One of the mysteries and highly debatedtopics in zoology is the evolutionary origin of the quite complex machinery of muscle cells in the Metazoa. In a collaboration between the groups of Ulrich Technau (Vienna), Bernie Degnan (Queensland), Evelyn Houliston (Paris), Gert Wörheide (Munich) and our group, we presented new insights in a recent publication in NATURE. A combination of comparative genomics, gene expression and morphological studies, our results point towrds the independent evolution of striated muscle cells in Cnidaria and Bilateria.
The part on our model sponge Tethya wilhelma was performed by Claire Larroux, Gert Wörheide (both LMU Munich), Jörg U. Hammel and Michael Nickel in a closer collaboration. We could show that although a muscle-type mysoin exists in the muscle-less sponge it seems to lack a role in the strong body contractility. While we have recently shown that contractility in demosponges is likely mainly mediated by pinacocytes, this particular myosin is not expressed by pinacocytes. Instead, the gene is expressed in a prominen, so far undescribed, cell type in the canal system. Functionally, this cell is thought to be internaly contractile, beeing able to open and close pore structures which penetrate the cell. Due to the specific position within the outflow opening of the sponges choanocyte chambers, the alteration of the pore diameter will strongly influence the water flow through the canal system. It seems likely that the sponge is able to control water flow specifically through this cell type and that the particular myosin is involved in this process.