Temporal and Spatial Expression of a Gene for the Nuclear Protein Hgv2 in Embryos and Adults of the Ascidian Halocynthia roretzi: ascidians/germinal vesicle/embryonic nuclear protein/gene expression).

The pHgv20 cDNA clone encodes an ascidian embryonic nuclear protein, Hgv2, that is closely related to the amphibian histone-binding protein N1. Genomic Southern blot analysis revealed the presence of two or more genes that hybridize with the Hgv2 probe under high-stringency conditions, although it remains to be determined whether or not each of them is actively expressed. On Northern blots prepared from embryos, a single, 2.3-kb Hgv2 mRNA was detectable during early stages of embryogenesis. The amount of Hgv2 mRNA gradually decreased after the 64-cell stages. Northern, Western and immunohistochemical analyses showed that the Hgv2 protein was not expressed exclusively in the oocyte: small amounts of the 2.3-kb mRNA and of the 83-kDa Hgv2 protein were detectable in the branchial sac of adult organisms. Weak but specific immunohistochemical staining was observed in the spermatocytes and/or spermatogonia. An Hgv2-specific antiserum reacted specifically with the 83-kDa protein on the Western blot of the testis. These results suggest that Hgv2 functions not only in embryonic cells but also in sperm precursor cells and some somatic cells.