Figure 1.—

Mechanisms that ensure the association of homologous chromosomes in Drosophila oocytes. Pink and blue are used to differentiate the homologous chromosomes (each composed of two sister chromatids). Sister-chromatid cohesion proteins are depicted as black horizontal lines. In Drosophila oocytes, cohesion proteins (SMC1, SMC3, and ORD) are highly enriched near the centromeres (white bilobed structures). Black stars represent heterochromatin-mediated pairing of homologous chromosomes that is required for accurate segregation of achiasmate bivalents. (A) Cohesion along the arms of sister chromatids provides an evolutionarily conserved mechanism to maintain the association of recombinant homologs until anaphase I. (B) In Drosophila oocytes, bivalents that lack a crossover rely on the achiasmate pathway to keep homologous chromosomes associated and thereby ensure their proper segregation. (Note that heterochromatin-mediated pairing of chiasmate homologs also occurs, but is not required for their accurate disjunction unless chiasmata are not maintained.)