Novel Insights Into Refugia at the Southern Margin of the Distribution Range of the Endangered Species Ulmus laevis

Riparian ecosystems, in long-time developed regions, are among the most heavilyimpacted by human activities; therefore, the distribution of tree riparian species, suchas Ulmus laevis, is highly affected. This phenomenon is particularly relevant at themargins of the natural habitat of the species, where populations are small and rare.In these cases, it is difficult to distinguish between relics or introductions, but it isrelevant for the restoration of natural habitats and conservation strategies. The aimof this study was to study the phylogeography of the southern distribution of thespecies. We sequenced the entire chloroplast (cp) genomes of 54 individuals fromfive sampled populations across different European regions to highlight polymorphismsand analyze their distribution. Thirty-two haplotypes were identified. All the sampledpopulations showed private haplotypes that can be considered an indicator of longterm residency, given the low mutation rate of organellar DNA. The network of allhaplotypes showed a star-like topology, and Serbian haplotypes were present in allbranches. The Balkan population showed the highest level of nucleotide and geneticdiversity. Low genetic differentiation between populations was observed but we founda significant differentiation among Serbia vs. other provenances. Our estimates ofdivergent time of U. laevis samples highlight the early split of above all Serbian individualsfrom other populations, emphasizing the reservoir role of white elm genetic diversity ofSerbian population