The impact of glacial and postglacial history on the genetic structure of Norway spruce and Siberian spruce

Mari Mette Tollefsrud*¹, Yoshiaki Tsuda², Jørn Henrik Søstebebø¹, Małgorzata Latałowa³, Laura Parducci², Thomas Källman², Jun Chen², Vladimir Semerikov⁴, Tore Skrøppa¹, Giovanni Giuseppe Vendramin⁵, Christoph Sperisen⁶, Martin Lascoux²

*Presenting author, ¹ Norwegian forest and landscape institute, Ås, Norway, ² Department of Ecology and Genetics, Evolutionary Biology Centre, Uppsala University, Sweden, ³Laboratory of Palaeoecology and Archaeobotany, Department of Plant Ecology, University of Gdańsk, Poland, ⁴Institute of Plant and Animal Ecology, Ural Branch of Russian Academy of Science, Ekaterinburg, Russia, ⁵Plant Genetics Institute, Division of Florence, National Research Council, Sesto Fiorentino, Italy, ⁶Swiss Federal Research Institute for Forest, Snow and Landscape Research (WSL), Switzerland.

*mari.mette.tollefsrud@skogoglandskap.no

During the Last Glacial Maximum, the boreal vegetation was greatly restricted. Climatic variation between regions had different impact on the glacial and postglacial history of tree species, resulting in contrasting distribution of genetic diversity. Norway spruce (Picea abies) and Siberian spruce (P. obovata) are two closely related species which parapatric ranges cover almost the entire boreal region of Eurasia; a vast region that experienced contrasting glacial histories. In the present study we combined extensive paleobotanical and genetic data to reconstruct the joint histories of the two species and to evaluate how their glacial and postglacial histories have affected their genetic structure.

Today, Norway spruce and Siberian spruce are clearly genetically differentiated in mitochondrial (mt) and nuclear SSR markers, suggesting that the two species had largely independent glacial histories. Nuclear SSR markers indicate the presence of hybrid individuals on both sides of the Urals and east-west longitudinal genetic structures indicate a wide zone of hybridization. The border for mtDNA is situated along the Ob River in Siberia. Along this river and eastwards, latitudinal genetic structures were weak. In Norway spruce, rather complex population genetic structures are revealed as a result of multiple refugia and contrasting recolonization patterns.

The current distribution of Norway spruce is divided into a southern and a northern domain. Coherent with the paleodata, both mtDNA and SSR loci suggest a long lasting separation between these two domains, which however,  did not preclude secondary contacts. Within the southern domain, mtDNA and paleodata suggest the presence of several refugia, a pattern that nuclear SSR loci fail to reveal probably reflecting pollen mediated gene flow. In the northern domain, the same data support the recolonization of Scandinavia during the mid Holocene from a large and scattered refugium located on the East European Plain. Recolonization took place along different migration routes, and diversity evolved differentially along these routes. The complex genetic structure at nuclear SSRs in the northern Norway spruce domain may be due to gene flow from the southern domain, gene flow from the hybrid zone along the Ural Mountains and expansion from a separate refugium along the Atlantic coast. The latter is suggested by ancient DNA, the presence of a Scandinavia endemic mitochondrial haplotype and possibly, the current structure at SSR loci, where the origin of a distinct genetic cluster in Central Scandinavia remains to be elucidated.

The implications of these findings for the response of the boreal forest to climate, forest management and breeding will be discussed.