The evolution of paired appendages in vertebrates: T-box genes in the zebrafish

The presence of two sets of paired appendages is one of the defining featur es of jawed vertebrates. We are interested in identifying genetic systems t hat could have been responsible for the origin of the first set of such app endages, for their subsequent duplication at a different axial level, and/o r for the generation of their distinct identities. It has been hypothesized that four genes of the T-box gene family (Tbx2-Tbx5) played important role s in the course of vertebrate limb evolution. To test this idea, we charact erized the orthologs of tetrapod limb-expressed T-box genes from a teleost, Danio rerio. Here we report isolation of three of these genes, tbx2, tbx4, and tbx5. We found that their expression patterns are remarkably similar t o those of their tetrapod counterparts. In particular, expression of tbx5 a nd tbx4 is restricted to pectoral and pelvic fin buds, respectively, while tbx2 can be detected at the anterior and posterior margins of the outgrowin g fin buds. This, in combination with conserved expression patterns in othe r tissues, suggests that the last common ancestor of teleosts and tetrapods possessed all four of these limb-expressed T-box genes (Tbx2-Tbx5), and th at these genes had already acquired, and have subsequently maintained, thei r gene-specific functions. Furthermore, this evidence provides molecular su pport for the notion that teleost pectoral and pelvic fins and tetrapod for e- and hindlimbs, respectively, are homologous structures, as suggested by comparative morphological analyses.