TGF-beta signalling is involved in torsion and shell positioning in the giant ramshorn snail Marisa cornuarietis (Gastropoda: Ampullariidae)

摘要

Shell internalization has evolved convergently multiple times in numerous molluscan clades, including various gastropod families. Previous experiments on the early ontogeny of the caenogastropod Marisa cornuarietis and of a pulmonate species revealed that short-term exposure to bivalent platinum ions during embryogenesis artificially inhibited torsion and induced shell internalization, thereby causing drastic alterations of the body plan. Instead of an external shell, the embryos developed a cone-shaped internal shell. While the molecular mechanisms behind such alterations of the body plan have not yet been identified, published literature points to the involvement of TGF- signalling in shell formation. In our study we blocked the TGF- pathway with the inhibitor SB 431542 and examined the embryonic development for torsion, shell internalization and malformation in M. cornuarietis. Most of the individuals without outer shells, induced by SB 431542, displayed a morphologically similar phenotype to those exposed to platinum, thus providing evidence for the involvement of TGF- signalling, not only in shell development, but also in shell positioning. Thus our work shows that changes in morphology result from inhibition of both torsion and differential outgrowth of the mantle epithelium and that unhampered TGF- signalling is required for these processes in the early embryonic development of M. cornuarietis. Consequently, modifications of the TGF- pathway could have been involved in the convergent evolution of internal shells in different molluscan clades.