Data from: The hidden side of a major marine biogeographic boundary: a wide mosaic hybrid zone at the Atlantic–Mediterranean divide reveals the complex interaction between natural and genetic barriers in mussels

The Almeria-Oran Front (AOF) is a recognised hotspot of genetic
differentiation in the sea. It is a barrier to dispersal and an ecological
boundary, which explain the position of genetic breaks. However, the
maintenance of genetic differentiation is likely reinforced by genetic
barriers. A general drawback of previous studies is an insufficient
density of sampling sites at the transition zone with a conspicuous lack
of samples from the southern coastline. We analysed the genetic structure
in the mussel Mytilus galloprovincialis with ancestry-informative loci. We
discovered a 600 km wide mosaic hybrid zone eastward of the AOF along the
Algerian coasts. This mosaic zone provides a new twist to our
understanding of the Atlantic-Mediterranean transition because it
demonstrates the two lineages can live in sympatry but hardly interbreed.
This implies some form of reproductive isolation must exist to maintain
the two genetic backgrounds locally cohesive. The zone ends with an abrupt
genetic shift at a barrier to dispersal in the Gulf of Bejaia. Simulations
in models that account for hydrodynamic features of the region support the
hypothesis that sister hybrid zones could have been differentially trapped
at two alternative barriers to dispersal or environmental boundaries. A
preponderantly unidirectional north-south gene flow next to the AOF can
also maintain a patch of an intrinsically maintained genetic background in
the south and the mosaic structure. Our results concur with the coupling
hypothesis that suggests natural barriers mostly explain the position of
genetic breaks while their maintenance must additionally require genetic
barriers.