The role of Nanos homologue in gametogenesis and blastogenesis with special reference to male germ cell formation in the colonial ascidian, Botryllus primigenus

We examined the structure, expression, and possible functions of a nanos homologue during gametogenesis and blastogenesis in the colonial ascidian Botryllus primigenus. An approximately 1.3-kb-long cDNA was cloned; it was termed BpNos since the deduced amino acid sequence (288 aa) contained 2 Nanos-like CCHC zinc finger motifs. Immature and mature male germ cells expressed BpNos most strongly, while loose aggregates of hemoblasts, multipotent epithelial cells (in developing buds) and a few coelomic cells in the hemocoel and tunic vessels weakly expressed BpNos. No signals were detected from female germ cells. To determine possible functions of BpNos, B. primigenus colonies were injected with BpNos short interfering (si)RNA. Buds developed normally, showing that BpNos plays a limited role in B. primigenus blastogenesis. However, the developing buds possessed no spermatogonia and spermatocytes in the testes, although oocytes developed normally. In the knockdown colonies, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling assay (TUNEL)-positive male germ cells were observed, suggesting that BpNos siRNA treatment might induce apoptosis. In conclusion, BpNos is a weak marker of germline precursor cells and multipotent somatic epithelial cells but a strong marker of spermatogonia and spermatocytes. A major function of BpNos may be the maintenance of male germline cells.