Meiotic Chromosome Behavior Influenced by Melanogaster Females Mutation-altered Disjunction in Drosophila

The effects of a female-specific meiotic mutation, altered disjunction (old: 361), are described. Although old females show normal levels of meiotic exchange, sexand 4th-chromosome nondisjunction occurs at an elevated level. A large proportion of the nondisjunction events is the result of nonhomologous disjunction of the sex and 4th chromosomes. These nonhomologous disjunction events, and probably all nondisjunction events occurring in old females, are the result of two anomalies in chromosome behavior: (1) X chromosomes derived from exchange tetrads undergo nonhomologous disjunction and (2) the 4th chromosomes nonhomologously disjoin from larger chromosomes. There is at best a marginal effect of old on the meiotic behavior of chromosomes 2 or 3. The results suggest that the old' gene product acts to prevent the participation of exchange X chromosomes and all 4th chromosomes in nonhomologous disjunction events. The possible role of old+ in current models of the disjunction process is considered. 0 account for the occurrence of nonhomologous disjunction, GRELL (1962a; T reviewed by GRELL 1976) proposed that two pathways of meiotic disjunction exist in Drosophila females. Homologs that have recombined will regularly disjoin via the exchange disjunction pathway. A second mode of disjunction, known as distributive disjunction, is also utilized. Only chromosomes that have not secured a disjunction partner by exchange are available for distributive disjunction. Thus, this mode is only used by the 4th chromosomes, which do not recombine, other chromosomes that have failed to recombine, and special rearrangements such as compound chromosomes. Unlike exchange disjunction, distributive disjunction partners are not established by homology; this system, therefore, supports the disjunction of both homologs and nonhomologs. Similarity in the sizes of chromosomes, at least for chromosomes approximately the size of the 4th chromosome, is important in establishing disjunctional relationships (GRELL 1964; MOORE and GRELL 1972). It is also known that distributive relationships are not limited to pairs of chromosomes-a metacentric chromosome will disjoin distributively from two acrocentric chromosomes (COOPER 1948; GRELL 1962b), and in many other genotypes trivalent formation is also evident (GRELL 1976). Present address: Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907 Genetics 102 503-524 November, 1982.