Northern Marsh Orchid - Dactylorhiza purpurella

The Northern Marsh-orchid, is widely distributed across the British Isles north of a line corresponding to the maximum extent of the Devensian ice sheet - roughly from The Wash to the coast in mid Wales. Often a large stocky plant, when growing in rabbit cropped turf it can be quite short. The flowers are usually densely packed in a columnar spike. The lip is shield-shaped, moderately deeply coloured purple, with bold magenta lines and dots.

D. purpurella has a plastid haplotype that is present in a consistently high frequency. This haplotype has probably derived from one that is  dominant in British populations of D. fuschsii. Unlike many other allopolyploid Dactylorhiza it retains both maternal ITS markers of D. fuchsii and the paternal ITS of D. incarnata. Thus it would seem that D. purpurella occurred as a result of a single alloploid event shortly  after the retreat of the ice sheets in Britain, and then spread outwards. It may be an endemic species, but there are examples resembling D. purpurella on the Norwegian coast. To have arrived there by land migration D. purpurella would have to have formed 10,000 years BP, but that seems to be at the oldest estimate of its age. The North Sea inundation of Doggerland would have resulted in a Norwegian coastline early in the process. Windblown seed remains a possibility, but genetic analysis may be needed to confirm the identity of these plants.

The reduced gene conversion of plastids in D. purpurella is also seen in another young and northern allopolyploid, D. sphagnicola. This species retains the plastids of both D. incarnata and D. maculata. The mechanism for this inconsistency is unknown but frequent hybridisation with D. incarnata with which it often grows, followed by introgression with parent population may be the answer.
When I went to Talacre dunes in North Wales I was greeted by a large purpurella colony. There were tall stately examples in the taller grass, but in the rabbit cropped turf, where the D. incarnata grew, there were plants typical of D. purpurella apart from being rather short and stunted. Could these be hybrids or partial hybrids with the dune forms of D. incarnata. While this seems a logical explanation, it also seems a little too convenient. The purpurella their also seemed to have good numbers of hybrids with D. fuchsii despite an apparently small population of the latter. However, it has been noted that D. purpurella colonies do carry an increased number of the specific D. incarnata genetic markers of those that grow at the same site.

The Welsh Marsh Orchid, cambrensis, originally considered to be part of D. majalis and then D. occidentalis, has now been reclassified as a sub-species of D. purpurella. In one study, the DNA Internal Transcribed Sequences of the Welsh Marsh-orchid did differ from the Northern Marsh-orchids studied. D. cambriensis had 63% ITS V and 37% ITS X, while for D. purpurella the figures were 25% and 75% respectively. Based purely on the ITS data D. cambriensis is as distinct from D. purpurella as D. praetissima is from D. traunsteinroides, but this is on limited sampling and just one factor. For me this suggests that cambriensis could be a distinct and local variety of D. purpurella, or a distinct entity.

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e less typical Northern Marsh-orchids on site.