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.

The evolutionary ecology of the major histocompatibility complex

“…Langefors et al (2001) found an association between certain MHC class IIb alleles and resistance to the bacterial pathogen Aeromonas salmonicida in experimentally challenged Atlantic salmon (Salmo salar). Lohm et al (2002) subsequently selectively mated the uninfected siblings of these fish, combining high-and low-resistance alleles within full-sibling families, thereby controlling for the potentially confounding effects of family. This study revealed a codominant pattern of resistance/susceptibility to A. salmonicida infection.…”

Associations between malaria and MHC genes in a migratory songbird

“…Allelic diversity will be maintained if different MHC alleles give protection against different parasites, and the composition of the parasite community varies on a temporal and/or spatial scale (Hedrick 2002). Direct associations between certain MHC alleles and resistance to specific diseases have been found in several studies on humans, on chickens (Gallus gallus domesticus), in mouse (Mus musculus) strains (Briles et al 1977;Hormaeche et al 1985;Nauciel et al 1988;Hill et al 1991;Plachy et al 1992;Hill 2001), in studies from a hatchery population of Atlantic salmon (Salmo salar; Langefors et al 2001;Lohm et al 2002) and in an unmanaged Soay sheep (Ovis aries) population (Paterson et al 1998). To our knowledge, however, there are no studies of specific MHC alleles and disease resistance in strictly natural and free-ranging avian populations.…”

Mhc supertypes confer both qualitative and quantitative resistance to avian malaria infections in a wild bird population

“…An alternative approach to discriminate advantageous alleles from disadvantageous alleles is to investigate the fitness consequences of the parasite strain (e.g. malaria) in terms of host survival rates [28][29][30]. If a mortality cost for infection during the acute stage can be shown in the population, then a positive association between prevalence of infection and the immunoallele would be interpreted as quantitative disease resistance.…”