CYTOGENETICS OF A UNIVALENT CHROMOSOME IN BS DROSOPHILA MELANOGASTER MALES

Bs males bearing a modified X:4 translocation in which one of the members, XPYL∙YS, is an invariable univalent show that the univalent-bearing gametes from males raised at 26.0 °C are recovered only half as frequently as from those raised at 18.0 °C (Zimmering, 1963).Such males were raised at 26.0 °C (Experiments I and II) and at 18.0 °C (Experiment III) and upon eclosion were mated to three yellow free-X females for 3 days at 26.0 °C. Testes of sib males were squashed and examined with phase optics to follow the meiotic behavior of the univalent. All tests were carried out over a 3-year period.The genetic results confirm those of Zimmering (1963) and show that the univalent is recovered with a frequency of 22.5 ± 0.9% and 18.5 ± 0.9% in Experiments I and II, whereas it is recovered with a frequency of 36.7 ± 1.6% in Experiment III. However, the cytological results show that while the univalent lags at A I with a frequency of 9.4, 28.7, and 32.2%, respectively, cells with and without XPYL∙YS are observed with equal frequency at M II and A II in all experiments. The post meiotic stages also appear to be normal at the level of the light microscope. Although negligible loss of the univalent is not excluded, it is clear that the frequency of XPYL∙YS lag is unrelated to temperature or to the distorted genetic ratios from males raised at 26.0 °C. Degeneration of XPYL∙YS-bearing sperm beyond the level of the light microscope remains as a possible explanation for the observed genetic distortion.