An investigation of oyster TGF-β receptor genes and their potential roles in early molluscan development

Though the roles of BMP signaling in development is studied extensively in insects and vertebrates, our knowledge of BMP signaling in molluscan development is limited. In the present study, we performed a genome-based analysis of TGF-β receptors in the Pacific oyster Crassostrea gigas and revealed that C. gigas possessed all five canonical members of the gene family, including three type I and two type II receptors. Whole mount in situ hybridization revealed that four receptor genes exhibited universal expression at the gastrula stage but cgi-bmprII mRNA was only expressed at the anterior lip of blastopore, indicating the regulation of BMP signaling through restricted expression of TGF-β receptors. Treatment of oyster embryos using a BMP signaling inhibitor (dorsomorphin) resulted in obvious changes of the expression of developmental regulatory genes. In particular, three dorsally expressed genes were inhibited and two genes expressed at the opposite side showed increased expression, indicating BMP signaling may function in dorsal-ventral (DV) patterning. Some of the influenced genes were potential shell-formation (pSF) genes, including the well-accepted pSF gene engrailed, and thus this indicates the development of shell field was affected by dorsomorphin treatment. Subsequent scanning electronic observation revealed morphological change of the posterior margin of the shell field. Taken together, these results indicate the roles of BMP signaling in both DV patterning and development of shell field in oyster embryo, which expands our knowledge of early molluscan development.