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

Abstract

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 polymorphism, 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 Ålesund, 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.

Historical human activities reshape evolutionary trajectories across both native and introduced ranges

“…While bi‐allelic markers do not provide information about the timing and magnitude of pulses of gene flow over short evolutionary timescales, the restricted natural dispersal capabilities of C. volutator and H. diversicolor , rare evidence of contemporary long‐distance connectivity, phylogenetic reticulation, and discord between genetic structure and geography are consistent with historical human‐mediated dispersal among estuarine habitats. Human transport can promote human‐mediated connectivity among introduced populations (Lacoursière‐Roussel et al., 2012; Reem, Mohanty, Katzir, & Rinkevich, 2013; Voisin, Engel, & Viard, 2005), and our results suggest that human endeavor may also facilitate connectivity among native populations.…”

Anthropogenic transport of species across native ranges: unpredictable genetic and evolutionary consequences

“…Human-mediated transport can promote artificial connectivity that translates into high levels of gene flow among populations that would otherwise be genetically differentiated. This is clearly exemplified by studies investigating introduced ranges of non-indigenous species [6,19,20]. Here, we compared genetic patterns in a region of the native range where human activities that are known to facilitate spread are prevalent.…”

A new record of Ascidiella scabra (Müller, 1776) (Ascidiacea, Phlebobranchia) in the southwestern Atlantic

“…Large commercial transoceanic vessels are wellknown vectors for the primary introduction of marine species into coastal habitats (Sylvester et al 2011;Reem et al 2013), while smaller recreational, fishing, and other types of crafts may aid subsequent, secondary spread (Zabin et al 2014;Leclerc et al 2020). According to official sources, the port of Mar del Plata is largely dominated by domestic traffic consisting of fishing and oil-tanker vessels (ca.…”