Genetic variation of a widespread subdominant tree species (Acer campestre L.) in Bosnia and Herzegovina

Tree species of minor commercial importance are mostly neglected both in large-scale genetic surveys and gene conservation programs. Gene conservation measures of forest trees focus on the preservation of intra-species genetic variability, which is a prerequisite of species' adaptive evolutionary potential enabling it to survive unfavourable conditions. Therefore, variation levels within populations as well as inter-population differentiation patterns need to be assessed under various geographical and evolutionary setups. We studied genetic variation of a widespread, but subdominant tree species, Acer campestre L., in 25 populations located close to the rear edge of the Holocene colonisation in Bosnia and Herzegovina at ten nuclear microsatellite loci. Genetic differentiation within this region was very weak, indicating a lack of strong barriers to gene flow despite range fragmentation. In spite of this, significant isolation by distance was revealed, along with isolation by environment related to temperature. We suggest phenological isolation as the explanation of isolation by environment. Moreover, the group of sub-Mediterranean populations appeared to diverge from the interior populations: they exhibit higher gene diversity, form a separate cluster in the neighbour-joining tree based on chord genetic distances, and show different proportions of gene pools identified by the Bayesian analysis of population structure. As the territory under study is located close to several important glacial refugia for tree species, an origin from different refugial populations may explain the pattern of weak but recognisable genetic differentiation between interior and sub-Mediterranean populations. A genetic bottleneck in several sub-Mediterranean populations was attributed to more recent colonisation events. The results suggest that genetic differentiation associated with the origin from different glacial refugia was not erased by gene flow even in rear-edge populations, and needs to be taken into consideration in gene conservation programs.