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What evolutionary processes maintain MHCIIβ diversity within and among populations of stickleback?
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  • Foen Peng,
  • Kimberly Ballare,
  • Hollis Woodard,
  • Stijn den Haan,
  • Daniel Bonick
Foen Peng
University of Connecticut
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Kimberly Ballare
University of California Santa Cruz
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Hollis Woodard
University of California Riverside
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Stijn den Haan
Central European University
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Daniel Bonick
University of Connecticut
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Abstract

Major Histocompatibility Complex (MHC) genes encode 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 is still an unsettled issue. 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 a thousand MHCIIβ alleles 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 alleles have the lowest parasite burden. Nor do we observe a rare-allele 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 MHCIIβ 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.

Peer review status:UNDER REVIEW

21 Aug 2020Submitted to Molecular Ecology
22 Aug 2020Assigned to Editor
22 Aug 2020Submission Checks Completed
01 Sep 2020Reviewer(s) Assigned