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Abstract: Divergence during the early stage of speciation can be driven by a population bottleneck via reduced gene flow and enhanced lineage sorting. In this study, we aimed to examine whether such bottlenecks occurred during the initial speciation of two closely related spruce species Picea asperata and P. crassifolia occurring on the Qinghai–Tibet Plateau (QTP). We analyzed sequences of three chloroplast, two mitochondrial DNA fragments and a further 13 nuclear loci from 216 individuals of the two species. Both species showed a low level of genetic diversity in contrast to other congeners occurring in the QTP and adjacent regions. The estimated population sizes of P. asperata and P. crassifolia are less than the ancestral population size before splitting. These results together with multiple statistical tests (Tajima's D, Fu and Li's D* and F*) suggest that these two species underwent recent bottlenecks. Based on approximate Bayesian computation (ABC), we also determined that the period of the population shrinkage was consistent with the interspecific divergence during the late Pleistocene. The reduced population sizes and the divergent selection may together have triggered the initial divergence under high gene flow between these two species. Our results therefore highlight the importance of climatic oscillations during the late Pleistocene in promoting speciation through changing demographic sizes of the ancestral species on the QTP and in adjacent regions. [ABSTRACT FROM AUTHOR]

• Many plant species comprising the present-day Arctic flora are thought to have originated in the high mountains of North America and Eurasia, migrated northwards as global temperatures fell during the late Tertiary period, and thereafter attained a circumarctic distribution. However, supporting evidence for this hypothesis that provides a temporal framework for the origin, spread and initial attainment of a circumarctic distribution by an arctic plant is currently lacking. • Here we examined the origin and initial formation of a circumarctic distribution of the arctic mountain sorrel (Oxyria digyna) by conducting a phylogeographic analysis of plastid and nuclear gene DNA variation. • We provide evidence for an origin of this species in the Qinghai-Tibet Plateau of southwestern China, followed by migration into Russia c. 11 million yr ago (Ma), eastwards into North America by c. 4 Ma, and westwards into Western Europe by c. 1.96 Ma. Thereafter, the species attained a circumarctic distribution by colonizing Greenland from both sides of the Atlantic Ocean. • Following the arrival of the species in North America and Europe, population sizes appear to have increased and then stabilized there over the last 1 million yr. However, in Greenland a marked reduction followed by an expansion in population size is indicated to have occurred during the Pleistocene. [ABSTRACT FROM AUTHOR]

The authors discuss the paper of Qi and colleagues which focuses on the response of relict genus Cercidiphyllum to climatic or geological changes using genetic information from chloroplast (cp) and nuclear DNAs. They mention that Qi and colleagues who used ecological niche modelling (ENM) estimated that the specie split from its sister lineage during late Cretaceous period based on cpDNA evidence. They state that the paper draws detailed pictured o the evolutionary history of Cercidiphyllum.

Trees are vital to the survival of numerous species and to forest ecosystem functioning. However, the current distribution, vulnerability to extinction, and conservation priorities of globally endangered trees are not well known. We mapped the global distribution of 1686 tree species listed as endangered on the International Union for the Conservation of Nature Red List and identified conservation priority for them based on species richness, life‐history traits, evolutionary distinctiveness, future climate change, and intensity of human activities. We also evaluated the impacts of various threats to these endangered tree species and evaluated the effectiveness of their protection based on the percentage of the species' range inside protected areas. The worldwide distribution of endangered trees, from the tropics through temperate zones, was uneven. Most endangered tree species were not protected in their native ranges, and only 153 species were fully protected. Hotspots of tree diversity occurred primarily in the tropics, and 79.06% of these were highly vulnerable to threats. We identified 253 areas of high priority for the conservation of endangered trees that are highly threatened and insufficiently protected. In particular, 43.42% of unprotected tree species in priority areas lacked recommended conservation measures or had no associated conservation plan. The priority conservation areas and unprotected trees we identified serve as a guideline for future management underpinning the post‐2020 global biodiversity framework. [ABSTRACT FROM AUTHOR]

