Plastid DNA haplotype variation in Dactylorhiza incarnata (Orchidaceae): evidence for multiple independent colonization events into Scandinavia

The early marsh orchid, Dactylorhiza incarnata (L.) Soo s. l., grows in medium-rich to rich fens and marshes over much of Europe and parts of Asia. The species is highly polymorphic and different forms may grow together at the same site. In the present study, I tested the hypothesis that these forms represent different migrant populations that have colonized Scandinavia independently of each other, possibly from different source areas. Accessions from Scandinavia and elsewhere were screened for variation at three size-variable plastid marker loci, one polyA repeat, one polyA-polyTA-polyT repeat and one 9 bp indel. Ten haplotypes were defined on basis on the combined variation pattern. The common occurrence of several haplotypes in southern Scandinavia and adjacent areas to the south and the east of the Baltic Sea suggests that D. incarnata has been dispersed on repeated occasions across the Baltic. Also, there was some correlation between haplotype composition and morphological form on the island of Gotland, in agreement with the independent colonization hypothesis. Material from northernmost Sweden, Finland and northwest Russia was fixed for a single widespread haplotype, indicating that populations in this area are located farther away from the Pleistocene refugia. Dactylorhiza incarnata ssp. lobelii from southwest Norway was characterized by a haplotype that was not encountered elsewhere in Scandinavia. Given its proximity to British populations dominated by the same haplotype, it is suggested that D. incarnata ssp. lobelii was established independently of the other Scandinavian populations, from coastal refugia located in western Europe.