Macro- and micro-geographic genetic variation in early fitness traits in populations of maritime pine (Pinus pinaster Ait.).

BACKGROUND AND AIMS: Assessing adaptive genetic variation and its spatial distribution is crucial to conserve forest genetic resources and manage species' adaptive potential. Macro-environmental gradients commonly exert divergent selective pressures that enhance adaptive genetic divergence among populations. Steep micro-environmental variation might also result in adaptive divergence at finer spatial scales, even under high gene flow, but it is unclear how often this is the case. Here, we assess genetic variation in early fitness traits among distant and nearby maritime pine (Pinus pinaster Ait.) populations, to investigate climatic factors associated with trait divergence, and to examine trait integration during seedling establishment. METHODS: Open pollinated seeds were collected from seven population pairs across the European species distribution, with paired populations spatially close (between <1 km up to 21 km) but environmentally divergent. Seeds were sown in semi-natural conditions at three environmentally contrasting sites, where we monitored seedling emergence, growth and survival. KEY RESULTS: At large spatial scales, we found significant genetic divergence among populations in all studied traits, with certain traits exhibiting association with temperature and precipitation gradients. Significant trait divergence was also detected between pairs of nearby populations. Besides, we found consistent trait correlations across experimental sites, notably heavier seeds and earlier seedling emergence were both associated with higher seedling survival and fitness over two years in all experimental conditions. CONCLUSIONS: We identified mean annual temperature and precipitation seasonality as potential drivers of P. pinaster population divergence in the studied early-life traits. Populations genetically diverge also at local spatial scales, potentially suggesting that divergent natural selection can override gene flow along local-scale ecological gradients. These results suggest the species exhibits substantial adaptive potential that has allowed it to survive and evolve under contrasting environmental conditions.

Common microgeographical selection patterns revealed in four European conifers.

Microgeographical adaptation occurs when the effects of directional selection persist despite gene flow. Traits and genetic loci under selection can then show adaptive divergence, against the backdrop of little differentiation at other traits or loci. How common such events are and how strong the selection is that underlies them remain open questions. Here, we discovered and analysed microgeographical patterns of genomic divergence in four European and Mediterranean conifers with widely differing life-history traits and ecological requirements (Abies alba MIll., Cedrus atlantica [Endl.] Manetti, Pinus halepensis Mill. and Pinus pinaster Aiton) by screening pairs from geographically close forest stands sampled along steep ecological gradients. We inferred patterns of genomic divergence by applying a combination of divergence outlier detection methods, demographic modelling, Approximate Bayesian Computation inferences and genomic annotation to genomic data. Surprisingly for such small geographical scales, we showed that selection is strong in all species but generally affects different loci in each. A clear signature of selection was systematically detected on a fraction of the genome, of the order of 0.1%-1% of the loci depending on the species. The novel modelling method we designed for estimating selection coefficients showed that the microgeographical selection coefficient scaled by population size (Ns) was 2-30. Our results convincingly suggest that selection maintains within-population diversity at microgeographical scales in spatially heterogeneous environments. Such genetic diversity is likely to be a major reservoir of adaptive potential, helping populations to adapt under fluctuating environmental conditions.

Genetic signatures of divergent selection in European beech (Fagus sylvatica L.) are associated with the variation in temperature and precipitation across its distribution range.

High genetic variation and extensive gene flow may help forest trees with adapting to ongoing climate change, yet the genetic bases underlying their adaptive potential remain largely unknown. We investigated range-wide patterns of potentially adaptive genetic variation in 64 populations of European beech (Fagus sylvatica L.) using 270 SNPs from 139 candidate genes involved either in phenology or in stress responses. We inferred neutral genetic structure and processes (drift and gene flow) and performed differentiation outlier analyses and gene-environment association (GEA) analyses to detect signatures of divergent selection. Beech range-wide genetic structure was consistent with the species' previously identified postglacial expansion scenario and recolonization routes. Populations showed high diversity and low differentiation along the major expansion routes. A total of 52 loci were found to be putatively under selection and 15 of them turned up in multiple GEA analyses. Temperature and precipitation related variables were equally represented in significant genotype-climate associations. Signatures of divergent selection were detected in the same proportion for stress response and phenology-related genes. The range-wide adaptive genetic structure of beech appears highly integrated, suggesting a balanced contribution of phenology and stress-related genes to local adaptation, and of temperature and precipitation regimes to genetic clines. Our results imply a best-case scenario for the maintenance of high genetic diversity during range shifts in beech (and putatively other forest trees) with a combination of gene flow maintaining within-population neutral diversity and selection maintaining between-population adaptive differentiation.

