ABSTRACT
American shad Alosa sapidissima, an anadromous clupeid, exhibits variation in reproductive strategies, including semelparity and iteroparity. It provides an excellent model for studying the behaviour of germ cells in anadromous fish during their migration from sea to river. The vasa gene was characterized in A. sapidissima as a germ-cell marker to elaborate the process of germ-cell development and differentiation in anadromous species. A complementary (c)DNA fragment of 819 bp, partial open reading frame (ORF), was cloned by degenerate PCR and named as ASvas. In adult A. sapidissima, vasa transcript was exclusively detected in gonads by reverse-transcription (RT)-PCR. Through chromogenic in situ hybridization, the vasa messenger (m)RNA was specifically detected in primordial germ cells (PGC) in embryos and germ cells at early stages in ovary and testis. Besides, the cellular distribution profile of Vasa protein also proved that vasa gene could be used as a germ-line marker to trace the PGCs migration during embryogenesis and the germ-cell differentiation during gametogenesis in A. sapidissima. During embryogenesis, the migrating PGCs were clearly detected at tail-bud stage and the PGCs reached the genital ridge at the stage of pre-hatching stage in A. sapidissima embryos. During gametogenesis, the Vasa protein was dynamically expressed in differentiating germ cells at different stages in adult gonads. As far as we know, this is the first report to demonstrate the PGCs migration and germ-cell differentiation through vasa gene expression in the anadromous species. The findings will pave a way for investigating germ-cell development and maturation in the A. sapidissima and other anadromous fish.
