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 histocompat ibility complex (Mhc) in nonmammalian vertebrates, we have amplified c omplete sequences of the polymorphic second (beta 1) and third (beta 2 ) exons of class II beta chain genes of songbirds, The pattern of nucl eotide substitution in the antigen-binding site of sequences cloned fr om three behaviorally and phylogenetically divergent songbirds [scrub jays (Aphelocoma coerulescens), red-winged blackbirds (Agelaius phoeni ceus), and house finches (Carpodacus mexicanus)] reveals that class II B genes of songbirds are subject to the same types of diversifying fo rces as those observed at mammalian class II loci. By contrast, the tr ee of avian class II B genes reveals that orthologous relationships ha ve not been retained as in placental mammals and that, unlike class II genes in mammals, genes in songbirds and chickens have had very recen t common ancestors within their respective groups, Thus, whereas the s elective forces diversifying class II B genes of birds are likely simi lar to those in mammals, their long-term evolutionary dynamics appear to be characterized by much higher rates of concerted evolution.