RIEGER, RM; LADURNER, P: What do we know about the origin and evolution of mesoderm among the lower Metazoa?

Ever since the classic studies of mesoderm by Oskar and Richard Hertwig (1882), the evolutionary origin of this germ layer has remained a central issue in animal phylogeny. Libbie Hyman (1951, 1959), Adolf Remane (1963), and Williard Hartmann (1963), placed this issue in a modern evolutionary context, and Salvini Plawen and Splechtna (1979) summarized evidence allowing the distinction between true mesoderm (as endomesoderm) and ectodermally derived mesoderm (ectomesoderm). Most recently, Edward Ruppert (1991) and Peter Ax (1996) have reopened discussion of the correspondence between true mesoderm in bilaterian metazoans and the occurrence of three tissue layers in all metazoan adults. Our presentation will briefly summarize key issues in the literature concerning the origin of mesoderm and discuss the usefulness of the distinction between ectomesenchyme and endomesoderm in the light of recent literature. We will present a cladogram of the basal metazoans and use it to discuss the origin and early evolution of mesoderm in the Bilateria. Such questions as “What was the original structure and function of mesodermal cells?” will be addressed, and we will summarize present knowledge of mesodermal genes of the lower Metazoa, especially of the lower Bilateria. Finally, we will provide evidence that understanding the histological organisation of mesodermal tissues remains key to deciphering the evolutionary origin of this last of the three germ layers in the Bilateria.