Contemporary evolution and phenotypic plasticity or genetic fixation: Rates of evolution and adaptive processes in populations under novel selection regimes
Final Report Abstract
The establishment of ex-situ cultures in botanic gardens as a method to safeguard endangered plant species is one strategy to conserve biodiversity. However, this approach to restoring extirpated species is challenging and needs sound biological and socio-economic knowledge as well as management skills. Nevertheless, for example the Convention on Biological Diversity in its “Global Strategy for Plant Conservation” aims to preserve at least 75% of threatened plant species in ex-situ collections, especially when immediate in-situ conservation cannot be guaranteed e.g. due to habitat degradation or land use change. To be an effective conservation measure, however, ex-situ cultures have to represent the full range of the adaptive potential of the endangered species including their resilience to future environmental conditions. Hence, the life-history as well as the phenotypic and genetic variation of the focal species have to be assessed in the wild and ideally also experimentally in reciprocal transplant and common garden studies. To assess the degree of rapid adaptation to botanic garden conditions and to evaluate the conservation value of ex-situ conserved species, we measured the degree of phenotypic divergence between ex-situ and their original wild-source populations of six annual to short-lived perennial plant species (Berteroa incana (L.) DC., Carlina vulgaris L., Corynephorus canescens (L.) P. Beauv., Jasione montana L., Melilotus officinalis (L.). Pall, and Silene otites (L.) Wibel. We conducted common garden and reciprocal transplant experiments using pairs of welldocumented ex-situ cultures from German botanic gardens and corresponding wild populations 3–31 years after population’ isolation and investigated genetic shifts by conducting population genetic analyses using neutral AFLP markers in ex-situ and wild populations of Carlina vulgaris and ddRADs in Silene otites. Our experiments revealed that genetic drift can lead to fitness decline in ex-situ cultivated plants, but these drift effects strongly depend on the conditions and cultivation history in the ex-situ facility. In addition, our data provided evidence that shifts in traits such as germination and flowering time, and a decrease in stress tolerance to drought and competition can reduce the conservation value of ex-situ collections. These threats associated with ex-situ conditions require more attention by researchers, curators and conservationists. Using molecular markers, we analysed the anonymous genetic divergence between pairs of populations in the species with most accessions, Carlina vulgaris. In this species, even though the cultivation time varied between 9–21 years only — corresponding to a maximum of 10 generations — all but the youngest ex-situ cultures were significantly low to moderate genetically differentiated from wild populations (Fst 0.06–0.14;). In Carlina vulgaris, we observed a consistently higher biomass production and frequency of flowering individuals in ex-situ cultures under botanic garden, but not under natural conditions in the field indicating a rapid adaptation of ex-situ plants to their cultivation conditions. Putative trait shifts in ex-situ populations of Melilotus officinalis (Fabaceae) that have been cultivated for 10–25 years were tested in a common garden under different experimental environmental conditions. In a series experiments, we detected significant pair-wise differences between descendants of ex-situ and wild populations. In summary, as first steps to increase the conservation value of ex situ plant collections we therefore suggest 1) reducing the duration of living ex-situ cultivated populations to a few generations (up to five in the case of Carlina vulgaris) in order to minimize maladaptive divergence of ex-situ plants from their natural original populations and to 2) establish conservation networks with replicated collections across gardens with a balanced seed contribution of mother plants to the next generation within a collection and 3) to avoid artificial selection e.g. by allowing natural rejuvenation to prevent or at least reduce genetic drift effects.
Publications
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(2015) Fitness decline and adaptation to novel environments in ex-situ plant collections: Current knowledge and future perspectives, Biological Conservation, 192, 394–401
Ensslin, A., Tschöpe, O., Burkart, M. & Joshi, J.
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(2017) Conservation genetic assessment on four different steppe plants in a small replica of a steppe ecosystem >30 years after establishment, Biodiversity and Conservation, 26, 2699–2716
Müller, C.M., Huwe, B., Wissemann, V., Joshi, J. & Gemeinholzer, B.
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(2018) Deutsche Botanische Gärten als künftige Quelle invasiver Pflanzenarten – eine Bewertung der Lebendsammlungen. Natur und Landschaft, 93; 23–27
Weber, E., Burkart, M. & Joshi, J.
