Western and eastern post-glacial migration pathways shape the genetic structure of sycamore maple (Acer pseudoplatanus L.) in Germany

Sycamore maple (Acer pseudoplatanus L.) is a broadleaved forest tree species mainly occurring in mountainous areas of Europe, preferring fresh to moist, deep, calcareous soils. As was the case with other important forest tree species, sycamore maple was absent from the area north of the Alps during the last ice age. Therefore, we investigated the origin of sycamore maple in this area and its genetic variation across the landscape as the result of its postglacial migration into this area. We compared our results with the current delineation of sycamore maple into regions of provenance in Germany. Altogether 1043 trees from Germany, but also, in few cases, from bordering countries, were analysed using 11 nuclear and one chloroplast microsatellite (SSR) markers. Both the analysis of chloroplast (cpDNA) haplotypes and the Bayesian cluster analysis based on nuclear SSR data revealed distinct genetic structures of sycamore maple in Germany, suggesting more than one origin. The spatial distribution of a particular cpDNA haplotype (haplotype '105') might be the result of a migration event with an eastern origin, probably along the Eastern Alps, which could have occurred during the Holocene. The other haplotype (haplotype '102') might be of (south-) western origin, representing a migration pathway along the Western Alps. This hypothesis is further supported by a Bayesian cluster analysis based on nuclear SSRs. We identified two clusters, one distributed in the western part of the study area and one in the eastern. Between them, a zone of genetic introgression occurs, where populations exhibit a mixed membership to both clusters. This zone of genetic admixture is narrower in the south. An analysis of the landscape shape confirms that average genetic differentiation per distance unit is higher in the south. This is also in agreement with higher genetic distances between regions of provenance in the south. The mountainous landscape of this region could have contributed to this higher genetic differentiation, by posing geographic barriers to gene flow. On the contrary, wide-scale seed movement, also beyond the limit of native range, as well as a flat relief could have resulted in a wider zone of admixture in the north. Based on our results, we argue that the current delineation of provenance regions for sycamore maple in Germany fits to the inferred patterns of genetic differentiation.