Evidence for environment-dependent introgression of adaptive genes between two red oak species with different drought adaptations

Conservation genomics provides insights into genetic resilience and adaptation of the endangered Chinese hazelnut, Corylus chinensis

Global climate change has increased concerns regarding biodiversity loss. However, many key conservation issues still required further research, including demographic history, deleterious mutation load, adaptive evolution, and putative introgression. Here we generated the first chromosome-level genome of the endangered Chinese hazelnut, Corylus chinensis, and compared the genomic signatures with its sympatric widespread C. kwechowensis-C. yunnanensis complex. We found large genome rearrangements across all Corylus species and identified species-specific expanded gene families that may be involved in adaptation. Population genomics revealed that both C. chinensis and the C. kwechowensis-C. yunnanensis complex had diverged into two genetic lineages, forming a consistent pattern of southwestern-northern differentiation. Population size of the narrow southwestern lineages of both species have decreased continuously since the late Miocene, whereas the widespread northern lineages have remained stable (C. chinensis) or have even recovered from population bottlenecks (C. kwechowensis-C. yunnanensis complex) during the Quaternary. Compared with C. kwechowensis-C. yunnanensis complex, C. chinensis showed significantly lower genomic diversity and higher inbreeding level. However, C. chinensis carried significantly fewer deleterious mutations than C. kwechowensis-C. yunnanensis complex, as more effective purging selection reduced the accumulation of homozygous variants. We also detected signals of positive selection and adaptive introgression in different lineages, which facilitated the accumulation of favorable variants and formation of local adaptation. Hence, both types of selection and exogenous introgression could have mitigated inbreeding and facilitated survival and persistence of C. chinensis. Overall, our study provides critical insights into lineage differentiation, local adaptation, and the potential for future recovery of endangered trees.

Genetic Structure of Natural Northern Range-Margin Mainland, Peninsular, and Island Populations of Northern Red Oak (Quercus rubra L.)

Plant populations at the leading edge of the species’ native range often exhibit genetic structure as a result of genetic drift and adaptation to harsh environmental conditions. Hence, they are likely to harbour rare genetic adaptations to local environmental conditions and therefore are of particular interest to understand climate adaptation. We examined genetic structure of nine northern marginal mainland, peninsular and isolated island natural populations of northern red oak (Quercus rubraL.), a valuable long-lived North American hardwood tree species, covering a wide climatic range, using 17 nuclear microsatellites. We found pronounced genetic differentiation of a disjunct isolated island population from all mainland and peninsular populations. Furthermore, we observed remarkably strong fine-scale spatial genetic structure (SGS) in all investigated populations. Such high SGS values are uncommon and were previously solely observed in extreme range-edge marginal oak populations in one other study. We found a significant correlation between major climate parameters and SGS formation in northern range-edge red oak populations, with more pronounced SGS in colder and drier regions. Most likely, the harsh environment in leading edge populations influences the density of reproducing trees within the populations and therefore leads to restricted overlapping of seed shadows when compared to more central populations. Accordingly, SGS was negatively correlated with effective population size and increased with latitude of the population locations. The significant positive association between genetic distances and precipitation differences between populations may be indicative of isolation by adaptation in the observed range-edge populations. However, this association was not confirmed by a multiple regression analysis including geographic distances and precipitation distances, simultaneously. Our study provides new insights in the genetic structure of long-lived tree species at their leading distribution edge.

