Distributive segregation

Abstract: In Drosophila melanogster females the segregation of nonexchange chromosomes is ensured by the distributive segregation system. The mutation noda specifically impairs distributive disjunction and induces nonexchange chromosomes to undergo nondisjunction, as well as both meiotic and mitotic chromosome loss. We report here the isolation of seven recessive X-linked mutations that are allelic to noda. As homozygotes, all of these mutations exhibit a phenotype that is similar to that exhibited by noda homozygotes. We have also used these mutations to demonstrate that nod mutations induce nonexchange chromosomes to nondisjoin at meiosis II. Our data demonstrate that the effects of noda on meiotic chromosome behavior are a general property of mutations at the nod locus. Several of these mutations exhibit identical phenotypes as homozygotes and as heterozygotes with a deficiency for the nod locus; these likely correspond to complete loss-of-function or null alleles. None of these mutations causes lethality, decreases the frequency of exchange, or impairs the disjunction of exchange chromosomes in females. Thus, either the nod locus defines a function that is specific to distributive segregation or exchange can fully compensate for the absence of the nod+ function.

Abstract: The basic aspects of the achiasmate distributive segregation system in Drosophila are briefly reviewed. Specific attention is given to the role of homology in specifying the patterns of distributive segregation. The genetic and molecular analysis of mutations affecting the system is also discussed. Finally, a consideration of the available evidence allows the presentation of a revised model of meiosis in Drosophila.