Plants have a remarkable capacity to adapt to local environmental conditions, which can result in ecotypic differentiation. Patterns of differentiation can, however, also be influenced by the extensive phenotypic plasticity exhibited by many plant species. In this study, we evaluated the distinctness of two putative ecotypes of the parasitic herb Rhinanthus angustifolius. We compared population means of characters commonly used to distinguish between the putative ecotypes after growing individuals of R. angustifolius with a variety of host species in a common garden. Resulting data were also pooled over environments to study how phenotypic plasticity affects the distinctness of ecotypes and individual populations. Except for node number, most of the characters were plastic. The pattern of differentiation was consistent with the existence of two, or possibly three, habitat‐related groups of populations; however, we observed considerable overlap in flowering time and morphological characters after pooling data across host environments. The results show that the complex phenological and morphological variation in R. angustifolius is caused by a combination of genetically determined ecotypic differentiation and plastic responses to the host environment and other factors. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2016, 180, 89–103.

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.

Analyses of intraspecific genetic structure can promote the conservation of genetic diversity of rare or declining plant species by enabling identification of proper management units. Here we investigate the genetic structure of the annual hemiparasitic herb Rhinanthus angustifolius to evaluate the genetic distinctness of two currently described subspecies and three habitat‐related groups of populations inferred from recent common‐garden data. Data from 11 nuclear microsatellite loci, obtained from 17 southern Swedish populations of R. angustifolius, were analyzed with a variety of population genetic techniques to examine how intraspecific genetic structure relates to taxonomy, habitat and geography. Neither the two subspecies nor the three recently identified population groups formed genetically coherent lineages. Rather, there was a clear isolation‐by‐distance pattern, pointing to a major role of geographically restricted gene flow in structuring the diversity at the putatively neutral microsatellite loci. Although uncertainties remain regarding the influence of homoplasy and possible introgression from the closely related R. minor, our microsatellite data argue against the recognition of previously identified subspecies or population groups as management units for conservation purposes. As currently described entities do not contain much unique genetic variation, we recommend conserving populations from a large geographical range, rather than populations from specific habitats, to preserve the long‐term evolutionary potential of the species.