Mechanisms of chromosome orientation revealed by two meiotic mutants in Drosophila melanogaster.

Two disjunction defective meiotic mutants, ord and mei-S332, each of which disrupts meiosis in both male and female Drosophila melanogaster, were analyzed cytologically and genetically in the male germ-line. It was observed that sister-chromatids are frequently associated abnormally during prophase I and metaphase I in ord. Sister chromatid associations in mei-S332 are generally normal during prophase I and metaphare I. By telophase I, sister chromatids have frequently precociously separated in both mutants. During the first division sister chromatids disjoin from one another frequently in ord and rarely in mei-S332. It is argued that the simplest interpretation of the observations is that each mutant is defective in sister chromatid cohesiveness and that the defect in ord manifests itself earlier than does the defect in mei-S332. In addition, based on these mutant effects, several conclusions regarding normal meiotic processes are drawn. (1) The phenotype of these mutants support the proposition that the second meiotic metaphase (mitotic-type) position of chromosomes and their equational orientation is a consequence of the equilibrium, at the metaphase plate, of pulling forces acting at the kinetochores and directed towards the poles. (2) Chromosomes which lag during the second meiotic division tend to be lost. (3) Sister chromatid cohesiveness, or some function necessary for sister chromatid cohesivenss, is required for the normal reductional orientation of sister kinetochores during the first meiotic division. (4) The kinetochores of a half-bivalent are double at the time of chromosome orientation during the first meiotic division. Finally, functions which are required throughout meiosis in both sexes must be considered in the pathways of meiotic control.