Within-litter covariance of allele-specific MHC heterozygosity, coccidian endoparasite load and growth is modulated by sibling differences in starting mass
Résumé
Although littermates in altricial mammals usually experience highly similar environmental conditions during early life, considerable
differences in growth and health can emerge among them. In a study on subadults of a European rabbit (Oryctolagus
cuniculus) population with low MHC polymorphism, we tested whether litter-sibling differences in endoparasitic coccidia
load 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 that
siblings with a lighter starting mass might be more susceptible to endoparasite infections and thus, negative effects of a more
unfavourable MHC constitution might be particularly pronounced in such individuals. Within-litter comparisons revealed
that animals with a lighter starting mass reached a relatively lower body mass in autumn. Furthermore, there were indications
for an allele-specific heterozygote advantage, as animals with heterozygous combinations of the allele Orcu-DRB*4
had relatively lower hepatic coccidia loads than their littermates with certain homozygous allele combinations. Consistent
with our hypothesis, significantly higher hepatic coccidia loads and tendentially lower autumn body masses in homozygous
compared 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, these
effects might lead to notable differences in fitness among litter siblings, as a low body mass and a high endoparasite burden
are key factors limiting young rabbits’ survival during winter.