A microsatellite-based estimation of clonal diversity and population subdivision in Zostera marina, a marine flowering plant

We examined the genetic population structure in eelgrass (Zostera marina L. ), the dominant seagrass species of the northern hemisphere, over spatial s cales from 12 km to 10 000 km using the polymorphism of DNA microsatellites . Twelve populations were genotyped for six loci representing a total of 67 alleles. Populations sampled included the North Sea (four), the Baltic Sea (three), the western Atlantic (two), the eastern Atlantic (one), the Medit erranean Sea (one) and the eastern Pacific (one). Microsatellites revealed substantial genetic variation in a plant group with low allozyme diversity. Average expected heterozygosities per population (monoclonal populations e xcluded) ranged from 0.32 to 0.61 (mean = 0.48) and allele numbers varied b etween 3.3 and 6.7 (mean = 4.7). Using the expected frequency of multilocus genotypes within populations, we distinguished ramets from genetic individ uals (i.e. equivalent to clones). Differences in clonal diversity among pop ulations varied widely and ranged from maximal diversity (i.e. all ramets w ith different genotype) to near or total monoclonality (two populations). A ll multiple sampled ramets were excluded from further analysis of genetic d ifferentiation within and between populations. All but one population were in Hardy-Weinberg equilibrium, indicating that Zostera marina is predominan tly outcrossing. From a regression of the pairwise population differentiati on with distance, we obtained an effective population size N-e of 2440-5000 . The overall genetic differentiation among eelgrass populations, assessed as rho (a standardized estimate of Slatkin's R-ST) was 0.384 (95% CI 0.34-0 .44, P < 0.001). Genetic differentiation was weak among three North Sea pop ulations situated 12-42 km distant from one another, suggesting that tidal currents result in an efficient exchange of propagules. In the Baltic and i n Nova Scotia, a small but statistically significant fraction of the geneti c variance was distributed between populations (rho = 0.029-0.053) at scale s of 15-35 km. Pairwise genetic differentiation between European population s were correlated with distance between populations up to a distance of 450 0 km (linear differentiation-by-distance model, R-2 = 0.67). In contrast, b oth Nova Scotian populations were genetically much closer to North Sea and Baltic populations than expected from their geographical distance (pairwise rho = 0.03-0.08, P < 0.01). A biogeographical cluster of Canadian with Bal tic/North Sea populations was also supported using a neighbour-joining tree based on Cavalli-Sforza's chord distance. Relatedness between populations may be very different from predictions based on geographical vicinity.