Adult survival in migratory caribou is negatively associated with MHC functional diversity

The genes of the major histocompatibility complex (MHC) are involved in
acquired, specific immunity in vertebrates. Yet, only a few studies have
investigated the fitness consequences of MHC gene diversity in wild
populations. Here, we looked at the association between annual survival
and body mass and MHC-DRB exon 2 (MHC-DRB) genetic diversity, obtained
from high-throughput sequencing, in two declining migratory caribou
(Rangifer tarandus) herds. To disentangle the potential direct and general
effects of MHC-DRB genetic diversity, we compared different indices of
diversity that were either based on DNA-sequence variation or on
physicochemical divergence of the translated peptides, covering a gradient
of allelic to functional diversity. We found that i) body mass was not
related to MHC-DRB diversity or genotype and that ii) adult survival
probability was negatively associated with PAM distance, a corrected
distance that considers the likelihood of each amino acid substitution to
be accepted by the processes of natural selection. In addition, we found
no evidence of fluctuating selection in time on MHC-DRB. We concluded that
direct effects were involved in the negative relationship between MHC
functional diversity and survival, although the mechanism underlying this
result remains unclear. A possible explanation could be that individuals
with higher MHC diversity suffer higher costs of immunity
(immunopathology). Further studies are needed to investigate this
hypothesis. Our results suggest that genetic diversity is not always
beneficial even in genes that are supposed to be strongly shaped by
balancing selection.