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

Abstract

When the ranges of closely-related lineages are large, and overlapping, we can often study introgression at many "replicated" contacts, with different locations and spatial scales. Here we analysed multiple contact zones of the M. edulis complex of marine mussel species, which represent a mosaic distribution of heterogeneously differentiated, semi-isolated genomes. Our aim was to contrast ongoing introgression at the heart of hybrid zones, with past introgression between similar parental populations, at increasing distance from the contact. Using a panel of ancestry-informative SNPs derived from a previous genomic study, we first confirm, with a broader sampling, that local introgression, affecting one but not all of the populations compared, is both widespread and heterogeneous across the genome. Some outlier loci show patterns of complete introgression in certain populations, and an absence of introgression in others. Genomic cline analyses reveal a globally high concordance among loci at a local scale, albeit with signals of asymmetric introgression at a few loci. Enhanced local introgression at specific loci is consistent with the early transfer of adaptive variants after contact, possibly including asymmetric bi-stable variants, or less loaded alleles. Given the mosaic structure of the M. edulis complex, with a succession of genetic barriers to gene flow, variants with enhanced introgression through one barrier can be trapped, maybe transiently, at the next barrier, confining introgression locally. This makes the Mytilus complex an ideal model of the heterogeneous porosity of species barriers.

Replicated anthropogenic hybridisations reveal parallel patterns of admixture in marine mussels

“…Naturally admixed Atl. M. galloprovincialis combine an old history of introgression during glacial oscillation periods (Fraïsse, Roux, et al, 2018;Roux et al, 2014) with ongoing local introgression from the native M. edulis populations in direct contact within the mosaic hybrid zone observed today Simon et al, 2019). Shared colonisation history cannot be the whole story, however, because we also found repeatable patterns between admixture events that must be considered independent.…”

“…Another study showed that the introduction on the Pacific coast of the USA was similarly composed by pure Med. M. galloprovincialis (Simon et al, 2019). Those observations preclude the idea that previously observed Med.…”

“…We aimed to examine mussel populations in ports, following the discovery of mussels with unexpected Med. M. galloprovincialis ancestry in the port of Cherbourg (France), as sampled in 2003 (Simon et al, 2019). Besides a new sampling in Cherbourg, we sampled seven additional ports and neighbouring natural populations.…”

“…Genotyping was subcontracted to LGC genomics (Hoddesdon, UK) and performed with the KASP™ array method (Semagn, Babu, Hearne, & Olsen, 2014). We used a set of ancestry informative SNPs developed previously (Simon et al, 2018;Simon et al, 2019). For cost reduction, we used a subset of SNPs that were sufficient for species and population delineation.…”

“…Finally, L3 defines sub-populations where the differentiation is mainly due to local introgression following historic contacts between lineages : (i) North-Eu. populations of M. edulis (edu_eu_north) were included (Simon et al, 2019). This lineage is present along the coast of Norway and meet with the South-Eu.…”