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 fi brinogen-like ligands on the VC (v-Themis-A and v-Themis-B) and cognate sperm-side hypervariable regioncontaining 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 Ca 2+ infl ux, 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 fl owering plants.