Evolution of T-cell receptor gamma and delta constant region and other T-cell-related proteins in the human-rodent-artiodactyl triplet.

In this paper we report a detailed comparative and evolutionary analysis of the sequences of constant T-cell receptor (Tcr) C gamma delta genes of artiodactyls compared to the homologous sequences of rodents and primates. Because of the frequency and physiological distribution of gamma delta T-cells in different animals, rodents and humans are defined as "gamma delta low" species and ruminants as "gamma delta high" species. Such a characteristic seems to be due to an adaptive role of gamma delta T-cell function. By analyzing the ruminant gene phylogeny of Tcr C gamma we were able to estimate the distance between cattle and sheep at 18 million years ago, a time that is in agreement with other nonmolecular estimates. For Tcr C gamma delta genes a peculiar phylogenetic relationship was found, with human and mouse clustering together and leaving artiodactyls apart. By using appropriate outgroups, the same phylogenetic pattern was obtained with other T-cell related sequences: namely, Tcr C alpha chain, CD3 gamma and delta invariant subunits. Interleukin-2. Interleukin-2 receptor alpha chain and Interleukin-1 beta with the exception of Tcr C beta chain and Interleukin-1 alpha. In contrast, the analysis of all other T-cell nonrelated genes, available in primary databases reveals a different tree, where primates and artiodactyls are sister taxa and rodents are apart in accordance with the current view of mammalian phylogeny. These data are relevant to important evolutionary issues. They show how misleading a phylogeny based on a single or on a few homologous genes may be. In addition they demonstrate that genes with correlated functions may evolve in a lineage specific manner probably in relation to environmental conditions.