Female reproductive cells of most species lack centrosomes, but how spindles form in their absence is poorly understood. Study of oocytes in Caenorhabditis elegans uncovers new steps in this process and reveals mechanisms required for acentrosomal spindle bipolarity via studies of two proteins, KLP-18/kinesin-12 and MESP-1.

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During mitosis, centrosomes serve as microtubule organizing centers that guide the formation of a bipolar spindle. However, oocytes of many species lack centrosomes; how meiotic spindles establish and maintain these acentrosomal poles remains poorly understood. Here, we show that the microtubule polymerase ZYG-9ch-TOG is required to maintain acentrosomal pole integrity in C. elegans oocyte meiosis; following acute depletion of ZYG-9 from pre-formed spindles, the poles split apart and an unstable multipolar structure forms. Depletion of TAC-1, a protein known to interact with ZYG-9 in mitosis, caused loss of proper ZYG-9 localization and similar spindle phenotypes, further demonstrating that ZYG-9 is required for pole integrity. However, depletion of ZYG-9 surprisingly did not affect the assembly or stability of monopolar spindles, suggesting that ZYG-9 is not required for acentrosomal pole structure per se. Moreover, fluorescence recovery after photobleaching (FRAP) revealed that ZYG-9 turns over rapidly at acentrosomal poles, displaying similar turnover dynamics to tubulin itself, suggesting that ZYG-9 does not play a static structural role at poles. Together, these data support a global role for ZYG-9 in regulating the stability of bipolar spindles and demonstrate that the maintenance of acentrosomal poles requires factors beyond those acting to organize the pole structure itself.

In Caenorhabditis elegans oocytes, kinesin-12/KLP-18 is the major force-generating motor that promotes spindle bipolarity. A combination of in vitro and in vivo approaches is used to gain insight into the mechanisms by which KLP-18 and its adaptor MESP-1 promote spindle assembly, and it is shown that KLP-18 is also continuously required to maintain bipolarity.