Isolation of hox genes from the scyphozoan Cassiopeia xamachana: Implications for the early evolution of hox genes

The isolation of Hox genes from two cnidarian groups, the Hydrozoa and Anth ozoa, has sparked hypotheses on the early evolution of Hox genes and a cons erved role for these genes for defining a main body axis in all metazoan an imals. We have isolated the first five Hox genes, Scox-1 to Scox-5, from th e third cnidarian class, the Scyphozoa. For all but one gene, we report ful l-length homeobox plus flanking sequences. Four of the five genes show clos e relationship to previously reported Cnox-1 genes from Hydrozoa and Anthoz oa. One gene, Scox-2, is an unambiguous homologue of Cnox-2 genes known fro m Hydrozoa, Anthozoa, and also Placozoa. Based on sequence similarity and p hylogenetic analyses of the homeobox and homeodomain sequences of known Hox genes from cnidarians, we suggest the presence of at least five distinct H ox gene families in this phylum, and conclude that the last common ancestor of the Recent cnidarian classes likely possessed a set of Hox genes repres enting three different families, the Cnox-1, Cnox-2, and Cnox-5 families. T he data presented are consistent with the idea that multiple duplication ev ents of genes have occurred within one family at the expense of conservatio n of the original set of genes, which represent the three ancestral Hox gen e families. (C) 1999 Wiley-Liss, Inc.