Evolutionary rate and genetic load in an emblematic Mediterranean tree following an ancient and prolonged population collapse.

Severe bottlenecks significantly diminish the amount of genetic diversity and the speed at which it accumulates (i.e., evolutionary rate). They further compromise the efficiency of natural selection to eliminate deleterious variants, which may reach fixation in the surviving populations. Consequently, expanding and adapting to new environments may pose a significant challenge when strong bottlenecks result in genetic pauperization. Herein, we surveyed the patterns of nucleotide diversity, molecular adaptation and genetic load across 177 gene-loci in a circum-Mediterranean conifer (Pinus pinea L.) that represents one of the most extreme cases of genetic pauperization in widespread outbreeding taxa. We found very little genetic variation in both hypervariable nuclear microsatellites (SSRs) and gene-loci, which translated into genetic diversity estimates one order of magnitude lower than those previously reported for pines. Such values were consistent with a strong population decline that began some ~1 Ma. Comparisons with the related and parapatric maritime pine (Pinus pinaster Ait.) revealed reduced rates of adaptive evolution (α and ωa ) and a significant accumulation of genetic load. It is unlikely that these are the result from differences in mutation rate or linkage disequilibrium between the two species; instead they are the presumable outcome of contrasting demographic histories affecting both the speed at which these taxa accumulate genetic diversity, and the global efficacy of selection. Future studies, and programs for conservation and management, should thus start testing for the effects of genetic load on fitness, and integrating such effects into predictive models.

The eastern part of the Fertile Crescent concealed an unexpected route of olive (Olea europaea L.) differentiation.

Background and Aims: Olive is considered a native plant of the eastern side of the Mediterranean basin, from where it should have spread westward along the Mediterranean shores, while little is known about its diffusion in the eastern direction. Methods: Genetic diversity levels and population genetic structure of a wide set of olive ecotypes and varieties collected from several provinces of Iran, representing a high percentage of the entire olive resources present in the area, was screened with 49 chloroplast and ten nuclear simple sequence repeat markers, and coupled with archaeo-botanical and historical data on Mediterranean olive varieties. Approximate Bayesian Computation was applied to define the demographic history of olives including Iranian germplasm, and species distribution modelling was performed to understand the impact of the Late Quaternary on olive distribution. Key Results: The results of the present study demonstrated that: (1) the climatic conditions of the last glacial maximum had an important role on the actual olive distribution, (2) all Iranian olive samples had the same maternal inheritance as Mediterranean cultivars, and (3) the nuclear gene flow from the Mediterranean basin to the Iranian plateau was almost absent, as well as the contribution of subspecies cuspidata to the diversity of Iranian olives. Conclusions: Based on this evidence, a new scenario for the origin and distribution of this important fruit crop has been traced. The evaluation of olive trees growing in the eastern part of the Levant highlighted a new perspective on the spread and distribution of olive, suggesting two routes of olive differentiation, one westward, spreading along the Mediterranean basin, and another moving towards the east and reaching the Iranian plateau before its domestication.

Adapting through glacial cycles: insights from a long-lived tree (Taxus baccata).

Despite the large body of research devoted to understanding the role of Quaternary glacial cycles in the genetic divergence of European trees, the differential contribution of geographic isolation and/or environmental adaptation in creating population genetic divergence remains unexplored. In this study, we used a long-lived tree (Taxus baccata) as a model species to investigate the impact of Quaternary climatic changes on genetic diversity via neutral (isolation-by-distance) and selective (isolation-by-adaptation) processes. We applied approximate Bayesian computation to genetic data to infer its demographic history, and combined this information with past and present climatic data to assess the role of environment and geography in the observed patterns of genetic structure. We found evidence that yew colonized Europe from the East, and that European samples diverged into two groups (Western, Eastern) at the beginning of the Quaternary glaciations, c. 2.2 Myr before present. Apart from the expected effects of geographical isolation during glacials, we discovered a significant role of environmental adaptation during interglacials at the origin of genetic divergence between both groups. This process may be common in other organisms, providing new research lines to explore the effect of Quaternary climatic factors on present-day patterns of genetic diversity.

