Fig. 5R

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, being separated from Appendicularia, into two lineages; one leading to stolidobranchian ascidians (Hr) (right side, middle) and the other leading to phlebobranchian (Ci) and aplousobranchian ascidians as well as Thaliacea (right side, upper) [33, 34]. By the divergence of the two evolutionary lineages, the Hox gene complement of the ancestral ascidian with each three of anterior, central and posterior genes must have been established. At the same time, early Hox gene cluster disintegration events must have occurred. Then, the Hox gene cluster further disintegrated in different patterns in the evolutionary lineages giving rise to Hr and Ci. 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.