Abstract
The Mytilus complex of marine mussel species forms a mosaic of hybrid zones, foundacross temperate regions of the globe. This allows us to study ‘replicated’ instancesof 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 theirfrequencies 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 theheterospecific allele, suggesting enhanced local introgression, or the local sweep ofa shared ancestral allele. Conversely, genomic cline analyses, treating local parentalpopulations as the reference, reveal a globally high concordance among loci, albeitwith 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 theintrogression locally. These results shed light on the decay of species barriers duringphases of contact.