Comparative analysis of Hox paralog group 2 gene expression during Nile tilapia (Oreochromis niloticus) embryonic development.

The hindbrain and pharyngeal arch-derived structures of vertebrates are determined, at least in part, by Hox paralog group 2 genes. In sarcopterygians, the Hoxa2 gene alone appears to specify structures derived from the second pharyngeal arch (PA2), while in zebrafish (Danio rerio), either of the two Hox PG2 genes, hoxa2b or hoxb2a, can specify PA2-derived structures. We previously reported three Hox PG2 genes in striped bass (Morone saxatilis), including hoxa2a, hoxa2b, and hoxb2a and observed that only HoxA cluster genes are expressed in PA2, indicative that they function alone or together to specify PA2. In this paper, we present the cloning and expression analysis of Nile tilapia (Oreochromis niloticus) Hox PG2 genes and show that all three genes are expressed in the hindbrain and in PA2. The expression of hoxb2a in PA2 was unexpected given the close phylogenetic relationship of Nile tilapia and striped bass, both of which are members of the order Perciformes. A reanalysis of striped bass hoxb2a expression demonstrated that it is expressed in PA2 with nearly the same temporal and spatial expression pattern as its Nile tilapia ortholog. Further, we determined that Nile tilapia and striped bass hoxa2a orthologs are expressed in PA2 well beyond the onset of chondrogenesis whereas neither hoxa2b nor hoxb2a expression persist until this stage, which, according to previous hypotheses, suggests that hoxa2a orthologs in these two species function alone as selector genes of PA2 identity.