Dr. Benjamin Dauphin

We trace the development of our understanding regarding the mating systems of moonworts (i.e., Botrychium sensu stricto), from the early recognition of inbreeding as the primary mode of sexual reproduction to the discovery of predominantly outcrossing populations in high mountain habitats. We propose an evolutionary trajectory in which both mating systems, in concert with mycorrhizal support, have been able to sustain Botrychium populations through changing environments. We present a depiction of the fern life cycle that portrays the positive role of inbreeding in speciation and evolution of Botrychium and its relevance to ferns and lycophytes in general.

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The effect of past environmental changes on the demography and genetic diversity of natural populations remains a contentious issue and has rarely been investigated across multiple, phylogenetically distant species. Here, we perform comparative population genomic analyses and demographic inferences for seven widely distributed and ecologically contrasting European forest tree species based on concerted sampling of 164 populations across their natural ranges. For all seven species, the effective population size, N_e, increased or remained stable over many glacial cycles and up to 15 million years in the most extreme cases. Surprisingly, the drastic environmental changes associated with the Pleistocene glacial cycles have had little impact on the level of genetic diversity of dominant forest tree species, despite major shifts in their geographic ranges. Based on their trajectories of N_e over time, the seven tree species can be divided into three major groups, highlighting the importance of life history and range size in determining synchronous variation in genetic diversity over time. Altogether, our results indicate that forest trees have been able to retain their evolutionary potential over very long periods of time despite strong environmental changes.

Nothofagus pumilio is a foundation tree species that inhabits a 2000-km-long range in the southern Andes, a region with two perpendicular environmental gradients: temperature and photoperiod (North–South), and precipitation (West–East). We investigated local adaptation patterns by searching for relationships between environmental clines and signatures of adaptation in candidate genes related to stress response, growth, and phenology. Using a paired site sampling design within a landscape genome analysis, we analyzed 493 adult N. pumilio trees in 20 sampling sites across the species' latitudinal range. We screened 47,336 single nucleotide polymorphism (SNP) loci in 1632 contigs (i.e., coding regions along the genome). Population structure and genetic diversity analyses preceded four genome scan analyses using genetic and environmental data. Population structure and genetic diversity are mainly oriented along the latitude axis. Genome scans identified 445 outlier SNPs, which are loci showing signatures of selection. Temperature and photoperiod variables were associated with notably more outliers than precipitation. However, the most frequent biological functions among genes were water deprivation response and cold response, suggesting that stress response is comprised of complex and polygenic traits that are affected by many environmental variables. Our findings suggest that N. pumilio shows signatures of local adaptation to extant climate conditions, including temperature, photoperiod, and precipitation. However, climate change is likely to alter existing relationships among environmental conditions to which this species is currently adapted. These changes may have unpredictable consequences for the species' future survival, adaptation potential, and the people who depend upon these forests.

Throughout the last centuries, the structure and genetic composition of forests have been strongly affected by forest management. Over 30% of European forests are artificially regenerated, very often using translocated forest reproductive material, among these species the Swiss stone pine (Pinus cembra L.). In the late nineteenth and early twentieth centuries, the species was largely used for artificial afforestation in the northern Alps. However, only a few planted trees have survived. Therefore, we aimed to evaluate if the historical afforestation of P. cembra in the northwestern Swiss Alps relied on allochthonous material. We sampled 12 sites, genotyping 11 nuclear microsatellites, to infer the spatial genetic structure of regional populations, to test for genetic differences between natural and planted stands, and to infer potential source regions of planted stands using reference samples covering the entire Alps. Population genetic structure analysis allowed us to distinguish planted from natural stands and to determine that forest reproductive material used for plantations was not of regional origin. We found similar levels of genetic diversity between natural and planted stands. Assignment tests revealed that reproductive material for planting was translocated to the study area from two source regions, i.e., near the border of Switzerland and Austria, and further to the East, between Austria and Italy. Our study shows how genetic tools may inform about historical transfer of forest reproductive material, which still may affect the population genetic make-up of regional occurrences, e.g., because of reduced natural regeneration.

