MOSHEL-LYNCH, S. M.: The conservation of regulatory genes and the evolution of molluscs

In Molluscs, the gastropods have undergone a great deal of change from the bilateral bauplan of their basal members. The investigation of regulatory gene pathways will facilitate our understanding of how this evolutionary diversification has taken place, particularly mesoderm formation and differentiation. While there are various mechanisms of mesodermal formation, the genes necessary for its differentiation have been conserved, illustrating genomic antiquity in the face of developmental diversification. Within the Spiralia, there is a dual origin of mesoderm, ectomesoderm and endomesoderm, both contributing to larval and adult mesoderm. Twist, a basic helix-loop-helix (bHLH) transcription factor, plays a major role in mesoderm formation in many organisms. Using the polymerase chain reaction we have identified a twist ortholog in various molluscs. We investigated the role of twist in the gastropod Ilyanassa obsoleta. Ilyanassa twist(Ily-twi) is expressed throughout development, initially present in the oocyte as a maternal transcript. After mesoderm determination, twist is localized in both ectomesodermal and endomesodermal derivatives. It appears that twist has been adopted early in evolution as a designator of mesoderm irrespective of the structural modes of gastrulation and mesoderm differentiation.