Fig. 3.

Evolution of the T-box gene family. The subfamilies or classes of genes that have been identified in the different animal groups are indicated in the boxes. There has been a remarkable conservation of transcription factors between lineages that have been evolving independently since the last common ancestor to metazoans, and many of the T-box gene families have their origin at the base of the tree. However, diversification at the onset of metazoan evolution is evident. T, which is the most ancient T-box gene, is represented in unicellular organisms, as is Tbx7/8, a class not present in Bilateria. Sponges have a diverse set of T-box genes including several that have not been retained in Bilateria, whereas Tbx6 subfamily genes apparently arose in Bilateria. Note that this diagram represents one possible order of divergence of phyla (Degnan et al., 2009; Sebe-Pedros et al., 2013).