Ectomycorrhizal (ECM) fungi serve key functions in forest ecosystems by supplying water and nutrients to tree hosts, yet mutualistic plant–fungi interactions are jeopardised by environmental alterations. Here, we discuss the great potential and current limitations of landscape genomics in investigating signatures of local adaptation in natural populations of ECM fungi.

Detecting the extrinsic selective pressures shaping genomic variation is critical for a better understanding of adaptation and for forecasting evolutionary responses of natural populations to changing environmental conditions. With increasing availability of geo-referenced environmental data, landscape genomics provides unprecedented insights into how genomic variation and underlying gene functions affect traits potentially under selection. Yet, the robustness of genotype–environment associations used in landscape genomics remains tempered due to various limitations, including the characteristics of environmental data used, sampling designs employed, and statistical frameworks applied. Here, we argue that using complementary or new environmental data sources and well-informed sampling designs may help improve the detection of selective pressures underlying patterns of local adaptation in various organisms and environments.

Aim: Knowing a species' response to historical climate shifts helps understanding its perspectives under global warming. We infer the hitherto unresolved postglacial history of Pinus cembra. Using independent evidence from genetic structure and demographic inference of extant populations, and from palaeoecological findings, we derive putative refugia and re-colonisation routes.
Location: European Alps and Carpathians.
Taxa: Pinus cembra.
Methods: We genotyped nuclear and chloroplast microsatellite markers in nearly 3000 individuals from 147 locations across the entire natural range of P. cembra. Spatial genetic structure (Bayesian modelling) and demographic history (approximate Bayesian computation) were combined with palaeobotanical records (pollen, macrofossils) to infer putative refugial areas during the Last Glacial Maximum (LGM) and re-colonisation of the current range.
Results: We found distinct spatial genetic structure, despite low genetic differentiation even between the two disjunct mountain ranges. Nuclear markers revealed five genetic clusters aligned East–West across the range, while chloroplast haplotype distribution suggested nine clusters. Spatially congruent separation at both marker types highlighted two main genetic lineages in the East and West of the range. Demographic inference supported early separation of these lineages dating back to a previous interstadial or interglacial c. 210,000 years ago. Differentiation into five biologically meaningful genetic clusters likely established during postglacial re-colonisation.
Main Conclusions: Combining genetic and palaeoecological evidence suggests that P. cembra primarily survived the LGM in 'cold period' refugia south of the Central European Alps and near the Carpathians, from where it expanded during the Late Glacial into its current Holocene 'warm period' refugia. This colonisation history has led to the distinct East–West structure of five genetic clusters. The two main genetic lineages likely derived from ancient divergence during an interglacial or interstadial. The respective contact zone (Brenner line) matches a main biogeographical break in the European Alps also found in herbaceous alpine plant species.

Delivering material selected for breeding purposes into the wild in the context of sustainable forest management might reduce the levels of genetic diversity of future forests in comparison to that of natural populations. Another consequence might be a reduction of their resilience under uncertain future climatic and socio-economic conditions if these new populations lack adaptability. Despite the long tradition of breeding activities in Europe, there is still a need to assess the impact of genetically enriched material on forests’ resilience. In this study, we address (1) the genetic diversity of selected material compared to its wild ancestors, and (2) how to enrich breeding material to support forests’ resilience under changing socio-environmental conditions. We analysed 16 study cases of selected material delivered from breeding activities in four European forest tree species (Pinus halepensis Mill., Pinus nigra J.F. Arnold, Pinus pinaster Ait. and Populus nigra L.) with different levels of breeding. To answer these two questions, we first assessed and compared the genetic diversity of selected material versus natural populations using both putatively neutral and adaptive (based on diverging selection) Single Nucleotide Polymorphisms (SNPs). We then suggest how to enrich these populations for resilience under future climatic conditions by defining a core collection for each species including material from populations that will likely disappear under future conditions. Thanks to the large SNP datasets available for our focal species, we were able to detect some trends in our data. Expected and observed heterozygosity values for selected populations were almost always identical. The selected material showed small but significant genetic differentiation from their original population and their inbreeding coefficient was generally lower. However, the level of genetic improvement (i.e. low vs high) was not correlated with the observed genetic differences between selected material and natural populations.The genetic characterization of natural populations distributed across the species range, and the future projection of their range stability, made it possible to identify core-collections that would significantly enrich breeding populations under uncertain future environmental conditions.