An Updated Infrageneric Classification of the North American Oaks (Quercus Subgenus Quercus): Review of the Contribution of Phylogenomic Data to Biogeography and Species Diversity

The oak flora of North America north of Mexico is both phylogenetically diverse and species-rich, including 92 species placed in five sections of subgenus Quercus, the oak clade centered on the Americas. Despite phylogenetic and taxonomic progress on the genus over the past 45 years, classification of species at the subsectional level remains unchanged since the early treatments by WL Trelease, AA Camus, and CH Muller. In recent work, we used a RAD-seq based phylogeny including 250 species sampled from throughout the Americas and Eurasia to reconstruct the timing and biogeography of the North American oak radiation. This work demonstrates that the North American oak flora comprises mostly regional species radiations with limited phylogenetic affinities to Mexican clades, and two sister group connections to Eurasia. Using this framework, we describe the regional patterns of oak diversity within North America and formally classify 62 species into nine major North American subsections within sections Lobatae (the red oaks) and Quercus (the white oaks), the two largest sections of subgenus Quercus. We also distill emerging evolutionary and biogeographic patterns based on the impact of phylogenomic data on the systematics of multiple species complexes and instances of hybridization.

Hybridization and introgression in sympatric and allopatric populations of four oak species

BACKGROUND

Hybridization and introgression are vital sources of novel genetic variation driving diversification during reticulated evolution. Quercus is an important model clade, having extraordinary diverse and abundant members in the Northern hemisphere, that are used to studying the introgression of species boundaries and adaptive processes. China is the second-largest distribution center of Quercus, but there are limited studies on introgressive hybridization.

RESULTS

Here, we screened 17 co-dominant nuclear microsatellite markers to investigate the hybridization and introgression of four oaks (Quercus acutissima, Quercus variabilis, Quercus fabri, and Quercus serrata) in 10 populations. We identified 361 alleles in the four-oak species across 17 loci, and all loci were characterized by high genetic variability (HE = 0.844-0.944) and moderate differentiation (FST = 0.037-0.156) levels. A population differentiation analysis revealed the following: allopatric homologous (FST = 0.064) < sympatric heterogeneous (FST = 0.071) < allopatric heterogeneous (FST = 0.084). A Bayesian admixture analysis determined four types of hybrids (Q. acutissima × Q. variabilis, Q. fabri × Q. serrata, Q. acutissima × Q. fabri, and Q. acutissima × Q. variabilis × Q. fabri) and their asymmetric introgression. Our results revealed that interspecific hybridization is commonly observed within the section Quercus, with members having tendency to hybridize.

CONCLUSIONS

Our study determined the basic hybridization and introgression states among the studied four oak species and extended our understanding of the evolutionary role of hybridization. The results provide useful theoretical data for formulating conservation strategies.

Molecular Research on Stress Responses in Quercus spp.: From Classical Biochemistry to Systems Biology through Omics Analysis

The genus Quercus (oak), family Fagaceae, comprises around 500 species, being one of the most important and dominant woody angiosperms in the Northern Hemisphere. Nowadays, it is threatened by environmental cues, which are either of biotic or abiotic origin. This causes tree decline, dieback, and deforestation, which can worsen in a climate change scenario. In the 21st century, biotechnology should take a pivotal role in facing this problem and proposing sustainable management and conservation strategies for forests. As a non-domesticated, long-lived species, the only plausible approach for tree breeding is exploiting the natural diversity present in this species and the selection of elite, more resilient genotypes, based on molecular markers. In this direction, it is important to investigate the molecular mechanisms of the tolerance or resistance to stresses, and the identification of genes, gene products, and metabolites related to this phenotype. This research is being performed by using classical biochemistry or the most recent omics (genomics, epigenomics, transcriptomics, proteomics, and metabolomics) approaches, which should be integrated with other physiological and morphological techniques in the Systems Biology direction. This review is focused on the current state-of-the-art of such approaches for describing and integrating the latest knowledge on biotic and abiotic stress responses in Quercus spp., with special reference to Quercus ilex, the system on which the authors have been working for the last 15 years. While biotic stress factors mainly include fungi and insects such as Phytophthora cinnamomi, Cerambyx welensii, and Operophtera brumata, abiotic stress factors include salinity, drought, waterlogging, soil pollutants, cold, heat, carbon dioxide, ozone, and ultraviolet radiation. The review is structured following the Central Dogma of Molecular Biology and the omic cascade, from DNA (genomics, epigenomics, and DNA-based markers) to metabolites (metabolomics), through mRNA (transcriptomics) and proteins (proteomics). An integrated view of the different approaches, challenges, and future directions is critically discussed.

