Assessing the structure and stability of the Mytilus mosaic hybrid zone in the South-West of the U.K. using a lagrangian biophysical transport model

Thomas Wright¹, Antony Knights², David Bilton¹, Louise Firth², Pete Cotton¹

¹University of Plymouth, UK - ²University College Cork, Ireland

Mosaic hybrid zones occur when taxa hybridise across a mosaic of habitat patches, producing a network of parent and hybrid populations. In mosaic hybrid zones, the distribution and abundance of parent and hybrid genotypes is expected to be underpinned by a combination of dispersal of individuals between and among populations (sensu connectivity) and selection. Variations in connectivity and/or selection between populations within the mosaic can result in a variety of outcomes, including mosaic hybrid zone collapse (introgression), hybrid speciation, hybrid zone stability and reinforcement. Understanding the evolutionary fate of mosaic hybrid zone taxa therefore depends on means of quantifying connectivity between populations, as well as selection acting within them. Here, using a lagrangian biophysical model, we characterise the connectivity pathways for two subspecies of the edible mussel, Mytilus edulis edulis and M. e. galloprovincialis that form a mosaic hybrid zone around the Southwest Coast of the U.K. Simulating connections between populations of parents and hybrids, we test the hypothesis that hybrid zone structure and abundance of subspecies is driven by connectivity. Structural components and high centrality populations are identified to reveal links between discrete patches in space and identify stepping-stone populations important to maintaining the hybrid zone’s overall structure. While this study is specific to the Mytilus complex, we show how we can predict the likelihood of hybridisation and detect early stages of sympatric divergence.