Data from: Pervasive introgression of MHC genes in newt hybrid zones

Variation in the vertebrate major histocompatibility complex (MHC) genes
is crucial for fighting pathogen assault. Because new alleles confer a
selective advantage, MHC should readily introgress between species, even
under limited hybridization. Using replicated transects through two hybrid
zones between strongly reproductively isolated European newts, Lissotriton
montandoni and L. vulgaris, we demonstrated recent and ongoing MHC class I
and II introgression in the Carpathian region. The extent of introgression
correlated with the age of contact. In the older zone, MHC similarity
between species within transects exceeded similarity between transects
within species, implying pervasive introgression –  a massive
exchange of MHC genes, not limited to specific variants. In simulations,
the observed pattern emerged under the combined action of balancing
selection and hybridization, but not when these processes acted
separately. Thus, massive introgression at advanced stages of divergence
can introduce novel and restore previously lost MHC variation, boosting
the adaptive potential of hybridizing taxa. In consequence, MHC genes may
be the last to stop introgressing between incipient species.