Meeting Abstract

S2-2.4  Thursday, Jan. 3  Head segments in developmental and evolutionary contexts KURATANI, Shigeru; CDB, RIKEN, Kobe, Japan saizo@cdb.riken.jp

The theory of head segmentation has been one of the central topics in vertebrate comparative morphology, which can now be treated in modern evolutionary developmental contexts. This theory first arose as a transcendental idea of morphology, more or less influenced by the morphological pattern of the vertebrate pharyngula or of the non-vertebrate chordate, amphioxus. In this scheme, presence of the head cavities, the epithelial mesodermal coelom differentiating into extrinsic eye muscles in some vertebrate embryos, was compared with somites in the trunk. Phylogenetically, these cavities are most conspicuously developing in the elasmobranchs, and only incompletely found in more crown gnathostomes. In cyclostomes no clear head cavities are found, and thus the cavities appear to be synapomorphy of gnathostomes on the phylogenetic tree. I also show, in the cyclostome embryos, that their head mesoderm is only secondarily regionalized by the presence of other embryonic structures. In the context of developmental constraint, the vertebrate body plan, as something distinct from that of ancestral chordates, can better be described as possessing unsegmented head mesoderm.