| Summary: | Activation is an essential process that accompanies fertilisation in all animals and heralds major cellular changes, most notably, resumption of the cell cycle. While activation involves wave-like oscillations in intracellular Ca<sup>2+</sup> concentration in mammals, ascidians and polychaete worms and a single Ca<sup>2+</sup> peak in fish and frogs, in insects, such as <em>Drosophila</em>, to date, it has not been shown what changes in intracellular Ca<sup>2+</sup> levels occur. Here, we utilise ratiometric imaging of Ca<sup>2+</sup> indicator dyes and genetically encoded Ca<sup>2+</sup> indicator proteins to identify and characterise a single, rapid, transient wave of Ca<sup>2+</sup> in the <em>Drosophila</em> egg at activation. Using genetic tools, physical manipulation and pharmacological treatments we demonstrate that the propagation of the Ca<sup>2+</sup> wave requires an intact actin cytoskeleton and an increase in intracellular Ca<sup>2+</sup> can be uncoupled from egg swelling, but not from progression of the cell cycle. We further show that mechanical pressure alone is not sufficient to initiate a Ca<sup>2+</sup> wave. We also find that processing bodies, sites of mRNA decay and translational regulation, become dispersed following the Ca<sup>2+</sup> transient. Based on this data we propose the following model for egg activation in <em>Drosophila</em>: exposure to lateral oviduct fluid initiates an increase in intracellular Ca<sup>2+</sup> at the egg posterior via osmotic swelling, possibly through mechano-sensitive Ca<sup>2+</sup> channels; a single Ca<sup>2+</sup> wave then propagates in an actin dependent manner; this Ca<sup>2+</sup> wave co-ordinates key developmental events including resumption of the cell cycle and initiation of translation of mRNAs such as <em>bicoid</em>.
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