Vitellogenesis in the sand shrimp, metapenaeus ensis: the contribution from the hepatopancreas-specific vitellogenin gene (MeVg2).

An additional vitellogenin gene (MeVg2) that is structurally different from MeVg1 was cloned and characterized from the shrimp Metapenaeus ensis. The MeVg2 gene consists of fewer exons-introns and is most likely evolved from the MeVg1 gene. The cDNA for MeVg2 is 8.0 kilobases (kb) in size, and the deduced MeVg2 precursor shared an overall 54% amino sequence identity to the MeVg1 gene of the same shrimp. As compared to the MeVg1 precursor, MeVg2 precursor consists of more potential subunit cleavage sites, suggesting that the precursor may be processed into many smaller subunits. The MeVg2 is expressed only in the hepatopancreas, and the expression level of MeVg2 in adult female increases from the early vitellogenic stage, reaching a maximum at the middle vitellogenic stage, and remains high toward the end of vitellogenic cycle. In addition to the 8-kb mRNA, smaller transcripts of 1.5-2.5 kb for MeVg2 were identified, and the 8-kb transcript only constitutes less than 10% of the overall MeVg2-derived transcripts. To confirm the presence of the small transcripts, we screened the shrimp hepatopancreas cDNA library and isolated two smaller MeVg2-specific cDNA clones. These clones shared greater than 99% overall identity to the corresponding C-terminal region of the MeVg2 precursor, suggesting that an alternative expression/ splicing of the MeVg2 gene occurred. By immunohistochemical analysis, vitellin-immunopositive signals were localized in the lumen and extracellular fraction of the hepatopancreas. Amino acid sequence determination of the tissue protein and secreted protein from the hepatopancreas revealed that the 76-kDa vitellogenin subunit is most likely processed into smaller-sized subunits. Taken together, these results suggest that the hepatopancreas is an important organ for the synthesis of vitellogenin and may contribute to vitellogenesis by producing a large quantity of smaller MeVg2 subunit for ovarian uptake.