Knowing which drivers affect the spatial distribution of hybridizing species and their admixed individuals on local or regional scale can leverage our understanding about processes that shape taxonomic diversity. Hybridizing white oak species (Quercus sect. Quercus) represent an ideal study system to elucidate which environmental factors determine their relative abundance and admixture levels within admixed forest stands. To elaborate these relationships, we used 58 species-diagnostic single-nucleotide polymorphism (SNP) markers and high-resolution topographic and soil data to identify the environmental factors associated with taxonomic composition of individuals and populations in 15 mixed stands of Q. petraea and Q. pubescens in the Valais, an inner-Alpine valley in Switzerland. At the individual tree level, generalized linear models (GLMs) explained a relatively small part of variation (R² = 0.32). At the population level, GLMs often explained a large part of variation (R² = 0.54-0.69) of the taxonomic indices. Mean taxonomic composition of the sites depended mainly on altitude and geographic position. Moreover, the more within-site variation we found in predictors related to topographic position, the higher was the average genetic admixture of single trees. Our results show that a multitude of topographic and edaphic factors affect the taxonomic composition and admixture levels of these two hybridizing oak species on local scale and that regional heterogeneity of these factors promote taxonomic diversity and admixture. Overall, our study highlights the prospects of using tailored genetic resources and high-resolution environmental data to understand and predict taxonomic composition in response to changing environments.

Ferns are the second most diverse group of land plants after angiosperms. Extant species occupy a wide range of habitats and contribute significantly to ecosystem functioning. Despite the importance of ferns, most taxa are poorly covered by genomic resources and within-species studies based on high-resolution markers are entirely lacking. The genus Botrychium belongs to the family Ophioglossaceae, which includes species with very large genomes and chromosome numbers (e.g., Ophioglossum reticulatum 2n = 1520). The genus has a cosmopolitan distribution with 35 species, half of which are polyploids. Here, we establish a transcriptome for Botrychium lunaria (L.) Sw., a diploid species with an extremely large genome of about ~19.0–23.7 Gb. We assembled 25,677 high-quality transcripts with an average length of 1,333 bp based on deep RNA-sequencing of a single individual. We sequenced 11 additional transcriptomes of individuals from two populations in Switzerland, including the population of the reference individual. Based on read mapping to reference transcript sequences, we identified 374,463 single nucleotide polymorphisms (SNPs) segregating among individuals for an average density of 14 SNPs per kilobase. We found that all 12 transcriptomes were most likely from diploid individuals. The transcriptome-wide markers provided unprecedented resolution of the population genetic structure, revealing substantial variation in heterozygosity among individuals. We also constructed a phylogenomic tree of 92 taxa representing all fern orders to ascertain the placement of the genus Botrychium. High-quality transcriptomic resources and SNP sets constitute powerful population genomic resources to investigate the ecology, and evolution of fern populations.

Burgundy truffles are heterothallic ascomycetes that grow in symbiosis with trees. Despite their esteemed belowground fruitbodies, the species' complex lifecycle is still not fully understood. Here, we present the genetic patterns in three natural Burgundy truffle populations based on genotyped fruitbodies, ascospore extracts and ectomycorrhizal root tips using microsatellites and the mating-type locus. Distinct genetic structures with high relatedness in close vicinity were found for females (forming the fruitbodies) and males (fertilizing partner as inferred from ascospore extracts), with high genotypic diversity and annual turnover of males, suggesting that ephemeral male mating partners are germinating ascospores from decaying fruitbodies. The presence of hermaphrodites and the interannual persistence of a few males suggest that persistent mycelia may sporadically also act as males. Only female or hermaphroditic individuals were detected on root tips. At one site, fruitbodies grew in a fairy ring formed by a large female individual that showed an outward growth rate of 30 cm per year, with the mycelium decaying within the ring and being fertilized by over 50 male individuals. While fairy ring structures have never been shown for truffles, the genetics of Burgundy truffle populations support a similar reproductive biology as those of other highly prized truffles.

