Climate and habitat type interact to influence contemporary dispersal potential in Prairie Smoke (Geum triflorum).

Understanding dispersal potential, or the probability a species will move a given distance, under different environmental conditions is essential to predicting species' ability to move across the landscape and track shifting ecological niches. Two important drivers of dispersal ability are climatic differences and variations in local habitat type. Despite the likelihood these global drivers act simultaneously on plant populations, and thus dispersal potential is likely to change as a result, their combined effects on dispersal are rarely examined. To understand the effect of climate and varying habitat types on dispersal potential, we studied Geum triflorum-a perennial grassland species that spans a wide range of environments, including both prairie and alvar habitats. We explored how the climate of the growing season and habitat type (prairie vs. alvar) interact to alter dispersal potential. We found a consistent interactive effect of climate and habitat type on dispersal potential. Across prairie populations, an increased number of growing degree days favored traits that increase dispersal potential or the probability of dispersing farther distances. However, for alvar populations, dispersal potential tended to decrease as the number of growing degree days increased. Our findings suggest that under continued warming, populations in prairie habitats will benefit from increased gene flow, while alvar populations will become increasingly segregated, with reduced potential to track shifting fitness optima.

Leveraging whole-genome sequencing to estimate telomere length in plants.

Changes in telomere length are increasingly used to indicate species' response to environmental stress across diverse taxa. Despite this broad use, few studies have explored telomere length in plants. Thus, evaluation of new approaches for measuring telomeres in plants is needed. Rapid advances in sequencing approaches and bioinformatic tools now allow estimation of telomere content from whole-genome sequencing (WGS) data, a proxy for telomere length. While telomere content has been quantified extensively using quantitative polymerase chain reaction (qPCR) and WGS in humans, no study to date has compared the effectiveness of WGS in estimating telomere length in plants relative to qPCR approaches. In this study, we use 100 Populus clones re-sequenced using short-read Illumina sequencing to quantify telomere length comparing three different bioinformatic approaches (Computel, K-seek and TRIP) in addition to qPCR. Overall, telomere length estimates varied across different bioinformatic approaches, but were highly correlated across methods for individual genotypes. A positive correlation was observed between WGS estimates and qPCR, however, Computel estimates exhibited the greatest correlation. Computel incorporates genome coverage into telomere length calculations, suggesting that genome coverage is likely important to telomere length quantification when using WGS data. Overall, telomere estimates from WGS provided greater precision and accuracy of telomere length estimates relative to qPCR. The findings suggest WGS is a promising approach for assessing telomere length and, as the field of telomere ecology evolves, may provide added value to assaying response to biotic and abiotic environments for plants needed to accelerate plant breeding and conservation management.

Los cambios en la longitud de los telómeros se utilizan cada vez más para indicar la respuesta de las especies al estrés ambiental en diversos taxones. A pesar de este amplio uso, pocos estudios han explorado la longitud de los telómeros en las plantas. Por lo tanto, es necesario evaluar nuevos enfoques para medir los telómeros en las plantas. Los rápidos avances en los enfoques de secuenciación y las herramientas bioinformáticas ahora permiten estimar el contenido de los telómeros a partir de datos de secuenciación del genoma completo (WGS), un indicador de la longitud de los telómeros. Si bien el contenido de los telómeros se ha cuantificado ampliamente mediante la reacción en cadena de la polimerasa cuantitativa (qPCR) y WGS en humanos, ningún estudio hasta la fecha ha comparado la efectividad de WGS para estimar la longitud de los telómeros en plantas en relación con los enfoques de qPCR. En este estudio, utilizamos cien clones de álamos (Populus) resecuenciados mediante secuenciación Illumina de lectura corta para cuantificar la longitud de los telómeros comparando tres diferentes enfoques bioinformáticos, Computel, K-seek y TRIP, además de qPCR. En general, las estimaciones de la longitud de los telómeros variaron según los diferentes enfoques bioinformáticos, pero la longitud de los telómeros estuvo altamente correlacionada entre los métodos para genotipos individuales. Se observó una correlación positiva entre las estimaciones de WGS y qPCR; sin embargo, las estimaciones de Computel mostraron la mayor correlación. Computel incorpora la cobertura del genoma en los cálculos de la longitud de los telómeros, lo que sugiere que la cobertura del genoma probablemente es importante para la cuantificación de la longitud de los telómeros cuando se utilizan datos de WGS. En general, las estimaciones de los telómeros de WGS proporcionaron mayor precisión y exactitud de las estimaciones de la longitud de los telómeros en relación con la qPCR. Los hallazgos sugieren que WGS es un enfoque prometedor para evaluar la longitud de los telómeros y, a medida que evoluciona el campo de la ecología de los telómeros, puede proporcionar un valor agregado para analizar la respuesta a ambientes bióticos y abióticos de las plantas necesarias para acelerar los programas de mejoramiento genético y conservación.

