Vertebrate Skeletal Development PDF Download

ABSTRACT: The phylogenetic relationships of the vertebrates were established largely based on anatomical characters, particularly those of the skeleton. Skeletal development has been well studied in higher vertebrates (e.g., mouse and chicken), however, little is known about the developmental mechanisms responsible for evolutionary origin of the vertebrate skeleton. Here I investigate cartilage development in two jawless vertebrates, hagfishes and lampreys, and in a sister group to the vertebrates, lancelets (amphioxus). I show that both lampreys and hagfishes have type II collagen based cartilage, suggesting that this type of cartilage was present in common ancestor of all crown-group vertebrates. My analysis of lancelets revealed the presence of an ancestral clade A fibrillar collagen (ColA) gene that is expressed in the notochord. The results suggest that, during the chordate-vertebrate transition, an ancestral Clade A fibrillar collagen gene underwent duplication and diversification, and this process may underlie the evolutionary origin of vertebrate skeletal tissues. By comparing the axial skeletal developmental genetic program in lamprey and catshark, I found that the network of sclerotomal genes is conserved in mediovental part of somites. These results suggested that lamprey possesses a sclerotome. The conservation of this sclerotomal gene network, the later chondrogenetic program, and the structure of lamprey arcualia and gnathostome vertebrae suggest that the arcualia develop from sclerotome. Phylogenetic analysis showed all of the axial skeletal genes examined here arose from gene duplication events. I propose that the sclerotome, like other vertebrate novelties may have resulted from the same gene duplication events. These duplications allowed differentiation of developmental control genes, which facilitated both subfunctionalization and acquisition of new expression domains.