Analysis of Gene Expression in Amphioxus: The Tbx1/10 Story

Navin R. Mahadevan
Washington University, St. Louis

T-box genes encode a family of transcription factors that share a highly conserved DNA-binding domain, the T-domain. These genes have been implicated in the regulation of numerous developmental processes in metazoans, and a number of genetic diseases have been attributed to their mutation in humans. Because T-box sequences are highly conserved between metazoans, confident orthology/paralogy relationships can be determined, so comparative studies allow the elucidation of their evolutionary origins, as well as the understanding of where and when novel functions have been gained. The cephalochordate, amphioxus, is an excellent model organism in which to study the evolutionary origins of developmental gene functions because its position on the evolutionary tree makes it the best available proxy for the last invertebrate ancestor of the vertebrates. Our work concerns the AmphiTbx1/10 gene which is orthologous to the two vertebrate genes, Tbx1 and Tbx10. We examined its expression pattern during embryonic and early larval development. AmphiTbx1/10 is first expressed in branchial arch endoderm and mesoderm of developing neurulae, and in a bilateral, segmented pattern in the ventral half of newly formed somites. Branchial expression is restricted to the first three branchial arches, and disappears completely by 4 days post fertilization. Ventral somitic expression is restricted to the first 10-12 somites, and is not observed in early larvae except in the most ventral mesoderm of the first three branchial arches. No expression can be detected by 4 days post fertilization. Integrating functional, phylogenetic and expression data from amphioxus and a variety of vertebrate model organisms, we have reconstructed the early evolutionary history of the Tbx1/10 subfamily of genes within the chordate lineage. We conclude that Tbx1/10-mediated branchial arch endoderm and mesoderm patterning functions predated the origin of neural crest, and that ventral somite specification functions predated the origin of vertebrate sclerotome, but that Tbx1 was later co-opted during the evolution of developmental programs regulating branchial neural crest and sclerotome migration..