Characterization and expression of a vasa homolog in the gonads and primordial germ cells of the striped catfish (Pangasianodon hypophthalmus).

The analysis of early gonadogenesis during larval development requires a molecular marker that is specifically expressed in the germ cell lineage, such as the vasa gene. In this study, we cloned and characterized vasa in the striped catfish (Pangasianodon hypophthalmus), and designated this as Phy-vasa. Phy-vasa contained all of the predicted consensus motifs that are shared among the vasa genes in other fish species, including RG and RGG repeats, ATPase motifs, and a DEAD-box, and phylogenetic analysis using various DEAD-box family proteins demonstrated that the Phy-vasa protein clustered within the Vasa family. Reverse transcription polymerase chain reaction (RT-PCR) indicated that Phy-vasa mRNA only occurred in the testis and ovary, and in situ hybridization showed that the gene was expressed only in the germ cells, with strong expression in the spermatogonia and oogonia. To investigate early gonadogenesis in catfish larvae, we undertook histological characterization and in situ hybridization using a Phy-vasa probe, which showed that migration of the primordial germ cells (PGCs) most commonly occurred in larvae at 2-10 days post-fertilization (dpf), the PGCs started to be surrounded by gonadal somatic cells at around 10-20 dpf, and rapid proliferation of the PGCs had begun by 30 dpf. These findings provide a valuable insight into early gonadal development in the striped catfish.