Natural hybrid zones have proven to be precious tools for understanding the origin and maintenance of reproductive isolation (RI) and therefore species. Most available genomic studies of hybrid zones ...using whole‐ or partial‐genome resequencing approaches have focused on comparisons of the parental source populations involved in genome admixture, rather than exploring fine‐scale patterns of chromosomal ancestry across the full admixture gradient present between hybridizing species. We have studied three well‐known European ‘replicate’ hybrid zones of Populus alba and P. tremula, two widespread, ecologically divergent forest trees, using up to 432 505 single‐nucleotide polymorphisms (SNPs) from restriction site‐associated DNA (RAD) sequencing. Estimates of fine‐scale chromosomal ancestry, genomic divergence and differentiation across all 19 poplar chromosomes revealed strikingly contrasting results, including an unexpected preponderance of F₁ hybrids in the centre of genomic clines on the one hand, and genomically localized, spatially variable shared variants consistent with ancient introgression between the parental species on the other. Genetic ancestry had a significant effect on survivorship of hybrid seedlings in a common garden trial, pointing to selection against early‐generation recombinants. Our results indicate a role for selection against recombinant genotypes in maintaining RI in the face of apparent F₁ fertility, consistent with the intragenomic ‘coadaptation’ model of barriers to introgression upon secondary contact. Whole‐genome resequencing of hybridizing populations will clarify the roles of specific genetic pathways in RI between these model forest trees and may reveal which loci are affected most strongly by its cyclic breakdown.
Abstract
Key message
Paternity analyses show that effective pollination of ash (
Fraxinus excelsior
L.) in a seed orchard and a floodplain forest affected by ash dieback is more likely to be ...facilitated by healthier males. Thereby, natural selection can have a positive effect on the health of future generations.
Context
Ongoing ash dieback and increasing fragmentation of ash populations may result in reduced pollen flow, which can reduce pollination success of future generations of ash trees. Therefore, it is essential to further improve our understanding of gene flow patterns, especially with respect to ash dieback.
Aims
In this study, paternity analyses were conducted in a seed orchard and a floodplain forest in Germany in 2018 to explain the relationship between pollination success and the health status of ash trees and distances of effective pollen transport.
Methods
Cambium samples (i.e., from twigs and stumps) were collected from 251 ash trees (putative father and mother trees) for genotyping, and the health status of each tree was documented using a scoring system to evaluate vitality. Additionally, seeds were harvested from 12 mother trees per site. Genetic analyses using nuclear microsatellites were performed to determine paternal trees. Paternities were assigned based on the likelihood model implemented in the Cervus 3.0.7 software.
Results
Our results showed that the average pollination distance was 76 m in the seed orchard and 166 m in the floodplain forest. In general, pollination success decreased substantially with increasing distance to the mother tree. Despite the dense tree cover in the floodplain forest, pollen were transported over long distances (greater than 550 m), suggesting that non-local sources also play a role in pollination. This is supported by the foreign pollen input identified in the seed orchard (66.5%). Self-pollination was detected only to a very small extent, and thus had no major influence on reproduction. In addition, both healthy and slightly diseased father trees showed similar mating success. However, this was not the case for the severely diseased ash trees (more than 50% of crown damage) because only a few offspring could be assigned to them. Nevertheless, in contrast to the floodplain forest, there was no significant correlation between damage classes and pollination success in the seed orchard.
Conclusion
Long-distance pollen transport contributes to the connectivity of ash trees in the landscape. Additionally, both healthy and slightly diseased fathers have a greater contribution to pollination, thus potentially improving the health of the next generation of ash trees. Moreover, gene flow between stepping stone populations is necessary to ensure the positive impact on the genetic diversity of ash populations in the future.
