Germ cell formation and reproduction in tunicates

Because tunicates are the closest living relatives of vertebrates within the phylum Chordata, understanding the mechanisms of tunicate germ cell formation is essential to infer this process in ancestral chordates. Vasa-localization to primordial germ cells (PGCs), PGC formation biased to the posterior side of embryos, and transcriptional silencing by phosphorylation of RNA polymerase II are the core PGC-forming events inherited from the shared ancestor of protostomes and deuterostomes. Neuropeptide- and peptidase-mediated regulations of oocyte maturation are examples of mechanisms observed in both vertebrates and tunicates. However, the pathways that activate and regulate these mechanisms, such as the hypothalamus-pituitary-gonad axis, are not always conserved between tunicates and vertebrates, suggesting that these animals achieved the same end-products via different molecular mechanisms. Tunicates are divided into the classes Ascidiacea, Thaliacea, and Larvacea, each of which exhibits specific characteristics in germ cell formation. Some ascidians form colonies of interconnected zooids, each having regenerative capacity sufficient to regrow the entire body, including gametes, from a tiny body piece. Larvaceans maintain their tadpole shape throughout their very short life cycle: they manage to complete their gamete formation using small gene sets encoded in the shortest genomes yet observed among non-parasitic animals. These group-specific characteristics are likely to reflect different strategies for ensuring survival in the respective environments.