A SPERM CHEMOATTRACTANT IS RELEASED FROM XENOPUS EGG JELLY DURING SPAWNING

Although the jelly layers surrounding amphibian eggs are known to be e ssential to fertilization, no biological activities have been reported for jelly macromolecules in any vertebrate. Here we provide evidence that the jelly surrounding Xenopus laevis eggs releases a small diffus ible protein into the surrounding media that serve as sperm chemoattra ctant. Using video microscopy we find that Xenopus sperm will preferen tially turn toward and contact a glass capillary filled with egg jelly extract. In experiments using a two-chamber bioassay device we find t hat egg jelly extracts are capable of stimulating sperm movement acros s a membrane barrier sixfold over controls. This activity is not obser ved in materials unrelated to egg jelly and the response of sperm to e gg jelly extract is clearly chemotactic rather than chemokinetic. A co ncentration gradient of the extract is absolutely necessary and the ch emotactic activity of the extract exhibits a biphasic dose dependence similar to that of chemotactic agents in other systems. We have been a ble to characterize the factor as being a heat-stable protein about 10 kDa in size. This study, therefore, provides the first clear evidence for a diffusible sperm chemoattractant in a nonhuman vertebrate, as w ell as the first demonstration of a physiological role for egg jelly m acromolecules in Xenopus fertilization. (C) 1998 Academic Press.