Osmolarity-regulated swelling initiates egg activation in Drosophila

Egg activation is a series of highly coordinated processes that prepare the mature oocyte for embryogenesis. Typically associated with fertilisation, 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 fertilisation. 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 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 (AQPs) and DEGenerin/Epithelial Na+ (DEG/ENaC) channels. Furthermore, through pharmacological and genetic disruption, we reveal a concentration-dependent requirement of Trpm channels to transport calcium, most likely from the perivitelline space, across the plasma membrane into the mature oocyte. Our data establishes osmotic pressure as the mechanism that initiates egg activation in Drosophila and is consistent with previous work from evolutionarily distant insects, including dragonflies and mosquitos, and shows remarkable similarities to the mechanism of egg activation in some plants.