Genomic organization, expression and evolution of gamma and beta TCR genes in Tursiops truncatus.

T cell receptors are heterodimers comprising alpha (TRA) and bet (TRB) or gamma (TRG) and delta (TRD) subunits, two combinations defining the two major lineages of T cells. alpha/beta T cells typically recognize peptide antigens presented on major histocompatibility complex (MHC) encoded molecules, while gamma/delta cells are either MHC restricted or resemble Ig in being able to bind free antigens. The bottlenosed dolphin is being sequenced by the Human Genome Sequencing Center (BCMHGSC) at the Baylor College of Medicine and the Broad Institute using a whole genome shotgun sequencing strategy (BioProject ID: 20367). We employed the NCBI draft genome assembly to identify the TRG and TRB loci of Tursiops truncatus. The dolphin gamma locus is the smallest and simplest of all the mammalian TRG loci as yet studied. It shows a gene cassette organization comprising two variable (V) genes, three joining (J) genes, and a single constant (C) gene. Expression analysis identified all the possible VJ rearrangements of the locus, although some transcripts are preferentially expressed. About half of the TRGV2 rearrangements originate transcripts unlikely to be functional due to stop codons in CDR3. Although a complete characterization was impossible because of gaps in the available assemblies, the dolphin beta locus is also very simple in comparison with human and ovine. We found that only four of the seven identified in the genome are TRBV expressed genes, with a significant bias toward expression of TRBV30, located immediately after the C gene in an inverted transcriptional orientation. However, there was no evidence of preferential rearrangement with regard to TRBD e TRBJ. Finally, multialignment analysis of TRGV and TRBV sequences confirmed, as expected, a close phylogenetic relationship between Tursiops truncatus and the Bovidae family of the Artiodactyla.