Halfway encounters: meeting points of colonization routes among the southern beeches Nothofagus pumilio and N. antarctica.

The Patagonian region is characterized by a complex biogeographic history, with evidence of deep phylogeographic breaks shared among species. Of particular interest to conservation is the nature of colonization and settlement patterns after the last glacial period, including the detection of secondary contact between different lineages and/or hybridization among related species around phylogeographic breaks. Here we studied population demography and past hybridization of two widespread tree species endemic to South America, Nothofagus pumilio and N. antarctica. Using 8 nuclear microsatellites we genotyped 41 populations of both species. Genetic variation and structure across the geographic region were evaluated within and among species and the past demographic history of hybridization between the two species was inferred using Approximate Bayesian Computation (ABC). Northern and southern lineages were identified in each species, and Bayesian clustering revealed their convergence at mid latitudes (42°S). Spatial genetic structure (SGS) also indicated the existence of a genetic discontinuity at these latitudes, which is in agreement with previous data from maternal DNA markers. Several populations around 42-44°S presented high levels of genetic diversity with a decrease toward southern populations. Even though the species are clearly differentiated (G'ST=0.335), admixed gene pools were observed in both species. Two independent runs of ABC suggested that inter species admixture-like patterns occurred within the timescale of the Last Glacial Maximum (around 20,000 BP). We also provide evidences of recent and bi-directional hybridization/introgression between the two Nothofagus species and describe features of the populations demography in the past. The settlement of a secondary contact zone in Nothofagus species around 42-44°S coincides with the phylogeographic breaks and hotspots of genetic diversity found in other plant and animal species in Patagonia, highlighting its importance as reservoir of diversity. The characterization of the population history of native species can contribute substantially to long-term conservation and management policies.

Resilience of genetic diversity in forest trees over the Quaternary.

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, Ne, 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 Ne 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.

Population structure of Cynara cardunculus complex and the origin of the conspecific crops artichoke and cardoon.

BACKGROUND AND AIMS: Globe artichoke and leafy cardoon, two crops within the same species Cynara cardunculus, are traditionally cultivated in the Mediterranean region and play a significant role in the agricultural economy of this area. The two cultigens have different reproductive systems: artichoke is generally vegetatively propagated, while leafy cardoon is seed propagated. The domestication events underlying the origin of both artichoke and cultivated cardoon from their wild relative and the area of occurrence are not yet fully understood. The aim of this study was to investigate population structure in wild cardoon, globe artichoke and leafy cardoon material and infer domestication events. METHODS: Thirty-five microsatellite (simple sequence repeat) markers, distributed in the C. cardunculus genome, and a large geographical and numerical sampling in southern Europe and North Africa were used to assess population structure and diversity. KEY RESULTS: The results suggest the presence of two distinct domestication events for artichoke and leafy cardoon, and also suggest a new possible scenario, with western wild cardoon having originated from cultivated cardoon escaped from cultivation. Evidence was found for a demographic bottleneck in the past history of globe artichoke. CONCLUSIONS: The results shed new light on the relationships between the three taxa of C. cardunculus and highlight relevant aspects on the evolution of domestication of two crops with a different reproductive system within the same species. It is proposed that the probable centre of origin of artichoke is located in southern Italy, probably Sicily.

Comparative mapping in the Fagaceae and beyond with EST-SSRs.

