title: Experimental evidence for major histocompatibility complex-allele-specific resistance to a bacterial infection.
abstract: The extreme polymorphism found at some major histocompatibility complex (MHC) loci is believed to be maintained by balancing selection caused by infectious pathogens. Experimental support for this is inconclusive. We have studied the interaction between certain MHC alleles and the bacterium Aeromonas salmonicida, which causes the severe disease furunculosis, in Atlantic salmon (Salmo salar L.). We designed full-sibling broods consisting of combinations of homozygote and heterozygote genotypes with respect to resistance or susceptibility alleles. The juveniles were experimentally infected with A. salmonicida and their individual survival was monitored. By comparing full siblings carrying different MHC genotypes the effects on survival due to other segregating genes were minimized. We show that a pathogen has the potential to cause very intense selection pressure on particular MHC alleles; the relative fitness difference between individuals carrying different MHC alleles was as high as 0.5. A co-dominant pattern of disease resistance/susceptibility was found, indicative of qualitative difference in the immune response between individuals carrying the high- and low-resistance alleles. Rather unexpectedly, survival was not higher among heterozygous individuals as compared with homozygous ones.
authors.affiliation: Department of Animal Ecology, Ecology Building, Lund University, SE-223 62 Lund, Sweden. jakob.lohm@zooekol.lu.se
authors.forename: Jakob
authors.initials: J
authors.lastname: Lohm
authors.forename: Mats
authors.initials: M
authors.lastname: Grahn
authors.forename: Asa
authors.initials: A
authors.lastname: Langefors
authors.forename: Øivind
authors.initials: Ø
authors.lastname: Andersen
authors.forename: Arne
authors.initials: A
authors.lastname: Storset
authors.forename: Torbjörn
authors.initials: T
authors.lastname: von Schantz
mesh_terms: D000333:Aeromonas / Q000276:immunology*; D000483:Alleles; D000818:Animals; D005393:Fish Diseases / Q000276:immunology / Q000382:microbiology; D005787:Gene Frequency; D005802:Genes, MHC Class II / Q000235:genetics; D020022:Genetic Predisposition to Disease / Q000235:genetics; D014644:Genetic Variation / Q000235:genetics; D016905:Gram-Negative Bacterial Infections / Q000276:immunology*; D008285:Major Histocompatibility Complex / Q000235:genetics*; D020317:Salmo salar / Q000235:genetics* / Q000276:immunology*