Elevational divergence and clinal variation in floral color and leaf chemistry in Silene vulgaris

The effect of global change on the expression and evolution of floral traits

BACKGROUND

Pollinators impose strong selection on floral traits, but other abiotic and biotic agents also drive the evolution of floral traits and influence plant reproduction. Global change is expected to have widespread effects on biotic and abiotic systems, resulting in novel selection on floral traits in future conditions.

SCOPE

Global change has depressed pollinator abundance and altered abiotic conditions, thereby exposing flowering plant species to novel suites of selective pressures. Here, we consider how biotic and abiotic factors interact to shape the expression and evolution of floral characteristics (the targets of selection), including floral size, colour, physiology, reward quantity and quality, and longevity, amongst other traits. We examine cases in which selection imposed by climatic factors conflicts with pollinator-mediated selection. Additionally, we explore how floral traits respond to environmental changes through phenotypic plasticity and how that can alter plant fecundity. Throughout this review, we evaluate how global change might shift the expression and evolution of floral phenotypes.

CONCLUSIONS

Floral traits evolve in response to multiple interacting agents of selection. Different agents can sometimes exert conflicting selection. For example, pollinators often prefer large flowers, but drought stress can favour the evolution of smaller flowers, and the size of floral organs can evolve as a trade-off between selection mediated by these opposing actors. Nevertheless, few studies have manipulated abiotic and biotic agents of selection factorially to disentangle their relative strengths and directions of selection. The literature has more often evaluated plastic responses of floral traits to stressors than it has considered how abiotic factors alter selection on these traits. Global change will likely alter the selective landscape through changes in the abundance and community composition of mutualists and antagonists and novel abiotic conditions. We encourage future work to consider the effects of abiotic and biotic agents of selection on floral evolution, which will enable more robust predictions about floral evolution and plant reproduction as global change progresses.

The impact of genetic similarity and environment on the flavonoids variation pattern of Cyclocarya paliurus

The leaves of Cyclocarya paliurus (Batalin) Iljinskaja, an endemic tree with a scattered distribution in subtropical China, are rich in flavonoids with beneficial, health-promoting properties. To understand the impact of environment and genetic similarity on the variation pattern of flavonoids in this species, we analyzed C. paliurus germplasm resources from 26 different populations previously sampled from the main distribution area. Environmental, genetic and biochemical data was associated by genetic structure analysis, non-parametric tests, correlation analysis and principal component analysis. We found that populations with higher flavonoid contents were distributed at higher elevations and latitudes and fell into two groups with similar genetic diversities. Significant accumulations of isoquercitrin and kaempferol 3-O-glucoside were detected in the higher flavonoid-content resources. In addition, the genetic clusters with higher flavonoid contents exhibited broader environmental-adaptive capacities. Even in the presence of environmental factors promoting C. paliurus flavonoid accumulation, only those populations having a specific level of genetic similarity were able to exploit such environments.

Population structure and natural selection across a flower color polymorphism in the desert plant Encelia farinosa

PREMISE

Clines-or the geographic sorting of phenotypes across continual space-provide an opportunity to understand the interaction of dispersal, selection, and history in structuring polymorphisms.

METHODS

In this study, we combine field-sampling, genetics, climatic analyses, and machine learning to understand a flower color polymorphism in the wide-ranging desert annual Encelia farinosa.

RESULTS

We find evidence for replicated transitions in disk floret color from brown to yellow across spatial scales, with the most prominent cline stretching ~100 km from southwestern United States into México. Because population structure across the cline is minimal, selection is more likely than drift to have an important role in determining cline width.

CONCLUSIONS

Given that the cline aligns with a climatic transition but there is no evidence for pollinator preference for flower color, we hypothesize that floret color likely varies as a function of climatic conditions.