Chasing the fitness optimum: temporal variation in the genetic and environmental expression of life-history traits for a perennial plant.

BACKGROUND AND AIMS: The ability of plants to track shifting fitness optima is crucial within the context of global change, where increasing environmental extremes may have dramatic consequences for life history, fitness, and ultimately population persistence. However, tracking changing conditions relies on the relationship between genetic and environmental variance, where selection may favour plasticity, the evolution of genetic differences, or both depending on the spatial and temporal scale of environmental heterogeneity. METHODS: Over three years, we compared the genetic and environmental components of phenological and life-history variation in a common environment for the spring perennial Geum triflorum. Populations were sourced from alvar habitats that exhibit extreme but predictable annual flood-desiccation cycles and prairie habitats that exhibit similar but less predictable variation in water availability. KEY RESULTS: Heritability was generally higher for early life-history (emergence probability) relative to later life-history traits (total seed mass), indicating that traits associated with establishment are under stronger genetic control relative to later life-history fitness expressions, where plasticity may play a larger role. This pattern was particularly notable in seeds sourced from environmentally extreme but predictable alvar habitats relative to less predictable prairie environments. Fitness landscapes based on seed source origin, largely characterized by varying water availability and flower production, described selection as the degree of maladaptation of seed source environment relative to the prairie common garden environment. Plants from alvar populations were consistently closer to the fitness optimum across all years. Annually, the breadth of the fitness optimum expanded primarily along a moisture gradient, with inclusion of more populations onto the expanding optimum. CONCLUSIONS: These results highlight the importance of temporally and spatially varying selection in life-history evolution, indicating plasticity may become a primary mechanism needed to track fitness for later life-history events within perennial systems.

Environmental contributions to the evolution of trait differences in Geum triflorum: Implications for restoration.

PREMISE: How the environment influences the distribution of trait variation across a species' range has important implications for seed transfer during restoration. Evolution across environments could influence fitness when individuals are transferred into new environments. Here, we evaluate the role the environment has had on the distribution of genetic variance for traits important to adaptation. METHODS: In a common garden experiment, we quantified trait differentiation for populations of Geum triflorum sourced from three distinct ecoregions and evaluated the ability of climate to predict trait variation. Populations were sourced from the Manitoba and Great Lake alvar ecoregions that experience predictable extremes in seasonal water availability and the prairie ecoregion which exhibits unpredictable changes in water availability. RESULTS: Plants sourced from alvar ecoregions exhibited smaller but more stomata and greater intrinsic water-use efficiency relative to prairie plant populations, supporting the evolution of ecotypic differences. Estimates of standing genetic variance and heritable genetic variation for quantitative traits suggest alvar populations have greater adaptive potential. However, low evolvability suggests all populations likely have limited capacity to evolve in response to environmental change. CONCLUSIONS: These results highlight the importance of the environment in influencing the evolution and distribution of genetic differences across populations used as seed sources for restoration. Additionally, these data may inform recommendations for seed transfer across novel environments and our expectations of populations' adaptive potential.

