Analysis Of The Chinese Alligator Tcr Alpha/Delta Loci Reveals The Evolutionary Pattern Of Atypical Tcr Delta/Tcr Mu In Tetrapods

Atypical TCR delta found in sharks, amphibians, birds, and monotremes and TCR mu found in monotremes and marsupials are TCR chains that use Ig or BCR-like variable domains (VH delta/V mu) rather than conventional TCR V domains. These unconventional TCR are consistent with a scenario in which TCR and BCR, although having diverged from each other more than 400 million years ago, continue to exchange variable gene segments in generating diversity for Ag recognition. However, the process underlying this exchange and leading to the evolution of these atypical TCR receptor genes remains elusive. In this study, we identified two TCR alpha/delta gene loci in the Chinese alligator (Alligator sinensis). In total, there were 144 V, 154 J alpha, nine J delta, eight D delta, two C alpha, and five C delta gene segments in the TCR alpha/delta loci of the Chinese alligator, representing the most complicated TCR alpha/delta gene system in both genomic structure and gene content in any tetrapod examined so far. A pool of 32 VH delta genes divided into 18 subfamilies was found to be scattered over the two loci. Phylogenetic analyses revealed that these VH delta genes could be related to bird VH delta genes, VH delta/V mu genes in platypus or opossum, or alligator VH genes. Based on these findings, a model explaining the evolutionary pattern of atypical TCR delta/TCR mu genes in tetrapods is proposed. This study sheds new light on the evolution of TCR and BCR genes, two of the most essential components of adaptive immunity.