1. Department of Organismal Biology and Anatomy, The University of Chicago, Chicago, Illinois 60637, USA
2. Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, Illinois 60605, USA

Correspondence to: Neil H. Shubin^1,2 Correspondence and requests for materials should be addressed to N.H.S. (Email: nshubin@uchicago.edu).

Comparative analyses of Hox gene expression and regulation in teleost fish and tetrapods support the long-entrenched notion that the distal region of tetrapod limbs, containing the wrist, ankle and digits, is an evolutionary novelty^{1, 2, 3, 4}. Data from fossils support the notion that the unique features of tetrapod limbs were assembled over evolutionary time in the paired fins of fish⁵. The challenge in linking developmental and palaeontological approaches has been that developmental data for fins and limbs compare only highly derived teleosts and tetrapods; what is lacking are data from extant taxa that retain greater portions of the fin skeletal morphology considered primitive to all bony fish^{6, 7}. Here, we report on the expression and function of genes implicated in the origin of the autopod in a basal actinopterygian, Polyodon spathula. Polyodon exhibits a late-phase, inverted collinear expression of 5' HoxD genes, a pattern of expression long considered a developmental hallmark of the autopod and shown in tetrapods to be controlled by a 'digit enhancer' region. These data show that aspects of the development of the autopod are primitive to tetrapods and that the origin of digits entailed the redeployment of ancient patterns of gene activity.

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