High genetic diversity and distinct origin of recently fragmented Scots pine (Pinus sylvestris L.) populations along the Carpathians and the Pannonian Basin

Historical evolutionary events highly affect the modern-day genetic structure of natural populations. Scots pine (Pinus sylvestris L.), as a dominant tree species of the Eurasian taiga communities following the glacial cycles of the Pleistocene, has survived in small, scattered populations at the range limits of its south-eastern European distribution. In this study, we examined genetic relationships, genetic divergence and demographic history of peripheral populations from central-eastern Europe, the Carpathian Mountains and the Pannonian Basin. Four hundred twenty-one individuals from 20 populations were sampled and characterized with both nuclear and chloroplast simple sequence repeat (SSR) markers. Standard population genetic indices, the degree of genetic differentiation and spatial genetic structure were analysed. Our results revealed that peripheral Scots pine populations retained high genetic diversity despite the recently ongoing fragmentation and isolation of the persisting relict populations. Analysis of molecular variance (AMOVA) showed 7% among-population genetic differentiation, and there was no isolation by distance among the island-like occurrences. Genetic discontinuities with strong barriers (99-100% bootstrap support) were identified in the Carpathians. Based on both marker types, populations of the Western Carpathians were delimited from those inhabiting the Eastern Carpathians, and two main genetic lineages were traced that most probably originate from two main refugia. One refugium presumably existed in the region of the Eastern Alps with the Hungarian Plain, while the other was probably found in the Eastern Carpathians. These findings are supported by recent palynological records. The strongest genetic structure was revealed within the Romanian Carpathians on the basis of both marker types. With only some exceptions, no signs of recent bottlenecks or inbreeding were detected. However, Carpathian natural populations of Scots pine are highly fragmented and have a small census size, though they have not yet been affected by genetic erosion induced by isolation.