Adaptive introgression of MHC genes across vertebrate hybrid zones

Interspecific introgression is a potentially important source of novel variation of adaptive significance. Although multiple cases of adaptive introgression are well documented, broader generalisations about its targets and mechanisms are lacking. Balancing selection, particularly when acting through rare allele advantage, is an evolutionary mechanism that should favour adaptive introgression. This is because introgressed alleles would be more likely to confer an immediate selective advantage, facilitating their establishment in the recipient species even in the face of strong genomic barriers to introgression. Vertebrate Major Histocompatibility Complex (MHC) genes are well-established targets of long-term balancing selection, so widespread adaptive MHC introgression is expected.