How noncrossover homologs are conjoined and segregated in Drosophila male meiosis I: Stable but reversible homolog linkers require a novel Separase target protein

摘要

Segregation of homologous chromosomes is the defining event of meiosis and is central to sex- ual reproduction. To ensure regular segregation, homologs must establish strong but reversible physical links capable of providing the resistance to poleward spindle forces needed for stable biorientation of homologous centromeres. What form do such linkages take? In the most widespread mechanism, the linkers are chiasmata, in which crossovers between homologous chromatids are anchored by sister chromatid cohesion distal to the crossover (see Fig 1A for details) [1,2]. The anchoring cohesin complexes are similar in structure and function to the pericentromeric cohesins that enable mitotic sister chromatids to biorient and segregate prop- erly. Cohesins are ring-shaped complexes that embrace pairs of sister chromatids at their inception and keep them together throughout the cell cycle until anaphase, when cleavage of the alpha-kleisin subunit (usually Rad21/SCC1 (Sister chromatid cohesion protein 1) or its meiosis-specific paralog Rec8) by Separase breaks the cohesin ring and releases the entrapped chromatids [1,2]. When activated at anaphase I, Separase specifically cleaves distal cohesins, thereby dissolving the chiasmata and releasing homologs (Fig 1A) [1]. Thus, chiasmata can be viewed as devices that repurpose sister chromatid cohesins as homolog linkers. This raises an important question—are there alternative mechanisms that link homologs directly, and if so, how do they work?