The roles of species' relatedness and climate of origin in determining optical leaf traits over a large set of taxa growing at high elevation and high latitude

Climate change is driving many mountain plant species to higher elevations and northern plant species to higher latitudes. However, various biotic or abiotic constraints may restrict any range shift, and one relevant factor for migration to higher elevations could be species' ability to tolerate high UV-doses. Flavonoids are engaged in photoprotection, but also serve multiple ecological roles. We compared plant optical leaf trait responses of a large set of taxa growing in two botanical gardens (French Alps and southern Finland), considering potential constraints imposed by the relatedness of taxa and the legacy of climatic conditions at plants' original collection sites. The segregation of optically measured leaf traits along the phylogeny was studied using a published mega-tree GBOTB.extended.tre for vascular plants as a backbone. For a subset of taxa, we investigated the relationship between climatic conditions (namely solar radiation, temperature and precipitation at a coarse scale) at the plants' original collection site and current trait values. Upon testing the phylogenetic signal (Pagel's λ), we found a significant difference but intermediate lambda values overall for flavonol or flavone index (I_flav) and anthocyanin index (I_ant), indicating that phylogenetic relatedness alone failed to explain the changes in trait values under a Brownian motion model of trait evolution. The local analysis (local indicator of phylogenetic association) indicated mostly positive autocorrelations for I_flav i.e. similarities in optically measured leaf traits, often among species from the same genus. We found significant relationships between climatic variables and leaf chlorophyll index (I_chl), but not I_flav, particularly for annual solar radiation. Changes in plants' I_flav across microhabitats differing in UV irradiance and predominately high F _v /F _m indicated that most plants studied had sufficient flexibility in photoprotection, conferred by I_flav, to acclimate to contemporary UV irradiances in their environment. While not explaining the mechanisms behind observed trait values, our findings do suggest that some high-elevation taxa display similar leaf flavonoid accumulation responses. These may be phylogenetically constrained and hence moderate plants' capacity to adjust to new combinations of environmental conditions resulting from climate change.

Pseudotargeted metabolomics revealed the adaptive mechanism of Draba oreades Schrenk at high altitude

Strong ultraviolet radiation and low temperature environment on Gangshika Mountain, located in the eastern part of the Qilian Mountains in Qinghai Province, can force plants to produce some special secondary metabolites for resisting severe environmental stress. However, the adaptive mechanism of Draba oreades Schrenk at high altitude are still unclear. In the current study, Draba oreades Schrenk from the Gangshika Mountain at altitudes of 3800 m, 4000 m and 4200 m were collected for comprehensive metabolic evaluation using pseudotargeted metabolomics method. Through KEGG pathway enrichment analysis, we found that phenylpropanoid biosynthesis, phenylalanine, tyrosine and tryptophan biosynthesis and phenylalanine metabolism related to the biosynthesis of flavonoids were up-regulated in the high-altitude group, which may enhance the environmental adaptability to strong ultraviolet intensity and low temperature stress in high altitude areas. By TopFc20 distribution diagram, the content of flavonoids gradually increased with the elevation of altitude, mainly including apigenin, luteolin, quercetin, hesperidin, kaempferol and their derivatives. Based on the random forest model, 10 important metabolites were identified as potential biomarkers. L-phenylalanine, L-histidine, naringenin-7-O-Rutinoside-4'-O-glucoside and apigenin related to the flavonoids biosynthesis and plant disease resistance were increased with the elevation of altitude. This study provided important insights for the adaptive mechanism of Draba oreades Schrenk at high altitude by pseudotargeted metabolomics.

Phenolic acids as chemotaxonomic markers able to differentiate the Euphrasia species

The objective of this study was to get the answer on the question, which phenolic compounds may serve as chemical markers in taxonomy of Euphrasia spp. (E. nemorosa, E. rostkoviana, E. stricta) collected from the wild. Moreover, it is still unknown how and which environmental parameters can impact on the quantity of polyphenols, flavonoids and phenolic acids. To the authors' knowledge, this is the first study that analyses such a broad spectrum of phenolic compounds in the genus Euphrasia and takes into account the habitat conditions of their synthesis. The species of Euphrasia differ significantly in the content of phenolic compounds. Euphrasia rostkoviana has the highest total content of polyphenols and flavonoids. Euphrasia stricta contains the largest amounts of phenolic acids, such as 5-O-caffeoylquinic, p-coumaric, protocatechuic and salicylic acid. It has been evidenced that the content of metabolites in the Euphrasia herb depends on a number of habitat parameters. The increased content of phenolic acids in E. stricta can be attributed to the fact that this species occupies drier habitats, with lower content of nitrogen and organic carbon compared to E. nemorosa and E. rostkoviana. The compounds that can be considered as chemotaxonomic markers are salicylic and protocatechuic acid for E. stricta, 5-O-caffeoylquinic acid for E. rostkoviana and ferulic acid for E. nemorosa. These findings provide theoretical and empirical basis for a chemotaxonomic classification of those species, which taken together with morphological characteristics should prevent misidentification.

