Evolution of the echinoderm Hox gene cluster

Extant echinoderms are members of an ancient and highly derived deuterostom e phylum. The composition and arrangement of their Hox gene clusters are co nsequently of interest not only from the perspective of evolution of develo pment, but also in terms of metazoan phylogeny and body plan evolution. Ove r the last decade numerous workers have reported partial Hox gene sequences from a variety of echinoderms. In this paper we used a combined methods ap proach to analyze phylogenetic relationships between 68 echinoderm Hox home odomain fragments, from species of five extant classes-two asteroids, one c rinoid, one ophiuroid, one holothuroid, and three echinoids. This analysis strengthens Mito and Endo's (2000) proposition that the ancestral echinoder m's Hox gene cluster contained at least eleven genes, including at least fo ur posterior paralogous group genes. However, representatives of all paralo gous groups are not known from all echinoderm classes. In particular, these data suggest that echinoids may have lost a posterior group Hox gene subse quent to the divergence of the echinoderm classes. Evolution of the highly derived echinoderm body plan may have been accompanied by class-specific du plication, diversification and loss of Hox genes.