Ascidian interspecific fertilization. III. Ultrastructural investigations of sperm-egg interaction.

Transmission electron microscope investigations on ascidian interspecific fertilization have been carried out on some crosses: Ascidia malaca female x Ascidiella aspersa male, Ascidia malaca female x Phallusia mammillata male, Ascidiella aspersa female x Ascidia malaca male. The morphological aspects of heterospecific sperm-egg interaction were compared with those of homospecific fertilization to complete the previous light and scanning electron microscope observations in this field. Sperm behaviour in homo- and heterospecific crosses mainly differs in the power of sperm passageway through the egg envelopes. Foreign spermatozoa, firstly interacting with the follicle cells (FCs), remain on the FC surface, leaving the interfollicular clefts empty. In homospecific fertilized eggs the initial events, also observed in hybrid crosses, are: attraction, entrapment and sperm mitochondrion translocation. But in homospecific crosses the FCs, by means of projections on their surface, facilitate the sperm path between the clefts for reaching the vitelline coat; the clefts are full of spermatozoa. These observations suggest a role of initial reproductive barrier for the FCs.