Long-term population genetic dynamics of the invasive ascidian Botryllus schlosseri, lately introduced to Puget Sound (Washington, USA) marinas

Invasive species are of increasing concern to biodiversity and the ecological functioning of a range of ecosystems, especially as the magnitude of biological invasions is increasing globally. The genetic structure of newly established populations may reveal insights into invasion processes, making population genetics an important tool for understanding current invasion pathways. Here, we studied newly established populations (non-existent < 10–20 years before the first sample) of the cosmopolitan alien ascidian Botryllus schlosseri in four Puget Sound marinas (Washington, USA) using eight polymorphic microsatellites. Up to seven sampling sessions over a period of 19 years revealed populations with fluctuating allelic richness (AR = 2.693–4.417) and expected heterozygosity (He = 0.362–0.589). The populations were well differentiated on spatial and temporal scales and were subject to moderate genetic drift (Fs’ = 0.027–0.071). The significant heterozygote deficiencies that were obtained, positive inbreeding coefficients (Fis), and population structure measures (Fst) revealed that no population was in Hardy-Weinberg equilibrium. Comparing these parameters with those from two Californian sites (Moss Landing and Santa Cruz, 1200 km south, invaded by Botryllus during the 1940s) revealed a connection between Moss Landing and Puget Sound, whereas Santa Cruz remained isolated. On the US West Coast scale, this study revealed no significant difference in introduced population dynamics between recently established populations and those established over 60 years ago, except for fewer alleles and lower He. When comparing ten worldwide sites, only a few microsatellite loci displayed strong regional differences. Globally, the Puget Sound Botryllus populations exhibit genetic characteristics of recently established populations, as they have the lowest number of alleles and lowest genetic indices, further emerging as one of the youngest B. schlosseri populations worldwide.