Postglacial-period and recent invasions shape the population genetics of botryllid ascidians along European Atlantic coasts

The colonial urochordate Botryllus schlosseri is a sedentary species of Mediterranean origin that became cosmopolitan, probably because of postglacial-period dispersal and human-mediated invasions of colonies attached to ship hulls. Here we studied microsatellite allele diversity of Atlantic coast populations from an area ranging from European regions south of the last glacial front to regions that had been permanently ice-covered. Gene diversity levels varied dramatically among populations residing in areas subject to different glacial conditions. Five populations from the Iberian Peninsula, in an area south of the last glacial front, as well as two populations from presumed refugia in Brittany, expressed high gene diversity values (expected heterozygosity [He]: 0.76-0.80; average number of alleles per locus [A]: 7.25-8.75). Two populations inhabiting areas that experienced permafrost conditions (Helgoland Island, Germany, and Plymouth, England) had intermediate values (He: 0.40-0.42; A: 3.0-4.0), whereas the Auchenmalg, Scotland, population, from an area previously covered by ice, showed a remarkably low value (He: 0.17; A: 1.75). Therefore, most European populations of B. schlosseri mirrored the movement of the ice front in the last ice age. A second population from the area that was covered by permanent ice (Lossiemouth, Scotland), however, had a high He of 0.61 and an intermediate A of 3.67. Results were compared with recent invasions (populations less than 200 years old) in the United States and New Zealand that had a higher degree of genetic variation than the European native populations established thousands of years ago. Given the overall dearth of studies on this subject, we suggest that in contemporary established Botryllus populations, gene diversity is affected by ecological factors, some of which can be traced directly to the last ice age. Other parameters of gene diversity are influenced by selection pressure, which might be more intense in northern regions.