Dominant Lethal and Inactivation Effects of Nitrogen Mustard on Habrobracon Sperm.

S early as 1911 0. HERTWIC demonstrated two types of behavior of irA radiated Amphibian sperm. One of these, the dominant lethal effect, resulted in death of embryos due to fusion of injured sperm nucleus with egg nucleus. The other, inactivation of sperm, stimulated the egg to develop gynogenetically since injured sperm entered it but took no part in development. PACKARD (1914) observed, in addition to these phenomena, a second type of inactivation in which heavily irradiated Nereis sperm failed to penetrate the egg. With this type likewise, eggs were activated to develop gynogenetically. MULLER (1927) used the partial sterility of irradiated Drosophila males as evidence “for the first time, of the occurrence of dominant lethal genetic changes, both in the X and in the other chromosomes.’’ The parasitic wasp Habrobracon juglandis was used for the present study. In the cross utilized about 62% of the eggs are fertilized and develop into diploid females ; 38% are not fertilized and develop parthenogenetically into haploid males. In this type of reproduction dominant lethal effects can be distinguished from sperm inactivation without cytological study. Any treatment of sperm which reduces hatchability at the expense of females, leaving number of males unchanged, has induced dominant lethal effects only. When hatchability is reduced to 38% and all survivors are males, there has been induced in each sperm at least one dominant lethal. When, after the attainment of complete dominant lethality, hatchability rises above 38% and all survivors are males, some sperm have been inactivated and are either entering the eggs without taking part in development or are failing to enter them. These facts are contrasted with expectation in a form like Drosophila in table 1. In making tests for distinguishing dominant lethal effects from inactivation, an advantage lies in the fact that polyspermy is rare in Habrobracon. About 1% of fertilized eggs have two sperm in them. Almost every fertilized egg therefore represents a test of a single sperm so that masking of inactivation by dominant lethality occurs rarely. STANCATI (1932) recognized the possibility of inactivation and the fact that it could be identified readily in Habrobracon but showed conclusively that after exposure of males to 2500 r, dominant lethal changes only were