Meeting Abstract

40.4  Jan. 6  Hox genes and paired appendages: Paddlefish put a new spin on the fin. DAVIS, M.C.*; DAHN, R.D.; SHUBIN, N.H.; Univ. of Chicago; Univ. of Chicago; Univ. of Chicago marcusd@uchicago.edu

Hox genes play an essential role in the development of vertebrate fins and limbs. During early appendage development in zebrafish and tetrapod (chick, mouse) model systems the pattern of Hox expression is highly conserved. However, at later stages there are a number of striking differences. For example, zebrafish and tetrapods both exhibit the same early pattern of 5� HoxD expression, but tetrapods possess a second phase of HoxD expression at later stages in the nascent autopod. Likewise, early HoxA expression appears conserved in each group, yet distinct spatial and inhibitory differences appear at later stages of limb development that are not observed in zebrafish fins. Based, in part, on these observations it has been proposed that the autopod is a developmental �novelty�. However, an alternative hypothesis is that zebrafish may have lost or modified portions of an ancestral Hox program that is retained in tetrapods. Here we demonstrate the presence of tetrapod-like biphasic HoxD expression in the paired fins of a basal ray-finned fish. Like zebrafish and tetrapods, the paddlefish Polyodon spathula exhibits an early posterior expression of 5� HoxD genes in nested domains. In contrast to zebrafish, Polyodon possesses a second phase of 5�HoxD expression in the distal fin bud. Furthermore, this expression exhibits the same inverted domains across the A-P axis of the distal appendage that has been observed in the tetrapod hand/foot. Together, these results suggest that the autopod is an elaboration or expansion of a previously existing developmental program and not a developmental novelty per se. These results also support the notion that zebrafish (and presumably teleosts in general) are derived in the loss or modification of aspects of late stage Hox expression during fin development.