Establishing how lineages with similar traits are phylogenetically related remains critical for understanding the origin of biodiversity on Earth. Floral traits in plants are widely used to explore phylogenetic relationships and to delineate taxonomic groups. The subtribe Swertiinae (Gentianaceae) comprises more than 350 species with high floral diversity ranging from rotate to tubular corollas and possessing diverse nectaries. Here we performed phylogenetic analysis of 60 species from all 15 genera of the subtribe Swertiinae sensu Ho and Liu, representing the range of floral diversity, using data from the nuclear and plastid genomes. Extensive topological conflicts were present between the nuclear and plastome trees. Three of the 15 genera represented by multiple species are polyphyletic in both trees. Key floral traits including corolla type, absence or presence of lobe scales, nectary type, nectary position, and stigma type are randomly distributed in the nuclear and plastome trees without phylogenetic correlation. We also revealed the likely ancient hybrid origin of one large clade comprising 10 genera with diverse floral traits. These results highlight the complex evolutionary history of this subtribe. The phylogenies constructed here provide a basic framework for further exploring the ecological and genetic mechanisms underlying both species diversification and floral diversity. [ABSTRACT FROM AUTHOR]

Angiosperms dominate the Earth's ecosystems and provide most of the basic necessities for human life. The major angiosperm clades comprise 64 orders, as recognized by the APG IV classification. However, the phylogenetic relationships of angiosperms remain unclear, as phylogenetic trees with different topologies have been reconstructed depending on the sequence datasets utilized, from targeted genes to transcriptomes. Here, we used currently available de novo genome data to reconstruct the phylogenies of 366 angiosperm species from 241 genera belonging to 97 families across 43 of the 64 orders based on orthologous genes from the nuclear, plastid, and mitochondrial genomes of the same species with compatible datasets. The phylogenetic relationships were largely consistent with previously constructed phylogenies based on sequence variations in each genome type. However, there were major inconsistencies in the phylogenetic relationships of the five Mesangiospermae lineages when different genomes were examined. We discuss ways to address these inconsistencies, which could ultimately lead to the reconstruction of a comprehensive angiosperm tree of life. The angiosperm phylogenies presented here provide a basic framework for further updates and comparisons. These phylogenies can also be used as guides to examine the evolutionary trajectories among the three genome types during lineage radiation. [ABSTRACT FROM AUTHOR]

Summary: Salt and drought impair plant osmotic homeostasis and greatly limit plant growth and development. Plants decrease stomatal aperture to reduce water loss and maintain osmotic homeostasis, leading to improved stress tolerance. Herein, we identified the C2H2 transcription factor gene OSMOTIC STRESS INDUCED C2H21 (OSIC1) from Populus alba var. pyramidalis to be induced by salt, drought, polyethylene glycol 6000 (PEG6000) and abscisic acid (ABA). Overexpression of OSIC1 conferred transgenic poplar more tolerance to high salinity, drought and PEG6000 treatment by reducing stomatal aperture, while its mutant generated by the CRISPR/Cas9 system showed the opposite phenotype. Furthermore, OSIC1 directly up‐regulates PalCuAOζ in vitro and in vivo, encoding a copper‐containing polyamine oxidase, to enhance H2O2 accumulation in guard cells and thus modulates stomatal closure when stresses occur. Additionally, ABA‐, drought‐ and salt‐induced PalMPK3 phosphorylates OSIC1 to increase its transcriptional activity to PalCuAOζ. This regulation of OSIC1 at the transcriptional and protein levels guarantees rapid stomatal closure when poplar responds to osmotic stress. Our results revealed a novel transcriptional regulatory mechanism of H2O2 production in guard cells mediated by the OSIC1‐PalCuAOζ module. These findings deepen our understanding of how perennial woody plants, like poplar, respond to osmotic stress caused by salt and drought and provide potential targets for breeding. [ABSTRACT FROM AUTHOR]