Reduced representation sequencing to understand the evolutionary history of Torrey pine (Pinus torreyana parry) with implications for rare species conservation.

Understanding the contribution of neutral and adaptive evolutionary processes to population differentiation is often necessary for better informed management and conservation of rare species. In this study, we focused on Pinus torreyana Parry (Torrey pine), one of the world's rarest pines, endemic to one island and one mainland population in California. Small population size, low genetic diversity, and susceptibility to abiotic and biotic stresses suggest Torrey pine may benefit from interpopulation genetic rescue to preserve the species' evolutionary potential. We leveraged reduced representation sequencing to tease apart the respective contributions of stochastic and deterministic evolutionary processes to population differentiation. We applied these data to model spatial and temporal demographic changes in effective population sizes and genetic connectivity, to identify loci possibly under selection, and evaluate genetic rescue as a potential conservation strategy. Overall, we observed exceedingly low standing variation within both Torrey pine populations, reflecting consistently low effective population sizes across time, and limited genetic differentiation, suggesting maintenance of gene flow between populations following divergence. However, genome scans identified more than 2000 candidate SNPs potentially under divergent selection. Combined with previous observations indicating population phenotypic differentiation, this indicates natural selection has probably contributed to the evolution of population genetic differences. Thus, while reduced genetic diversity, small effective population size, and genetic connectivity between populations suggest genetic rescue could mitigate the adverse effects of rarity, evidence for adaptive differentiation suggests genetic mixing could disrupt adaptation. Further work evaluating the fitness consequences of inter-population admixture is necessary to empirically evaluate the trade-offs associated with genetic rescue in Torrey pine.

One hundred years into the study of ecotypes, new advances are being made through large-scale field experiments in perennial plant systems.

A hundred years after Turesson first clearly described how locally adaptive variation is distributed within species, plant biologists are making major breakthroughs in our understanding of mechanisms underlying adaptation from local populations to the scale of continents. Although the genetics of local adaptation has typically been studied in smaller reciprocal transplant experiments, it is now being evaluated with whole genomes in large-scale networks of common garden experiments with perennial switchgrass and poplar trees. These studies support the hypothesis that a complex combination of loci, both with and without adaptive trade-offs, underlies local adaptation and that hybridization and adaptive introgression play a key role in the evolution of these species. Future studies incorporating high-throughput phenotyping, gene expression, and modeling will be used to predict responses of these species to climate change.

The relevance of pedigrees in the conservation genomics era.

Over the past 50 years conservation genetics has developed a substantive toolbox to inform species management. One of the most long-standing tools available to manage genetics-the pedigree-has been widely used to characterize diversity and maximize evolutionary potential in threatened populations. Now, with the ability to use high throughput sequencing to estimate relatedness, inbreeding, and genome-wide functional diversity, some have asked whether it is warranted for conservation biologists to continue collecting and collating pedigrees for species management. In this perspective, we argue that pedigrees remain a relevant tool, and when combined with genomic data, create an invaluable resource for conservation genomic management. Genomic data can address pedigree pitfalls (e.g., founder relatedness, missing data, uncertainty), and in return robust pedigrees allow for more nuanced research design, including well-informed sampling strategies and quantitative analyses (e.g., heritability, linkage) to better inform genomic inquiry. We further contend that building and maintaining pedigrees provides an opportunity to strengthen trusted relationships among conservation researchers, practitioners, Indigenous Peoples, and Local Communities.

Testing for evolutionary change in restoration: A genomic comparison between ex situ, native, and commercial seed sources of Helianthus maximiliani.

