Egg activation is a series of highly coordinated processes that prepare the mature oocyte for embryogenesis. Typically associated with fertilization, egg activation results in many downstream outcomes, including the resumption of the meiotic cell cycle, translation of maternal mRNAs and cross-linking of the vitelline membrane. While some aspects of egg activation, such as initiation factors in mammals and environmental cues in sea animals, have been well-documented, the mechanics of egg activation in insects are less well-understood. For many insects, egg activation can be triggered independently of fertilization. In Drosophila melanogaster, egg activation occurs in the oviduct resulting in a single calcium wave propagating from the posterior pole of the oocyte. Here we use physical manipulations, genetics and live imaging to demonstrate the requirement of a volume increase for calcium entry at egg activation in ex vivo mature Drosophila oocytes. The addition of water, modified with sucrose to a specific osmolarity, is sufficient to trigger the calcium wave in the mature oocyte and the downstream events associated with egg activation. We show that the swelling process is regulated by the conserved osmoregulatory channels, aquaporins and DEGenerin/Epithelial Na⁺ channels. Furthermore, through pharmacological and genetic disruption, we reveal a concentration-dependent requirement of transient receptor potential M channels to transport calcium, most probably from the perivitelline space, across the plasma membrane into the mature oocyte. Our data establish osmotic pressure as a mechanism that initiates egg activation in Drosophila and are consistent with previous work from evolutionarily distant insects, including dragonflies and mosquitos, and show remarkable similarities to the mechanism of egg activation in some plants.

卵子激活是一系列高度协调的过程，为成熟的卵母细胞准备胚胎发生。通常与受精相关，卵子激活会导致许多下游结果，包括减数分裂细胞周期的恢复、母体 mRNA 的翻译和卵黄膜的交联。虽然卵子激活的某些方面，例如哺乳动物的启动因子和海洋动物的环境线索，已被充分记录，但昆虫卵子激活的机制却鲜为人知。对于许多昆虫来说，卵激活可以独立于受精触发。在黑腹果蝇，卵子激活发生在输卵管中，导致从卵母细胞后极传播的单个钙波。在这里，我们使用物理操作、遗传学和实时成像来证明在体外成熟的果蝇卵母细胞中卵子激活时钙进入的体积需要增加。添加用蔗糖修饰至特定渗透压的水足以触发成熟卵母细胞中的钙波和与卵子激活相关的下游事件。我们表明，肿胀过程受保守的渗透压调节通道、水通道蛋白和 DEGenerin/上皮 Na ^{+的调节。}渠道。此外，通过药理学和遗传破坏，我们揭示了瞬时受体电位 M 通道的浓度依赖性要求，以将钙（很可能从卵周空间）转运到成熟的卵母细胞中。我们的数据将渗透压确定为在果蝇中启动卵子激活的机制，并且与进化上遥远的昆虫（包括蜻蜓和蚊子）先前的研究结果一致，并且与某些植物的卵子激活机制具有显着的相似性。