Taxonomic and genetic assessment of disjunct populations of the threatened plant Arnica montana (Asteraceae) from central France

To optimize genetic restoration, not only the amount of genetic diversity is important, but also genetic differentiation among populations. When genetic divergence is too high, maladaptation and outbreeding depression may compromise long-term population evolutionary potential. The declining, self-incompatible, clonally-propagating mountainous herb Arnica montana (Asteraceae) shows wide phenotypic variation through its range. Two subspecies (atlantica and montana) were described based on nuclear and plastid markers and on sesquiterpene lactone composition. Taxonomic uncertainties remain about disjunct, low elevational, and ecologically marginal populations in central France (Val-de-Loire region). We investigated genetic diversity and structure using four plastid markers and nine polymorphic microsatellite markers and quantified helenalin and dihydrohelenalin compounds in flowers, to compare Val-de-Loire populations with low-elevation populations from Ardennes (northern France, Belgium and Luxembourg) ascribed to A. m. subsp. montana. Small Val-de-Loire populations retained genetic diversity similar to the larger restored Ardennes populations, so that they may be used as seed sources for local translocations, with source mixing to take the small number of compatible mates due to high clonality into account. Genetic differentiation between Val-de-Loire and Ardennes populations suggests isolation-by-distance, and founding and genetic drift effects. Val-de-Loire populations share the same phylogeographic history and chemotype as Ardennes populations, and can thus be ascribed to A. m. subsp. montana. The wide chemotypic variation among and within populations suggests phenotypic plasticity related to site environment and climatic conditions, but also genetically-based local adaptation. The preservation of such ecologically marginal populations appear important in a context of climate change and increased eutrophication.