Adaptive divergence generates distinct plastic responses in two closely related Senecio species

The evolution of plastic responses to external cues allows species to maintain fitness in response to the environmental variations they regularly experience. However, it remains unclear how plasticity evolves during adaptation. To test whether distinct patterns of plasticity are associated with adaptive divergence, we quantified plasticity for two closely related but ecologically divergent Sicilian daisy species (Senecio, Asteraceae). We sampled 40 representative genotypes of each species from their native range on Mt. Etna and then reciprocally transplanted multiple clones of each genotype into four field sites along an elevational gradient that included the native elevational range of each species, and two intermediate elevations. At each elevation, we quantified survival and measured leaf traits that included investment (specific leaf area), morphology, chlorophyll fluorescence, pigment content, and gene expression. Traits and differentially expressed genes that changed with elevation in one species often showed little changes in the other species, or changed in the opposite direction. As evidence of adaptive divergence, both species performed better at their native site and better than the species from the other habitat. Adaptive divergence is, therefore, associated with the evolution of distinct plastic responses to environmental variation, despite these two species sharing a recent common ancestor.

Transcriptomic and metabolomic analyses reveal the altitude adaptability and evolution of different-colored flowers in alpine Rhododendron species

Understanding the molecular mechanisms and evolutionary process of plant adaptation to the heterogeneous environment caused by altitude gradients in plateau mountain ecosystems can provide novel insight into species' responses to global changes. Flower color is the most conspicuous and highly diverse trait in nature. Herein, the gene expression patterns, evolutionary adaptation and metabolites changes of different-colored flowers of alpine Rhododendron L. species along altitude gradients were investigated based on a combined analysis of transcriptomics and metabolomics. Differentially expressed genes were found to be related to the biosynthesis of carbohydrates, fatty acids, amino acids and flavonoids, suggesting their important roles in the altitude adaptability of Rhododendron species. The evolution rate of high-altitude species was faster than that of low-altitude species. Genes related to DNA repair, mitogen-activated protein kinase and ABA signal transduction, and lipoic acid and propanoate metabolism were positively selected in the flowers of high-altitude Rhododendron species and those associated with carotenoid biosynthesis pathway, ABA signal transduction and ethylene signal transduction were positively selected in low-altitude species. These results indicated that the genes with differentiated expressions or functions exhibit varying evolution during the adaptive divergence of heterogeneous environment caused by altitude gradients. Flower-color variation might be attributed to the significant differences in gene expression or metabolites related to sucrose, flavonoids and carotenoids at the transcription or metabolism levels of Rhododendron species. This work suggests that Rhododendron species have multiple molecular mechanisms in their adaptation to changing environments caused by altitude gradients.

Altitudinal Variation of Metabolites, Mineral Elements and Antioxidant Activities of Rhodiola crenulata (Hook.f. & Thomson) H.Ohba

Rhodiolacrenulata (Hook.f. & Thomson) H.Ohba is an alpine medicinal plant that can survive in extreme high altitude environments. However, its changes to extreme high altitude are not yet clear. In this study, the response of Rhodiola crenulata to differences in altitude gradients was investigated through chemical, ICP-MS and metabolomic methods. A targeted study of Rhodiola crenulata growing at three vertical altitudes revealed that the contents of seven elements Ca, Sr, B, Mn, Ni, Cu, and Cd, the phenolic components, the ascorbic acid, the ascorbic acid/dehydroascorbate ratio, and the antioxidant capacity were positively correlated with altitude, while the opposite was true for total ascorbic acid content. Furthermore, 1165 metabolites were identified: flavonoids (200), gallic acids (30), phenylpropanoids (237), amino acids (100), free fatty acids and glycerides (56), nucleotides (60), as well as other metabolites (482). The differential metabolite and biomarker analyses suggested that, with an increasing altitude: (1) the shikimic acid-phenylalanine-phenylpropanoids-flavonoids pathway was enhanced, with phenylpropanoids upregulating biomarkers much more than flavonoids; phenylpropanes and phenylmethanes upregulated, and phenylethanes downregulated; the upregulation of quercetin was especially significant in flavonoids; upregulation of condensed tannins and downregulation of hydrolyzed tannins; upregulation of shikimic acids and amino acids including phenylalanine. (2) significant upregulation of free fatty acids and downregulation of glycerides; and (3) upregulation of adenosine phosphates. Our findings provide new insights on the responses of Rhodiola crenulata to extreme high altitude adversity.

