Local adaptation in European populations of Arabidopsis lyrata (Brassicaceae)

We studied local adaptation to contrasting environments using an organism that is emerging as a model for evolutionary plant biology-the outcrossing, perennial herb Arabidopsis lyrata subsp. petraea (Brassicaceae). With reciprocal transplant experiments, we found variation in cumulative fitness, indicating adaptive differentiation among populations. Nonlocal populations did not have significantly higher fitness than the local population. Experimental sites were located in Norway (alpine), Sweden (coastal), and Germany (continental). At all sites after one year, the local population had higher cumulative fitness, as quantified by survival combined with rosette area, than at least one of the nonlocal populations. At the Norwegian site, measurements were done for two additional years, and fitness differences persisted. The fitness components that contributed most to differences in cumulative fitness varied among sites. Relatively small rosette area combined with a large number of inflorescences produced by German plants may reflect differentiation in life history. The results of the current study demonstrate adaptive population differentiation in A. lyrata along a climatic gradient in Europe. The studied populations harbor considerable variation in several characters contributing to adaptive population differentiation. The wealth of genetic information available makes A. lyrata a highly attractive system also for examining the functional and genetic basis of local adaptation in plants.     

Quantifying latitudinal clines to light responses in natural populations of Arabidopsis thaliana (Brassicaceae)

Evidence of adaptation in Arabidopsis thaliana (Brassicaceae) phenotypic traits has rarely been shown. We demonstrate latitudinal clines in two A. thaliana traits: hypocotyl responses to red and far-red light. Natural populations of A. thaliana were sampled along a latitudinal gradient from southern to northern Norway. Seeds from maternal families within each population were subjected to 1 wk of constant red, far-red, blue, white, and dark treatments. Hypocotyl lengths were measured for each maternal family within each population. Significant variability within and among populations in hypocotyl responses for the various treatments was found. There was a significant latitudinal cline in hypocotyl responses for red and far-red treatments, with northern populations being more de-etiolated than southern populations. These results suggest that northern populations are more responsive to red and far-red light than southern populations. Thus, differentiation of seedling traits in natural populations of A. thaliana seems in part to be mediated by the phytochrome pathway. There was no correlation between hypocotyl responses and flowering time for any treatment. This suggests that flowering time variability and variability in hypocotyl responses may not be governed by genes shared between the pathways, such as those involved in photoreception or the circadian clock.     

The colorless flavonoids of Arabidopsis thaliana (Brassicaceae). I. A model system to study the orthodihydroxy structure

The orthodihydroxy structure may mediate flavonoid coloration or antioxidant activity, making it of interest to floral or health food industries, respectively. In the first of two companion papers, the authors demonstrate a model system to study the orthodihydroxy structure in colorless flavonoids of Arabidopsis, with tools similar to those used upon the colored flavonoids. The system includes a fluorescent staining procedure to visualize colorless orthodihydroxy flavonoids at a subcellular level and to screen seedlings for rare genetic recombinants. The system also includes mutant lines to study the synthesis of colorless orthodihydroxy flavonoids, their potential role as antioxidants, and their potential risk as oxidative mutagens.     

Zinc tolerance and accumulation in metallicolous and nonmetallicolous populations of Arabidopsis halleri (Brassicaceae)

Zinc tolerance was investigated in five populations of Arabidopsis halleri (syn.: Cardaminopsis halleri ) raised from seeds collected from contaminated and uncontaminated sites. Tolerance was measured by determining the concentration which inhibited root growth (EC₁₀₀). A. halleri populations from contaminated and uncontaminated sites were found to be Zn-tolerant compared with the Zn-nontolerant species Arabidopsis thaliana and A. lyrata subsp. petraea . At very high Zn concentrations, populations of A. halleri from uncontaminated sites were slightly less Zn-tolerant than those from contaminated sites. These observations support the hypothesis that in A. halleri , Zn tolerance is largely a constitutive property. One population from an uncontaminated site and one population from a contaminated site were studied for Zn uptake. Zinc content was measured in shoots and roots using a colorimetric test under laboratory conditions. The results showed that whatever their origin, individuals from both populations are Zn accumulators compared with the nonaccumulator species A. thaliana . Moreover, the population from the uncontaminated area accumulated Zn in its shoots and roots more quickly than the population from the contaminated site. These results suggest that, in A. halleri , Zn accumulation to very high concentration is a constitutive property.     

The colorless flavonoids of Arabidopsis thaliana (Brassicaceae). II. Flavonoid 3' hydroxylation and lipid peroxidation

The 3' hydroxylation of kaempferol forms quercetin with an orthodihydroxy structure having two neighboring hydroxyl groups that could theoretically chelate with metal ions and mediate oxidative phenomena. Colorless flavonoids were purified by high pressure liquid chromatography (HPLC) and screened by diode array analysis. The accumulation of quercetin derivatives in Arabidopsis was coordinately regulated with flavonoid biosynthesis in a chalcone isomerase mutant having reduced flux through the biosynthetic pathway, but not within differing wild-type tissues, where seedling, floral, and leaf tissue have a reduced ratio of quercetin to kaempferol derivatives, respectively. The accumulation of lipid peroxidation products in kaempferol proficient mutant seedlings was indistinguishable from that in quercetin proficient wild-type seedlings, leaving no evidence for the role of quercetin antioxidants. However, laser scanning confocal microscopy revealed quercetin derivatives lining the tonoplast of diphenylboric acid 2-aminoethyl ester-stained Arabidopsis seedling tissue and floral papillae, and Norfluorazon induced oxidative stress decreased the most lipophilic of HPLC purified quercetin derivatives. Its potential involvement with lipophyllic oxidative phenomena may warrant further study.     

