Population genetic structure and modes of dispersal for the colonial ascidian Botryllus schlosseri along the Scandinavian Atlantic coasts

The colonial ascidian Botryllus schlosseri is a well-known cosmopolitan invader of sheltered temperate marine communities which has garnered major scientific attention. We analyzed modes of dispersal and population genetic structures for 11 populations of B. schlosseri along the Scandinavian coasts, using 5 microsatellite loci. The analysis revealed high poly - morphism, resulting in 108 different alleles (of which 58 were private alleles), positive correlations between the number of sites shared by specific alleles and their mean frequencies, and lower genetic diversity values than in previously studied worldwide populations. A complex network of gene flow among sampled populations was revealed, with 2 clades, southeastern and northwestern, and higher genetic variation in the latter clade due to either restricted gene flow or more intensive genetic drift. A detailed analysis of allele frequencies revealed possible ancestral alleles. By using Bayesian analysis, 9 previously studied populations from Britain and European Atlantic coasts were compared, encompassing a single geographical entity along thousands of kilometers from Gibraltar (36° 8' N) to Alesund, Norway (62° 29' N). Results showed a high connectivity among distant localities, most probably due to extensive human-mediated transport. This refutes isolation by distance, with a higher intensity of gene flow among Scandinavian sites compared to the other European sites. Bayesian clustering computation assembled the whole data set of 19 populations into 14 clusters and 2 major northern and southern clades