Cytogenetic analysis of homozygous segregation distorter males of Drosophila melanogaster.

Certain homozygous SD males are nearly sterile. Sterility is not due to aneuploid gametes--no significant second chromosome nondisjunction was found in matings to attached-2 and mei-S332 females. Some fourth chromosome aneuploidy was observed here, and in the cytological work. Otherwise, cytology of the meiotic divisions was essentially normal. Early spermatid bundles are normal, sperm head counts approximating the normal 64. In the later, coiled bundle stage, one observes less than 30 heads many of which are grossly abnormal: twisted, club-like, or globular. In double mating experiments, SD/SD sperm did not displace normal sperm introduced first. In the reverse experiment, sperm (or fluid) from SD/SD males markedly reduced capacity of the females to store and utilize sperm from normal males, as scored from progeny and by counts of stored sperm. No sperm were seen in the storage organs of females imseminated first by SD/SD then by normal males. Many females refuse such a second mating. Our observations are quantitatively different from those with heterozygous SD males, but qualitatively similar, supporting the view that the near sterility of homozygous SD males arises from a mechanism of sperm dysfunction like that in SD/+ males.