The receptor of a T lymphocyte (TCR) recognizes nonself antigens in the com
pany of major histocompatibility complex (MHC) molecules presented to it by
the antigen-presenting cell. The variable region of TCR is encoded by eith
er a concatenation of variable region (TCR-V), diversity region (TCR-D), an
d joining region (TCR-J) genes, or a concatenation of TCR-V and TCR-J genes
. The TCR-V genes exist as a multigene family in vertebrate species. Here w
e study the evolutionary relationships of TCR-V genes from humans, sheep, c
attle, rabbits, mice, and chicken. These six species can be classified into
two groups according to the frequency of gamma delta T-cells in their peri
pheral T-cell populations. The "gamma delta low" group of species includes
humans and mice, in which gamma delta T-cells constitute very limited porti
on of the T-cell population. The "gamma delta high'' group includes sheep,
cattle, rabbits, and chicken, in which gamma delta T-cells comprise up to 6
0% of the T-cell population. Here, we compiled TCR-V sequences from the six
species and conducted a phylogenetic analysis. We identified various TCR-V
gene subgroups based on the analysis. We found that humans and mice have r
epresentatives from nearly all of the subgroups identified, while other spe
cies have lost subgroups to different extent. Therefore, the ya low species
have a high degree of diversity of TCR-V genes, while ya high species all
have limited diversity of TCR-V genes. This pattern is similar to that foun
d for immunoglobulin variable region (IGV) genes.