Fig. 2.

Diagrammatic representation of adaptive evolution of immune supertypes. Supertypes (STs) evolve an epitope/paratope space within a population that results in a balanced polymorphism during Red Queen co-evolution. For simplicity in depicting interactions, we visualise all alleles present in the population. Depending on the number of MHC loci, individuals possesses just a subset of these alleles and STs. Immune alleles (dots) that are closely related are presented in coloured networks (blue, green, red and orange). Alleles of the same ST cover an area in the epitope/paratope space depicted by coloured ellipses. (Non-focal STs are depicted in grey) a STs in a gene pool have evolved to cover the entire epitope space with little overlap. b The loss of one allele from the population (e.g., due to drift or positive selection on an alternative allele) opens a hole in the paratope space (black area) that becomes exploited by parasites with matching epitope. c Selection favours new alleles with a paratope that covers the hole, but only rarely are these substitutions made by alleles from a different ST (red allele covering the hole left by the loss of the blue allele). This causes the STs to ‘wobble’ in the epitope/paratope space. Nevertheless, changes in the paratope of STs are restricted by the presence of neighbouring STs, effectively resulting in a form of balancing selection. Hence, STs remain conserved over evolutionary time, despite the Red Queen arms race and the high turnover of their constituent alleles (see ‘Results’ section)