Ultrastructural investigations on sperm penetration and gamete fusion in the ascidians Boltenia villosa and Phallusia mammillata

Sperm-egg interactions during fertilization were studied by scanning and transmission electron microscopy in the two ascidian species, Boltenia villosa (collected in 1990 near Friday Harbor, Washington, USA) and Phallusia mammillata (collected in 1990 near Brest, France). Emphasis was put on sperm-egg vitelline coat (VC) and sperm-egg plasma membrane interaction. In both species the sperm activation as revealed by translocation of the mitochondrion was exhibited by sperm binding firmly to or penetrating the VC. Vesicles are present in the sperm apex of both species. Their persistence in late stages of VC penetration suggests for them a function rather in Sperm-egg fusion than in VC penetration. In P. mammillata sperm which are about to make contact and to start fusion with the egg plasma membrane, two types of vesicles, one located at the tip and the other slightly behind the apical cap, were detected. Function and fate of these prominent vesicles remain to be determined. Membrane fusion starts between the sperm membrane of the cap-like structure and short microvilli of the egg surface and then proceeds simultaneously at several sites of the apical region. In B. villosa conspicuous microvilli which are formed at the contact site and interdigitate with the corkscrew-like sperm represent fusion sites on the egg. In both ascidian species sperm are incorporated into the egg at an acute angle. Once fully embodied the sperm lie close and almost parallel to the egg membrane. Our findings are compared with observations in Ciona intestinalis, the only other ascidian species in which Sperm-egg interactions have been thoroughly investigated so far. We suggest that observed differences in the fusion processes are based on morphological differences in sperm structure which reflect phylogenetic modifications.