SCEMAMA, Jean-Luc*; LE PABIC, Pierre; STELLWAG, Ed; East Carolina University; East Carolina University; East Carolina University: The evolutionary developmental divergence of vertebrate Hox paralog group 2 gene expression.

Segmental identity of structures located in the hindbrain and branchial arches of vertebrates are determined, at least in part, through the action of Hox paralog group 2 (PG2) genes. Despite an overall similarity in their function during development, Hox PG2 genes from evolutionarily divergent lineages have distinctly different spatio-temporal expression patterns, particularly in comparisons between actinopterygians and sarcopterygians. Results from our laboratory, including the cloning and expression analysis of an unprecedented three Hox PG2 genes (Hoxa2a, a2b, and b2a) from Morone saxatilis (striped bass), show that the trans-orthologs Hoxa2a and a2b are differentially expressed in both rhombomeres and branchial arch tissue whereas their paralog�s expression (Hoxb2a) is restricted solely to the rhombomeres. The pattern of Hox PG2 expression documented for M. saxatilis contrasts sharply with that of Danio rerio (zebrafish) and mammals but appears to be similar to that of Gallus gallus (chicken). Our results indicate that while the Hox PG2 gene complement in actinopterygians is greater than that of sarcopterygians, expression patterns in these two major vertebrate lineages are similarly divergent, indicative that an increased number of genes does not lead to concordant increases in expression pattern divergence. Our results also reveal a considerable plasticity in the Hox gene-related evolutionary developmental specification of vertebrate hindbrain and branchial arch structures, at least some of which may underlie the variation in morphology observed in these divergent organisms.