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.
Desertification is a major threat to biodiversity in arid areas of the world, in part because many organisms in these regions are already existing at or near the limits of their movement and physiology. Here, we used molecular data to investigate patterns of persistence and dispersal in an ecologically and economically important carpenter bee (Xylocopa grisescens Lepeletier) found throughout the semiarid Caatinga region of Brazil. We used a genome-wide approach (Restriction-site associated DNA sequencing, RAD-seq) to gather genetic data (>83,000 SNPs) from bees sampled from eight sites within an arid region subject to desertification in Northeastern Brazil. We observed low levels of population genetic diversity and differentiation across the study region, despite data collection from sites up to 300 km distant. Additionally, we detected evidence of a relatively severe genetic bottleneck occurring an estimated 60 years ago. Our data suggest that population genetic patterns of X. grisescens have been shaped by human-mediated changes in the Caatinga, but these patterns have also been heavily influenced by biological characteristics of this species, such as their relatively high capacity for movement.
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