Differential contributions of β-tubulin isotypes to acentrosomal oocyte meiosis in C. elegans.

Oocyte meiotic spindles must achieve bipolarity and segregate chromosomes in the absence of centrosomes. Here we use high-resolution immunofluorescence microscopy and live imaging to investigate the differential contributions of β-tubulin isotypes (TBB-1 and TBB-2) to assembly and function of acentrosomal spindles in Caenorhabditis elegans oocytes. By combining strains with altered β-tubulin isotype composition with mutations affecting microtubule-crosslinking motor KLP-18 and/or mutations affecting katanin-mediated microtubule severing, we show that TBB-1 and TBB-2 make distinct contributions to promoting spindle bipolarity. Further, by measuring multiple spindle features in wild-type and β-tubulin isotype substitution strains, we reveal contributions of isotype composition to spindle morphology, kinetics of anaphase chromosome separation and maintenance of spindle structural integrity under stress. Together, our data support a model in which β-tubulin isotype composition helps to maintain a balance between microtubule crosslinking and severing activities during oocyte meiosis. We further propose that this balance is crucial for establishing spindle bipolarity, maintaining spindle structures, and modulating the dynamics of chromosome separation.