Restriction fragment-length polymorphisms of class II gene sequences in mice expressing minor structural variants of I-A(k) and I-A(p1)

Serologic and structural analyses of the I-A molecules expressed among a large collection of wild mouse-derived H-2 haplotypes has led to the definition of 'families' of I-A alleles which encode antigenically similar molecules that are identical in more than 90% of their tryptic peptides. Two of these families, denoted the I-A(k) and I-A (p) families, consist of 10 I-A alleles which encode I-A molecules whose structures are closely related to either I-A(p) or I-A(k). The evolutionary relationships of the I-A alleles in these families were assessed by a molecular analysis of their genomic structures. The A(α) and A(β) alleles within these I-A families were compared by analysis of restriction fragment-length polymorphisms (RFLP) detected at high stringency by Southern blot hybridization with DNA probes specific for either A(α) or A(β). The polymorphic restriction enzyme sites detected in this survey were distributed over more than 7 kb of genomic DNA surrounding each gene. Because both A(α) and A(β) are encoded by about 700 bp of exon DNA, the majority of the restriction enzyme sites assayed by this RFLP analysis reflect polymorphisms in noncoding regions. The DNA sequence homologies of these alleles were estimated from the RFLP results with seven restriction endonucleases by calculating the fraction homologous value as defined previously. The results indicate that evolutionarily dissimilar I-A alleles can encode I-A molecules with very similar structures. The five I-A alleles in the I-A(k) family could be divided into two discrete groups, denoted K1 and K2, on the basis of their restriction fragment (RF) genotypes. The RF genotypes of alleles within each group shared more than 80% of the restriction fragments for both A(α) and A(β). In contrast, the RF genotypes of alleles in group K1 differed extensively from those in group K2, indicating that alleles in these separate groups may not be evolutionarily closely related. These observations suggest that gene conversion or intragenic recombinational events may have been involved in the evolution of groups K1 and K2 in the I-A(k) family. The RF genotypes of alleles in the I-A(p) family demonstrated a close evolutionary relationship among all but two of the alleles. These two alleles encoded I-A molecules whose structures were the least related to I-A(p) of any of the alleles in the I-A(p) family. RFLP analysis of the I-A(p) and I-A(k) families demonstrate that the evolutionary relationships defined for A(β) alleles coincide with those defined for A(α). This result indicates that alleles at these two loci accumulate mutations during their divergence in a coordinate rather than an independent manner, suggesting that alleles of A(α) and A(β) co-evolve as an A(α)A(β) gene duplex in natural mouse populations.