Global warming is pushing populations outside their range of physiological tolerance. According to the environmental envelope framework, the most vulnerable populations occur near the climatic edge of their species' distributions. In contrast, populations from the climatic center of the species range should be relatively buffered against climate warming. We tested this latter prediction using a combination of linear mixed effects and machine learning algorithms on an extensive, citizen-scientist generated dataset on the fruitbody productivity of the Burgundy (aka summer) truffle (Tuber aestivum Vittad.), a keystone, ectomycorrhizal tree-symbiont occurring on a wide range of temperate climates. T. aestivum's fruitbody productivity was monitored at 3-week resolution over up to 8 continuous years at 20 sites distributed in the climatic center of its European distribution in southwest Germany and Switzerland. We found that T. aestivum fruitbody production is more sensitive to summer drought than would be expected from the breadth of its species' climatic niche. The monitored populations occurring nearly 5°C colder than the edge of their species' climatic distribution. However, interannual fruitbody productivity (truffle mass year⁻¹) fell by a median loss of 22% for every 1°C increase in summer temperature over a site's 30-year mean. Among the most productive monitored populations, the temperature sensitivity was even higher, with single summer temperature anomalies of 3°C sufficient to stop fruitbody production altogether. Interannual truffle productivity was also related to the phenology of host trees, with ~22 g less truffle mass for each 1-day reduction in the length of the tree growing season. Increasing summer drought extremes are therefore likely to reduce fruiting among summer truffle populations throughout Central Europe. Our results suggest that European T. aestivum may be a mosaic of vulnerable populations, sensitive to climate-driven declines at lower thresholds than implied by its species distribution model.

Climate change is increasing the frequency and intensity of drought events in many boreal forests. Trees are sessile organisms with a long generation time, which makes them vulnerable to fast climate change and hinders fast adaptations. Therefore, it is important to know how forests cope with drought stress and to explore the genetic basis of these reactions. We investigated three natural populations of white spruce (Picea glauca) in Alaska, located at one drought-limited and two cold-limited treelines with a paired plot design of one forest and one treeline plot. We obtained individual increment cores from 458 trees and climate data to assess dendrophenotypes, in particular the growth reaction to drought stress. To explore the genetic basis of these dendrophenotypes, we genotyped the individual trees at 3000 single nucleotide polymorphisms in candidate genes and performed genotype–phenotype association analysis using linear mixed models and Bayesian sparse linear mixed models. Growth reaction to drought stress differed in contrasting treeline populations. Therefore, the populations are likely to be unevenly affected by climate change. We identified 40 genes associated with dendrophenotypic traits that differed among the treeline populations. Most genes were identified in the drought-limited site, indicating comparatively strong selection pressure of drought-tolerant phenotypes. Contrasting patterns of drought-associated genes among sampled sites and in comparison to Canadian populations in a previous study suggest that drought adaptation acts on a local scale. Our results highlight genes that are associated with wood traits which in turn are critical for the establishment and persistence of future forests under climate change.

Motivation: Trait variation within species can reveal plastic and/or genetic responses to environmental gradients, and may indicate where local adaptation has occurred. Here, we present a dataset of rangewide variation in leaf traits from seven of the most ecologically and economically important tree species in Europe. Sample collection and trait assessment are embedded in the GenTree project (EU‐Horizon 2020), which aims at characterizing the genetic and phenotypic variability of forest tree species to optimize the management and sustainable use of forest genetic resources. Our dataset captures substantial intra‐ and interspecific leaf phenotypic variability, and provides valuable information for studying the relationship between ecosystem functioning and trait variability of individuals, and the response and resilience of species to environmental changes.
Main types of variable contained: We chose morphological and chemical characters linked to trade‐offs between acquisition and conservation of resources and water use, namely specific leaf area, leaf size, carbon and nitrogen content and their ratio, and the isotopic signature of stable isotope ¹³C and ¹⁵N in leaves.
Spatial location and grain: We surveyed between 18 and 22 populations per species, 141 in total, across Europe.
Time period: Leaf sampling took place between 2016 and 2017.
Major taxa and level of measurement: We sampled at least 25 individuals in each population, 3,569 trees in total, and measured traits in 35,755 leaves from seven European tree species, i.e. the conifers Picea abies, Pinus pinaster and Pinus sylvestris, and the broadleaves Betula pendula, Fagus sylvatica, Populus nigra and Quercus petraea.
Software format: The data files are in ASCII text, tab delimited, not compressed.