Removal of the block to self-fertilization by low-calcium artificial seawater in the ascidian .

Ascidians (Urochordate) are hermaphroditic marine invertebrates that release sperm and eggs to the surrounding seawater. However, several ascidians, including and show strict self-sterility due to a self/nonself-recognition mechanism in the interaction between sperm and the vitelline coat (VC) of the eggs. We have previously reported that sperm intracellular Ca level drastically increased immediately after sperm binding to the VC of self eggs but not nonself eggs in type A, which was potently inhibited by lowering the external Ca concentration, suggesting that sperm Ca influx occurs after sperm self-recognition on the VC. Here, we investigated whether self-sterility was abolished by lowering the external Ca concentration in The results showed that the block to self-fertilization was removed by low-Ca (∼1 mM) seawater without decreasing the fertilization rate. Such an effect was not observed with Mg or K. These results led us to conclude that a low-Ca environment is sufficient to block the self-recognition signal upon fertilization. As low-Ca seawater showed no effect on self-sterility, we propose that the mechanism of self-sterility in must be distinctive from that in .