Abstract:
Metazoan body plans are built from complex repertoires of specialized cells assembled into functional structures. To achieve this complexity many bilaterian embryos undergo some form of metameric segmentation, which subdivides groups of cells into discrete blocks along the major body axis. Vertebrate somitogenesis is a classic example of metameric development, wherein paraxial mesoderm is sequentially partitioned into epithelial blocks whose axial identities are assigned by differential Hox gene expression. Cnidaria, as the sister phyla to Bilateria, until recently was not thought to exhibit segmentation. Remarkably, recent work demonstrated that an ancient segmentation program uses Hox genes to subdivide endo-mesodermal cells along the directive body axis in the cnidarian subphylum Anthozoa (corals and sea anemones). This thesis investigates potentially key factors in cnidarian segment morphogenesis that also regulate vertebrate somitogenesis: the transcription factor Paraxis and the integrin family of proteins. To characterize paraxis function in the cnidarian model Nematostella, short hairpin-mediated knockdowns and CRISPR induced mutations in the paraxis ortholog were analyzed. In vertebrates, Paraxis drives somite epithelialization; however, examination of knockdowns and CRISPR-mediated mutations in Nematostella embryos did not reveal a similar role. Since integrin signaling is essential in cell-to-cell interactions and may be transcriptionally regulated by Paraxis, the integrin repertoire of Nematostella was characterized, and it was found that specific subunits were restricted to endo-mesodermal tissue, suggesting a potential role in the segmentation program. Together, these observations suggest that while a core set of conserved regulatory genes were present in the common ancestor of cnidarians and bilaterians, some sub-functionalization may have occurred in the 600 million years since their divergence.
