Norway spruce (Picea abies [L.] Karst.) is a broadly distributed European conifer tree whose history has been intensively studied by means of fossil records to infer the location of full-glacial refugia and the main routes of postglacial colonization. Here we use recently compiled fossil pollen data as a template to examine how past demographic events have influenced the species' modern genetic diversity. Variation was assessed in the mitochondrial nad1 gene containing two minisatellite regions. Among the 369 populations (4876 trees) assayed, 28 mitochondrial variants were identified. The patterns of population subdivision superimposed on interpolated fossil pollen distributions indicate that survival in separate refugia and postglacial colonization has led to significant structuring of genetic variation in the southern range of the species. The populations in the northern range, on the other hand, showed a shallow genetic structure consistent with the fossil pollen data, suggesting that the vast northern range was colonized from a single refugium. Although the genetic diversity decreased away from the putative refugia, there were large differences between different colonization routes. In the Alps, the diversity decreased over short distances, probably as a result of population bottlenecks caused by the presence of competing tree species. In northern Europe, the diversity was maintained across large areas, corroborating fossil pollen data in suggesting that colonization took place at high population densities. The genetic diversity increased north of the Carpathians, probably as a result of admixture of expanding populations from two separate refugia.
Norway spruce of northern Europe expanded at the end of the last glacial out of one refugium in Russia. To provide a detailed insight into how the genetic structure in the northern European lineage of this species has been shaped by postglacial migration, recurrent pollen flow and marginality, we here compare variation at seven highly variable nuclear microsatellite loci in 37 populations (1715 trees) with mitochondrial DNA variation. Microsatellite diversity was high (H E ¼ 0.640) and genetic differentiation was low (F ST ¼ 0.029). The microsatellite structure supported a mitochondrial DNA (mtDNA)-based hypothesis of two migration routes out of a single Russian refugium; one northwestern over Finland to northern Scandinavia, and one southwestern across the Baltic Sea into southern Scandinavia. Microsatellite diversity was maintained along the southwestern migration routes, whereas a significant decrease was observed towards the north. In contrast, the mtDNA diversity suggested higher amounts of historical gene flow towards the north than along the southwestern migration route. This suggests that the loss of nuclear diversity after postglacial colonization has been efficiently replenished by pollen-mediated gene flow in the south. Towards the north, smaller effective population size because of more limited seed and pollen production may have caused decreased nuclear diversity and increased inbreeding, reflecting the ecological marginality of the species in the north.
The temperature during maternal reproduction affects adaptive traits in progenies of Norway spruce ( Picea abies (L) Karst.). Seed production in a cold environment advances bud set and cold acclimation in the autumn and dehardening and flushing in spring, whereas a warm reproductive environment delays timing of these traits. We repeated crosses between the same parents and produced seeds under contrasting temperatures. Elevated temperatures were applied at different time points from female meiosis to embryogenesis, followed by full-sib progeny tests in common environments. We measured timing of terminal bud formation, cold acclimation in the autumn and transcription levels of conifer phytochromes PhyO, PhyN , PhyP , and the class IV chitinase PaChi4 in these tests. No progeny differences were found that could be related to temperature differences during prezygotic stages and fertilization. In contrast, progeny performance was strongly associated with the degree-days from proembryo to mature seeds. Progenies with a warm embryonic history formed terminal buds later, were less hardy and expressed lower transcription levels of the Phy and PaChi4 genes. We hypothesize that temperature during zygotic embryogenesis and seed maturation regulates an 'epigenetic memory' in the progeny, involving differential expression of genes that may regulate bud phenology, cold acclimation and embryogenesis in Norway spruce.
Summary• Adaptive traits in Picea abies (Norway spruce) progenies are influenced by the maternal temperatures during seed production. Here, we have extended these studies by testing the effects of maternal photoperiod and temperature on phenology and frost hardiness on progenies.• Using eight phytotron rooms, seeds from three unrelated crosses were made in an environmental 2 × 2 factorial combination of long and short days and high and low temperatures. The progenies were then forced to cease growth rapidly at the end of the first growing season.• An interactive memory effect was expressed the second growth season. Progenies from high temperature and short days, and from low temperatures and long days, started growth later in spring, ceased shoot growth later in summer, grew taller and were less frost hardy in the autumn than their full siblings from low temperatures and short days, and from high temperatures and long days.• Norway spruce has developed a memory mechanism, regulating adaptive plasticity by photoperiod and temperature, which could counteract harmful effects of a rapidly changing climate.
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