BURTON, Patrick M*; PANG, Kevin; KRONE, Cassandra; MARTINDALE, Mark; FINNERTY, John; University of Maryland; University of Hawaii; Boston University; University of Hawaii; Boston University: An Endodermal Origin of Mesoderm

Mesoderm played a critical role in the radiation of triploblastic bilaterians, allowing the evolution of more complex body plans than their diploblastic predecessors. The diploblastic phylum Cnidaria represents the likely sister group to triploblasts. Members of this phylum possess two distinct tissue layers separated by mesoglea consisting primarily of extracellular matrix. In some cnidarians, including the starlet sea anemome, Nematostella vectensis, the mesoglea also contains a limited number of cells. These cells are endodermal in origin. We investigated the expression of eight genes from Nematostella whose triploblast homologs are implicated in mesodermal specification and the differentiation of mesodermal cell types (twist, snail-a, snail-b, otx, forkhead, mef2, a GATA transcription factor and a LIM transcription factor). Expression of these genes is largely restricted to the endoderm, suggesting that (1) these genes play a role in endoderm specification and patterning and (2) the mesoderm and endoderm of triploblastic animals are derived from the endoderm of diploblastic animals. In Nematostella, as in other Cnidaria, an individual adult animal may by formed through three distinct processes. With respect to germ layer specification, regeneration and asexual fission are quite distinct from embryogenesis, as both the endoderm and ectoderm exist at the outset. Comparing expression of developmental genes across all three developmental contexts can provide insight into their developmental role. For example, forkhead and otx expression during fission and regeneration suggests that this gene plays a role in endodermal patterning, not just endodermal specification.