Are there Ubiquitous Parasite-driven Major Histocompatibility Complex Selection Mechanisms in Gray Mouse Lemurs? |
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Authors: | Nina Schwensow Manfred Eberle Simone Sommer |
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Institution: | 1.Department of Evolutionary Genetics,Leibniz Institute for Zoo and Wildlife Research (IZW),Berlin,Germany;2.Department of Behavioral Ecology & Sociobiology,Leibniz Institute for Primate Research (DPZ),G?ttingen,Germany |
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Abstract: | A major goal of evolutionary biology is to understand how selection drives local adaptation. For example, the major histocompatibility
complex (MHC) plays an important role in the immune system, and high levels of MHC variation are thought to be a form of adaptation
in natural populations. Individual MHC composition may influence parasite resistance via advantages associated with 1) heterozygosity,
because heterozygotes recognize a broader range of different antigens than homozygotes (heterozygote advantage); 2) highly
variable amino acid sequences in MHC alleles, allowing individuals to bind a broader spectrum of parasite-derived peptides
(divergent-alleles advantage, a mechanistic variant of the heterozygote advantage model); or 3) specific MHC alleles (rare
allele advantage or frequency dependent selection). We investigated relationships between gastrointestinal nematode burden
and both adaptive immune gene variability (MHC class II DRB) and neutral microsatellites in free-living gray mouse lemurs
(Microcebus murinus) native to a dry deciduous forest population in western Madagascar to test these hypotheses. The individual MHC composition
was related to parasite infestation. Specific MHC alleles were involved in parasite resistance and the presence of common
alleles negatively influenced infestation intensity. We found no support for the heterozygote advantage hypothesis, but we
did find support for the divergent-MHC allele advantage hypothesis: Individuals with very divergent MHC alleles carried fewer
and less intense nematode infestations than individuals with more similar alleles in the more variable dry deciduous forest
population. These results indicate that intestinal parasites are important selection pressures under natural conditions and
suggest that different selection mechanisms are not mutually exclusive. In contrast, we detected no association between neutral
overall individual genetic diversity (measured via 17 microsatellites) and parasite load. Finally, we investigated the ubiquity
of parasite-driven selection mechanisms by comparing our results with a previous study of a mouse lemur population from the
climatically different littoral forest in southeastern Madagascar, ca. 500 km away. This revealed that different specific MHC alleles were involved in parasite resistance in the 2 habitats, showing
that gene-parasite associations are not consistent between populations. |
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