An Influence of Ploidy on the Time of Expression of Dominant Lethal Mutations in Habrobracon.

HEN Habrobracon eggs are X-rayed at metaphase I and subsequently fertiw l i z e d , the decreased hatchability is attributed to dominant lethal mutation, in the same sense in which the term is used when sperm are irradiated. This is justified by WHITING'S demonstration (1946, 1948, 1949, 1950, 1955) that cytoplasmic injury is not lethal a t the doses employed. In unfertilized eggs, which develop as haploids, both dominant and recessive lethals are expressed, whereas only the dominant effects are expressed in the diploid zygotes. Hence the viable proportion should, in general, be increased if the eggs are fertilized after irradiation. An early experiment of WHITING (19458) showed dominant lethality to be the most frequent consequence of X irradiation of the oocytes, and the expected difference due to recessive lethals was not statistically significant. In further experiments, to be reported here, significant differences have been found, and these can theoretically be used to estimate the frequency of recessive lethals. This frequency can also be obtained by HEIDENTHAL'S method (1952), a test for heterozygosity of the FI females. Thus the recessive embryo lethals induced in oocytes can be estimated from two different data; first, from the daerence in hatchability of fertilized and unfertilized eggs, and second, from the proportion of FI females found by later testing to be heterozygous for such recessive lethals. When these two estimates are made within the same experiment, a curious discrepancy is noted: the apparent frequency of recessive embryo lethals is much greater by the first than by the second method. It will be shown that the disagreement results from the presence of a class of dominant lethals expressed before hatching in haploids, but after hatching in diploids. This leads to underestimation of the dominant lethality in diploids, and a corresponding overestimate of recessive lethals when the hatchabilities of haploids and diploids are compared after irradiation in metaphase I. When eggs are irradiated while in prophase, no such discrepancy is observed. This indicates a somewhat different basis for dominant lethality, depending on the stage irradiated. Aside from the delayed expression of certain dominant lethals, no anomalous effect of the unirradiated chromosome set need be assumed in order to explain the results. Thus it is shown that the introduction of an unirradiated chromosome set, within the time limitations of the technique, does not change the frequency of dominant or recessive lethal mutations in an irradiated set. Although this point may a t first seem trivial, on closer scrutinyit