Combined effects of bottlenecks and selfing in populations of Corella eumyota, a recently introduced sea squirt in the English Channel

The success of an exotic species depends notably on its capacity to initiate a new population from a few individuals, to survive genetic bottlenecks and to adapt locally. Species with multiple reproductive strategies (e.g. mixed‐mating system with both self‐ and cross‐fertilization) can be efficient colonizers. Herein we focus on Corella eumyota , an exotic ascidian that has rapidly invaded English Channel coasts in recent years. Interestingly, this brooding hermaphroditic ascidian is capable of self‐fertilization in the laboratory. We developed 12 microsatellite markers from an enriched library of genomic DNA to investigate the level of inbreeding and selfing in two putatively native populations (South Africa, N = 34, and New Zealand, N = 28) and to examine if founder effects were possibly associated with its recent introduction in two French populations (Perros‐Guirec, N = 22 and Brest; N = 25). Genetic polymorphism was very low in both native populations (i.e. less than 60% of the loci were polymorphic) and even lower in the introduced populations, one of which was monomorphic at all loci, suggesting a recent bottleneck. F is and a new method based on multi‐locus heterozygosity were used to provide estimates of inbreeding. A high selfing rate was inferred in the South Africa population with both methods ( s = 0.90), whereas in the other native population (New Zealand) a lower but significant estimate of selfing rate ( s = 0.29) was obtained with the multi‐locus method. This variability of population selfing rate might be explained by a mixed‐mating system, allowing C. eumyota to reproduce through inbreeding and outbreeding according to mating possibilities; this trait may have favoured the rapid establishment of new populations in Europe.