Globally imperiled ecosystems often depend upon collection, propagation, and storage of seed material for use in restoration. However, during the restoration process demographic changes, population bottlenecks, and selection can alter the genetic composition of seed material, with potential impacts for restoration success. The evolutionary outcomes associated with these processes have been demonstrated using theoretical and experimental frameworks, but no study to date has examined their impact on the seed material maintained for conservation and restoration. In this study, we compare genomic variation across seed sources used in conservation and restoration for the perennial prairie plant Helianthus maximiliani, a key component of restorations across North American grasslands. We compare individuals sourced from contemporary wild populations, ex situ conservation collections, commercially produced restoration material, and two populations selected for agronomic traits. Overall, we observed that ex situ and contemporary wild populations exhibited similar genomic composition, while four of five commercial populations and selected lines were differentiated from each other and other seed source populations. Genomic differences across seed sources could not be explained solely by isolation by distance nor directional selection. We did find evidence of sampling effects for ex situ collections, which exhibited significantly increased coancestry relative to commercial populations, suggesting increased relatedness. Interestingly, commercially sourced seed appeared to maintain an increased number of rare alleles relative to ex situ and wild contemporary seed sources. However, while commercial seed populations were not genetically depauperate, the genomic distance between wild and commercially produced seed suggests differentiation in the genomic composition could impact restoration success. Our results point toward the importance of genetic monitoring of seed sources used for conservation and restoration as they are expected to be influenced by the evolutionary processes that contribute to divergence during the restoration process.

Seed morphological traits as a tool to quantify variation maintained in ex situ collections: a case study in Pinus torreyana.

Understanding the within- and among-population distribution of trait variation within seed collections may provide a means to approximate standing genetic variation and inform plant conservation. This study aimed to estimate population- and family-level seed trait variability for existing seed collections of Torrey pine (Pinus torreyana), and to use these data to guide sampling of future collections. We quantified variation in 14 seed morphological traits and seedling emergence within and among Torrey pine populations. Using a simulation-based approach, we used estimates of within-population variance to assess the number of maternal families required to capture 95 % of trait variation within each existing seed collection. Substantial structure was observed both within and among Torrey pine populations, with island and mainland seeds varying in seed size and seed coat thickness. Despite morphological differences, seedling emergence was similar across populations. Simulations revealed that 83 % and 71 % of all maternal families within island and mainland seed collections respectively needed to be resampled to capture 95 % of seed trait variation within existing collections. From a conservation perspective, our results indicate that to optimize genetic diversity captured in Torrey pine seed collections, maximizing the number of maternal families sampled within each population will be necessary.

Using environmental and geographic data to optimize ex situ collections and preserve evolutionary potential.

Maintenance of biodiversity through seed banks and botanical gardens, where the wealth of species' genetic variation may be preserved ex situ, is a major goal of conservation. However, challenges can persist in optimizing ex situ collections if trade-offs exist among cost, effort, and conserving species evolutionary potential, particularly when genetic data are not available. We evaluated the genetic consequences of population preservation informed by geographic (isolation by distance [IBD]) and environmental (isolation by environment [IBE]) distance for ex situ collections for which population provenance is available. We used 19 genetic and genomic data sets from 15 plant species to assess the proportion of population genetic differentiation explained by geographic and environmental factors and to simulate ex situ collections prioritizing source populations based on pairwise geographic distance, environmental distance, or both. Specifically, we tested the impact prioritizing sampling based on these distances may have on the capture of neutral, functional, or putatively adaptive genetic diversity and differentiation. Individually, IBD and IBE explained limited population genetic differences across all 3 genetic marker classes (IBD, 10-16%; IBE, 1-5.5%). Together, they explained a substantial proportion of population genetic differences for functional (45%) and adaptive (71%) variation. Simulated ex situ collections revealed that inclusion of IBD, IBE, or both increased allelic diversity and genetic differentiation captured among populations, particularly for loci that may be important for adaptation. Thus, prioritizing population collections based on environmental and geographic distance data can optimize genetic variation captured ex situ. For the vast majority of plant species for which there is no genetic information, these data are invaluable to conservation because they can guide preservation of genetic variation needed to maintain evolutionary potential within collections.

