THE TIME OF X-RAY INDUCTION OF CROSSOVERS AND OF TRANSLOCATIONS IN DROSOPHILA MELANOGASTER MALES

Abstract

HEN crossovers are induced in Drosophila melanogaster males by X-ray neous crossovers or to translocations. A spontaneous crossover, as it occurs in females , is defined as an equal exchange between homologous chromatids which presumably occurs during the first meiotic prophase. On the other hand, translo-cations, either spontaneous or induced, are usually considered to be the result of exchanges between nonhomologous chromatids; these exchanges are not necessarily equal. It has been demonstrated that X-ray treatment of adult flies will, after a time lapse, increase the frequency of recovered crossovers (MAVOR 1923) and of recovered translocations (MULLER and ALTENBURG 1930). While studying X-ray effects on crossing over, MULLER (1925, 1926) noticed that the marked regions on the long autosomes showed different susceptibilities to treatment, i.e., the effect was concentrated in the spindle attachment region. MULLER also observed, upon comparing his results with those of MAVOR (MAVOR 1923, MAVOR and SVENSON 1924), that the heavier doses used by MAVOR produced a stronger effect in the central region than did the light doses used by MULLER. It was later found that X-rays would also induce crossovers in males as well as in females (FRIESEN 1933, 1934). FRIESEN observed that various genotypes responded unequally to induction of crossing over, but that when crossover offspring did occur, they carried cytologically normal chromosomes. The susceptible region was again the spindle attachment region, as had been shown earlier for females, and FRIESEN accepted this as an indication that the mechanism was identical in both sexes. FRIESEN'S observations on regions of induction were confirmed by PATTERSON and SUCHE (1934). The possibility that induced crossovers might occur in the gonia1 cell prior to meiosis was suggested by WHITTINGHILL (1937) on the basis of his study of crossovers induced by heat treatment of larval stages. This same suggestion had been made by FRIESEN (1936, 1937) on the basis of clustering of crossovers obtained from males ten or more days after treatment. FRIESEN felt that crossing over was an exchange of homologous loci which showed a specific attraction for each other. The occurrence of o6gonial crossing over in females was demonstrated W treatment, the question arises as to whether they are more akin to sponta-Condensation of a thesis submitted to the faculty of the