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Proteins encoded by antigen-processing genes (APGs) provide major histocompatibility complex (MHC) class I (MHC-I) with antigenic peptides. In mammals, polymorphic multigenic  MHC-I  family is served by monomorphic APGs, whereas in certain nonmammalian species both  MHC-I  and APGs are polymorphic and coevolve within stable haplotypes. Coevolution was suggested as an ancestral gnathostome feature, presumably enabling only a single highly expressed classical  MHC-I  gene. In this view coevolution, while optimizing some aspects of adaptive immunity, would also limit its flexibility by preventing the expansion of classical  MHC-I  into a multigene family. However, some nonmammalian taxa, such as salamanders, have multiple highly expressed  MHC-I  genes, suggesting either that coevolution is relaxed or that it does not prevent the establishment of multigene  MHC-I . To distinguish between these two alternatives, we use salamanders (30 species from 16 genera representing six families) to test, within a comparative framework, a major prediction of the coevolution hypothesis: the positive correlation between  MHC-I  and APG diversity. We found that  MHC-I  diversity explained both within-individual and species-wide diversity of two APGs,  TAP1  and  TAP2 , supporting their coevolution with  MHC-I , whereas no consistent effect was detected for the other three APGs ( PSMB8 ,  PSMB9 , and  TAPBP ). Our results imply that although coevolution occurs in salamanders, it does not preclude the expansion of the  MHC-I  gene family. Contrary to the previous suggestions, nonmammalian vertebrates thus may be able to accommodate diverse selection pressures with flexibility granted by rapid expansion or contraction of the  MHC-I  family, while retaining the benefits of coevolution between  MHC-I  and  TAPs .

Coevolution between MHC Class I and Antigen-Processing Genes in Salamanders