Uso de Datos Ambientales y Geográficos para Optimizar las Colecciones Ex Situ y Preservar el Potencial Evolutivo Resumen El mantenimiento de la biodiversidad por medio de bancos de semillas y jardines botánicos, en donde la riqueza de la variación genética de las especies puede preservarse ex situ, es una de las principales metas de la conservación. Sin embargo, los obstáculos para la optimización de las colecciones ex situ pueden persistir si existen compensaciones entre el costo, el esfuerzo y la conservación del potencial evolutivo de la especie, particularmente cuando no están disponibles los datos genéticos. Evaluamos las consecuencias genéticas que tiene la preservación de una población fundamentada en la distancia geográfica (aislamiento por distancia [APD]) y ambiental (aislamiento por entorno [APE]) para las colecciones ex situ cuyo origen poblacional está disponible. Usamos 19 conjuntos de datos genéticos y genómicos de 15 especies de plantas para evaluar la proporción de la diferenciación genética de la población explicada por los factores geográficos y ambientales y para simular las colecciones ex situ que priorizan a las poblaciones de origen con base en la distancia geográfica por pares, la distancia ambiental, o ambas. Específicamente, comprobamos el impacto que puede tener la priorización de muestreos basados en estas distancias sobre la captura de la diferenciación y la diversidad genética neutral, funcional o putativamente adaptativa. De manera individual, el APD y el APE explicaron las diferencias limitadas en la genética poblacional en todas las clases de marcadores genéticos (APD, 10-16%; APE, 1-5.5%). En conjunto, ambos tipos de aislamiento explicaron una proporción sustancial de las diferencias en la genética poblacional para la variación funcional (45%) y adaptativa (71%). La simulación de colecciones ex situ reveló que la inclusión del APD, APE o ambos incrementó la diversidad de alelos y la diferenciación genética capturada entre las poblaciones, particularmente para los loci que pueden ser importantes para la adaptación. Por lo tanto, la priorización de las colecciones poblacionales basadas en los datos de distancia geográfica o ambiental pueden optimizar la variación genética capturada ex situ. Para la mayoría de las especies de plantas para las cuales no hay información genética, estos datos son indispensables para la conservación porque pueden dirigir la preservación de la variación genética necesaria para mantener el potencial evolutivo dentro de las colecciones.

The importance of quantitative trait differentiation in restoration: landscape heterogeneity and functional traits inform seed transfer guidelines.

For widely distributed species, understanding the scale over which genetic variation correlates to landscape structure and composition is critical. Particularly within the context of restoration, the evolution of genetic differences may impact success if seeds are maladapted to the restoration environment. In this study, we used Geum triflorum to quantify the scale over which genetic differences for quantitative traits important to adaptation have evolved, comparing the proportion of variance attributed to broad regional- and local population-level effects. Geum triflorum is a widely distributed species spanning a range of environments, including alvar and prairie habitats, which have extreme regional differences in soil-moisture availability. Alvar habitats are regions of thin soil over limestone that experience substantial seasonal variation in water availability, from flooding to desiccation annually. This contrasts with prairie habitats, whose deeper soils mitigate irregular flood-desiccation cycles. Using a common garden experiment, we evaluated 15 traits broadly grouped into three trait classes: resource allocation, stomatal characteristics, and leaf morphological traits for individuals sourced from prairie and alvar environments. We quantified the proportion of trait variance explained by regional- and population-scale effects and compared the proportion of regional- and population-trait variances explained across trait classes. Significant regional differentiation was observed for the majority of quantitative traits; however, population-scale effects were equal or greater than regional effects, suggesting that important genetic differences may have evolved across the finer population scale. Stomatal and resource allocation trait classes exhibited substantial regional differentiation relative to morphological traits, which may indicate increased strength of selection for stomatal and resource allocation traits relative to morphological traits. These patterns point towards the value in considering the scale over which genetic differences may have evolved for widely distributed species and identify different functional trait classes that may be valuable in establishing seed transfer guidelines.

