Contrasting histories of avian and mammalian Mhc genes revealed by class II B sequences from songbirds

To explore the evolutionary dynamics of genes in the major histocompatibility complex (Mhc) in non-mammalian vertebrates, we have amplified complete sequences of the polymorphic second (β1) and third (β2) exons of class II β chain genes of songbirds. The pattern of nucleotide substitution in the antigen-binding site of sequences cloned from three behaviorally and phylogenetically divergent songbirds [scrub jays (Aphelocoma coerulescens), red-winged blackbirds (Agelaius phoeniceus), and house finches (Carpodacus mexicanus)] reveals that class II B genes of songbirds are subject to the same types of diversifying forces as those observed at mammalian class II loci. By contrast, the tree of avian class II B genes reveals that orthologous relationships have not been retained as in placental mammals and that, unlike class II genes in mammals, genes in songbirds and chickens have had very recent common ancestors within their respective groups. Thus, whereas the selective forces diversifying class II B genes of birds are likely similar to those in mammals, their long-term evolutionary dynamics appear to be characterized by much higher rates of concerted evolution.