BACKGROUND: Genetic markers and linkage mapping are basic prerequisites for comparative genetic analyses, QTL detection and map-based cloning. A large number of mapping populations have been developed for oak, but few gene-based markers are available for constructing integrated genetic linkage maps and comparing gene order and QTL location across related species. RESULTS: We developed a set of 573 expressed sequence tag-derived simple sequence repeats (EST-SSRs) and located 397 markers (EST-SSRs and genomic SSRs) on the 12 oak chromosomes (2n = 2x = 24) on the basis of Mendelian segregation patterns in 5 full-sib mapping pedigrees of two species: Quercus robur (pedunculate oak) and Quercus petraea (sessile oak). Consensus maps for the two species were constructed and aligned. They showed a high degree of macrosynteny between these two sympatric European oaks. We assessed the transferability of EST-SSRs to other Fagaceae genera and a subset of these markers was mapped in Castanea sativa, the European chestnut. Reasonably high levels of macrosynteny were observed between oak and chestnut. We also obtained diversity statistics for a subset of EST-SSRs, to support further population genetic analyses with gene-based markers. Finally, based on the orthologous relationships between the oak, Arabidopsis, grape, poplar, Medicago, and soybean genomes and the paralogous relationships between the 12 oak chromosomes, we propose an evolutionary scenario of the 12 oak chromosomes from the eudicot ancestral karyotype. CONCLUSIONS: This study provides map locations for a large set of EST-SSRs in two oak species of recognized biological importance in natural ecosystems. This first step toward the construction of a gene-based linkage map will facilitate the assignment of future genome scaffolds to pseudo-chromosomes. This study also provides an indication of the potential utility of new gene-based markers for population genetics and comparative mapping within and beyond the Fagaceae.

Imprints of selection in peripheral and ecologically marginal central-eastern European Scots pine populations.

Knowledge of the molecular mechanisms underlying the stress response in plants is essential to understand evolutionary processes that result in long-term persistence of populations. Populations inhabiting marginal ecological conditions at the distribution range periphery may have preserved imprints of natural selection that have shaped functional genetic variation of the species. Our aim was to evaluate the extent of selection processes in the extremely fragmented, peripheral and isolated populations of Scots pine in central-eastern Europe. Autochthonous populations of the Carpathian Mts. and the Pannonian Basin were sampled and drought stress-related candidate genes were re-sequenced. Neutrality tests and outlier detection approaches were applied to infer the effect and direction of selection. Populations retained high genetic diversity by preserving a high number of alleles and haplotypes, many of them being population specific. Neutrality tests and outlier detection highlighted nucleotide positions that are under divergent selection and may be involved in local adaptation. The detected genetic pattern confirms that natural selection has played an important role in shaping modern-day genetic variation in marginal Scots pine populations, allowing for the long-term persistence of populations. Selection detected at functional regions possibly acts to maintain diversity and counteract the effect of genetic erosion.

WOODIV, a database of occurrences, functional traits, and phylogenetic data for all Euro-Mediterranean trees.

Trees play a key role in the structure and function of many ecosystems worldwide. In the Mediterranean Basin, forests cover approximately 22% of the total land area hosting a large number of endemics (46 species). Despite its particularities and vulnerability, the biodiversity of Mediterranean trees is not well known at the taxonomic, spatial, functional, and genetic levels required for conservation applications. The WOODIV database fills this gap by providing reliable occurrences, four functional traits (plant height, seed mass, wood density, and specific leaf area), and sequences from three DNA-regions (rbcL, matK, and trnH-psbA), together with modelled occurrences and a phylogeny for all 210 Euro-Mediterranean tree species. We compiled, homogenized, and verified occurrence data from sparse datasets and collated them on an INSPIRE-compliant 10 × 10 km grid. We also gathered functional trait and genetic data, filling existing gaps where possible. The WOODIV database can benefit macroecological studies in the fields of conservation, biogeography, and community ecology.

The GenTree Platform: growth traits and tree-level environmental data in 12 European forest tree species.

