Genetic structure of a regionally endangered orchid, the dark red helleborine (Epipactis atrorubens) at the edge of its distribution.

The genetic structure and diversity of species is determined by both current population dynamics and historical processes. Population genetic structure at the edge of the distribution is often expected to differ substantially from populations at the centre, as these edge populations are often small and fragmented. In addition, populations located in regions that have experienced repeated glaciations throughout the Pleistocene, may still carry imprints from the genetic consequences of frequent distribution shifts. Using chloroplast DNA sequences and nuclear microsatellite markers we studied the genetic structure of Epipactis atrorubens at the northern edge of its distribution. Contrary to populations in the centre of the distribution, populations at the northern range are regionally endangered as they are small and disjunct. Sequence data of 2 chloroplast loci and allelic data from 6 nuclear microsatellite markers were obtained from 297 samples from Finland, Estonia and Russia. We sought for genetic indicators of past population processes, such as post-glacial colonisation history of E. atrorubens. As expected, we observed low genetic variation, in terms of numbers of substitutions, haplotypes and alleles, and significant levels of differentiation, especially pronounced in the chloroplast DNA. These features suggest that the edge populations could be prone to extinction.