title: Natural selection on individual variation in tolerance of gastrointestinal nematode infection.
abstract: Hosts may mitigate the impact of parasites by two broad strategies: resistance, which limits parasite burden, and tolerance, which limits the fitness or health cost of increasing parasite burden. The degree and causes of variation in both resistance and tolerance are expected to influence host-parasite evolutionary and epidemiological dynamics and inform disease management, yet very little empirical work has addressed tolerance in wild vertebrates. Here, we applied random regression models to longitudinal data from an unmanaged population of Soay sheep to estimate individual tolerance, defined as the rate of decline in body weight with increasing burden of highly prevalent gastrointestinal nematode parasites. On average, individuals lost weight as parasite burden increased, but whereas some lost weight slowly as burden increased (exhibiting high tolerance), other individuals lost weight significantly more rapidly (exhibiting low tolerance). We then investigated associations between tolerance and fitness using selection gradients that accounted for selection on correlated traits, including body weight. We found evidence for positive phenotypic selection on tolerance: on average, individuals who lost weight more slowly with increasing parasite burden had higher lifetime breeding success. This variation did not have an additive genetic basis. These results reveal that selection on tolerance operates under natural conditions. They also support theoretical predictions for the erosion of additive genetic variance of traits under strong directional selection and fixation of genes conferring tolerance. Our findings provide the first evidence of selection on individual tolerance of infection in animals and suggest practical applications in animal and human disease management in the face of highly prevalent parasites.
authors: Department of Animal and Plant Sciences, University of Sheffield, Sheffield, United Kingdom.
authors: Adam D
authors: AD
authors: Hayward
authors: Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom; Institute of Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom; Centre for Immunity, Infection and Evolution, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom.
authors: Daniel H
authors: DH
authors: Nussey
authors: Centre for Ecology and Conservation, University of Exeter, Cornwall Campus, Penryn, Cornwall, United Kingdom.
authors: Alastair J
authors: AJ
authors: Wilson
authors: Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom.
authors: Camillo
authors: C
authors: Berenos
authors: Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom.
authors: Jill G
authors: JG
authors: Pilkington
authors: Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom; Institute of Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom; Centre for Immunity, Infection and Evolution, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom.
authors: Kathryn A
authors: KA
authors: Watt
authors: Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom.
authors: Josephine M
authors: JM
authors: Pemberton
authors: Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey, United States of America.
authors: Andrea L
authors: AL
authors: Graham
mesh_terms: D000818:Animals; D001835:Body Weight; D005260:Female; D005767:Gastrointestinal Diseases / Q000469:parasitology* / Q000473:pathology; D006790:Host-Parasite Interactions; D008297:Male; D009349:Nematode Infections / Q000469:parasitology* / Q000473:pathology; D010270:Parasite Egg Count; D059208:Parasite Load; D010273:Parasitic Diseases, Animal / Q000473:pathology; D012098:Reproduction; D012641:Selection, Genetic; D012756:Sheep / Q000502:physiology; D012757:Sheep Diseases / Q000469:parasitology