Résumé
Genes of the Major Histocompatibility Complex (MHC) play a central role in
adaptive immune responses of vertebrates. They exhibit remarkable
polymorphism, often crossing species boundaries with similar alleles or
allelic motifs shared across species. This pattern may reflect parallel
parasite-mediated selective pressures, either favouring the long
maintenance of ancestral MHC allelic lineages across successive speciation
events by balancing selection (‘trans-species polymorphism’), or
alternatively favouring the independent emergence of functionally similar
alleles post-speciation via convergent evolution. Here we investigate the
origins of MHC similarity across several species of dwarf and mouse lemurs
(Cheirogaleidae). We examined MHC class II variation in two highly
polymorphic loci (DRB, DQB) and evaluated the overlap of gut-parasite
communities in four sympatric lemurs. We tested for parasite-MHC
associations across species to determine whether similar parasite
pressures may select for similar MHC alleles in different species. Next,
we integrated our MHC data with those previously obtained from other
Cheirogaleidae to investigate the relative contribution of convergent
evolution and co-ancestry to shared MHC polymorphism by contrasting
patterns of codon usage at functional versus neutral sites. Our results
indicate that parasites shared across species may select for functionally
similar MHC alleles, implying that the dynamics of MHC-parasite
co-evolution should be envisaged at the community level. We further show
that balancing selection maintaining trans-species polymorphism, rather
than convergent evolution, is the primary mechanism explaining shared MHC
sequence motifs between species that diverged up to 30 million years ago.