Data from: High levels of genetic diversity in marginal populations of the marsh orchid Dactylorhiza majalis ssp. majalis

Because of harsh conditions, suboptimal habitat quality and poor
connectivity to other populations, plant populations at the margin of a
distribution are expected to be less genetically diverse, but to be more
divergent from each other than populations in the centre of a
distribution. In northern Europe, northern marginal populations may also
be younger than populations further to the south, and may have had less
time to accumulate genetic diversity by mutation and gene flow. However,
orchids have very small seeds, which are easily dispersed long distances
by wind, and orchids are therefore expected to show less differentiation
between marginal and central populations than other groups of seed plants.
Here, we analysed whether Scandinavian populations of the tetraploid marsh
orchid Dactylorhiza majalis ssp. majalis differ from Central European
populations in genetic diversity patterns. A total of 220 plants from
eight Central European and ten Scandinavian populations was examined for
variation at five nuclear microsatellite loci, nuclear ITS and 13
polymorphic sites in noncoding regions of the plastid genome. The total
genetic diversity was slightly lower in Scandinavia than in Central
Europe, both in plastid and nuclear markers, but the differences were
small. Also, the Scandinavian populations were less diverse and somewhat
more strongly differentiated from each other than the Central European
ones. Dactylorhiza majalis ssp. majalis has apparently colonized
Scandinavia on multiple independent occasions and from different source
areas in the south. Seed flow between Scandinavian populations has still
not fully erased the patterns imprinted by early colonization. Our results
suggest that marginal populations of orchids may be as important as
central ones in preserving genetic diversity through Pleistocene glacial
cycles. We also predict that orchids with their light seeds are better
adapted than many other plants to respond to future climate changes by
dispersing into new suitable areas.