Squamate reptiles may have compensated for the lack of γδTCR with a duplication of the TRB locus

Squamate reptiles are amongst the most successful terrestrial vertebrate lineages, with over 10,000 species across a broad range of ecosystems. Despite their success, squamates are also amongst the least studied lineages immunologically. Recently, a universal lack of γδ T cells in squamates due to deletions of the genes encoding the T cell receptor (TCR) γ and δ chains was discovered. Here, we begin to address how the loss of γδ T cells may have impacted the evolution of the squamate immune system. Using the skink <i>Tiliqua rugosa</i>, we found that squamates have not significantly increased the complexity of conventional T cell receptor beta (TCRβ or <i>TRB</i>) chain V regions compared to that of the nearest living squamate relative, the tuatara, <i>Sphenodon punctatus</i> or other amniotes. Our analyses include a putative new TCR locus. This novel locus contains V, D, and J gene segments that undergo V(D)J recombination, albeit with a limited number of gene segments in most squamate species. Based on conserved residues, the predicted protein chain would be expected to form a heterodimer with TCRα. This new TCR locus appears to be derived from an ancient duplication of the <i>TRB</i> locus and is homologous to the recently described T cell receptor epsilon (<i>TRE</i>). <i>TRE</i> is absent from the genomes of the tuatara and all Archosaurs examined and appears squamate specific.