On the hybrid origin of Dactylorhiza brennensis and implications for the taxonomy of allotetraploid Dactylorhiza

Dactylorhiza brennensis is an allotetraploid member of the Dactylorhiza majalis s.lat. complex, which comprises orchids that combine two genomes from the D. incarnata s.lat. lineage and two genomes from the D. maculata s.lat. lineage (incl. D. maculata subsp. fuchsii). The distribution of D. brennensis is restricted to central France. It was originally described as a subspecies of D. elata, which is distributed further to the south, but it has also been indicated that it resembles the northern D. majalis subsp. integrata (syn. D. praetermissa) and may be related to this taxon as well. On basis of detailed morphometric analysis, it has also been argued that it merits recognition as a separate species. Here, we studied Dactylorhiza brennensis for variation at nuclear and plastid molecular markers and made a comparison with potentially related taxa. We show that D. brennensis combines genes from D. majalis subsp. sesquipedalis (syn. D. elata subsp. sesquipedalis) and D. majalis subsp. integrata, and that it originated by hybridization between already established allotetraploids, rather than by a separate allopolyploidization event from the parental D. maculata and D. incarnata lineages. We also show that D. brennensis integrates the plastid genomes of both parental allotetraploids, which indicates that it has evolved by merger of populations, rather than by hybridization between single individuals. However, the hypothesis that it has hybridized with local population of D. maculata subsp. fuchsii could not be substantiated. Since D. brennensis and its parents belong to the same species according to the biological species concept, we provide the combination D. majalis subsp. brennensis (E.Nelson) D.Tyteca & Hedrén for those who use this concept as basis for taxonomic classification