Chromosome mis-segregation during meiosis leads to aneuploidy, and hence to reduced fertility and birth defects. The frequency of aneuploidy increases in older mothers. Various mechanisms have been proposed to account for this correlation between maternal age and chromosome segregation defects, most placing the emphasis on errors occurring in meiosis I (MI). Keith Jones and colleagues (p. 199) now use sophisticated live imaging of young and aged mouse oocytes to follow individual bivalents through MI to metaphase II (metII) arrest. In aged oocytes, they observe defects in MI such as weakly attached bivalents and lagging chromosomes during anaphase, but surprisingly find no significant defects in chromosome congression or bivalent segregation. Instead, their data suggest that aneuploidy in aged oocytes is primarily a result of premature separation of dyads during meiosis II, likely during assembly of the metII spindle. The authors propose that these errors are a result of cohesion loss during MI, but - in contrast to previous proposals - that the major segregation errors only occur during meiosis II.
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