HANDRIGAN, G. R.; WASSERSUG, R. J.; Dalhousie University, Halifax, Canada: The case of the anuran backbone: Missing vertebrae and genetic culprits

Nearly all anuran (frog and toad) species have less than a dozen discrete vertebral elements, some as few as 8. They are thus the vertebrates with the least vertebrae. Paradoxically, anuran vertebrae arise from a much greater complement of approximately 50 somites. The majority of these somites, found within the tail, never ossify and are resorbed along with other caudal structures during metamorphosis. Two key issues emerge: 1) Which genes direct the global patterning events that have expanded the somitic complement of the tail at the expense of the trunk? 2) What precludes the caudal somites from forming cartilage and bone? Towards answering these questions, we have profiled the expression of orthologs of both growth/differentiation factor 11 (XlGdf11) and paired-box gene 1 (XlPax1) in the African clawed frog Xenopus laevis by RT-PCR, Northern blot, and in situ hybridization. Gdf11, which encodes a potent posteriorizing signal, regulates vertebrate Hox gene expression and, in turn, the establishment of morphological boundaries within the axial skeleton. Pax1 controls cell proliferation and differentiation in the sclerotome, the bone-forming portion of the somite. XlGdf11 is first upregulated during neurulation and transcripts are subsequently localized to the tail bud, the site of somitogenesis and caudal body development. XlPax1 transcripts are noted in the developing sclerotome as well as the pharyngeal pouches and limb. We suggest that heterochronic shifts in the expression of both genes may contribute to the unique axial morphology of anurans. Preliminary RNA injection and transgenic studies are underway to explore the effects of perturbing the expression of each gene on vertebral column development in anurans.