The evolution of spring fen ecotypes in Rhinanthus: genetic evidence for parallel origins in Scandinavia after the last ice age

Locally adapted ecotypes can constitute an important part of the biodiversity, especially in young floras with few endemic species. However, the origins, distinctness and conservation value of many ecotypes remain uncertain because genetic data are lacking or no common-garden study has been carried out. In the present study, we evaluated the distinctness and genetic structure of a phenotypically deviating morph of Rhinanthus angustifolius, growing in calcareous spring fens on the Baltic island of Gotland. Using data from a common-garden experiment and analyses of nuclear microsatellite variation, we compared fen populations on Gotland with conspecific populations from habitats more typical of the study species. We also included the fen specialist R. osiliensis from the Baltic island of Saaremaa in the molecular analyses to make further inferences about the origin of the Gotlandic fen morph. Our data indicate that the Gotlandic fen populations constitute a phenotypically and genetically distinct ecotype that most likely has evolved at least two times on Gotland after the last ice age. In congruence with previous studies, we also infer that fen ecotypes have evolved independently on Gotland and Saaremaa. We propose a varietal status for the Gotlandic fen ecotype and give recommendations for the conservation of this taxon.