Asynapsis and mutability in Drosophila melanogaster.

Abstract

From analysis of salivary gland configurations and patterns of nondisjunction, it is estimated that the highly rearranged Second Multiple 5 (SM5) chromosome does not pair extensively with its homologue. The frequency of chromosome II lethal mutations was significantly higher among sperm from asynaptic SM5/+ males (19.5%) than among sperm from control males (8.6%) six to eight days after treatment with 1000 r of x rays. These sperm batches are interpreted as having been in or near meiosis at the time of irradiation. This sensitivity of SM5/+ males was found not to extend to the X chromosome after irradiation, indicating that such males do not have an inherently high mutability. The rate of spermiogenesis or sperm maturation in SM5/+ males and control males was compared by scoring induced crossovers, fecundity, and egg hatchability during single-day mating periods after irradiation. No difference in the pattern of these effects was observed, so that the maturation time of both groups of males is ap parently the same. The interpretation that asynapsis is an important factor during meiosis is supported by these findings. It is suggested that during meiosis the synapsis of homologous chromosomes favors the repair of chromosomal disruptions over the formation of rearrangements,more » and that asynapsis may greatly increase the frequency of lethal mutations. (auth)« less