Kobresia pygmaea meadows are typical of Tibetan Plateau landscapes in the 3,000 to 5,500 m elevation range and constitute the most extensive alpine ecosystem in the world. Kobresia pygmaea forms turf mats that stabilize the surface and shelter the underlying soils from water erosion. Large tracts of the Plateau, however, exhibit signs of ongoing degradation of the turf and erosion of the underlying soil. Despite the crucial role played by K. pygmaea turf mats in the stabilization of the headwaters of major Asian rivers, the mechanisms responsible for their degradation remain poorly investigated. Here we develop a process-based model of land degradation of Tibetan Plateau slopes, which accounts for (i) turf cracking, (ii) water flow concentration in the cracks, (iii) crack widening by scouring, and (iv) sheet-flow erosion. As expected, soil erosion increases with the slope and drainage area (hence the observation of stronger erosion in relatively steep downhill sites). Model simulations indicate that with a sensible set of parameters representative of soil and hydrologic conditions in the region, Tibetan Plateau landscapes are vulnerable to turf mat degradation and soil erosion. As soon as polygonal cracks develop, water flow widens them until the landscape is completely barren. At this point sheet flow eventually erodes the mineral soil leaving behind a highly degraded landscape. [ABSTRACT FROM AUTHOR]

•The effects of Quaternary climatic oscillations on the demography of organisms vary across regions and continents. In taxa distributed in Europe and North America, several paradigms regarding the distribution of refugia have been identified. By contrast, less is known about the processes that shaped the species' spatial genetic structure in areas such as the Himalayas, which is considered a biodiversity hotspot. Here, we investigated the phylogeographic structure and population dynamics of Primula tibetica by combining genomic phylogeography and species distribution models (SDMs). • Genomic data were obtained for 293 samples of P. tibetica using restriction site-associated DNA sequencing (RADseq). Ensemble SDMs were carried out to predict potential present and past distribution ranges. • Four distinct lineages were identified. Approximate Bayesian computation analyses showed that each of them have experienced both expansions and bottlenecks since their divergence, which occurred during or across the Quaternary glacial cycles. The two lineages at both edges of the distribution were found to be more vulnerable and responded in different ways to past climatic changes. • These results illustrate how past climatic changes affected the demographic history of Himalayan organisms. Our findings highlight the significance of combining genomic approaches with environmental data when evaluating the effects of past climatic changes. [ABSTRACT FROM AUTHOR]

The coexistence of closely related species is key to understanding the nature of biodiversity hotspots where in situ diversification has yielded rich communities of close relatives. Limiting similarity predicts that co‐occurring species are differentiated in their niches; identifying the axes of differentiation in sympatric close relatives can thus help reveal the eco‐evolutionary dynamics of community assembly. For flowering plants, these axes may be temporal (related to reproductive phenology) or morphological (related to functional traits).We collected fine‐scale data on abundance, morphology and phenology over a flowering season for 34 species of Rhododendron (Ericaceae) spanning a 2700 m elevation gradient in a nature reserve in the eastern Hengduan Mountains, China. We used null models to test for patterns of clustering versus overdispersion in species' abundances, phylogenetic relatedness and functional traits across sites, and applied joint distribution models to examine the correlates of pairwise associations.We found that species tended to be spatially aggregated, indicating that communities are not strongly structured by competitive exclusion. At higher elevation sites, species tended to be vegetatively more similar (clustered) and closely related. Environmental variables, including climate and topography, were strong predictors of species' ranges. No evidence of niche differentiation was detected along spatial or morphological (functional) axes, but along the temporal axis, the phenology of co‐occurring species showed significant divergence, and was less phylogenetically conserved compared to morphological traits.Synthesis. Local communities of Rhododendron in its centre of diversity are structured by environmental filtering and the effects of limiting similarity. Evidence for the latter is apparent in the pervasive phenological divergence of co‐occurring species, likely driven by reproductive interference from shared pollinators. The evolutionary lability of flowering time appears to render it the quickest path to coexistence for recently diverged species that experience secondary contact in this biodiversity hotspot. [ABSTRACT FROM AUTHOR]

