The genetic dynamics of the rapid and recent colonization of Denmark by Arion lusitanicus (Mollusca, Pulmonata, Arionidae)

We describe the genetic dynamics of the recent establishment of the ‘Iberian slug’, Arion lusitanicus J. Mabille 1868, in Denmark and compare its population structure to two other members of the ‘large Arion complex’, Arion ater ater , native to Denmark, and Arion ater rufus , introduced into Denmark in the early 1900s. Assaying allozyme polymorphism at seven enzyme loci, we found that: (1) None of the three taxa reproduce primarily by self-fertilization. Differences among loci and colonies in the pattern of deviation from Hardy–Weinberg equilibrium are most consistent with isolate mixing and perhaps with low amounts of selfing. (2) For both A. lusitanicus and A. a. rufus , gene diversity is lower in Danish colonies than in southern German colonies, implying population bottlenecks in the establishment of Danish colonies. (3) Significant linkage disequilibrium values usually involve the same three loci, viz. PGI, MDH-1 and MDH-2, suggesting physical linkage among these loci. (4) For both A. a. rufus and A. lusitanicus , the overall gene frequencies from Denmark and southern Germany are homogeneous, while variation among colonies within these regions ranges from around 15 to 28% for the three taxa. This indicates strong, local population genetic subdivision but with little restriction to gene flow from possible source areas. The heterogeneity in measures of diversity and differentiation indicates that population structure for all three taxa is dominated by ongoing founder effects, local extinction/colonisation dynamics, and genetic drift processes.