From transects to transcripts: Teasing apart the architecture of reproductive isolation.

Understanding the processes underlying speciation has long been a challenge to evolutionary biologists. This spurs from difficulties teasing apart the various mechanisms that contribute to the evolution of barriers to reproduction. The study by Rafati et al. () in this issue of Molecular Ecology combines spatially explicit whole-genome resequencing with evaluation of differential gene expression across individuals with mixed ancestry to associate the genomic architecture of reproductive barriers with expression of reproductive incompatibilities. In a natural hybrid zone between rabbit subspecies, Oryctolagus cuniculus cuniculus and O. c. algirus (Figure ), Rafati et al. () use landscape-level patterns of allele frequency variation to identify potential candidate regions of the genome associated with reproductive isolation. These candidate regions are used to test predictions associated with the genomic architecture of reproductive barriers, including the role of structural rearrangements, enrichment of functional categories associated with incompatibilities, and the contribution of protein-coding versus regulatory changes. A lack of structural rearrangements and limited protein-coding changes in candidate regions point towards the importance of regulatory variation as major contributors to genetic incompatibilities, while functional enrichments indicate overrepresentation of genes associated with male infertility. To quantify phenotypic expression of proposed incompatibilities, the authors assess gene expression of experimental crosses. Extensive misregulation of gene expression within the testes of backcross hybrids relative to F1 and parental individuals provides an important link between genotype and phenotype, validating hypotheses developed from assessment of genomic architectures. Together, this work shows how pairing natural hybrid zones with experimental crosses can be used to link observations in nature to mechanistic underpinnings that may be tested experimentally.

Genetic conservation and management of the California endemic, Torrey pine (Pinus torreyana Parry): Implications of genetic rescue in a genetically depauperate species.

Rare species present a challenge under changing environmental conditions as the genetic consequences of rarity may limit species ability to adapt to environmental change. To evaluate the evolutionary potential of a rare species, we assessed variation in traits important to plant fitness using multigenerational common garden experiments. Torrey pine, Pinus torreyana Parry, is one of the rarest pines in the world, restricted to one mainland and one island population. Morphological differentiation between island and mainland populations suggests adaptation to local environments may have contributed to trait variation. The distribution of phenotypic variances within the common garden suggests distinct population-specific growth trajectories underlay genetic differences, with the island population exhibiting substantially reduced genetic variance for growth relative to the mainland population. Furthermore, F1 hybrids, representing a cross between mainland and island trees, exhibit increased height accumulation and fecundity relative to mainland and island parents. This may indicate genetic rescue via intraspecific hybridization could provide the necessary genetic variation to persist in environments modified as a result of climate change. Long-term common garden experiments, such as these, provide invaluable resources to assess the distribution of genetic variance that may inform conservation strategies to preserve evolutionary potential of rare species, including genetic rescue.

The joint influence of photoperiod and temperature during growth cessation and development of dormancy in white spruce (Picea glauca).

