Allelic Polymorphism of T-cell Receptor Constant Domains is Widespread in Fishes

T-cell receptor chains contain membrane-proximal constant domains of the immunoglobulin superfamily that are relatively invariant in mammalian species. In contrast, recent studies in the bicolor damselfish have demonstrated surprising allelic polymorphism in the TCR alpha ( A) and TCR beta ( B) "constant" (C) domain genes. This report extends these initial observations beyond Perciformes to two other orders of teleost fishes. Studies in both the Atlantic cod and zebrafish show high levels of polymorphism in the TCRA constant genes. Levels of 13% and 15% amino acid nonidentity were found within cod and zebrafish, respectively. Evolutionary analysis of codon usage suggests that positive selection maintains the high number of TCRAC alleles in these fish populations. Additionally, investigation of a TCRB constant gene from the Beau Gregory, a sister species of the bicolor damselfish, shows no evidence of transpecies maintenance of constant region alleles. These data argue that the T-cell receptor constant domain is being employed by many vertebrates in a manner inconsistent with our current understanding, and may indicate unheralded complexity in signal transduction through the TCR/CD3 complex.