Information concerning the evolution of T lymphocyte receptors (TCR) can be deciphered from that part of the molecule that recognizes antigen presented by major histocompatibility complex (MHC), namely the variable (V) regions. The genes that code for these variable regions are found within the TCR loci. Here, we describe a study of the evolutionary origin of V genes that code for the α and β chains of the TCR loci of mammals. In particular, we demonstrate that most of the 35 TRAV and 25 TRBV conserved genes found in Primates are also found in other Eutheria, while in Marsupials, Monotremes, and Reptiles, these genes diversified in a different manner. We also show that in mammals, all TRAV genes are derived from five ancestral genes, while all TRBV genes originate from four such genes. In Reptiles, the five TRAV and three out of the four TRBV ancestral genes exist, as well as other V genes not found in mammals. We also studied the TRGV and TRDV loci from all mammals, and we show a relationship of the TRDV to the TRAV locus throughout evolutionary time.

We studied the V exons of 14 rodent species obtained from whole genome sequencing (WGS) datasets. Compared to other mammals, we found an increase in the number of immunoglobulin (IG) V genes in the heavy (IGH) and kappa chain (IGK) loci. We provide evidence for a reduction genes in lambda chain (IGL) locus, disappearing entirely in one of the species(Dipodomys ordii). We show relationships amongst the V genes of the T-cell receptors (TR) found in primates, possessing ortholog sequences between them. As compared with other mammals, there is an increase in the number of TRAV genes within rodents. Such an increase within this locus is caused by duplication events involving a few putative V genes. This duplication phenomenon does not occur in the TRBV locus. In those species that underwent an expansion of TRAV genes, we found that they also have a correspondingly larger number of MHC Class I genes. The results suggest that selective pressures have conditioned the expansion of V genomic repertoire the TRA, IGK and IGH loci during the diversification process of rodents.