Comparing raccoon MHC diversity in native and introduced ranges: evidence for the importance of functional immune diversity for adaptation and survival in novel environments.

The adaptive potential of invasive species is related to the genetic
diversity of the invader, which is influenced by genetic drift and natural
selection. Typically, the genetic diversity of invaders is studied with
neutral genetic markers, however, the expectation of reduced diversity has
not been consistently supported by empirical studies. Here, we describe
and interpret genetic diversity at both neutral microsatellite loci and
the immune related MHC-DRB locus of native and invasive populations of
raccoon to better understand of how drift and selection impact patterns of
genetic diversity during the invasion process. We found that despite the
loss of many MHC alleles in comparison with native populations, functional
MHC supertypes are preserved in the invasive region. In the native raccoon
population the number of supertypes within individuals was higher than
expected under a neutral model. The high level of individual functional
divergence may facilitate the adaptation to local conditions in the
invasive range. These results suggest that selection is driving divergent
allele combinations despite drift causing allelic loss. In the invasive
populations, we also detected increased population structure at
microsatellites compared to the MHC locus, further suggesting that
balancing selection is acting on adaptively important regions of the
raccoon genome. Finally, we found that alleles known to exhibit resistance
to rabies in the native range, Prlo-DRB*4, Prlo-DRB*16 and Prlo-DRB*102,
were the most common alleles in the European populations, suggesting
directional selection is acting on this locus. Our research shows
empirical support for the importance of functional immune diversity for
adaptation and survival in novel environments.