The increase in species number from poles to the equator is one of the most fundamental patterns in ecology. Although several hypotheses have been proposed, there is a lack of consensus on the mechanisms underlying this pattern. While most hypotheses provide plausible explanation for high tropical diversity of tropical clades, it is unclear if similar mechanisms drive the diversity of extra‐tropical clades. Here, we investigated the environmental drivers influencing the diversity pattern of a Mediterranean plant genus Medicago and dissect their effects across continents and biomes. We compiled a comprehensive dataset on the distribution of all Medicago species and mapped their distribution at the spatial resolution of 100 × 100 km2. We used generalized linear models to quantify the relative effects of environmental factors on the richness patterns of Medicago and its two life forms. Next, using geographically weighted regression, we explored the variation in richness–environment relationship across latitude. We found that Quaternary climate change and environmental energy were important explanatory variables, and their effects were consistent at global, continental and biome scales. However, we found contrasting effect of environmental energy in temperate and tropical regions, with species richness increasing with energy in the temperate and decreasing in the tropics. We also found strong affinity of Medicago species to its ancestral Mediterranean climatic niche. Our results suggest that the spatial variation in Medicago richness patterns may be strongly influenced by heat tolerance as opposed to the tropical clades that depend upon cold tolerance. This dependency on heat tolerance can be attributed to strong niche conservatism exhibited by Medicago species. Our findings provide new insights into the richness‐energy hypothesis and suggest that high environmental energy may not necessarily yield high species diversity but may also lower species diversity, especially of extra‐tropical clades towards the tropics. [ABSTRACT FROM AUTHOR]

Summary: The Qinghai–Tibet Plateau (QTP) sensu lato (sl), comprising the platform, the Himalaya and the Hengduan Mountains, is characterized by a large number of endemic plant species. This evolutionary cradle may have arisen from explosive species diversification because of geographic isolation. However, gene flow has been widely detected during the speciation processes of all groups examined, suggesting that natural selection may have also played an important role during species divergence in this region. In addition, natural hybrids have been recovered in almost all species‐rich genera. This suggests that numerous species in this region are still 'on the speciation pathway to complete reproductive isolation (RI)'. Such hybrids could directly develop into new species through hybrid polyploidization and homoploid hybrid speciation (HHS). HHS may take place more easily than previously thought through alternate inheritance of alleles of parents at multiple RI loci. Therefore, isolation, selection and hybridization could together have promoted species diversification of numerous plant genera on the QTP sl. We emphasize the need for identification and functional analysis of alleles of major genes for speciation, and especially encourage investigations of parallel adaptive divergence causing RI across different lineages within similar but specific habitats in this region. [ABSTRACT FROM AUTHOR]

Summary: Achnatherum splendens Trin. (Gramineae) is a constructive species of the arid grassland ecosystem in Northwest China and is a major forage grass. It has good tolerance of salt and drought stress in alkaline habitats. Here, we report its chromosome‐level genome, determined through a combination of Illumina HiSeq sequencing, PacBio sequencing and Hi‐C technology. The final assembly of the ~1.17 Gb genome sequence had a super‐scaffold N50 of 40.3 Mb. A total of 57 374 protein‐coding genes were annotated, of which 54 426 (94.5%) genes have functional protein annotations. Approximately 735 Mb (62.37%) of the assembly were identified as repetitive elements, and among these, LTRs (40.53%) constitute the highest proportion, having made a major contribution to the expansion of genome size in A. splendens. Phylogenetic analysis revealed that A. splendens diverged from the Brachypodium distachyon–Hordeum vulgare–Aegilops tauschii subclade around 37 million years ago (Ma) and that a clade comprising these four species diverged from the Phyllostachys edulis clade ~47 Ma. Genomic synteny indicates that A. splendens underwent an additional species‐specific whole‐genome duplication (WGD) 18–20 Ma, which further promoted an increase in copies of numerous saline–alkali‐related gene families in the A. splendens genome. By transcriptomic analysis, we further found that many of these duplicated genes from this extra WGD exhibited distinct functional divergence in response to salt stress. This WGD, therefore, contributed to the strong resistance to salt stress and widespread arid adaptation of A. splendens. [ABSTRACT FROM AUTHOR]

Summary: The abscisic acid (ABA) signalling pathway is involved in the plant response to osmotic stress caused by drought and/or salinity. Although the ABA signalling pathway has been elucidated in Arabidopsis, it remains elusive in woody poplars. In this study, genome‐wide analyses of U‐box genes in poplars revealed that a U‐box E3 ubiquitin ligase gene, PalPUB79, is significantly induced following drought, salinity and ABA signalling. PalPUB79 overexpression enhanced drought tolerance in transgenic poplars, while PalPUB79 RNAi lines were more sensitive to drought. PalPUB79 positively regulated ABA signalling pathway. Furthermore, PalPUB79 interacted with PalWRKY77, a negative transcriptional regulator of ABA signalling, and mediated its ubiquitination for degradation, therefore counteracting its inhibitory effect on PalRD26 transcription. However, the finding that PalWRKY77 negatively regulates PalPUB79 expression was indicative of a negative feedback loop between PalWRKY77 and PalPUB79 during ABA signalling in poplar. These findings provide novel insight into the mechanism through which PalPUB79 enhances the ABA‐mediated stress response in woody poplars. [ABSTRACT FROM AUTHOR]

