Visualization of the biphasic calcium wave during fertilization in Caenorhabditis elegans using a genetically encoded calcium indicator

Fertilization is a critical step in development, yet internal fertilization events are notoriously difficult to visualize. Taking advantage of the calcium response that is a hallmark of sperm-egg fusion, we adapted the genetically encoded calcium indicator jGCaMP7s to visualize the moment of fertilization in Caenorhabditis elegans using fluorescence. We termed this tool the 'CaFE' reporter, for 'calcium during fertilization in C. elegans'. The CaFE reporter produced a robust signal that recapitulated the previously reported, biphasic nature of the calcium wave and had no significant deleterious effects on worm physiology or fecundity. Calcium waves were not observed at the restrictive temperature in the spe-9(hc88) strain, in which sperm can still trigger meiotic maturation but can no longer fuse with the oocyte. Demonstrating the utility of the CaFE reporter, we analyzed polyspermy induced by inhibition of egg-3 via RNAi and observed late calcium waves in the uterus. This finding provides support to the idea that calcium release is not restricted to the first sperm fusion event during polyspermy. Establishment of the CaFE reporter in the genetically tractable and optically transparent worm provides a powerful tool to dissect the oocyte-to-embryo transition inside a living animal.