What Maintains Flower Colour Variation within Populations?

Natural selection acts on phenotypic trait variation. Understanding the mechanisms that create and maintain trait variation is fundamental to understanding the breadth of diversity seen on Earth. Flower colour is among the most conspicuous and highly diverse traits in nature. Most flowering plant populations have uniform floral colours, but a minority exhibit within-population colour variation, either discrete (polymorphic) or continuous. Colour variation is commonly maintained by balancing selection through multiple pollinators, opposing selection regimes, or fluctuating selection. Variation can also be maintained by heterozygote advantage or frequency-dependent selection. Neutral processes, or a lack of selection, may maintain variation, although this remains largely untested. We suggest several prospective research directions that may provide insight into the evolutionary drivers of trait variation.

Geographic Variation in Floral Color and Reflectance Correlates With Temperature and Colonization History

Petal color variation within species is common and may be molded by abiotic or biotic selection pressures, or neutral population structure. For example, darker flowers may be favored in cooler environments because they absorb more solar radiation, elevating the temperature of reproductive structures. Additionally, flower color may evolve to attract the dominant or most efficient pollinator type in a given population. Here, we evaluate geographic variation in petal coloration across the range of Campanula americana in Eastern North America and test whether color covaries with abiotic factors, the pollination community, and genetic structure established through post-glacial expansion. Consistent with other studies, flowers from cooler, higher latitude populations were less reflective across the UV-NIR spectrum than those from warmer populations. Local temperature explained variation in petal reflectance better than the pollinator community or colonization history. Petal color perceived by trichromatic bee pollinators displayed a strong longitudinal pattern but was unassociated with climatic factors and the pollinator community. Instead, pollinator-perceived color was tightly correlated with the geographic distance from C. americana's glacial refugium. In total, abiotic conditions appear to shape large-scale geographic variation in the intensity of petal reflectance while genetic structure is the strongest driver of pollinator-perceived petal coloration. This study highlights the importance of abiotic factors and historical processes associated with range expansion as major evolutionary forces shaping diversity of flower coloration on large geographic scales.

Pollen colour morphs take different paths to fitness

Colour phenotypes are often involved in communication and are thus under selection by species interactions. However, selection may also act on colour through correlated traits or alternative functions of biochemical pigments. Such forms of selection are instrumental in maintaining petal colour diversity in plants. Pollen colour also varies markedly, but the maintenance of this variation is little understood. In Campanula americana, pollen ranges from white to dark purple, with darker morphs garnering more pollinator visits and exhibiting elevated pollen performance under heat stress. Here, we generate an F2 population segregating for pollen colour and measure correlations with floral traits, pollen attributes and plant-level traits related to fitness. We determine the pigment biochemistry of colour variants and evaluate maternal and paternal fitness of light and dark morphs by crossing within and between morphs. Pollen colour was largely uncorrelated with floral traits (petal colour, size, nectar traits) suggesting it can evolve independently. Darker pollen grains were larger and had higher anthocyanin content (cyanidin and peonidin) which may explain why they outperform light pollen under heat stress. Overall, pollen-related fitness metrics were greater for dark pollen, and dark pollen sires generated seeds with higher germination potential. Conversely, light pollen plants produce 61% more flowers than dark, and 18% more seeds per fruit, suggesting a seed production advantage. Results indicate that light and dark morphs may achieve fitness through different means-dark morphs appear to have a pollen advantage whereas light morphs have an ovule advantage-helping to explain the maintenance of pollen colour variation.

