A review of suggested mechanisms of MHC odor signaling

Although an individual’s mix of MHC immune genes determines its resistance, findingMHC-dependent mate choice occurred by accident in inbred mice. Inbred mice prefer MHC dissimilarmates, even when the choice was restricted to urine. It took decades to find the info-chemicals, whichhave to be as polymorphic as the MHC. Microbiota were suggested repeatedly as the origin ofthe odor signal though germ-free mice maintained normal preference. Different versions of the‘carrier hypothesis’ suggested MHC molecules carry volatiles after the bound peptide is released.Theory predicted an optimal individual MHC diversity to maximize resistance. The optimallycomplementary mate should be and is preferred as several studies show. Thus, the odor signal needsto transmit the exact information of the sender’s MHC alleles, as do MHC ligand peptides but notmicrobiota. The ‘MHC peptide hypothesis’ assumes that olfactory perception of the peptide ligandprovides information about the MHC protein in a key-lock fashion. Olfactory neurons react onlyto the anchors of synthesized MHC peptides, which reflect the binding MHC molecule’s identity.Synthesized peptides supplemented to a male’s signal affect choice in the predicted way, however,not when anchors are mutated. Also, the human brain detects smelled synthesized self-peptidesas such. After mate choice, the lottery of meiosis of randomly paired oocyte and sperm haplotypeswould often produce MHC non-optimal offspring. In sticklebacks, eggs select MHC-compatiblesperm, thus prefer the best combination close to the population optimum