Interspecific introgression of MHC genes in Triturus newts: Evidence from multiple contact zones

The major histocompatibility complex (MHC) genes are central to the
adaptive immune response in vertebrates. Selection generally maintains
high MHC variation because the spectrum of recognised pathogens depends on
MHC polymorphism. Novel alleles favoured by selection originate by
interallelic recombination or de novo mutations but may also be acquired
by introgression from related species. However, the extent and prevalence
of MHC introgression remain an open question. In this study, we tested for
MHC introgression in six hybrid zones formed by six Triturus newt species.
We sequenced and genotyped the polymorphic second exons of the MHC class I
and II genes and compared their interspecific similarity at various
distances from the centre of the hybrid zone. We found evidence for
introgression of both MHC classes in the majority of examined hybrid
zones, with support for a more substantial class I introgression.
Furthermore, the overall MHC allele sharing outside of hybrid zones was
elevated between pairs of Triturus species with abutting ranges,
regardless of the phylogenetic distance between them. No effect of past
hybrid zone movement on MHC allele sharing was found. Finally, using
previously published genome-wide data, we demonstrated that MHC
introgression was more extensive than genome-wide introgression,
supporting its adaptive potential. Our study thus provides evidence for
the prevalence of MHC introgression across multiple Triturus hybrid zones,
indicating that MHC introgression between divergent hybridising species
may be widespread and adaptive.