One of the biological inventions associated with the invasion of land by fish is the posession of a limb skeleton with wrists and digits. This distal segment of limbs, known as the autopod, has been the part a great adaptive diversification of tetrapods. What are the biological mechanisms that drive the formation of the autopod and what portions are present in fish? Studies of mice reveal that 5’ member of Hoxa and Hoxd gene families play an important role in specification of the autopod: 1. a separate and distinct phase of expression is correlated with the specification, 2. knockout phenotypes result in the specific loss of the autopod, and 3. cell lineage analyses reveal that 5’ hox expressing cells definitively mark the autopod compartment. We show that the autopod compartiment has deep phenotypic, genomic and embryological roots in fish. Fossil data, such as that from the Devonian sarcopterygian, Tiktaalik roseae, reveal that the autopod compartment had antecedents in fish fins. Moreover, patterns of gene expression and regulation have deep similarities to tetrapods— fish and tetrapod enhancers share conserved synteny and functions despite 400my of independent evolution. Knockout phenotypes (employing Crispr/Cas and cell lineage reveal that the cells that form the wrists and fingers form the fin rays in fish fins. These insights offer new perspectives into the origin and development of the skeletal patterns of vertebrate paired appendages.
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