THE EFFECTS OF URETHANE, SODIUM MONOHYDROGEN ARSENATE AND SELENOCYSTINE ON CROSSING-OVER IN DROSOPHILA MELANOGASTER

The effects of 0–25 mM urethane, 0–50 μM selenocystine and 0–100 μM sodium monohydrogen arsenate on marker-exchange frequencies have been studied along a region of the X chromosome of Drosophila melanogaster marked by y, cv, v and f. Clear and consistent effects seen in concentration curves were usually but not always found significant in analyses of variance. Urethane concentration curves rose to a higher level at 0.5 to 3 mM and dropped to control levels between 10 and 25 mM. It is proposed that this reversibility was due to a competition between two categories of lesions mimicking natural recombination sites, those on unpaired regions of the chromosome competing with those on already paired regions for recombination-repair enzymes. Selenocystine affected exchange frequencies mainly toward the ends of the unmarked region, especially y – cv, negatively from 2 to 10 μM and positively above 10 μM. These effects are interpreted as being mediated by selenocystine control over restriction of synaptic pairing to terminal regions, especially y – cv. Interactions between urethane and selenocystine in two-chemical treatments satisfactorily support the above explanations for both the urethane and selenocystine effects. Sodium monohydrogen arsenate effects, tentatively attributed to the arsenate ion, differed markedly from those of the other chemicals: "arsenate" concentration curves for single-exchange classes tended to be broadly convex and those for double-exchange classes concave, while interactions with urethane tended to be synergistic or neutral except in one exchange class (that for single exchange in y – cv). No satisfactory explanation of the arsenate effects has yet been found. At 25 mM only, urethane caused male-specific, 95% pupal mortality.