MHC-mediated local adaptation in reciprocally translocated Chinook salmon
Most Pacific salmonid populations have faced significant population declines over the past 30 years. In order to effectively conserve and manage these populations, knowledge of the evolutionary adaptive state of individuals and the scale of adaptation across populations is needed. The vertebrate major histocompatibility complex (MHC) represents an important adaptation to parasites, and genes encoding for the MHC are widely held to be undergoing balancing selection. However, the generality of balancing selection across populations at MHC loci is not well documented. Using Chinook salmon (Oncorhynchus tshawytscha) from two populations, we follow the survival of full-sib family replicates reared in their natal river and reciprocally transplanted to a foreign river to examine selection and local adaptation at the MHC class I and II loci. In both populations, we found evidence of a survivorship advantage associated with nucleotide diversity at the MHC class I locus. In contrast, we found evidence that MHC class II diversity was disadvantageous in one population. There was no evidence that these effects occurred in translocated families, suggesting some degree of local adaptation at the MHC loci. Thus, our results implicate balancing selection at the MHC class I but potentially differing selection across populations at the class II locus.
KeywordsMajor histocompatibility complex Chinook salmon Adaptive divergence Nucleotide divergence Balancing selection Directional selection Survival
Many thanks to L. Clint and D. Ewart for permission to conduct this research in the Big Qualicum and Quinsam hatchery facilities. We also thank all of the staff at the Big Qualicum, Quinsam, and Rosewall hatcheries (Department of Fisheries and Oceans Canada, Pacific Region) for their significant logistical support throughout the duration of the experiment. Thanks to S. Garner for assistance setting up crosses, to V. Padmanabhan for assistance with the microsatellite genotyping, and to R. Hepburn and K. Ambacher for assistance with DNA extraction. A. Lachance provided access to the SSCP equipment. Eric Anderson and four anonymous reviewers provided helpful comments on the manuscript. Funding was provided by a Natural Sciences and Engineering Research Council of Canada (NSERC) PGS D Scholarship and an Ontario Graduate Scholarship in Science and Technology to MLE, NSERC Discovery/Accelerator grants and Canadian Foundation for Innovation/Ontario Innovation Trust grants to BDN, and NSERC Discovery and Canada Research Chair grants to DDH.
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