Genetic analysis of a mutation defective in chromosome partner choice

Abstract

This report describes the isolation and characterization of Aberrant X segregation (Axs), a dominant female-specific meiotic mutation in Drosophila melanogaster. Axs has little or no effect on the frequency or distribution of exchange, or on the disjunction of exchange bivalents. Nonexchange X chromosomes nondisjoin at high frequencies in Axs females which is shown to result from an Axs-induced defect in distributive segregation. Fourth chromosome nondisjunction in Axs is observed only in the presence of high frequencies of nonexchange X chromosomes and is argued to be the result of improper X-4 associations within the distributive system. Indeed, in XX females bearing a compound 4 chromosome, the frequency of nonhomologous disjunction of the X chromosomes from the compound 4 accounts for at least 80% of the total X nondisjunction observed. Axs diminishes or ablates the capacity of nonexchange X chromosomes to form trivalents in females bearing either a Y or a small free duplication for the X, and also impairs compound X from Y segregation. The effect of Axs on these segregations parallels the defects observed for homologous nonexchange X segregation. In addition to its dramatic effects on X chromosome disjunction, Axs exerts a similar effect on the segregation of the major autosomes. It is concluded that Axs defines a locus required for proper homolog disjunction within the distributive system.