Dispersal mechanisms for zebra mussels: population genetics supports clustered invasions over spread from hub lakes in Minnesota

Invasion genetic studies show great promise for inferring sources, pathways and vectors of spread, but have to date focused on introductions from native ranges at continental scales and larger. Here we present genetic analyses of post-introduction (i.e. secondary) spread of zebra mussels at the scale of a US state, Minnesota (MN). We genotyped 9 microsatellite DNA loci in 2047 zebra mussels collected from 40 lakes and 4 rivers that geographically and chronologically span the MN invasion. We analyzed genetic variation, geographic differentiation, and genetic clustering of populations across water bodies, and performed invasion-scenario contrasts using Approximate Bayesian Computation. Our population genetic analyses revealed that the pattern of spread of zebra mussels to inland lakes did not conform to the hub-lake model that is often proposed for this species. Consistent with a stratified dispersal model, lake rich regions were colonized from afar, followed by spread within regions by short-distance dispersal, but high-traffic hub lakes made no contribution. For the first time, we obtained evidence about the pattern of spread of zebra mussels to inland lakes that is not based on inference from analysis of boater movements; rather it derives from direct evaluation of genetic attributes of invasive populations. Local introductions overwhelmed more distant ones in three independently invaded regions, suggesting a need to understand the lake colonization process and identify the vectors responsible.