Environment-dependent introgression from Quercus dentata to a coastal ecotype of Quercus mongolica var. crispula in northern Japan

Introgression from one species in a specific environment to another may facilitate colonization of the environment by the recipient species. However, such environment-dependent introgression has been clarified in limited plant taxa. In northern Japan, there are two interfertile oak species: Quercus dentata (Qd) in coastal areas and Q. mongolica var. crispula (Qc) in inland areas. However, at higher latitudes where Qd is rare, a coastal Qc ecotype with Qd-like traits is distributed in the coastal areas. We distinguished inland Qc, coastal Qc, and coastal Qd populations based on genome-wide genotypes and multitrait phenotypes and verified introgression from coastal Qd to coastal Qc using reduced library sequencing. Genotypes and phenotypes differed among the populations, and coastal Qc was intermediate between inland Qc and coastal Qd. The ABBA-BABA test showed introgression from coastal Qd to coastal Qc. In coastal Qc, we found various stages of introgression after the first generation of backcross but detected no genomic regions where introgression was enhanced. Overall, we show evidence for introgression from a coastal species to an ecotype of an inland species, which has colonized the coastal environment. It remains unclear whether introgressed alleles are selected in the coastal environment.

Abiotic genetic adaptation in the Fagaceae

Fagaceae can be found in tropical and temperate regions and contain species of major ecological and economic importance. In times of global climate change, tree populations need to adapt to rapidly changing environmental conditions. The predicted warmer and drier conditions will potentially result in locally maladapted populations. There is evidence that major genera of the Fagaceae are already negatively affected by climate change-related factors such as drought and associated biotic stressors. Therefore, knowledge of the mechanisms underlying adaptation is of great interest. In this review, we summarise current literature related to genetic adaptation to abiotic environmental conditions. We begin with an overview of genetic diversity in Fagaceae species and then summarise current knowledge related to drought stress tolerance, bud burst timing and frost tolerance in the Fagaceae. Finally, we discuss the role of hybridisation, epigenetics and phenotypic plasticity in adaptation.

Ancient introgression drives adaptation to cooler and drier mountain habitats in a cypress species complex

Introgression may act as an important source of new genetic variation to facilitate the adaptation of organisms to new environments, yet how introgression might enable tree species to adapt to higher latitudes and elevations remains unclear. Applying whole-transcriptome sequencing and population genetic analyses, we present an example of ancient introgression from a cypress species (Cupressus gigantea) that occurs at higher latitude and elevation on the Qinghai-Tibet Plateau into a related species (C. duclouxiana), which has likely aided the latter species to extend its range by colonizing cooler and drier mountain habitats during postglacial periods. We show that 16 introgressed candidate adaptive loci could have played pivotal roles in response to diverse stresses experienced in a high-elevation environment. Our findings provide new insights into the evolutionary history of Qinghai-Tibet Plateau plants and the importance of introgression in the adaptation of species to climate change.

Influence of a climatic gradient on genetic exchange between two oak species

PREMISE

In plant groups with limited intrinsic barriers to gene flow, it is thought that environmental conditions can modulate interspecific genetic exchange. Oaks are known for limited barriers to gene flow among closely related species. Here, we use Quercus as a living laboratory in which to pursue a fundamental question in plant evolution: Do environmental gradients restrict or promote genetic exchange between species?

METHODS

We focused on two North American oaks, the rare Quercus dumosa and the widespread Q. berberidifolia. We sampled intensively along a contact zone in California, USA. We sequenced restriction site-associated DNA markers and measured vegetative phenotype. We tested for genetic exchange, the association with climate, and the effect on phenotype.

