Affecting Nonexchange Chromosome Segregation

A new meiotic mutation, moremight (mzur) was identified by screening for new mutations that act as dominant enhancers of the dosage-sensitive Drosophila melanogaster female meiotic mutant, nod”? mwr is a recessive meiotic mutant, specifically impairing the segregation of nonexchange chromosomes. Cytological evidence suggests that the meitoic defect in mwr/mw females is in homologue recognition because the chromosomes appear to be misaligned on an intact spindle. The mwrmutation was recovered during a screen of random P-element insertions on a chromosome with a single insertion located at 50C. The P-element insertion is a recessive female-sterile mutation. While excision of the P element from the mwr-bearing chromosome partially relieves the female sterility, the excisions retain the dominant nodDm-enhancing activity. The mzur meiotic phenotype maps very close to the female-sterile P insertion. Thus the mzur locus appears to encode a function required for partner recognition i meiosis, although its relationship to the neighboring female-sterile mutation remains to be elucidated. T HE purpose of meiosis is to allow the parent to supply exactly one copy of each chromosome pair to the offspring. This process must be precise because aneuploidy has severe consequences for developing embryos and is often lethal. In most organisms, the two disjoining homologues undergo exchange (physically manifested as chiasmata), forming a physical connection between them. The bivalents are correctly positioned by balancing the poleward forces exerted by the spindle and spindle-based motor proteins of the dynein and kinesin families with the polar ejection force (mainly chiasmata) (NICKLAS 1974; MURRAY and SZOSTAK 1985). Although exchange is generally sufficient to ensure proper alignment and disjunction of homologues (reviewed by HAWLEY 1989), Drosophila females and other organisms have evolved mechanisms for segregating nonexchange (achiasmate) chromosomes, referred to historically in Drosophila as the “distributive system” (GRELL 1962). Nonexchange chromosomes first choose partners. Recent evidence suggests that homologues will generally pair with each other, while nonhomologues will disjoin from each other based on similarities in size and shape (HAWLEX et al. 1993a). While nonhomologue pairs are physically paired before segregation in Saccharomyces cerevisiae (LOIDL et al. 1994), in Drosophila females nonhomologues are not paired but homologues are associated throughout prophase I whether or not they have an exchange (DERNBURG et al. 1996). Heterochromatin appears to mediate the pairing between Addressfor cmrespondence: Rebekah Rasooly, Department of Biological Sciences, St. John’s University, 8000 Utopia Parkway, Jamaica, NY 11439. E-mail: [email protected] Genetics 144: 1725-1734 (December, 1996) chromosomes. In the case of exchange chromosomes, discrete sites of intercalary heterochromatin distributed along the length of the chromosome appear to act as pairing sites (HAWLEY 1980). For nonexchange h o m e logues, the centromeric heterochromatin appears to be the site of pairing (DERNBURG et al. 1996; KARPEN et al.