Timely responses to environmental cues enable the synchronization of phenological life-history transitions essential for the health and survival of north-temperate and boreal tree species. While photoperiodic cues will remain persistent under climate change, temperature cues may vary, contributing to possible asynchrony in signals influencing developmental and physiological transitions essential to forest health. Understanding the relative contribution of photoperiod and temperature as determinants of the transition from active growth to dormancy is important for informing adaptive forest management decisions that consider future climates. Using a combination of photoperiod (long = 20 h or short = 8 h day lengths) and temperature (warm = 22 °C/16 °C and cool = 8 °C/4 °C day/night, respectively) treatments, we used microscopy, physiology and modeling to comprehensively examine hallmark traits of the growth-dormancy transition-including bud formation, growth cessation, cold hardiness and gas exchange-within two provenances of white spruce [Picea glauca (Moench) Voss] spanning a broad latitude in Alberta, Canada. Following exposure to experimental treatments, seedlings were transferred to favorable conditions, and the depth of dormancy was assessed by determining the timing and ability of spruce seedlings to resume growth. Short photoperiods promoted bud development and growth cessation, whereas longer photoperiods extended the growing season through the induction of lammas growth. In contrast, cool temperatures under both photoperiodic conditions delayed bud development. Photoperiod strongly predicted the development of cold hardiness, whereas temperature predicted photosynthetic rates associated with active growth. White spruce was capable of attaining endodormancy, but its release was environmentally determined. Dormancy depth varied substantially across experimental treatments suggesting that environmental cues experienced within one season could affect growth in the following season, which is particularly important for a determinate species such as white spruce. The joint influence of these environmental cues points toward the importance of including local constant photoperiod and shifting temperature cues into predictive models that consider how climate change may affect northern forests.

Adaptive introgression as a resource for management and genetic conservation in a changing climate.

Current rates of climate change require organisms to respond through migration, phenotypic plasticity, or genetic changes via adaptation. We focused on questions regarding species' and populations' ability to respond to climate change through adaptation. Specifically, the role adaptive introgression, movement of genetic material from the genome of 1 species into the genome of another through repeated interbreeding, may play in increasing species' ability to respond to a changing climate. Such interspecific gene flow may mediate extinction risk or consequences of limited adaptive potential that result from standing genetic variation and mutation alone, enabling a quicker demographic recovery in response to changing environments. Despite the near dismissal of the potential benefits of hybridization by conservation practitioners, we examined a number of case studies across different taxa that suggest gene flow between sympatric or parapatric sister species or within species that exhibit strong ecotypic differentiation may represent an underutilized management option to conserve evolutionary potential in a changing environment. This will be particularly true where advanced-generation hybrids exhibit adaptive traits outside the parental phenotypic range, a phenomenon known as transgressive segregation. The ideas presented in this essay are meant to provoke discussion regarding how we maintain evolutionary potential, the conservation value of natural hybrid zones, and consideration of their important role in adaptation to climate.

The role of climate adaptation in colonization success in Arabidopsis thaliana.

Understanding the genetic mechanisms that contribute to range expansion and colonization success within novel environments is important for both invasion biology and predicting species-level responses to changing environments. If populations are adapted to local climates across a species' native range, then climate matching may predict which genotypes will successfully establish in novel environments. We examine evidence for climate adaptation and its role in colonization of novel environments in the model species, Arabidopsis thaliana. We review phenotypic and genomic evidence for climate adaptation within the native range and describe new analyses of fitness data from European accessions introduced to Rhode Island, USA, in spring and fall plantings. Accessions from climates similar to the Rhode Island site had higher fitness indicating a potential role for climate pre-adaptation in colonization success. A genomewide association study (GWAS), and genotypic mean correlations of fitness across plantings suggest the genetic basis of fitness in Rhode Island differs between spring and autumn cohorts, and from previous fitness measurements in European field sites. In general, these observations suggest a scenario of conditional neutrality for loci contributing to colonization success, although there was evidence of a fitness trade-off between fall plantings in Norwich, UK, and Rhode Island. GWAS suggested that antagonistic pleiotropy at a few specific loci may contribute to this trade-off, but this conclusion depended upon the accessions included in the analysis. Increased genomic information and phenotypic information make A. thaliana a model system to test for the genetic basis of colonization success in novel environments.

Genetic and morphological structure of a spruce hybrid (Picea sitchensis x P. glauca) zone along a climatic gradient.

