Concerted evolution of two Mhc class II B loci in pheasants and domestic chickens

The major histocompatibility complex (Mhc) of the ring-necked pheasant contains two polymorphic Mhc class II B genes. We show here, by screening of a cDNA library and RT-PCR from RNA, that both of these loci, Phco-DAB1 and Phco-DAB2, normally are transcribed in the spleen. They differ mainly in the 3' untranslated (UT) region, with the transcript lengths, not including the poly(A) tails, being 1,100 nt for DAB1 and 955 nt for DAB2. These two loci are orthologous to the B-LBI and B-LBII loci of the domestic chicken, respectively. DAB1 and DAB2 therefore seem to have evolved from a duplication before the split of the evolutionary lineages leading to the pheasant and the domestic chicken ca. 20 MYA. This is the first report of an orthologous relationship between avian Mhc genes. Yet, the third exons of DAB1 and DAB2 were identical in all available sequences and differed at 10 positions from the exon 3 sequences of B-LBI/B-LBII. The species-specific exon 3 suggests that DAB1 and DAB2 are subject to concerted evolution, i.e., interlocus genetic exchange. The exon 2 sequences show characteristic polymorphism, with hypervariable segments occurring in different combinations in different alleles. Given the divergence in the 3'UT region, the finding of the same exon 2 sequence at both the DAB1 and the DAB2 loci in one of the pheasant haplotypes also suggests that interlocus genetic exchange does occur. Accordingly, the exon 2 sequences tended to cluster irrespective of locus in the phylogenetic analyses. Genetic exchange simultaneously involving both exon 2 and exon 3 may be facilitated by the short length of the intervening intron (< 100 bp) in pheasants and domestic chickens compared with, e.g., humans (about 3 kb).