Allorecognition and Lysin Systems During Ascidian Fertilization

Abstract

Ascidians (primitive chordates) are hermaphroditic animals that release sperm and eggs almost simultaneously, but several species, including Halocynthia roretzi and Ciona intestinalis, show strict self-sterility. In H. roretzi, a 70-kDa vitelline coat (VC) protein consisting of 12 EGF-like repeats (HrVC70) appears to be a promising candidate for the self/non-self-recognition (or allorecognition) system during gamete interaction. After sperm recognizes the VC as non-self, the sperm extracellular ubiquitin-proteasome system appears to degrade HrVC70, allowing sperm to penetrate through the VC with the aid of sperm trypsin-like proteases. In C. intestinalis, egg-side highly polymorphic fibrinogen-like ligands on the VC (v-Themis-A and v-Themis-B) and cognate sperm-side hypervariable region-containing polysystin-1-like receptors (s-Themis-A and s-Themis-B) seem to be responsible for allorecognition in gamete interaction. Recently, we noticed that a novel pair of v-Themis-B2 and s-Themis-B2 and an acid-extractable VC protein called Ci-v-Themis-like may take part in gamete interaction or allorecognition. When sperm recognizes the VC as self, the sperm undergoes a drastic Ca2+ influx, which is one of the major intracellular self-recognition responses within sperm, resulting in sperm detachment from the VC or in sperm becoming quiescent. These allorecognition systems and self-recognition responses within sperm are very similar to the self-incompatibility system in flowering plants.