PREMISE OF THE STUDY: Historic colonization and contemporary evolutionary processes contribute to patterns of genetic variation and differentiation among populations. However, separating the respective influences of these processes remains a challenge, particularly for natural hybrid zones, where standing genetic variation may result from evolutionary processes both preceding and following contact, influencing the evolutionary trajectory of hybrid populations. Where adaptation to novel environments may be facilitated by interspecific hybridization, teasing apart these processes will have practical implications for forest management in changing environments. METHODS: We evaluated the neutral genetic architecture of the Picea sitchensis (Sitka spruce) × P. glauca (white spruce) hybrid zone along the Nass and Skeena river valleys in northwestern British Columbia using chloroplast, mitochondrial, and nuclear microsatellite markers, in combination with cone morphological traits. KEY RESULTS: Sitka spruce mitotype "capture", evidenced by this species dominating the maternal lineage, is consistent with earlier colonization of the region by Sitka spruce. This "capture" differs from the spatial distribution of chloroplast haplotypes, indicating pollen dispersal and its contribution to geographic structure. Genetic ancestry, based on nuclear markers, was strongly influenced by climate and geography. Highly parallel results for replicate transects along environmental gradients provide support for the bounded hybrid superiority model of hybrid zone maintenance. • CONCLUSIONS: This broad-scale analysis of neutral genetic structure indicates the importance of historic and contemporary gene flow, environmental selection, and their interaction in shaping neutral genetic variation within this hybrid zone, informative to seed transfer development and reforestation for future climates.

Genomic and phenotypic architecture of a spruce hybrid zone (Picea sitchensis × P. glauca).

Interspecific hybridization may enhance the capacity of populations to adapt to changing environments, and has practical implications for reforestation. We use genome-wide estimates of admixture and phenotypic traits for trees in a common garden to examine the extent and direction of gene flow across a Picea hybrid zone, testing assumptions of the bounded hybrid superiority and tension zone models of hybrid zone maintenance. Seeds were collected from the ecological transition zone spanning from maritime to continental climates across the Picea sitchensis-P. glauca contact zone, and 721 trees were planted in a common garden experiment within the hybrid zone. Individuals were genotyped using a panel of 384 candidate-gene single nucleotide polymorphisms (SNPs) putatively associated with adaptive traits in Picea, and phenotyped at age ten for height and autumn cold hardiness. Low interspecific heterozygosity in hybrids indicated that intrinsic reproductive barriers were too weak to prevent widespread recombination, although introgression appeared asymmetric with P. sitchensis dominating the zone. Whereas marker-based hybrid index was strongly correlated with climate and geography, phenotypic traits exhibited weak or no significant clines. Our results indicated that exogenous selection appeared to play a strong role in the distribution and structure of this hybrid zone, indicative of an environmentally determined bounded hybrid superiority model of hybrid zone maintenance, although endogenous mechanisms could not be ruled out. This study provides insight into the mechanisms underlying adaptation across ecologically transitional hybrid zones that will ultimately provide an additional tool in managing these economically important tree species.

Differential introgression reveals candidate genes for selection across a spruce (Picea sitchensis × P. glauca) hybrid zone.

Differential patterns of introgression between species across ecological gradients provide a fine-scale depiction of extrinsic and intrinsic factors that contribute to the maintenance of species barriers and adaptation across heterogeneous environments. Introgression was examined for 721 individuals collected from the ecological transition zone spanning maritime to continental climates within the Picea sitchensis-Picea glauca contact zone using a panel of 268 candidate gene single nucleotide polymorphisms. Geographic clines showed a strong spatial relationship between allele frequencies and both distance from the ocean along major rivers and mean annual precipitation, indicating a strong role for environmental selection. Interspecific patterns of differentiation using outlier tests revealed three candidate genes that may be targets of long-term divergent selection between the parental species, although contemporary genomic clines within the hybrid zone suggested neutral patterns of introgression for these genes. This study provides a fine-scale analysis of locus-specific introgression, identifying a suite of candidate loci that may be targets of extrinsic or intrinsic selection, with broad application in understanding local adaptation to climate.