The ongoing fragmentation of ash populations due to ash dieback requires an effective gene flow between individuals; thus, investigations on ash pollen transport are essential. In this study, ...comprehensive aerobiological field experiments at two seed plantations in Baden-Württemberg were conducted in 2019 and 2020 in order to study the influence of phenology and meteorology (especially wind) on pollen transport using self-constructed gravimetric pollen traps located 1.5 and 5 m a.g.l. Our main objectives were to investigate the local scale dispersion of ash pollen and to evaluate the recommended distance (i.e., 400 m) from seed plantations to other ash trees according to the German Forest Reproduction Act. Our results showed a link between pollen transport and meteorology, the onset of phenological development, and the topography of the plantation. The plantation at Schorndorf was characterized by a slope and associated cold air flows, suggesting that this could be a factor contributing to higher pollen levels at the downslope traps. In addition, in many cases, the cardinal direction associated with the highest pollen impaction was also identical with the predominant wind direction. Analyzing pollen data for single traps in detail, we found that the highest total pollen catch (31%) was measured outside the plantations in 2019, a year with very low flower development. In contrast, most pollen (33%) was caught within the plantation in 2020, which presented a much stronger pollen year than 2019 (with a factor of 11 regarding total sums). This indicates, in the lower pollen year, a potential higher contribution of trees from outside the plantation, and thus it can be recommended that seed harvesting of ash trees in the plantations should preferentially take place in full mast years. Interestingly, the total pollen deposition in Emmendingen at 5 m height showed little difference compared to the traps at 1.5 m height, but there was a large temporal difference pointing to vertical variations in pollen availability. In general, we found that ash pollen was transported for a larger distance than 400 m, but the amount of pollen decreased substantially with increasing distance. At a distance of 200 m, there was already approx. 50% less pollen captured from the air. However, even at a distance of 500 m, more than 10% of the pollen was still captured. In order to ensure cross-pollination of healthy ash trees, the distance of ash individuals or stands should not be too large, and there should be no spatial separation (e.g., by conifer stands).
In the present paper we studied the genetic diversity and genetic structure of five Norway spruce (Picea abies (L.) Karsten) natural populations situated in Serbia, belonging to the southern lineage ...of the species at the southern margin of the species distribution range. Four populations occur as disjunct populations on the outskirts of the Dinaric Alps mountain chain, whereas one is located at the edge of Balkan Mountain range and, therefore, can be considered as ecologically marginal due to drier climatic conditions occurring in this region. Due to the negative effect of biotic and abiotic stress factors, the sustainability of these populations is endangered, making conservation of their genetic resources one of the key measures of Norway spruce persistence in Serbia under climatic changes. The insight on genetic diversity and genetic structure of the studied spruce populations can provide the information required for the initiation of programs aimed at the conservation and utilization of spruce genetic resources at the rear edge of species environmental limits. Norway spruce genetic variation and population genetic structure were estimated using eight EST-SSR markers. The results showed that mean expected heterozygosity was 0.616 and allelic richness 10.22. Genetic differentiation among populations was low (Fst = 0.007). No recent bottleneck effect or isolation by distance were detected. Bayesian clustering, obtained with STRUCTURE, grouped the populations into two genetic clusters, whereas UPGMA analysis distinguished three main groups approximately in line with the geographic area of occurrence. Based on the study results and the EUFORGEN Pan-European strategy for genetic conservation of forest trees, the establishment of additional dynamic gene conservation units must be considered in Serbia in order to protect the adaptive and neutral genetic diversity of the species.
In the Romanian Carpathians, Silver fir covers about 5% of the forest area and is the second most important conifer species. Although there are a number of genetic studies concerning the distribution ...of genetic diversity of Abies alba in Europe, populations from the south-eastern limit of the distribution range have been studied less. The aim of the present study was to assess the genetic diversity and differentiation in 36 silver fir populations along the Carpathian Mountains in Romania, using seven microsatellites loci. High levels of genetic diversity (He = 0.779 to 0.834 and AR = 11.61 to 14.93) were found in all populations. Eastern Carpathians populations show higher levels of diversity, both in allelic richness and expected heterozygosity and higher degrees of genetic differentiation compared to southern populations. Bayesian clustering analysis revealed the existence of two genetically distinct groups for silver fir populations, one larger cluster which comprises the Inner Eastern Carpathians, Curvature Carpathians, South Carpathians and the Banat Mountains and the second cluster contained most of the North and Outer Eastern Carpathians population. Both AMOVA and Barrier analysis supported genetic differentiation among geographical provenance regions. The high genetic diversity of silver fir populations from the eastern limit of its distribution provide high potential to mitigate the negative effects of climate warming being valuable genetic resources in the context of global change. The distribution pattern of genetic variation at local, regional and country scale could and should be considered for the preservation of the forest genetic resources.
