Toward an evolutionary genomics of the avian Mhc

We review recent developments in the ongoing study of the evolution of the Mhc gene family in birds, with emphasis on class II B genes and results from songbirds obtained in our laboratory. Southern blots suggest a surprising diversity in Mhc class II gene number among various songbird species (Passeriformes). We have sequenced similar to 30 kb contigs from Mhc-bearing cosmid clones from two species, red-winged blackbirds (Agelaius phoeniceus) and house finches (Carpodacus mexicanus), whose demography, lifetime reproductive success, epizootics, parasitology and mate choice are among the best studied for natural populations of birds. Of three genes cloned from these species, only one appears strongly polymorphic, and one (from the house finch) is likely a pseudogene. All are similar in structure to those in chickens, albeit with introns intermediate in length between chickens and mammals. Phylogenetic analysis of available class II B peptide-binding region exons suggests that the overwhelming long-term force operating on avian genes sampled thus far has been post-speciation gene duplication and/or concerted evolution. These and other results suggest that the evolution of class II B genes in birds conforms to a mixture of several models of multigene family evolution proposed for the mammalian Mhc, incorporating ongoing homogenization, duplication and pseudogene formation. Large-scale sequencing studies in these and other species, though still in their infancy, will prove invaluable for studying the comparative structures of avian Mhcs, as well as patterns of selection, mutation and linkage disequilibrium at several scales.