AFLP markers show low levels of clonal propagation and high genotypic diversity in the rare, southernmost populations of Linnaea borealis L. (Caprifoliaceae) in the Western Alps.

In plants, clonal propagation is a common reproductive strategy in parallel to sexual reproduction. It has both advantages and drawbacks, and the potential complete loss of sexual reproduction causes serious conservation concerns, especially because population maintenance then only relies on adult survival and low genetic diversity leads to decreased adaptive potential. We investigated the rare, southernmost populations of the mostly circumboreal twinflower Linnaea borealis, located in the Western Alps. Based on 105 AFLP markers and 118 leaf samples, including replicates, we estimated the genetic similarity threshold above which samples belong to a single clone. Although the species is known for extensive clonal propagation, we observed high genotypic diversity within the seven studied populations and almost all samples were genetically distinct. Nevertheless, some clonal samples were detected in two populations, separated by up to 180 m. We found a strong genetic differentiation among populations (overall Fst = 0.38), which was congruent with the previously documented high plastid diversity in the region. We therefore hypothesize that Alpine populations are relicts of the Quaternary glacial periods, when the species probably survived at these lower latitudes before colonizing Northern Europe. Regarding conservation, our results suggest that most extant plants result from sexual reproduction and that populations are not highly threatened. Nevertheless, since clones can be very long-lived and almost no seedlings were observed in recent years, events of sexual reproduction may be ancient. The current reproductive dynamics should therefore be studied to estimate e.g. pollinators activity, proportions of flowering plants, and seed set.