Meeting Abstract
How an autopod with morphologically distinct digits evolved during the fin to limb transition is one of the fundamental questions of comparative vertebrate anatomy. Recently published data for Hox family transcription factors provide new insights into this question, revealing that 1) certain enhancers regulating autopod HoxD expression are primitive to tetrapods; and 2) an inverted, collinear late-phase of 5’ HoxD expression, once thought specific to tetrapods, is present in more basal taxa including paddlefish (an actinopterygian) and catshark (a chondrichthyan). Notably, any comparison of gene expression between paddlefish and other gnathostomes is complicated by a whole genome duplication (est. 42MYA) endemic to the paddlefish lineage. Here we present new in-situ hybridization results for 5’ HoxD members in paddlefish, revealing variation in late phase expression not described previously. Additionally, we use 3’UTR probes specific to alpha and beta paralogues of HoxD13 to test if both are expressed during paired fin development, and whether their relative position and timing of onset differ. Finally, we characterize the expression of Evx2 and LNP, two non-Hox genes that reside at the 5’ end of the HoxD cluster. In tetrapods, homologues of Evx2 and LNP are expressed during autopod development and are regulated by the same enhancers that govern the transcription of 5’ HoxD genes. We hypothesize that a similar expression profile between 5’ HoxD genes and their non-Hox neighbors in paddlefish would be consistent with shared enhancers, as in tetrapods. Our new data will be discussed in the context of the emerging viewpoint that the evolutionary origin of the autopod involved minor modifications of ancient patterns of gene expression and their underlying regulatory landscape.
