Selection, drift, and introgression shape MHC polymorphism in lizards

The major histocompatibility complex (MHC) has long served as a model for the evolution of adaptive geneticdiversity in wild populations. Pathogen-mediated selection is thought to be a main driver of MHC diversity, but it remains elusive to what degree selection shapes MHC diversity in complex biogeographical scenarios where other evolutionary processes (e.g. genetic driftand introgression) may also be acting. Here we focus on two closely related green lizard species, Lacerta trilineataand L. viridis, to address the evolutionary forces acting on MHC diversity in populations with different biogeographic structure. We characterized MHC class Iexon 2 and exon 3, and neutral diversity (microsatellites), to study the relative importance of selection, drift, and introgressionin shaping MHC diversity. As expected, positive selection was a significant force shaping the high diversity of MHC genes in both species. Moreover, introgressionsignificantly increased MHC diversity in mainland populations, with a primary direction of gene flow from L. viridis to L. trilineata. Finally, we found significantly fewer MHC alleles in island populations, but maintained MHC sequence and functional diversity, suggesting that positive selection counteracted the effect of drift. Overall, our data support that different evolutionary processes govern MHC diversity in different biogeographical scenarios: positive selection occurs broadly while introgressionacts in sympatryand drift when the population sizes decrease.