Meeting Abstract

S2-1.1  Jan. 4  Craniofacial evolution from a developmental perspective KURATANI, Shigeru; Center for Developmental Biology, RIKEN, Kobe, Japan saizo@cdb.riken.jp

In the craniofacial development of vertebrates, some developmental stages are conserved across species, representing particular developmental constraints such as the Hox code clearly expressed in pharyngula. By this stage, primarily unsegmented cephalic mesoderm is subdivided by embryonic structures like otocysts and pharyngeal pouches into several domains. This mesodermal regionalization is shared by all the vertebrate species. The oral developmental program is also constrained to some extent, as both its morphology and the the Hox-code-default state of this region are well conserved among vertebrate embryos. These features do not by themselves explain the evolution of jaws, but should be regarded as a prerequisite for evolutionary diversification of the mandibular arch. By comparing the morphology of pharyngula between the lamprey and gnathostomes, it has become clear that the oral pattern is not entirely identical, in particular the differentiation of the rostral ectomesenchyme is shifted between these animals. Therefore, the jaw seems to have arisen as an evolutionary novelty, by overriding ancestral constraints, a process in which morphological homologies are partially lost. This change involves the heterotopic shift of tissue interaction, which appears to have been preceded by the transition from monorhiny to diplorhiny, as well as separation of the hypophysis. When gene expression patterns are compared between the lamprey and gnathostomes, cell-autonomously functioning genes tend to be associated with identical cell types or equivalent anatomical domains, whereas growth factor-encoding genes have changed their expression domains during evolution. Thus the heterotopic evolution may be based on changes in the regulation of signalling molecule-encoding genes.