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.