Data from: Introgression between invasive and native blue mussels (genus Mytilus) in the central California hybrid zone.

The ecological and genetic factors determining the extent of introgression between species in secondary contact zones remain poorly understood. Here, we investigate the relative importance of isolating barriersand the demographic expansion of invasive Mytilus galloprovincialison the magnitude and the direction of introgression with the native M. trossulus in a hybrid zone incentral California. We use double-digest restriction-site associated DNA sequencing (ddRADseq) to genotype 1,337 randomly-selected single nucleotide polymorphismsand accurately distinguish early and advanced generation hybrids for the first time in the central California Mytilus spp. hybrid zone. Weaklevels of introgression were observed in both directions but wereslightly more prevalent from the native M. trossulus into the invasive M. galloprovincialis. Few early and advanced backcrossed individuals were observed across the hybrid zone confirming the presence of strong barriers to interbreeding.Heterogeneous patterns of admixture across the zone of contact were consistent with the colonization history of M. galloprovincialis with more extensive introgression in northern localities furthest away from the putative site of introduction in southern California. These observations reinforce the importance of dynamic spatial and demographic expansionsin determining patterns of introgression between close congeners, even in those with high dispersal potential and well-developed reproductive barriers. Our results suggest that the threat posed by invasive M. galloprovincialis is more ecological than genetic as it has, and continues to, displace the native M. trossulus from much of central and southern California.