The major histocompatibility complex of tassel-eared squirrels

摘要

The extent of polymorphism and the rate of divergence of class I and class II sequences mapping to the mammalian major histocompatibility complex (MHC) have been the subject of experimentation and speculation. To provide further insight into the evolution of the MHC we have initiated the analysis of two geographically isolated subspecies of tassel-eared squirrels. In the preceding communication we described the number and polymorphism of TSLA class I and class II sequences in Kaibab squirrels (S. aberti kaibabensis), which live north of the Grand Canyon. In this report we present a parallel analysis of Abert squirrels (S. aberti aberti), which live south of the Grand Canyon in northern Arizona. Genomic DNA from 12 Abert squirrels was digested with restriction enzymes, electrophoresed, blotted, and hybridized with DRα, DRβ, DQα, DQβ, and HLA-B7 probes. The results of these hybridizations were remarkably similar to those obtained in Kaibab squirrels. The majority of class I and class Il bands were identical in size and number, suggesting that Abert and Kaibab squirrels have not significantly diverged in theTSLAcomplex despite their geographical separation. Relative polymorphism of class 11 sequences was similar to that observed with Kaibab squirrels:βsequences exhibited higher polymorphism than a sequences. As in Kaibab squirrels, a number of α andβsequences were apparently carried on the same fragments. In comparison to class IIβsequences, there was limited polymorphism in class I sequences, although a diverse number of class I genotypes were observed. Attempts to identify segregatingTSLAhaplotypes were futile in that the only families of sequences with concordant distributions were DQαand DQβ. These observations and those obtained with Kaibab squirrels suggest that the present-dayTSLAhaplotypes of both subspecies are derived from a limited number of common, progenitor haplotypes through repeated intra-TSLAreco