over in the male of Drosophila melanogaster has T been reported by PATTERSON and SUCHE (1934), who concluded that i t occured in the growth stages of the spermatocytes. This conclusion was based on the assumption that, were the crossing over taking place in pre-spermatocyte stages (that is, spermatogonia or stem cells), its rate should be highest in the youngest age irradiated, since a larger proportion of the gametes to be sampled would be represented by such stages. They foand the rate in 40-52 hour larvae to be some six times greater than the rate in 2 4 3 6 hour larvae. FRIESEN (1934) has likewise reported the production of crossing over in the male by irradiation. In a later paper (1936), on the basis of the occurrence of clusters of similar crossovers in the progeny of individual males, he concluded that crossing over must be occurring in a stage earlier than the spermatocyte; he assumed it to be spermatogonial crossing over. If FRIESEN is correct in assuming that crossing over in one cell results in all of the crossovers recovered in one cluster, then in some cases the large size of the clusters makes it likely that crossing over is being induced in one of the apical stem cells. An attempt will be made in this paper to explain the occurrence of clusters without assuming spermatogonial (or stem cell) crossing over. Both PATTERSON and SUCHE, and FRIESEN (1937) describe the crossover chromosomes as being normal, with few exceptions. The few lethals found were not associated with the point of break. FRIESEN'S studies included examination of the crossover chromosomes in the salivary glands. On the basis of the absence of lethal position effects, PATTERSON and SUCIIE concluded that induced crossing over could not be the effect of chromosome breakage by X-rays, but rather is due to a possible alteration of the rate of maturation. However, if the crossover chromatids are broken a t identical loci, the absence of lethal position effects might be expected.Thus if the breakage is produced by irradiation in the same manner as in aberrations, the absmce o€ lethal position effects would show such effects to be the result of rearrangement and not of breakage. If induced crossing over is due to orthodox chromosome brezkage, and if the majority of the crossover chromosomes recovered are essentially normal as reported, then the possibility suggests itself that the breakage of the two chromatids involved is accomplished by a single ionizing particle. The primary purpose of this paper is to open up the question of the relationship between dosage and frequency of induced crossing over to see if this relationship indicates breakage by a single particle.