Analysis of the regulation of meiosis-specific chromosome segregation

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During mitosis, sister kinetochores attach to microtubules that extend to opposite spindle poles and pull the chromatids apart at anaphase (equational segregation). A multisubunit complex called cohesin, including Rad21/Scc1, plays a crucial role in sister chromatid cohesion and equational segregation at mitosis. Meiosis I differs from mitosis in having a reductional pattern of chromosome segregation, in which sister kinetochores are attached to the same spindle. During meiosis, Rad21/Scc1 is largely replaced by its meiotic counterpart, Rec8. If Rec8 is inactivated in fission yeast, meiosis I is shifted from reductional to equational division. However, the reason rec8Δ cells undergo equational rather than random division has not been clarified. Here we have shown that the equational segregation at meiosis I depends on substitutive Rad21, which relocates to the centromeres if Rec8 is absent. Therefore, cohesin is a crucial determinant of the attachment manner of kinetoclores to the spindle microtubules at meiosis I in fission yeast. It is well known that sister chromatid cohesion must persist at centromeres throughout the first meiotic division (meiosis I) when only arm cohesion is destroyed. The factors that protect centromeric cohesion during meiosis I have remained, elusive. Here we identify Sgo1 (Shugoshin), a protector of the centromeric Rec8 cohesin in fission yeast, and a homologue in budding yeast. We also suggest that shugoshin is widely conserved among eukaryotes. Moreover, we identify Sgo2, a paralogue of shugoshin in fission yeast, which is required for faithful mitotic chromosome segregation. These findings provide insights into the evolution of meiosis and kinetocho regulation at mitosis and meiosis.