Our results revealed that the three populations are genetically distinct, differing both in the clonal structure and in the level of genetic polymorphism. Olsen et al. (2004)

claim that the North Sea and western Baltic populations of eelgrass, occupying the central part of its range, should exhibit higher allelic richness than those at the limits of the species’ distribution. The situation we found Ivacaftor cost in the Baltic seems to be somewhat different. The GB population, the nearest to the ‘differentiation hotspot’, has the lowest allelic richness and a much more explicit clonal structure, while in the CB population, situated close to the limits of the eelgrass range in the Baltic, no clones were spotted among 24 individuals and the allelic PD332991 richness was similar to that observed in the North Sea populations (Figure 1). The low genetic polymorphism of the GB population is understandable, given that this population dramatically decreased in size in the 1990s as a result of the bay’s eutrophication (Munkes 2005). The

high level of genetic polymorphism in the CB population is more difficult to explain, however. This population is much more variable than several other populations located further north still, off the coast of Finland (Olsen et al. 2004). These populations are regarded as being at the ‘leading edge’ of the species range (Olsen et al. 2004). The genetic polymorphism of the CB population could have been higher because of the set of 12 markers we used, as against the nine msDNA loci used by Olsen et al. (2004). However, the additional analysis of genetic polymorphism that we performed by testing the nine markers used by Olsen et al. (2004) (data not

shown) showed that it was immaterial whether nine or 12 loci were analysed. One can assume that Cudema Bay, being the southernmost part of the Gulf of Finland, was colonised by eelgrass much earlier than the rest of the gulf. We did not find any correlation between geographical and genetic distance (data not shown). The pairwise FST values are lower between Chloroambucil the PB and CB than between the PB and GB populations, which are located much closer to each other. The STRUCTURE analysis ( Figure 3) showed that the genetic characteristics of the GB and CB populations are quite different, whereas the PB population is intermediate. This may suggest that a small-scale gene flow occurred between the three populations. The Baltic Sea is known for its strong currents, frequently changing direction depending on the strength and direction of winds. The long-distance dispersal of eelgrass shoots over the open water, caused by currents or wind, has already been observed ( Reusch, 2002 and Harwell and Orth, 2002). The differences we observed in the genetic structure of the three populations most probably result from their adaptation to local environmental conditions and their history.