Chapter 9 Self- and Nonself-Recognition of Gametes in Ascidians

Abstract

Ascidians (Tunicata) are hermaphroditic sessile marine invertebrates, which release sperm and eggs nearly simultaneously to the surrounding seawater during the spawning season. To avoid inbreeding, several species, including Halocynthia roretzi (Stolidobranchia) and Ciona intestinalis type A (Ciona robusta) (Phlebobranchia) possess a self-sterility system. In H. roretzi, a 70-kDa vitelline coat (VC) protein consisting of 12 EGF-like repeats with polymorphisms, designated as VC70, appears to be involved in gamete interaction and also in self/nonself-recognition. A cysteine-rich secretory protein, designated as Urabin and a type II transmembrane serine protease-1, called TTSP-1, are candidate sperm-borne binding partners for VC70. In C. intestinalis type A, on the other hand, a fibrinogen-like VC protein, v-Themis-A and v-Themis-B, and sperm PKDREJ-like protein, s-Themis-A and s-Themis-B, are highly polymorphic among individuals and these proteins appear to play a pivotal role in self/nonself-recognition during gamete interaction. It was recently suggested that three pairs of v-Themis and s-Themis genes (s/v-Themis-A, s/v-Themis-B, and s/v-Themis-B2) are responsible for this system. After sperm attachment to the VC of self-eggs, drastic Ca2+ influx is elicited, resulting in sperm vigorous movement on the VC followed by sperm detachment from the VC, or by cessation in sperm motility. The C-terminal Ca2+-permeable cation channel domain in s-Themis-B/B2 may be involved in Ca2+ influx. Although s/v-Themis homologous genes with polymorphisms were detected in the genome of H. roretzi, it is not known whether s/v-Themis is involved in self/nonself-recognition of H. roretzi. Since flowering plants utilize family specific S-determinant proteins for self-incompatibility, the mechanism of self/nonself-recognition may be different between stolidobranch and phlebobranch ascidians.