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 s...