The vertebrate connexin family.

Connexins are chordate-specific transmembrane proteins that can form gap junctional channels between adjacent cells. With the progress in vertebrate genome sequencing, it is now possible to reconstruct the main lines in the evolution of the connexin family from fishes to mammals. Four connexin groups are only found in fishes. Otherwise, the differences between fishes and mammals can be explained by two gene losses (Cx39.9 and Cx43.4) after the divergence of the Reptilia, and three gene duplications (the generation of Cx26 and 30 from a preCx26/30 sequence, Cx30.3 and 31.1 from a preCx30.3/ 31.1 sequence, and Cx31.3 from an uncertain origin). Orthologs of most connexins can be found throughout the vertebrates from fishes to mammals. As judged from the recently defined connexins in tunicates, the original connexin might be related to the ortholog groups of Cx36, 39.2, 43.4, 45 or 47.