SEGREGATION-DISTORTION AND REGULARLY NONFUNCTIONAL PRODUCTS OF SPERMATOGENESIS IN DROSOPHILA MELANOGASTER

Abstract

A cytological analysis was made of the mode of action of Segregation-Distorter, a second-chromosome locus which demonstrates an unusually high frequency of recovery from heterozygous males. Meiosis and sperm development were found to be without any visible abnormalities, and it was shown that the SDf-bearing chromosome was included in 50 percent of motile sperm. When the number of sperm transferred in an insemination was less than the storage capacity of the female (ca. 700) all sperm entered the storage organs. Counts of stored sperm and of progeny recovered from comparable females showed that only one half of the sperm are able to fertilize eggs; the nonfunctional sperm all carry the SDf- bearing chromosome. Where the number of sperm transferred is greater than the storage capacity or where the numbers are very small, competitive inclusion results in a departure from the 2:1 ratio of stored sperm to progeny. These same relationships are found with control wild-type males. The conclusion was made that D. melanogaster regularly forms two functional and two nonfunctional sperm from each primary spermatocyte. The determination of these two classes of sperm occurs at the first meiotic division, where an inequality of the two spindle poles is proposed. The mode of operation of SD is regarded to be a specific orientation at metaphase I relative to this polarity, such that the SD-bearing homologue of the second chromosome bivalent moves to that pole, leading to the production of two functional gametes.