Variation in plastid genomes in the gynodioecious species Silene vulgaris

BACKGROUND

Gynodioecious species exist in two sexes - male-sterile females and hermaphrodites. Male sterility in higher plants often results from mitonuclear interaction between the CMS (cytoplasmic male sterility) gene(s) encoded by mitochondrial genome and by nuclear-encoded restorer genes. Mitochondrial and nuclear-encoded transcriptomes in females and hermaphrodites are intensively studied, but little is known about sex-specific gene expression in plastids. We have compared plastid transcriptomes between females and hermaphrodites in two haplotypes of a gynodioecious species Silene vulgaris with known CMS candidate genes.

RESULTS

We generated complete plastid genome sequences from five haplotypes S. vulgaris including the haplotypes KRA and KOV, for which complete mitochondrial genome sequences were already published. We constructed a phylogenetic tree based on plastid sequences of S. vulgaris. Whereas lowland S. vulgaris haplotypes including KRA and KOV clustered together, the accessions from high European mountains diverged early in the phylogram. S. vulgaris belongs among Silene species with slowly evolving plastid genomes, but we still detected 212 substitutions and 112 indels between two accessions of this species. We estimated elevated Ka/Ks in the ndhF gene, which may reflect the adaptation of S. vulgaris to high altitudes, or relaxed selection. We compared depth of coverage and editing rates between female and hermaphrodite plastid transcriptomes and found no significant differences between the two sexes. We identified 51 unique C to U editing sites in the plastid genomes of S. vulgaris, 38 of them in protein coding regions, 2 in introns, and 11 in intergenic regions. The editing site in the psbZ gene was edited only in one of two plastid genomes under study.

CONCLUSIONS

We revealed no significant differences between the sexes in plastid transcriptomes of two haplotypes of S. vulgaris. It suggests that gene expression of plastid genes is not affected by CMS in flower buds of S. vulgaris, although both sexes may still differ in plastid gene expression in specific tissues. We revealed the difference between the plastid transcriptomes of two S. vulgaris haplotypes in editing rate and in the coverage of several antisense transcripts. Our results document the variation in plastid genomes and transcriptomes in S. vulgaris.

Co-occurrence among three divergent plant-castrating fungi in the same Silene host species

The competitive exclusion principle postulates that different species can only coexist in sympatry if they occupy distinct ecological niches. The goal of this study was to understand the geographical distribution of three species of Microbotryum anther-smut fungi that are distantly related but infect the same host plants, the sister species Silene vulgaris and S. uniflora, in Western Europe. We used microsatellite markers to investigate pathogen distribution in relation to host specialization and ecological factors. Microbotryum violaceo-irregulare was only found on S. vulgaris at high elevations in the Alps. Microbotryum lagerheimii could be subdivided into two genetically differentiated clusters, one on S. uniflora in the UK and the second on S. vulgaris in the Alps and Pyrenees. The most abundant pathogen species, M. silenes-inflatae, could be subdivided into four genetic clusters, co-occurring in the Alps, the UK and the Pyrenees, and was found on both S. vulgaris and S. uniflora. All three fungal species had high levels of homozygosity, in agreement with the selfing mating system generally observed in anther-smut fungi. The three pathogen species and genetic clusters had large range overlaps, but occurred at sites with different elevations, temperatures and precipitation levels. The three Microbotryum species thus do not appear to be maintained by host specialization or geographic allopatry, but instead may occupy different ecological niches in terms of environmental conditions.

Geographic variation in pollen color is associated with temperature stress

The evolution of flower color, especially petal pigmentation, has received substantial attention. Less understood is the evolutionary ecology of pollen pigmentation, though it varies among and within species and its biochemical properties affect pollen viability. We characterize the distribution of pollen color across 24 populations of the North American herb Campanula americana, and assess the degree to which this variation is genetically based. We identify abiotic factors that covary with pollen color and test whether germination of light and dark pollen is differentially affected by variable temperature and UV. Pollen color varies from white to deep purple in C. americana and is genetically determined. There was a longitudinal cline whereby pollen was darkest in western populations. Accounting for latitudinal variation, western populations experience elevated temperature and UV irradiance. Germination of light-colored pollen was reduced by 60% under high temperature, but dark pollen was unaffected. Exposure to UV reduced germination of light and dark pollen similarly. The cline in pollen color across the range may reflect adaptation to heat stress. This study supports thermal tolerance as a novel function of pollen pigmentation and contributes to growing evidence that abiotic factors can drive floral diversity.