Genetic analysis of revertant alleles of Axs(D), a meiotic mutation in Drosophila melanogaster

Abstract

The regular segregation of achiasmate chromosomes during the first meiotic division in Drosophila melanogaster females is ensured by two distinct segregational systems. The segregation of achiasmate homologs is assured by the maintenance of heterochromatic pairing; while the segregation of heterologous chromosomes is ensured by a separate mechanism that may not require physical association. {dollar}Axs\sp{lcub}D{rcub}{dollar} (Aberrant X segregation) is a semi-dominant mutation that specifically impairs the segregation of achiasmate homologs; heterologous achiasmate segregations are not affected in that achiasmate homologs participate in heterologous achiasmate segregations in {dollar}Axs\sp{lcub}D{rcub}{dollar} mutants. Two intragenic revertants of the {dollar}Axs\sp{lcub}D{rcub}{dollar} mutation were isolated, {dollar}Axs\sp{lcub}r2{rcub}{dollar} and Axs{dollar}\sp{lcub}r3{rcub},{dollar} that exhibited a recessive meiotic phenotype identical to that observed in {dollar}Axs\sp{lcub}D{rcub}{dollar} females. These revertants were further characterized as loss-of-function mutations; therefore they defined a gene necessary for homologous achiasmate segregation during meiosis. A third revertant, {dollar}Axs\sp{lcub}r1{rcub},{dollar} exhibited no meiotic phenotype as a homozygote, but induced {dollar}Axs\sp{lcub}D{rcub}{dollar}-like nondisjunction when heterozygous to {dollar}Axs\sp{lcub}r2{rcub},{dollar} and enhanced the {dollar}Axs\sp{lcub}D{rcub}{dollar} mutation. Interactions and the functional relationship with a similar mutation, ald (altered disjunction), were genetically characterized. Also described is an affect of {dollar}Axs\sp{lcub}D{rcub}{dollar} on chiasmate chromosomes in that exchange X chromosomes were found among the secondary exceptions from XXY mothers. The recent findings of the interaction with the ncd (non-claret disjunctional) gene and the cytology of {dollar}Axs\sp{lcub}D{rcub}{dollar} during meiotic prophase to metaphase in Stage 14 oocytes is discussed. Finally, a model for {dollar}Axs\sp+{dollar} is proposed in which the gene product is required for the normal separation of paired achiasmate homologs. In the absence of {dollar}Axs\sp+{dollar} function, the homologs are often unable to separate from each other and behave as a single segregational unit that is then free to segregate from heterologous chromosomes.