Genetic and morphological variation of two local allotetraploid orchids, Dactylorhiza baltica and D. ruthei

Dactylorhiza baltica (Klinge) Nevski and D. ruthei (M. Schulze ex Ruthe) Soó are taxa of unclear taxonomic status and often distinguished based on a few morphological characters. Their classification as distinct species or infraspecific taxa is problematic and author dependent. More than 300 specimens from nine populations of D. baltica and 10 of D. ruthei were analysed. Three data sets were used (36 morphological characters, 10 plastid DNA markers and five nuclear microsatellites) to assess the difference between D. baltica and D. ruthei. Both taxa are distinct young allopolyploids, but share the same genetic inheritance and ploidy level, which means that they are not genetically uniquely defined, but can be identified using a full set of diagnostic morphological traits. They are the result of common evolution from the same pair of parental species with a common evolutionary and post-glacial history, but arose independently at a similar time in the region of the Baltic Sea. Therefore, the observed patterns of genetic variation and quite distinct differences in morphology, as well as the common mechanisms of origin and evolution of these allopolyploid taxa, provide arguments for assigning the same taxonomic status to both D. baltica and D. ruthei. It is postulated that populations that have integrated within the same allopolyploid gene pool and that appear to be historically related to each other should be considered infraspecific taxa, within the D. majalis s.l. group as D. majalis subsp. baltica (Klinge) H. Sund. and D. majalis subsp. ruthei (M. Schulze ex Ruthe) H. Kretzschmar, respectively.