SHOOK, D.S. *; KELLER, R.; University of Virginia, Charlottesville: Variation among Amphibians of Morphogenetic Mechanisms Driving Gastrulation

Vertebrate embryos use a great variety of mechanisms to drive morphogenesis during gastrulation and neurulation, as they reorganize a simple ball or sheet of cells into a complex tube that has most of the features of the adult body plan. Understanding the pattern behind these variations throughout the vertebrate, and ultimately the entire metazoan phylogeny is essential to understanding the evolution of development. We study variations in these mechanisms within the amphibians, with the goal of answering several questions: What is the breadth of variation? What are the constraints on this variation, and what are they based on? Do particular variations in morphogenic mechanism correspond to particular variations in the intrinsic and extrinsic embryonic environment, such as embryonic architecture, rate of development, degree of predation or temperature and moisture content of the egg habitat? And do particular variants of one morphogenic mechanism preclude or require particular variants in other mechanisms? By addressing these questions, we gain a broader understanding of the rules of morphogenesis, and their relationship to particular embryonic architectures and environments. Examples of this approach will be presented. The frogs Xenopus laevis and Epipedobates tricolor vary in their expression of convergent extension during gastrulation, suggesting that they must use different mechanisms to produce the force for closing their blastopore. And Xenopus and two species of Ambystoma show variations in the presence of presumptive mesoderm in the superficial layer, with correlated differences in the mechanism driving blastopore closure and involution, which none-the-less produce the same force. The relationship between these variations and embryonic architecture and environment will also be discussed.