The role of the chromosomal passenger complex and condensin complex in meiotic chromosome cohesion and segregation

Abstract

The canonical mitotic cell cycle is modified in metazoans to achieve a variety of developmental goals. Among these, meiotic divisions reduce the DNA content of the cell, haploid gametes fuse and reenter mitotic cycling, and mitoses of early embryogenesis, in many animals, employ a rapid cell cycle that lacks gap phases. Much less is understood about regulation of these developmentally regulated cell cycles, than about canonical mitosis, because they cannot be studied in tissue culture systems and because mutations that specifically affect these processes are rare. This thesis describes a screen to characterize mutants that display defects in these programs in Drosophila melanogaster. This model system allowed use of powerful tools for genetic and cell biological analyses of these mutants. Nineteen mutants with defects in this developmental window were screened and placed them into five phenotypic classes, including mutants that displayed defects in fertilization, pronuclear fusion, and mitosis and DNA synthesis in the early divisions of embryogenesis. Among these mutations, one was mapped and molecularly characterized as an allele of the passenger complex component incenp.

(cont.) This allele was used to investigate roles of the passenger complex in meiosis in both male and female Drosophila. incenp mutant males displayed missegregation of sex chromosomes in both meiosis I and meiosis II, and this is due, at least in part, to premature loss of cohesion between sister chromatids. incenp is required for proper localization of MEI-S332, an essential protector of meiotic centromere cohesion, in spermatocytes. MEI-S332 is phosphorylated by Aurora B/INCENP in vitro at a specific Aurora B target site. Mutation of this site disrupts MEI-S332 localization to the centromere, suggesting a model in which phosphorylation of MEI-S332 by the passenger complex is required for proper localization of MEI-S332, and thereby required for maintenance of sister-chromatid cohesion. In female meiosis, incenp is required for maintenance of the synaptonemal complex and for proper formation of the metaphase I chromosome configuration. Characterization of female-sterile alleles of the condensin component dcap-g revealed a role for the condensin complex in disassembly of the synaptonemal complex and also for metaphase I chromosome behavior, but in a manner distinct from the role played by incenp.

(cont.) The differences between the meiotic roles of dcap-g and incenp are striking, given that both have characterized roles in chromosome condensation in mitosis and that the passenger complex is required for localization of the condensin complex in several systems. In addition, ord, another player in meiotic chromosome condensation that also has essential roles in meiotic cohesion, interacts genetically with incenp.