Summary: It is critically important for plants to control the trade‐off between normal growth and pathogen immunity. However, the underlying molecular mechanism remains largely unknown. Here we report such a mechanism controlled by WRKY70 and its partner CHYR1 in Arabidopsis.We found that both levels of the WRKY70 target gene SARD1 and the phosphorylated forms of WRKY70 were increased in WRKY70OE plants upon Pst DC3000 infection. Mechanistically, phosphorylation of WRKY70 at Thr22 and Ser34 occurs, which then activates SARD1 expression through binding to a WT box.Phosphorylated WRKY70 is degraded by 26S proteasome via CHYR1 when resuming normal growth after infection. In addition, nonphosphorylated WRKY70 represses SARD1 expression by binding to both W (inhibitory activity site) and WT (active activity site) boxes.The binding of WRKY70 to alternative cis‐elements of SARD1 through a phosphorylation‐mediated switch controlled by CHYR1 contributes to modulating the balance between immunity and growth. [ABSTRACT FROM AUTHOR]

Summary: High salinity, one of the most widespread abiotic stresses, inhibits photosynthesis, reduces vegetation growth, blocks respiration and disrupts metabolism in plants. In order to survive their long‐term lifecycle, trees, such as Populus species, recruit the abscisic acid (ABA) signaling pathway to adapt to a saline environment. However, the molecular mechanism behind the ABA‐mediated salt stress response in woody plants remains elusive. We have isolated a WRKY transcription factor gene, PalWRKY77, from Populus alba var. pyramidalis (poplar), the expression of which is repressed by salt stress. PalWRKY77 decreases salt tolerance in poplar. Furthermore, PalWRKY77 negatively regulated ABA‐responsive genes and relieved ABA‐mediated growth inhibition, indicating that PalWRKY77 is a repressor of the ABA response. In vivo and in vitro assays revealed that PalWRKY77 targets the ABA‐ and salt‐induced PalNAC002 and PalRD26 genes by binding to the W‐boxes in their promoters. In addition, overexpression of both PalNAC002 and PalRD26 could elevate salt tolerance in transgenic poplars. These findings reveal a novel negative regulation mechanism for the ABA signaling pathway mediated by PalWRKY77 that results in more sensitivity to salt stress in poplar. This deepens our understanding of the complex responses of woody species to salt stress. Significance Statement: This work reveals a novel negative regulation mechanism for the abscisic acid (ABA) signaling pathway mediated by PalWRKY77 that results in more sensitivity to salt stress in poplar. This deepens our understanding of the complex responses of woody species to salt stress and of the relationship between ABA signaling and salt stress responses. [ABSTRACT FROM AUTHOR]

Summary: Tolerance of hypoxia is essential for most plants, but the underlying mechanisms are largely unknown. Here we show that adaptation to submergence induced hypoxia in Arabidopsis involves up‐regulation of RAP2.2 through interactive action of WRKY33 and WRKY12.WRKY33‐ or WRKY12‐overexpressing plants showed enhanced resistance to hypoxia. Y2H, BiFC, Co‐IP and pull‐down experiments confirmed the interaction of WRKY33 with WRKY12. Genetic experiments showed that RAP2.2 acts downstream of WRKY33/WRKY12.WRKY33 and WRKY12 can bind to and activate RAP2.2 individually. Genetic and molecular experiments demonstrate that the two WRKYs can synergistically enhance activation towards RAP2.2 to increase hypoxia tolerance. WRKY33 expression is increased in RAP2.2‐overexpressing plants, indicating a feedback regulation by RAP2.2 during submergence process, which was corroborated by EMSA, ChIP, dual‐LUC and genetic experiments.Our results show that a regulatory cascade module involving WRKY33, WRKY12 and RAP2.2 plays a key role in submergence induced hypoxia response of Arabidopsis and illuminate functions of WRKYs in hypoxia tolerance. See also the Editorial on this article by Sasidharan et al., 229: 5–7. [ABSTRACT FROM AUTHOR]

