A HIGHLY POLYMORPHIC MICROSATELLITE IN THE CLASS-II EB GENE ALLOWS TRACING OF MAJOR HISTOCOMPATIBILITY COMPLEX EVOLUTION IN MOUSE

A hallmark of major histocompatibility complex (MHC) genes is their ex traordinarily high level of polymorphism. Polymorphic residues on MHC molecules determine which peptide ligands they bind and present to eff ector T lymphocytes. Although the genetic mechanisms responsible for M HC polymorphism have been delineated, the timetable and the pathway of their diversification remain unclear. To trace MHC evolution, we have characterized a highly polymorphic microsatellite containing tandem r epeats (TRs) of two tetra-nucleotide units, TGGA and GGCA, located at the 3' end of the second intron in the class II Eb gene of mouse. On t he basis of length as well as sequence variations, 11 TR alleles were defined in 55 inbred mouse strains, which included MHC recombinant hap lotypes and haplotypes derived from different subspecies of mouse. In this extensive sampling, a striking concordance was observed between t he serologically identified class II proteins and the associated TR al leles. Examination of several strains carrying the same MHC haplotypes as well as strains carrying recombinant MHC haplotypes indicates that TR alleles are extremely stable. These observations suggest that TR p olymorphism predates the separation of various subspecies of mouse. On the basis of sequence divergence, a genealogical tree has been constr ucted to depict evolution of the different TR alleles. Finally, eviden ce is presented that suggests this microsatellite polymorphism is gene rated by slipped-strand mispairing during DNA replication.