THE GENETIC-ANALYSIS OF ACHIASMATE SEGREGATION IN DROSOPHILA-MELANOGASTER .3. THE WILD-TYPE PRODUCT OF THE AXS GENE IS REQUIRED FOR THE MEIOTIC SEGREGATION OF ACHIASMATE HOMOLOGS

The regular segregation of achiasmate chromosomes in Drosophila melano gaster females is ensured by two distinct segregational systems. The s egregation of achiasmate homologs is assured by the maintenance of het erochromatic pairing; while the segregation of heterologous chromosome s is ensured by a separate mechanism that may not require physical ass ociation. Axs(D) (Aberrant X segregation) is a dominant mutation that specifically impairs the segregation of achiasmate homologs; heterolog ous achiasmate segregations are not affected. As a result, achiasmate homologs frequently participate in heterologous segregations at meiosi s I. We report the isolation of two intragenic revertants of the Axs(D ) mutation (Axs(r2) and Axs(r3)) that exhibit a recessive meiotic phen otype identical to that observed in Axs(D)/Axs(D) females. A third rev ertant (Axs(r1)) exhibits no meiotic phenotype as a homozygote, but a meiotic defect is observed in Axs(r1)/Axs(r2) females. Therefore mutat ions at the Axs(D) locus define a gene necessary and specific for homo logous achiasmate segregation during meiosis. We also characterize the interactions of mutations at the Axs locus with two other meiotic mut ations (ald and ncd). Finally, we propose a model in which Axs+ is req uired for the normal separation of paired achiasmate homologs. In the absence of Axs+ function, the homologs are often unable to separate fr om each other and behave as a single segregational unit that is free t o segregate from heterologous chromosomes.