SUMMARY: Species are becoming extinct at unprecedented rates as a consequence of human activity. Hence it is important to understand the evolutionary dynamics of species with already small population sizes. Populus ilicifolia is a vulnerable poplar species that is isolated from other poplar species and is uniquely adapted to the Tropics. It has a very limited size, reproduces partly clonally and is therefore an excellent case study for conservation genomics. We present here the first annotated draft genome of P. ilicifolia, characterize genome‐wide patterns of polymorphisms and compare those to other poplar species with larger natural ranges. P. ilicifolia experienced a more prolonged and severe decline of effective population size (Ne) and signs of genetic erosion than any other poplar species with which it was compared. At present, the species has the lowest genome‐wide genetic diversity, the highest abundance of long runs of homozygosity, high inbreeding levels as well as a high overall accumulation of deleterious variants. However, more effective purging of severely deleterious variants and adaptation to the Tropics may have contributed to its survival. Hence, in spite of its limited genetic variation, it is certainly worth pursuing the conservation efforts of this unique species. Significance Statement: Whole‐genome sequencing of Populus ilicifolia, a vulnerable and isolated poplar tree from the Tropics that can reproduce both sexually and asexually, was used to explore how population history and reproduction affected its genetic characteristics, genetic load, and its potential for tropical adaptation. [ABSTRACT FROM AUTHOR]

Summary: Phylogenetic analysis is complicated by interspecific gene flow and the presence of shared ancestral polymorphisms, particularly those maintained by balancing selection. In this study, we aimed to examine the prevalence of these factors during the diversification of Populus, a model tree genus in the Northern Hemisphere.We constructed phylogenetic trees of 29 Populus taxa using 80 individuals based on re‐sequenced genomes. Our species tree analyses recovered four main clades in the genus based on consensus nuclear phylogenies, but in conflict with the plastome phylogeny. A few interspecific relationships remained unresolved within the multiple‐species clade because of inconsistent gene trees. Our results indicated that gene flow has been widespread within each clade and also occurred among the four clades during their early divergence.We identified 45 candidate genes with ancient polymorphisms maintained by balancing selection. These genes were mainly associated with mating compatibility, growth and stress resistance.Both gene flow and selection‐mediated ancient polymorphisms are prevalent in the genus Populus. These are potentially important contributors to adaptive variation. Our results provide a framework for the diversification of model tree genus that will facilitate future comparative studies. [ABSTRACT FROM AUTHOR]

Summary: Nonbifurcating divergence caused by introgressive hybridization is continuously reported for groups of closely related species. In this study, we aimed to reconstruct the genome‐scale classification of deep lineages of the conifer genus Picea, establish their phylogenetic relationships and test the bifurcating hypothesis between deeply branching lineages based on genomic data.We sequenced the transcriptomes of 35 individuals of 27 taxa covering all main lineages of the genus. Four major lineages, comprising three to 12 taxa each, largely consistent with morphological evidence, were recovered across the coalescent and integrated nuclear phylogeny. However, many of the individual gene trees recovered contradict one another. Moreover, the well‐supported coalescent tree inferred here differs from previous studies based on various DNA markers, with respect to topology and inter‐lineage relationships.We identified the shared polymorphisms between four major lineages. ABBA–BABA tests confirmed the inter‐lineage gene flow and thus violated the bifurcating divergence model. Gene flow occurred more frequently between lineages distributed in the same continent than those disjunct between continents.Our results indicate that introgression and nonbifurcating diversification apply, even between deeply branching lineages of the conifer genus Picea. [ABSTRACT FROM AUTHOR]

