Southern Marsh Orchid - Dactylorhiza praetermissa

The Southern Marsh Orchid, D. praetermissa, is the widespread Marsh Orchid across England and Wales but more likely to be found in southern areas, and it also occurs in northern continental Europe - in the Low Countries and France for example. Its British distribution in fact is the counterpart to the Northern Marsh Orchid though there is a fair degree of this species’ northern range and across Wales that overlaps with D. purpurella. In fact the extent of the last glaciation across Britain more or less coincides with the boundary between D. praetermissa in the south and D. purpurella in the north. They are now overlapping to some extent and this may be due to recent expansion by either species.

It is surmised that the polyploid event that lead to D. praetermissa occurred in southern Europe while the north was still covered by the ice sheets. 26,000 years ago would not be an bad estimate. It is likely that the continental Broad-leaved Marsh Orchid, D. majalis, also evolved in the same broad area and time period. Both were limited to the Mediterranean refugal area during the last glaciation, migrating north as the ice retreated about 12,000 years ago, but they would have occupied different habitats. D praetermissa is quite a lowland plant, while D. majalis prefers montane environments. They would have had their own evolutionary paths and their northward migration would have happened at different times.

It is rather surprising, to me at least, that this species wasn’t actually recognised until 1914. In fact its Latin name means ‘overlooked’. But even in as recent times as 1947 a comprehensive book, The Handbook of British Flora, put all Marsh Orchids as one species, Orchis latifolia, and made no little distinctions as to subspecies or below. It merely said that the species appearance was highly variable. We now know better! The flowers of D. praetermissa are rather paler than D. purpurella with lip markings less noticeable and often a fine spotting. Whilst pure bred populations are unlikely due to past hybridisations and introgression, identification should be straightforward unless one is looking at a hybrid swarm.

More recently it has been discovered that the colonies labeled D. traunsteineroides from Norfolk and southern England (based on both appearance and habitat) are very close genetically to D. praetermissa. In fact the southern ‘traunsteineroides’ plants, whilst having a likeness for Schoenus fens, have a D. praetermissa genetic make-up, often indistinguishable from the regular D. praetermissa nearby. This may be a hybridisation effect, but as stated earlier, the allopolyploids do not seem to routinely hybridise in the wild. Consequently these southern populations have been reclassified as a subspecies - D praetermissa schoenophila. Could it be that the genes that both allow a Marsh Orchid to colonise fens and give them a rather ‘skinny’ appearance have come from the same ancestral population which contributed to both species’ polyploid event (as well as the other similar continental species such as D. traunsteineri on the continent)? Could this be more of a genomics than genetics issue? Methylation of certain DNA bases could be the basis of this phenomenon, explaining how the skinny plants are able to pop-up and prosper where others can not.
This reclassification probably also applies to populations labeled D. traunsteroides growing in fens in northern France and the Low Countries, but that requires confirmation. The effect of this change is to make the range of D. purpurella to overlap that of D. traunsteinoides. The significance of this is discussed later.

There seems to be a disagreement regarding whether D. praetermissa is an old or recently formed allopolyploid species. Perhaps it is a matter of relative age. It is nowhere near as young as D. purpurella, having populated Britain before 9,000 years BP when this island was still part of the continental mainland. Whether it arose south of the Alps some 15-20,000 years BP is more open to discussion. Nearly half the UK samples in a large analysis carried the ITS VI marker, with most also carrying the typical D. fuchsii ITS markers. This is largely absent from other allopolyploids or D. fuchsii, but is found in D. saccifera, a D. fuchsii-like plant of Croatia, Greece and Turkey (nowadays). It seems unlikely that D. saccifera was the ovule plant for the original polyploid event resulting in D. praetissima (but that cannot be ruled out), but it does suggest ancient hybridisation between the two species. Thus D. pratermissa may have arisen in the Balkan area before migrating beyond the Alps as the ice retreated. Could D. pratermissa be an amalgam of two similar species; one with D. fuchsii and the other with D. saccifera as the ovule parent? 

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