Fig. 5.

A proposed scheme for Hox gene cluster disintegration during ascidian evolution. The last common ancestor for cephalochordates, tunicates, and vertebrates (represented as Chordata) possessed a single Hox gene cluster consisting of three anterior (red, orange, and yellow), five central (green) and three ancestral posterior genes (blue). After the ancestral cephalochordate diverged, the tunicate ancestor (represented as Tunicata), in turn, diverged from the vertebrate lineage. At this stage, the ancestral tunicate must have experienced extensive changes in the genome, and the Hox gene cluster disintegration started, losing one or two central Hox genes. The ancestral tunicate subsequently evolved into two lineages, and in turn, diverged into Stolidobranchia and Appendicularia lineages (right side, upper) and Phlebobranchia, Aplousobranchia and Thaliacea lineages (right side, lower) [27]. The Hox gene complement of the ancestral tunicate with each three of anterior, central and posterior genes must have been established by the divergence of the two evolutionary lineages stated above. At the same time, early Hox gene cluster disintegration events must have occurred. In one of the two resultant evolutionary lineages, the ancestral stolidobranchial ascidian (Hr) emerged, being separated from the larvacean lineage. In the other evolutionary lineage, the ancestral phlebobranchial ascidian (Ci) emerged, being separated from Aplousobranchia and Thaliacea lineages. The Hox gene cluster subsequently disintegrated in different patterns in the two evolutionary lineages. White or gray ovals indicate Hox genes, probably of the central Hox gene group origin (see text). The Hox gene complement of Oikopleura dioica, consisting of two anterior, one central, and six posterior genes, and that of amphioxus, consisting of 15 members, are schematically represented