Species delimitation through which infers species boundaries is emerging as a major work in modern systematics. Hop-hornbeam species in Ostrya (Betulaceae) are well known for their hard and heavy woods. Five species were described in China and their interspecific delimitations remain unclear. In this study, we firstly explored their distributions in all recorded field sites distributed in China. We then selected 110 samples from 22 natural populations of five species from this genus and one type specimen of O. yunnanensis, for molecular barcoding analyses. We sequenced four chloroplast (cp) DNA fragments ( trnH-psbA, trnL-trnF, rps16, and trnG) and the nuclear internal transcribed spacer ( ITS) region for all samples. Sequence variations of Ostrya from four cp DNA fragments identified three groups that showed no correspondence to any morphological delimitation because of the incomplete lineage sorting and/or possible interspecific introgression in the history. However, phylogenetic analyses of ITS sequence variations discerned four species, O. japonica, O. rehderiana, O. trichocarpa, and O. multinervis while O. yunnanensis nested within O. multinervis. Morphological clustering also discerned four species and showed the complete consistency with molecular evidence. Moreover, our phylogenetic analyses-based ITS sequence variations suggested that O. trichocarpa comprised an isolated lineage different from the other Eurasian ones. Based on these results, hop-hornbeams in China should be treated as four separate species. Our results further highlight the importance of ITS sequence variations in delimitating and discerning the closely related species in plants. [ABSTRACT FROM AUTHOR]

Because of introgressive hybridization, closely related species can be more similar to each other in areas of range overlap (parapatry or sympatry) than in areas where they are geographically isolated from each other (allopatry). Here, we report the reverse situation based on nuclear genetic divergence between two fir species, Abies chensiensis and Abies fargesii, in China, at sites where they are parapatric relative to where they are allopatric. We examined genetic divergence across 126 amplified fragment length polymorphism ( AFLP) markers in a set of 172 individuals sampled from both allopatric and parapatric populations of the two species. Our analyses demonstrated that AFLP divergence was much greater between the species when comparisons were made between parapatric populations than between allopatric populations. We suggest that selection in parapatry may have largely contributed to this increased divergence. [ABSTRACT FROM AUTHOR]

• Because of heterogeneous topographies, high-mountain areas could harbor a significant pool of cryptic forest refugia (glacial microrefugia unrecognized by palaeodata), which, as a result of poor accessibility, have been largely overlooked. The juniper forests of the southern Tibetan Plateau, with one of the highest tree lines worldwide, are ideal for assessing the potential of high-mountain areas to harbor glacial refugia. • Genetic evidence for Last Glacial Maximum (LGM) endurance of these microrefugia is presented using paternally inherited chloroplast markers. Five-hundred and ninety individuals from 102 populations of the Juniperus tibetica complex were sequenced at three polymorphic chloroplast regions. • Significant interpopulation differentiation and phylogeographic structure were detected ( GST = 0.49, NST = 0.72, NST > GST, P < 0.01), indicating limited among-population gene flow. Of 62 haplotypes recovered, 40 were restricted to single populations. These private haplotypes and overall degrees of diversity were evenly spread among plateau and edge populations, strongly supporting the existence of LGM microrefugia throughout the present distribution range, partly well above 3500 m. • These results mark the highest LGM tree lines known, illustrating the potential significance of high-mountain areas for glacial refugia. Furthermore, as the close vicinity of orographic rear-edge and leading-edge populations potentially allows gene flow, surviving populations could preserve the complete spectrum of rear-edge and leading-edge adaptations. [ABSTRACT FROM AUTHOR]

• The homoploid hybrid species Pinus densata is restricted to alpine habitats that exceed the altitude range of its two parental species, Pinus tabulaeformis and Pinus yunnanensis. Alpine habitats usually generate cold-induced water stress in plants. To understand the ecological differentiation between these three species, we examined their physiological responses to drought stress. • Potted seedlings of three species were subjected to low, mild, moderate and severe water stress in an automatic-controlled glasshouse. Fifteen indicators of fitness were measured for each species in each treatment, and most of these decreased as drought increased. • Pinus densata exhibited higher fitness than both parental species in terms of total dry mass production (TDM) and long-term water use efficiency (WUEL) across all treatments; several other ecophysiological traits were also extreme but not across every treatment, and not always in the highest stress treatment. • These results indicate that extreme characters that have become well fixed in P. densata, confer a faster seedling growth rate and more efficient water use, which in turn should confer increased drought tolerance. These traits of P. densata likely promoted its ecological separation from its parental species and facilitated its successful colonization and establishment in high-altitude habitats. [ABSTRACT FROM AUTHOR]