Human and pathogen genotype-by-genotype interactions in the light of coevolution theory

Antagonistic coevolution (i.e., reciprocal adaptation and counter-adaptation) between hosts and pathogens has long been considered an important driver of genetic variation. However, direct evidence for this is still scarce, especially in vertebrates. The wealth of data on genetics of susceptibility to infectious disease in humans provides an important resource for understanding host-pathogen coevolution, but studies of humans are rarely framed in coevolutionary theory. Here, I review data from human host-pathogen systems to critically assess the evidence for a key assumption of models of host-pathogen coevolution-the presence of host genotype-by-pathogen genotype interactions (G×G). I also attempt to infer whether observed G×G fit best with "gene-for-gene" or "matching allele" models of coevolution. I find that there are several examples of G×G in humans (involving, e.g., ABO, HBB, FUT2, SLC11A1, and HLA genes) that fit assumptions of either gene-for-gene or matching allele models. This means that there is potential for coevolution to drive polymorphism also in humans (and presumably other vertebrates), but further studies are required to investigate how widespread this process is.