Asymmetric divisions that yield daughter cells of different sizes are frequent during early embryogenesis, but the importance of such a physical difference for successful development remains poorly understood. Here, we investigated this question using the first division of C. elegans embryos, which yields a large AB cell and a small P₁ cell. We equalized AB and P₁ sizes using acute genetic inactivation or optogenetic manipulation of the spindle positioning protein LIN-5. We uncovered that only some embryos tolerated equalization, and that there was a size asymmetry threshold for viability. Cell lineage analysis of equalized embryos revealed an array of defects, including faster cell cycle progression in P₁ descendants, as well as defects in cell positioning, division orientation and cell fate. Moreover, equalized embryos were more susceptible to external compression. Overall, we conclude that unequal first cleavage is essential for invariably successful embryonic development of C. elegans.

中文翻译：

---

线虫受精卵的物理不对称分裂确保胚胎发生始终成功


在早期胚胎发生过程中，产生不同大小子细胞的不对称分裂很常见，但这种物理差异对于成功发育的重要性仍然知之甚少。在这里，我们使用线虫胚胎的第一次分裂来研究这个问题，该胚胎产生一个大的 AB 细胞和一个小的 P ₁细胞。我们通过对纺锤体定位蛋白 LIN-5 进行急性基因失活或光遗传学操作来平衡 AB 和 P ₁的大小。我们发现，只有一些胚胎能够耐受均等，并且存在生存能力的尺寸不对称阈值。均衡胚胎的细胞谱系分析揭示了一系列缺陷，包括 P ₁后代细胞周期进展更快，以及细胞定位、分裂方向和细胞命运方面的缺陷。此外，均衡的胚胎更容易受到外部压力的影响。总的来说，我们得出的结论是，不平等的第一次卵裂对于线虫胚胎发育的成功至关重要。