Evolutionary diversification of class II P loci in the Mhc of the mole-rat Spalax ehrenbergi

The class II region of the major histocompatibility complex (Smh) in the mole rat, Spalax ehrenbergi, consists of only two gene families, P and Q, instead of the four families (P, O, Q, and R) found in all other mammals studied to date. The Spalax P family consists of at least four beta and three alpha genes or gene fragments. In DNA-hybridization experiments, two of the beta genes behave as bona fide P-family members in that they hybridize strongly with human DP beta probes and hybridize weakly with probes specific for other class II gene families. The other two beta genes, on the other hand, hybridize weakly with human DP beta probes and nearly as well with human DQ beta probes. To determine the evolutionary relationships among these P-like genes, we have sequenced one of them. The sequence reveals, on the basis of its organization, that the gene clearly belongs to the P family, yet, on the basis of its nucleotide sequence, it is only slightly more similar to human DP than to human DQ genes. These results indicate that in the Spalax the P family of genes split into two subfamilies, PA and PB. For unknown reasons, one of these subfamilies (PB) retained more similarity to the Q gene family than did the other (PA).