SHOOK, David*; KELLER, Ray; Univ. of Virginia, Charottesville: Evolution of Vertebrate Mesoderm Morphogenesis

The Chordate lineage shows a evolutionary trend from mesoderm internalization by budding off epithelial tubes, as in ascidians and amphioxus, to internalization by ingression, as in the amniotes. Urodele amphibians appear to be an interesting intermediate in this transition, using an epithelial folding mechanism to internalize axial mesoderm and using ingression through a bilateral primitive streak to internalize paraxial mesoderm. Medio-lateral and anterior-posterior mesoderm patterning through the chordates appears to be relatively conserved, as do the fundamental global morphogenic movements: epiboly, internalization, emboly, convergence and extension. What is not conserved is the initial embryonic architecture of the fertilized egg, and the specific cellular mechanisms used to drive mesoderm morphogenesis. Amphibians in particular, probably because of their broad range of reproductive strategies, show diverse embryonic architectures across their class and appear to use a broad range of cellular behaviors, making them an interesting comparative group. How then are conserved patterning mechanisms superimposed on divergent architectures and the cellular mechanisms used for rearranging them? If morphogenesis is a simple consequence of fate, then how has the patterning machinery evolved to accommodate a different starting point, and the use of different cellular mechanisms, to none-the-less wind up at a common end point? Or if morphogenesis is specified (at least somewhat) independently of fate, as at least two examples indicate, at what point in the developmental hierarchy does patterning diverge from morphogenesis? In either case, it appears that morphogenesis is far more labile than patterning.