Meeting Abstract

S5.6  Monday, Jan. 5  Molecular identification of a sclerotome in lampreys and sharks: implications for the origin of the vertebral column ZHANG, GuangJun*; COHN, Martin/J.; Koch Institute, MIT; Zoology Department, University of Florida zhanggj@mit.edu

Origin of the vertebrae is one of the central stories of vertebrate evolution. Vertebral elements first appeared in the jawless fish, although their embryonic origin remains unclear Early comparative and embryonic data suggested it may be derivative of the sclerotome, similar to the situation in amniotes. The only molecular data on lamprey Pax1/9 suggested sclerotome may not present in lampreys. While, the major cartilage matrices in jawless fishes have been reported to be non-collagenous, instead their cartilage is made of elastin-like molecules, named lamprin and myxinin. Thus the embryonic origin of the lamprey axial skeleton, termed arcualia, remains a puzzle. Very recently we demonstrated that agnathans cartilage matrix is composed type II collagen like gnathostomes. Concurrently, Col2a1a is expressed in the paraxial mesoderm. This led us to hypothesize that lampreys may possess a sclerotome that might contribute to the development of arcualia. We investigated seven sclerotome marker genes and their expression patterns in lamprey. We also re-examed the PmPax1/9A expression patterns in four lamprey species including the Japanese lamprey, along with a new cloned Pax1/9 homologous gene, PmPax1/9B. Except PmPax1/9B , all the sclerotome marker genes including the PmPax1/9A are expressed in the medioventral part of the somite, which is the position of sclerotome in amniotes. Ontogenetically, our data strongly suggested that lamprey do possess sclerotome and that it contributes to the development of arculia, the earliest vertebral elements. The phylogenetic analysis of these sclerotome markers revealed that all of them underwent a gene duplication event, probably as a part of whole genome duplication. And the origin of these sclerotome markers may account the phylogenetic origin of vertebral columns.