Gonadogenesis and expression pattern of the vasa gene in the sea cucumber Apostichopus japonicus during early development.
Vasa has been extensively used as a germ-line marker to trace the origin and migration pathway of primordial germ cells (PGCs) in many organisms, but little work has been reported on vasa genes and the origin of PGCs in holothurians. Using in situ hybridization and immunohistochemistry, vasa mRNA and protein of the sea cucumber Apostichopus japonicus (Aj-vasa) was detected in the cytoplasm of the unfertilized egg and was equally distributed in the cytoplasm of early embryos, from the two-cell embryo to the blastula, indicating that Aj-vasa mRNA is maternally supplied. Later, expression of both Aj-vasa mRNA and protein centralizes gradually in newly organized structures from blastula to five-tentacle larva, and then is restricted to PGC-like cells of the original gonad in juveniles with 0.1-cm body length. The structure of the gonad develops further from a simple tubular gonad in 0.5-cm-length juveniles to a branched gonad in 3-cm-length juveniles. Our findings showed that the maternal supply of the vasa gene products in A. japonicus is different from that in sea urchin Strongylocentrotus purpuratus, of echinoderm, and suggested that the specialization of PGCs is an epigenesis mechanism in A. japonicus.