Central European populations of silver fir ( Abies alba Mill.) grow under a relatively wide amplitude of environmental conditions. Assuming that forest tree stands are locally adapted, the use of ...forest reproductive material from sites with limited water availability is expected to increase drought tolerance in future forests. At the same time, genetic diversity is of utmost importance as the basis of adaptation to a changing environment. Currently, a selection of potential sources for climate-smart reproductive material of silver fir is under way in Southern Germany. It is based on a multidisciplinary approach elucidating the genetic variation, site conditions, as well as tree resilience based on a dendroecological approach. The analysis encompasses a large number of stands representing the whole range of the species’ ecological requirements. The population genetic analysis based on molecular markers (nuclear microsatellites) provided important information concerning the gene pool of the species in Southern Germany. On one hand, it revealed genetic differentiation and transition zones between western and eastern clusters. On the other hand, the results indicated gradients and regional variation of genetic diversity. These patterns can be explained by post glacial recolonization and the peripheral character of the species at the northern limit of its distribution. Together with the outcomes of the site condition and dendroecological approaches, the results of the genetic analysis will be used to inform future provenance recommendations.
Corylus colurna is considered as important tree species under climate change for dry and warm conditions in Central Europe and was overused because of its valuable wood. Therefore Turkish hazel is ...now present only in small isolated populations and is protected under IUCN. Genetic conservation of this tree species plays a key role in future sustainable forest development. Turkish hazel co-occurs with Common hazel (C. avellana) in its whole distribution area and may form hybrids. To differentiate between the pure species and their hybrids, cross-species amplifying markers are required. In this study we have evaluated existing simple sequence repeat (SSR) markers using altogether 128 samples of C. avellana and C. colurna. Fifteen nuclear SSRs have generated easy to-score alleles in the two species and 13 of them were highly polymorphic. For all 15 markers the mean allele number, average observed heterozygosity, genetic diversity and polymorphism information index were high. The two most polymorphic SSRs were L1.10 and CaT-B501 with 19 and 16 alleles, respectively. Structure analysis proved the differentiation of the two species C. avellana and C. colurna. No hybridization was detected in the analysed populations. Results also indicated that C. colurna from Balkan Peninsula and Asia Minor belong to separate groups. Our study presents highly polymorphic, easy to score, ready to use SSR-multiplexes, which can be applied in population genetics and gene conservation studies.
European beech, Fagus sylvatica L., is one of the most important and widespread deciduous tree species in Central Europe and is widely managed for its hard wood. The complete DNA sequence of the ...mitochondrial genome of Fagus sylvatica L. was assembled and annotated based on Illumina MiSeq reads and validated using long reads from nanopore MinION sequencing. The genome assembled into a single DNA sequence of 504,715 bp in length containing 58 genes with predicted function, including 35 protein-coding, 20 tRNA and three rRNA genes. Additionally, 23 putative protein-coding genes were predicted supported by RNA-Seq data. Aiming at the development of taxon-specific mitochondrial genetic markers, the tool SNPtax was developed and applied to select genic SNPs potentially specific for different taxa within the Fagales. Further validation of a small SNP set resulted in the development of four CAPS markers specific for Fagus, Fagaceae, or Fagales, respectively, when considering over 100 individuals from a total of 69 species of deciduous trees and conifers from up to 15 families included in the marker validation. The CAPS marker set is suitable to identify the genus Fagus in DNA samples from tree tissues or wood products, including wood composite products.
The conservation and sustainable use of forests and forest genetic resources (FGR) is a challenging task for scientists and foresters. Forest management practices can affect diversity on various ...levels: genetic, species, and ecosystem. Understanding past natural disturbance dynamics and their level of dependence on human disturbances and management practices is essential for the conservation and management of FGR, especially in the light of climate change. In this review, forest management practices and their impact on genetic composition are reviewed, synthesized, and interpreted in the light of existing national and international forest monitoring schemes and concepts from various European projects. There is a clear need and mandate for forest genetic monitoring (FGM), while the requirements thereof lack complementarity with existing forest monitoring. Due to certain obstacles (e.g., the lack of unified FGM implementation procedures across the countries, high implementation costs, large number of indicators and verifiers for FGM proposed in the past), merging FGM with existing forest monitoring is complicated. Nevertheless, FGM is of paramount importance for forestry and the natural environment in the future, regardless of the presence or existence of other monitoring systems, as it provides information no other monitoring system can yield. FGM can provide information related to adaptive and neutral genetic diversity changes over time, on a species and/or on a population basis and can serve as an early warning system for the detection of potentially harmful changes of forest adaptability. In addition, FGM offers knowledge on the adaptive potential of forests under the changing environment, which is important for the long-term conservation of FGR.
•Sycamore maple in Central Europe (Germany) shows a longitudinal genetic structure.•Postglacial recolonization might account for this pattern of genetic variation.•An introgression zone points to ...genetic exchange after secondary contact.•Genetic exchange has been less in the South; mountains might have limited gene flow.•Genetic differentiation was observed among certain regions of provenance.
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.
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