Hox gene expression in teleost fins and the origin of vertebrate digits

HOX genes are essential for growth and patterning of the tetrapod limb skeleton1–5. Mice mutant for the Hoxd-13gene have an important delay in morphogenesis owing to reduced proliferation2. Based on the appearance of atavisms in such mice, we suggested that modifications of Hox gene regulation may have been a source of morphological variation during the evolution of tetrapod limbs2,6. Pectoral and pelvic fins are homologous to fore- and hind-limbs, respectively. To compare the relative importance of Hox genes during fin versus limb morphogenesis, we cloned zebrafish (Danio rerio) HoxD and HoxA complex genes and analysed their expression during fin development. The results suggest a scheme for the fin–limb transition in which the distal autopods (digits) are neomorphic structures produced by unequal proliferation of the posterior part of an ancestral appendix.

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