Invasion without a bottleneck: Microsatellite variation in natural and invasive populations of the brown mussel Perna perna (L).

Population-level genetic diversity of the brown mussel Perna perna was investigated using nuclear microsatellite markers in 6 natural and 6 invasive populations. A total of 448 individuals from 12 populations spanning the natural and introduced ranges of the brown mussel were scored for 2 polymorphic microsatellite loci. Wright's hierarchical F statistics ( F(ST)), Hardy-Weinberg equilibrium, Nei's genetic distance, and other descriptive statistics were used to quantify geographic population subdivision, and to estimate the number of migrants per generation. F(ST) values (0.007-0.042) revealed that genetic partitioning among populations was low. Microsatellite data revealed a slight difference in observed heterozygosity and no statistically significant differences in expected heterozygosity or allelic diversity between natural and introduced populations. Effective numbers of migrants ( N(em)) per generation ranged from 6 to 35 individuals. The potential significance of an invasive species with high genetic variation in terms of the risk of establishment and conservation implications is discussed.