Fig. 10

A model of tenascin evolution. A single tenascin gene is proposed to have appeared early in the chordate lineage, perhaps from building blocks found in the genome of a common ancestor with hemichordates, whereas TNC-like and TNR-like paralogs are proposed to have originated following a whole genome duplication (WGD) event early in the evolution of vertebrates. Additional WGD events lead to the appearance of additional tenascin (TNC- and TNR-like) paralogs in agnathans. The appearance of TNX in a common ancestor of cartilaginous and bony fishes is attributed to duplication of TNR through the second WGD in vertebrates. TNX has been lost in chimaeras (holocephalans) but is found in all other fishes and tetrapods. A local duplication of TNR is proposed to have led to the appearance of TNW (also known as TNN) in an ancestor of lobe-fined and ray-finned fishes, which roughly coincides with the appearance of bone. Thus, four tenascin paralogs are found in sarcopterygians (lobe-finned fishes and tetrapods), with the exceptions of the West African lungfish, and the tetraploid clawed frog Xenopus laevis. Additional WGD events in the ray-finned fishes results in the appearance of numerous paralogs of TNC, TNR, TNW and TNX. See text for further details