Characterization of Hox genes in the bichir, Polypterus palmas

It has been suggested that the increase in the number of Hox genes may have been one of the key events in vertebrate evolution. Invertebrates have one Hox cluster, while mammals have four. Interestingly, the number of Hox gene clusters is greater in the teleost fishes, zebrafish and medaka, than in mouse and human. The greater number of Hox clusters in the teleosts suggests that Hox gene duplication events have occurred during the radiation of ray-finned fishes. The question is when the Hox gene duplication event(s) that lead to seven Hox clusters in the teleosts actually occurred. We have addressed this question by studying the Hox genes in the bichir, Polypterus palmas. A preliminary PCR-estimation of the number of Hox genes suggests that Polypterus has five different Hox9 cognate group genes, which may be an indication of more than four Hox clusters in the bichir.