BACKGROUND: Progress in the field of evolutionary forest ecology has been hampered by the huge challenge of phenotyping trees across their ranges in their natural environments, and the limitation in high-resolution environmental information. FINDINGS: The GenTree Platform contains phenotypic and environmental data from 4,959 trees from 12 ecologically and economically important European forest tree species: Abies alba Mill. (silver fir), Betula pendula Roth. (silver birch), Fagus sylvatica L. (European beech), Picea abies (L.) H. Karst (Norway spruce), Pinus cembra L. (Swiss stone pine), Pinus halepensis Mill. (Aleppo pine), Pinus nigra Arnold (European black pine), Pinus pinaster Aiton (maritime pine), Pinus sylvestris L. (Scots pine), Populus nigra L. (European black poplar), Taxus baccata L. (English yew), and Quercus petraea (Matt.) Liebl. (sessile oak). Phenotypic (height, diameter at breast height, crown size, bark thickness, biomass, straightness, forking, branch angle, fructification), regeneration, environmental in situ measurements (soil depth, vegetation cover, competition indices), and environmental modeling data extracted by using bilinear interpolation accounting for surrounding conditions of each tree (precipitation, temperature, insolation, drought indices) were obtained from trees in 194 sites covering the species' geographic ranges and reflecting local environmental gradients. CONCLUSION: The GenTree Platform is a new resource for investigating ecological and evolutionary processes in forest trees. The coherent phenotyping and environmental characterization across 12 species in their European ranges allow for a wide range of analyses from forest ecologists, conservationists, and macro-ecologists. Also, the data here presented can be linked to the GenTree Dendroecological collection, the GenTree Leaf Trait collection, and the GenTree Genomic collection presented elsewhere, which together build the largest evolutionary forest ecology data collection available.

Author Correction: The GenTree Dendroecological Collection, tree-ring and wood density data from seven tree species across Europe.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The GenTree Dendroecological Collection, tree-ring and wood density data from seven tree species across Europe.

The dataset presented here was collected by the GenTree project (EU-Horizon 2020), which aims to improve the use of forest genetic resources across Europe by better understanding how trees adapt to their local environment. This dataset of individual tree-core characteristics including ring-width series and whole-core wood density was collected for seven ecologically and economically important European tree species: silver birch (Betula pendula), European beech (Fagus sylvatica), Norway spruce (Picea abies), European black poplar (Populus nigra), maritime pine (Pinus pinaster), Scots pine (Pinus sylvestris), and sessile oak (Quercus petraea). Tree-ring width measurements were obtained from 3600 trees in 142 populations and whole-core wood density was measured for 3098 trees in 125 populations. This dataset covers most of the geographical and climatic range occupied by the selected species. The potential use of it will be highly valuable for assessing ecological and evolutionary responses to environmental conditions as well as for model development and parameterization, to predict adaptability under climate change scenarios.

One species, many terpenes: matching chemical and biological diversity.

Volatile terpenes have been proposed as chemotaxonomic markers, despite the strong environmental control on their synthesis. To clarify whether chemical profiles match biological diversity, cork oak, a monoterpene-emitting species that has been bred by humans and frequently hybridizes with other oaks, is a useful case-study. Analysis of the available genetic information in cork oak provenances suggests that volatile terpenes might indeed suitably track geographical diversity even at the intraspecific level. Phylogeographical diversity does not reflect chemical diversity in other evergreen oaks that have not been intensively bred. Breeding for productive traits might therefore drive selection for terpene diversity, in turn modulating important adaptive mechanisms against biotic and abiotic stressors.

Molecular cloning, characterisation and expression of two catalase genes from peach.

Two cDNA clones encoding catalase (Cat1 and Cat2) from peach [Prunus persica (L.) Batsch] were identified, that show homologies to other plant catalases. The nucleotide sequences of the two coding regions showed 88% identity to each other. The amino acid sequences predicted from the two full-length clones showed the highest homology to a catalase from cotton and Nicotiana plumbaginifolia L. and included C-terminal tri-peptides typical of those used to target proteins to peroxisomes. Southern hybridisation analysis suggested the existence of two catalase genes in peach. The expression of Cat1 and Cat2 was determined in seeds, vegetative tissue, leaves during the seasonal cycle and in leaves in response to light / dark treatments. Cat1 had high levels of expression only in leaf tissue and was responsive to light and seasonal changes. Cat2 had high levels of expression in in vitro shoots and was also responsive to seasonal changes, but not to light. In situ hybridisations to leaf tissue indicated that the expression of Cat1 was localised mainly in palisade cells, while Cat2 mRNA was present in the vascular tissue. The results of the expression analysis and in situ hybridisation suggest a role for Cat1 in photorespiration and for Cat2 in stress responses.