The Major Histocompatibility Complex, Sexual Selection, and Mate Choice

To maintain sexual reproduction, recombination of good genes through selective mate choice must achieve a twofold genetic benefit in each generation. “Fragrant” immune genes of the major histocompatibility complex (MHC) allow the choosy sex to complement her own set of alleles with a more or less diverse set of male alleles to reach an optimal number of different MHC alleles for the offspring. The optimal complement from the partner should include those MHC alleles that provide resistance against the current parasites, which could be revealed by the expression of costly secondary sexual characters. This maximizes resistance to ever-changing infectious diseases. Because the advantage of sex must be produced through recombination, assortative mating should combine currently advantageous MHC alleles. Preferring just MHC dissimilar mates is only a best-of-bad-job rule. MHC ligand peptides may be the natural “perfume” that reveals a potential partner's MHC genetics probably in all vertebrates. Perfumes may mimick MHC related signals.