Meeting Abstract
The morphology of the vertebrate head skeleton is highly plastic, with the number, size, shape, and position of its components varying dramatically between groups. While this evolutionary flexibility has been key to vertebrate success, its developmental basis is poorly understood. The larval head skeleton of the frog Xenopus laevis possesses a unique combination of ancestral tetrapod features and anuran-specific novelties. We built a detailed gene expression map of the early head mesenchyme in X. laevis, focusing on transcription factor families with known functions in vertebrate head skeleton development. While we observed broad conservation of gene expression between X. laevis and other gnathostomes, we also found several divergent features that correlate to lineage-specific novelties. We noted a conspicuous change in dlx1/2 expression in the second pharyngeal arch, presaging the differentiation of the asymmetrical hyoid arch skeleton. In the future mandible we observed a shift in the expression of the joint inhibitor barx1, and new expression of the joint marker gdf5, suggesting that the anuran-specific infrarostral cartilage evolved by partitioning of Meckel’s cartilage with a new paired joint. We posit that changes in the expression of downstream regulators of skeletal differentiation, like barx1 and gdf5, are an important mechanism by which the number and positioning of skeletal elements can be altered during evolution.
