Major histocompatibility complex variation and age-specific endoparasite load in subadult European rabbits

Genes of the major histocompatibility complex (MHC) play a fundamental role in the vertebrate immune response and are amongst the most polymorphic genes in vertebrate genomes. It is generally agreed that the highly polymorphic nature of the MHC is maintained through host–parasite co-evolution. Two nonexclusive mechanisms of selection are supposed to act on MHC genes: superiority of MHC heterozygous individuals (overdominance) and an advantage for rare MHC alleles. However, the precise mechanisms and their relative importance are still unknown. Here, we examined MHC dependent parasite load in European rabbits (Oryctolagus cuniculus) from a distinct population with low MHC diversity (three alleles, six genotypes). Using a multivariate approach, we tested for associations of individual MHC class II DRB constitution and the rabbits’ intestinal burden with nematodes and coccidia. Rabbits having a particular allele showed lower infestations with hepatic coccidia (E. stiedai). However, a comparison of all six genotypes in the population revealed that carriers of this allele only benefit when they are heterozygous, and furthermore, MHC heterozygosity in general did not affect individual parasite load. In conclusion, this study suggests an immunogenetic basis of European rabbit resistance to hepatic coccidiosis, which can strongly limit survival to maturity in this species. Our study gives a complex picture of MHC–parasite correlations, unveiling the limits of the classical hypotheses of how MHC polymorphism is maintained in natural systems

Within-litter covariance of allele-specific MHC heterozygosity, coccidian endoparasite load and growth is modulated by sibling differences in starting mass

International audienceAlthough littermates in altricial mammals usually experience highly similar environmental conditions during early life, considerabledifferences in growth and health can emerge among them. In a study on subadults of a European rabbit (Oryctolaguscuniculus) population with low MHC polymorphism, we tested whether litter-sibling differences in endoparasitic coccidiaload and body mass at the end of the vegetation period were associated with within-litter differences in starting body mass(measured around 2 weeks prior to weaning) and in immune-genetic (MHC class II DRB) constitution. We hypothesized thatsiblings with a lighter starting mass might be more susceptible to endoparasite infections and thus, negative effects of a moreunfavourable MHC constitution might be particularly pronounced in such individuals. Within-litter comparisons revealedthat animals with a lighter starting mass reached a relatively lower body mass in autumn. Furthermore, there were indicationsfor an allele-specific heterozygote advantage, as animals with heterozygous combinations of the allele Orcu-DRB*4had relatively lower hepatic coccidia loads than their littermates with certain homozygous allele combinations. Consistentwith our hypothesis, significantly higher hepatic coccidia loads and tendentially lower autumn body masses in homozygouscompared to heterozygous individuals for the allele Orcu-DRB*4 were evident in initially lighter but not in heavier siblings,suggesting synergistic effects between an unfavourable MHC constitution and a light starting mass. Taken together, theseeffects might lead to notable differences in fitness among litter siblings, as a low body mass and a high endoparasite burdenare key factors limiting young rabbits’ survival during winter