Cohesin is required for meiotic spindle assembly independent of its role in cohesion in C. elegans

Abstract

Accurate chromosome segregation requires a cohesin-mediated physical attachment between chromosomes that are to be segregated apart, and a bipolar spindle with microtubule plus ends emanating from exactly two poles toward the paired chromosomes. We asked whether the striking bipolar structure of C. elegans meiotic chromosomes is required for bipolarity of acentriolar female meiotic spindles by analyzing mutants that lack cohesion between chromosomes. Both a spo-11, rec-8, coh-3, coh-4 quadruple mutant and a spo-11, rec-8 double mutant entered M phase with single chromatids lacking any cohesion. However, the quadruple mutant formed an apolar spindle whereas the double mutant formed a bipolar spindle that segregated chromatids into two roughly equal masses. Residual non-cohesive COH-3/4-dependent cohesin on single chromatids of the double mutant was sufficient to recruit haspin- dependent Aurora B kinase, which regulated the localization of the spindle-assembly factors CLASP-2 and kinesin-13 to mediate bipolar spindle assembly in the apparent absence of chromosomal bipolarity. These results demonstrate that cohesin is essential for spindle assembly and chromosome segregation independent of its role in sister chromatid cohesion.