How do species barriers decay? concordance and local introgression in mosaic hybrid zones of mussels

The Mytilus complex of marine mussel species forms a mosaic of hybrid
zones, found across temperate regions of the globe. This allows us to
study "replicated" instances of secondary contact between
closely-related species. Previous work on this complex has shown that
local introgression is both widespread and highly heterogeneous, and has
identified SNPs that are outliers of differentiation between lineages.
Here, we developed an ancestry-informative panel of such SNPs. We then
compared their frequencies in newly-sampled populations, including samples
from within the hybrid zones, and parental populations at different
distances from the contact. Results show that close to the hybrid zones,
some outlier loci are near to fixation for the heterospecific allele,
suggesting enhanced local introgression, or the local sweep of a shared
ancestral allele. Conversely, genomic cline analyses, treating local
parental populations as the reference, reveal a globally high concordance
among loci, albeit with a few signals of asymmetric introgression.
Enhanced local introgression at specific loci is consistent with the early
transfer of adaptive variants after contact, possibly including asymmetric
bi-stable variants (Dobzhansky-Muller incompatibilities), or haplotypes
loaded with fewer deleterious mutations. Having escaped one barrier,
however, these variants can be trapped or delayed at the next barrier,
confining the introgression locally. These results shed light on the decay
of species barriers during phases of contact.