RESULTS

There is evidence for genetic exchange between the species. Admixed plants are found in areas of intermediate climate, while less admixed plants are found at the extremes of the climatic gradient. Genetic and phenotypic patterns are out of phase in the contact zone; some plants display the phenotype of one species but are genetically associated with another.

CONCLUSIONS

Our results support the hypothesis that a strong climatic gradient can promote genetic exchange between species. The overall weak correlation between genotype and phenotype in the contact zone between the species suggests that genetic exchange can lead to the breakdown of trait combinations used to define species. This incongruency predicts ongoing problems for conservation of Q. dumosa, with implications for conservation of other oaks.

An admixture of Quercus dentata in the coastal ecotype of Q. mongolica var. crispula in northern Hokkaido and genetic and environmental effects on their traits

In northern Japan, coastal oak forests consist of Quercus dentata (Qd) on the coastal side and Q. mongolica var. crispula (Qc) on the inland side. In the forests of northern Hokkaido, Qd is rare, and a coastal ecotype of Qc with some Qd-like traits grows on the coastal side. To reveal the genetic background of this ecotype, nuclear microsatellite genotypes in closely related oak taxa were obtained from the Eurasian continent, Sakhalin, and Hokkaido. The clustering of these genotypes suggests an admixture of Qd in the coastal ecotype of Qc. Next, we evaluated the effects of admixture and coastal stress on the leaf and shoot traits of Qc and Qd along coastal-inland gradients in northern Hokkaido. The admixture of Qd in Qc was quantified by the Qd ancestry proportions. Coastal stress causes bud mortality in the upper parts of shoots and was quantified by the survival patterns of buds in shoots. The genetic and environmental effects on the traits at Qd-abundant and Qd-rare sites were estimated using linear mixed models. The genetic effect was detected in all traits. Both genetic and environmental effects were detected in most traits. Some traits differed between Qd-abundant and Qd-rare sites in addition to these effects, indicating more Qd-like traits at Qd-rare sites. The findings suggest that an admixture of Qd characterizes the genetic background of the coastal ecotype of Qc and that not only the coastal stress but also the genetic background is responsible for the leaf and shoot traits of Qc and Qd in northern Hokkaido.

Diversification, adaptation, and community assembly of the American oaks (Quercus), a model clade for integrating ecology and evolution

Contents Summary 669 I. Model clades for the study and integration of ecology and evolution 670 II. Oaks: an important model clade 671 III. Insights from the history of the American oaks for understanding community assembly and ecosystem dominance 673 IV. Bridging the gap between micro- and macroevolutionary processes relevant to ecology 679 V. How do we reconcile evidence for adaptive evolution with niche conservatism and long-term stasis? 682 VI. High plasticity and within-population genetic variation contribute to population persistence 683 VII. Emerging technologies for tracking functional change 685 VIII. Conclusions 685 Acknowledgements 686 References 686 SUMMARY: Ecologists and evolutionary biologists are concerned with explaining the diversity and composition of the natural world and are aware of the inextricable linkages between ecological and evolutionary processes that maintain the Earth's life support systems. Yet examination of these linkages remains challenging due to the contrasting nature of focal systems and research approaches. Model clades provide a critical means to integrate ecology and evolution, as illustrated by the oaks (genus Quercus), an important model clade, given their ecological dominance, remarkable diversity, and growing phylogenetic, genomic, and ecological data resources. Studies of the clade reveal that their history of sympatric parallel adaptive radiation continues to influence community assembly today, highlighting questions on the nature and extent of coexistence mechanisms. Flexible phenology and hydraulic traits, despite evolutionary stasis, may have enabled adaptation to a wide range of environments within and across species, contributing to their high abundance and diversity. The oaks offer fundamental insights at the intersection of ecology and evolution on the role of diversification in community assembly processes, on the importance of flexibility in key functional traits in adapting to new environments, on factors contributing to persistence of long-lived organisms, and on evolutionary legacies that influence ecosystem function.