The Occurrence of X-Ray Induced Dominant Lethal Mutations in Habrobracon.

H E PARASITIC wasp, Habrobracon, has been considered of special T value in connection with studies of dominant lethals because of its somewhat exceptional method of breeding. In “out-crosses,” now called three-allele crosses because they involve triple alleles a t the sex locus, all fertilized eggs develop into females, unfertilized ones into males (WHITING 1937a, 1940, 1943). It follows, then, that if a sperm bearing one or more dominant lethals fertilizes an egg, a potential female is lost. Statistically the result of an appreciable number of such events would be a depression in the number of females. But the number of males should remain unaltered, since the frequency with which they occur is determined by the proportion of cases in which sperm fail to enter the eggs, and not, therefore, by the constitution of the sperm. Sex-ratios, then, would be expected to vary in accordance with the incidence of dominant lethal-bearing sperm produced by the father. Several authors have made use of the foregoing argument (STANCATI 1932; BISHOP 1937; WHITING 1937b, 1938; MAXWELL 1938). The results of their experiments have agreed in showing that the numbers of females could be reduced or even eliminated in three-allele crosses of females by males bearing sperm in which dominant lethals had been induced by X-radiation. WHITING’S data indicated that a t dosages of about 10,000 r units, practically every sperm contained a t least one dominant lethal change (number of F1=.07 per $2 per day). At lower dosages ( 2 5 0 0 r, 5000 r, 7500 r), the numbers of female progeny per day varied inversely with increasing dosage. The present work was undertaken with the purpose of supplementing the known data by securing larger numbers of progeny and estimates for dosages a t smaller intervals. This would allow the construction of a curve showing the relationship between dosage and mortality of females. The author is grateful to lk P. w. WHITING for proposing the project and for materials and helpful suggestions during the course of the work.