The Vasa gene encoding an ATP-dependent RNA helicase protein member of the DEAD-box family, is one of the important regulatory factors which determines the germline cells development.In the present study,homolog cloning strategy and SMART RACE technique were used to isolate and clone the Vasa gene of U.unicinctus, a sole commercial species of Xenopenusta.The full length of the Vasa cDNA sequence is 4 080 bp,comprising an open reading frame(ORF)of 2 322 bp encoding a polypeptide of 773 amino acids,sharing nine conserved motifs of all DEAD-box family proteins.Phylogenetic analysis showed the deduced amino acids sequence contributes to the VASA relate proteins.The temporal expression analysis of Vasa mRNA using RT-PCR and in situ hybridization techniques indicated Vasa mRNA was present in unfertilized eggs,fertilized eggs and 2-8 cell embryos,suggesting a maternally-provided characteristic.The expression level decreased obviously at blastula stage,and kept the low level from blastulas to worm-like larvae and juveniles. In situ hybridizations howed that Uu-Vasa mRNA is distributed equally in the cytoplasm of early embryos from unfertilized egg,fertilized egg to blastula. At gastrula stage, Uu-Vasa mRNA concentrated on endoderm and mesoderm of gastrula.In rtochophore, the Vasa positive signals are located in cells of alimentary canal.During the somite larvae,the hybridization signals appear in the cells of body segment membrane and alimentary canal.When developing to worm-like larvae, Un-Vasa is detected in abdominal bristle of the head region and posterior region of intestine.The location of positive signals in the posterior region of intestine is coincident with that of gonad in the future,and it was deduced that U.unicinctus gonad maybe initializes to form in worm-like larva.Our results provide primary data to generate new insight into the origin and differentiation of germ cells as well as the gonad genesis and differentiation in U.unicinctus.