What evolutionary processes maintain MHC II diversity within and among populations of stickleback?

Major Histocompatibility Complex (MHC) genes code for proteins that recognize foreign protein antigens to initiate T-cell-mediated adaptive immune responses. They are often the most polymorphic genes in vertebrate genomes. How evolution maintains this diversity remains of debate. Three main hypotheses seek to explain the maintenance of MHC diversity by invoking pathogen-mediated selection: heterozygote advantage, frequency-dependent selection, and fluctuating selection across landscapes or through time. Here, we use a large-scale field parasite survey in a stickleback metapopulation to test predictions derived from each of these hypotheses. We identify over 1000 MHC II beta variants (alleles spanning paralogous genes) and find that many of them covary positively or negatively with parasite load, suggesting that these genes contribute to resistance or susceptibility. However, despite our large sample-size, we find no evidence for the widely cited stabilizing selection on MHC heterozygosity, in which individuals with an intermediate number of MHC variants have the lowest parasite burden. Nor do we observe a rare-variant advantage, or widespread fluctuating selection across populations. In contrast, we find that MHC diversity is best predicted by neutral genome-wide heterozygosity and between-population genomic divergence, suggesting neutral processes are important in shaping the pattern of metapopulation MHC diversity. Thus, although MHC II beta is highly diverse and relevant to the type and intensity of macroparasite infection in these populations of stickleback, the main models of MHC evolution still provide little explanatory power in this system.

Automated summary

The study found high diversity in the MHC II beta genes in a stickleback population, with many of them positively or negatively correlating with parasite load, suggesting an impact on resistance or susceptibility. However, no evidence was found supporting the hypotheses of MHC heterozygote advantage, frequency-dependent selection, or fluctuating selection. Instead, MHC diversity was found to be more influenced by neutral processes.