The polymorphic early marsh orchids, Dactylorhiza incarnata s.l. (Orchidaceae), at Lough Gealain, Ireland

Populations of Dactylorhiza incarnata (early marsh orchids) with spotted leaves appear in widely separate parts of Europe including Scandinavia and the Baltic area, the Alps, and Britain and Ireland. Forms with spotted leaves have often been segregated as a separate subspecies, D. incarnata subsp. cruenta. Although previous studies have indicated that the taxon is heterogeneous, it is not known how spotted forms from Britain and Ireland are related to unspotted forms from the same area and how they are related to spotted populations from other parts of Europe. Here, we performed a detailed genetic analysis of the mixed population of spotted and unspotted early marsh orchids at Lough Gealain, County Clare, Ireland in an attempt to shed some light on these questions. A total of 27 plants (11 spotted and 16 unspotted) was examined for genetic variation patterns at three nuclear microsatellite loci and three plastid loci, including one 9-bp deletion and two plastid microsatellites. Spotted and unspotted morphs at Lough Gealain were clearly genetically differentiated from each other, and the total genetic diversity at the site was much better explained by separation into morphs than by separation into subsites along the lake. However, there was still some overlap between morphs at both nuclear microsatellite loci and in plastid haplotypes indicating that at least restricted gene flow occurs. Within each morph, plants growing at close distance from each other seemed to be more closely related than plants growing at larger distances, but the effect was not significant. The spotted morph was affected by some inbreeding, whereas the unspotted morph was not. The spotted morph at Lough Gealain does not agree any more closely with spotted forms of D. incarnata sensu lato from other parts of Europe than with unspotted forms. Both morphs of D. incarnata growing at Lough Gealain contain some fraction of plastid haplotypes that do not occur in any form of the species in the Baltic area, but that are widespread in western Europe, in the Alps, and in Asia Minor. Our results are in agreement with the commonly adopted hypothesis that local segregates of D. incarnata sensu lato may represent genetically divergent groups with restricted gene flow between them. Such patterns could be consistent over local or regional scales, but should not be extrapolated to wider geographic areas in the absence of good supporting molecular data. Spotted forms of D. incarnata growing in different parts of Europe could at most be allocated the rank of variety or forma.