Co-evolution with chicken class I genes

The concept of co‐evolution (or co‐adaptation) has a long history, but application at molecular levels (e.g., ‘supergenes’ in genetics) is more recent, with a consensus definition still developing. One interesting example is the chicken major histocompatibility complex (MHC). In contrast to typical mammals that have many class I and class I‐like genes, only two classical class I genes, two CD1 genes and some non‐classical Rfp‐Y genes are known in chicken, and all are found on the microchromosome that bears the MHC. Rarity of recombination between the closely linked and polymorphic genes encoding classical class I and TAPs allows co‐evolution, leading to a single dominantly expressed class I molecule in each MHC haplotype, with strong functional consequences in terms of resistance to infectious pathogens. Chicken tapasin is highly polymorphic, but co‐evolution with TAP and class I genes remains unclear. T‐cell receptors, natural killer (NK) cell receptors, and CD8 co‐receptor genes are found on non‐MHC chromosomes, with some evidence for co‐evolution of surface residues and number of genes along the avian and mammalian lineages. Over even longer periods, co‐evolution has been invoked to explain how the adaptive immune system of jawed vertebrates arose from closely linked receptor, ligand, and antigen‐processing genes in the primordial MHC.