Induction of Dominant Lethal Effects by X-Radiation in Habrobracon.

ULLER in 1927 stated that the “partial sterility” of X-rayed males M of Drosophila wasdue,at least to an appreciable extent, to dominant lethal changes induced in the sperm. It was impossible, however, to determine whether the eggs were actually “fertilized.” In Drosophila the result would be the same; the eggs would fail to develop, whether dominant lethals were induced or the sperm were inactivated so that they could not penetrate the eggs. MULLER attributed a shift in the sex ratio by reduction in the number of females to lethals in the X chromosome. STANCATI (193 2) presented evidence that dominant lethals were induced by X-radiation of sperm in Habrobracon. In this wasp in which males develop by haploid parthenogenesis, any reduction in percentage of females due to inactivation of sperm should be compensated by increase in number of male offspring, “step-sons’’ of the treated males. The number of “stepsons” should remain constant, however, despite the decrease in females if such decrease were due to dominant lethals alone, the number of eggs fertilized remaining constant. STANCATI’S material was complicated by the presence of highly inviable biparental males which are now known to replace a variable number of females when the parents are closely related. Grouping all biparentals, males and females, together, STANCATI reported, 64.8 percent of biparentals in fraternities from untreated sires, while only 24.3 percent of biparentals occurred in bisexual fraternities from treated sires. In addition to these there were I I I I sisterless males in 48 unisexual fraternities, “step-sons” of treated males. The three groups: controls, bisexual fraternities from treated and unisexual from treated males, show as average numbers of offspring per vial: 9.64, 6.16 and 4.94 respectively, indicating that decrease in biparentals is not compensated by increase in impaternate males as it is when sperm are lacking. It has also been shown (GREB 1933 and WHITING 1935) that reduction in percentage of females occurs in Habrobracon if mated females are X-rayed. This may be due to a greater proportion of dominant lethal chromosomal abnormalities induced in the sperm than in the eggs. WHITING (1936), using parental stocks that were unrelated to each other, has recently reported dominant lethals resulting from treatment of sperm with neutrons. In this case biparental males were lacking, all zygotes being potentially female-producing. The number of females was markedly decreased without any compensating increase in that of impaternate males.