Ascidian Sexual Reproductive Strategies: Mechanisms of Sperm-Egg Interaction and Self-Sterility

Ascidians (protochordate) are hermaphrodites that release sperm and eggs nearly simultaneously. However, self-fertilization is prohibited by self-sterility mechanisms during interaction between sperm and the vitelline coat (VC) of the eggs in many ascidian species, including Ciona robusta(former name: Ciona intetsinalis type A) and Halocynthia roretzi. A recent genetic study in C. robusta revealed that two tightly linked gene pairs in loci A and B, i.e., the sperm PKDREJ-like receptor s-Themis-A and the VC fibrinogen-like ligand v-Themis-A, and s-Themis-B and v-Themis-B, which include highly variable regions among individuals, are responsible for self-recognition. Sperm recognizes an egg as a self-egg when both alleles of s/v-Themis-A and s/v-Themis-B possess the same haplotypes. When attached to the VC of self-eggs, acute and drastic Ca2+ influx takes place in the sperm head and flagella probably via the Ca2+-conducting cation channel in the C-terminal region of s-Themis-B, which results in sperm detachment from the VC or decrease in sperm motility. We recently identified v-Themis-like, an acid-extractable VC protein with no allelic polymorphism, as a new candidate that participates in self-sterility. This self-sterility mechanism is closely related to the self-incompatibility systems in angiosperms. A different ascidian, Halocynthia roretzi, utilizes a different self/nonself-recognition system during fertilization, using an EGF-like repeat-containing VC protein, HrVC70. Moreover, the genome database of H. roretzi contains four pairs of s/v-Themis homologs. These gene products may also play a role in self-sterility in this species. This chapter describes the historic and current understandings of the mechanisms of gamete interaction and self/nonself-recognition in ascidian fertilization.