T-cell receptor genes in tassel-eared squirrels (Sciurus aberti) - I. Genetic polymorphism and divergence in the Abert and Kaibab subspecies

The role of environmental factors in the evolution and maintenance of diversity of antigen receptor gene families which participate in the immune response in mammals is inadequately understood. In order to elucidate the impact of these factors, we have undertaken the analysis of these gene families in the tassel-eared squirrel (Sciurus aberti) which has been separated into discrete subspecies by geographic barriers and whose food resources can be quantitated for estimating environmental quality. In this communication we describe the initial analysis of the complexity and polymorphism of sequences related to T-cell receptor (Tcr) α and β chain genes in two subspecies, Sciurus aberti aberti (Abert) and Sciurus aberti kaibabensis (Kaibab) which have identical habitats and are separated by the Grand Canyon in Arizona, USA. Genomic blot analysis of 60 Abert and 62 Kaibab individuals collected over a 3-year period was performed with mouse Tcrb and Tcra cDNA probes. Sequences homologous to Tcrb-C, Tcrb-J1, and Tcrb-J2 genes were observed in all individuals from both subspecies; although Tcrb-J1 fragments were monomorphic, Tcrb-C and Tcrb-J2 fragments were polymorphic with both species- and subspecies-specific sequences. A single, monomorphic Tcra-C fragment was observed in addition to multiple Tcra-V fragments homologous to the mouse Tcra-V1 subfamily. Abert samples exhibited greater numbers of Tcra-V1 fragments as well as greater polymorphism than Kaibab samples. Heterozygosity estimates of Tcrb-C and Tcra-V1 sequences were determined for annually collected samples and compared with the yearly estimates of availability of hypogeous fungi, one of the major diet items of tassel-eared squirrels. In the Kaibab annual collections, Tcra-V1 heterozygosity declined with the decline in food resource, whereas heterozygosity of Tcrb-C sequences was inversely related to food resources. Similarly, a reduction in food resource for Abert squirrels in 1985 coincided with an increase in Tcrb-C heterozygosity in the same year. These results suggest that the diversity of gene families which participate in the immune response in mammals may be affected by environmental factors.