Divergence of Cortical Force-Generating Mechanisms Underlies Differences in Spindle Behavior between C. elegans and C. inopinata

Microtubule-dependent pronuclear migration and mitotic spindle positioning are fundamental processes during the first embryonic division in many animals. In the one-cell embryo of Caenorhabditis elegans , these events are regulated by well-characterized pulling forces acting on astral microtubules, including cortical forces mediated by the Gα-GPR-LIN-5 dynein complex. Although the overall framework of these dynamics is conserved, recent studies have revealed substantial interspecies variation in their regulation. Here, we investigated nuclei and mitotic spindle behaviors in one-cell embryos of Caenorhabditis inopinata , the closest known relative of C. elegans , using live-cell imaging and functional perturbation. We found that C. inopinata embryos exhibit altered pronuclear migration, reduced anaphase spindle oscillations, and slower centrosome diffusion during telophase compared with C. elegans . These differences suggest weaker cortical pulling forces. Functional analyses using RNA interference showed that GPR retains its essential role in force generation, whereas the contribution of the microtubule depolymerizing kinesin KLP-7 is reduced in C. inopinata . Our results point to evolutionary changes in microtubule-regulated spindle dynamics, and provide insight into how conserved cellular processes can diversify through subtle changes in their underlying mechanisms.