Landscape structure, clonal propagation, and genetic diversity in Scandinavian populations of Arabidopsis lyrata (Brassicaceae)

Colonization history, landscape structure, and environmental conditions may influence patterns of neutral genetic variation because of their effects on gene flow and reproductive mode. We compared variation at microsatellite loci within and among 26 Arabidopsis lyrata populations in two disjunct areas of its distribution in northern Europe (Norway and Sweden). The two areas probably share a common colonization history but differ in size (Norwegian range markedly larger than Swedish range), landscape structure (mountains vs. coast), and habitat conditions likely to affect patterns of gene flow and opportunities for sexual reproduction. Within-population genetic diversity was not related to latitude but was higher in Sweden than in Norway. Population differentiation was stronger among Norwegian than among Swedish populations (F(ST) = 0.23 vs. F(ST) = 0.18). The frequency of clonal propagation (proportion of identical multilocus genotypes) increased with decreasing population size, was higher in Norwegian than in Swedish populations, but was not related to altitude or substrate. Differences in genetic structure are discussed in relation to population characteristics and range size in the two areas. The results demonstrate that the possibility of clonal propagation should be considered when developing strategies for sampling and analyzing data in ecological and genetic studies of this emerging model species.     

Time scales of divergence and speciation among natural populations and subspecies of Arabidopsis lyrata (Brassicaceae)

? Premise of the study: Plant populations that face new environments adapt and diverge simultaneously, and both processes leave footprints in their genetic diversity. Arabidopsis lyrata is an excellent species for studying these processes. Pairs of populations and subspecies of A. lyrata represent different stages of divergence. These populations are also known to be locally adapted and display various stages of emerging reproductive isolation. ? Methods: We used nucleotide diversity data from 19 loci to estimate divergence times and levels of diversity among nine A. lyrata populations. Traditional distance-based methods and model-based clustering analysis were used to supplement pairwise coalescence-based analysis of divergence. ? Key results: Estimated divergence times varied from 130000 generations between North American and European subspecies to 39000 generations between central European and Scandinavian populations. In concordance with previous studies, the highest level of diversity was found in Central Europe and the lowest in North America and a diverged Russian Karhum?ki population. Local adaptation among Northern and central European populations has emerged during the last 39000 generations. Populations that are reproductively isolated by prezygotic mechanisms have been separated for a longer time period of ～70000 generations but still have shared nucleotide polymorphism. ? Conclusions: In A. lyrata, reproductively isolated populations started to diverge ～70000 generations ago and more closely related, locally adapted populations have been separate lineages for ～39000 generations. However, based on the posterior distribution of divergence times, the processes leading to reproductive isolation and local adaptation are likely to temporally coincide.     

Biomarker responses in human populations: towards a worldwide map.

The discipline of epidemiology studies the determinants of diseases in human populations, identifies causes, determines outcomes and develops prevention strategies. Traditional epidemiology is most useful for studies of acute, relatively common diseases with short incubation periods but less so for studies of chronic low incidence diseases with long incubation periods. Molecular epidemiology, which employs biological responses or biomarkers as surrogates of exposures or effects, can help with the latter. For this reason, there is a great interest in developing and validating biomarkers. DNA damage underlies an important group of chronic diseases with long incubation periods, i.e., cancer. Biomarkers may measure the exposures that induce the DNA damage, the damage itself, or individual susceptibility to damage. Before they can be used for human population research, however, these measures must be validated. Biomarker validation critically depends on field studies. This is accomplished through transitional epidemiological studies that 'bridge the gap' between laboratory and field. Transitional epidemiological studies are of three varieties: (i) Developmental, (ii) Characterization, and (iii) Applied. Biomarkers are the dependent variables in transitional studies. An international network of laboratories for human population monitoring requires yet another dimension for validation, i.e., the comparability of results among laboratories must be determined. This will be achieved by sample sharing projects, with workshops to compare results. Only then can results in one population be compared with results in another. Interlaboratory standardization of assays for biomarkers validated by transitional studies will have far-reaching benefits. It will allow development of worldwide databases of background values for the various biomarkers-or biomarker maps. This, in turn, will facilitate problem identification and eventually constitute the baselines for area-specific population monitoring. Biomarker databases so developed can be compared with worldwide databases for cancer and heritable diseases, validating the former as statistical surrogates of the latter.     

Environmental and genetic effects on flowering differences between northern and southern populations of Arabidopsis lyrata (Brassicaceae)

Arabidopsis lyrata (Brassicaceae) is a close outcrossing relative of A. thaliana. We examine flowering time variation of northern and southern A. lyrata populations in controlled environmental conditions, in a common garden experiment with A. thaliana, and in the field. Southern populations of A. lyrata flowered earlier than northern ones in all environmental conditions. Individuals from southern populations were more likely to flower in short days (14 h light) than northern ones, and all populations had a higher probability of flowering and flowered more rapidly in long days (20 h). The interaction of population and day length significantly affected flowering probability, and flowering time in one of two comparisons. The common garden experiment demonstrated differences between populations in the response to seed cold treatment, but growth chamber experiments showed no vernalization effect after 4 wk of rosette cold treatment. In a field population in Norway, a high proportion of the plants flowered in each year of the study. The plants progressed to flowering more rapidly in the field and common garden than in the growth chamber. The genetic basis of these flowering time differences here can be further studied using A. thaliana genetic tools.     

Reaction norms of Arabidopsis. I. Plasticity of characters and correlations across water,nutrient and light gradients

The univariate and multivariate study of variation for phenotypic plasticity is central to providing a clear understanding of hypotheses about the genetic control and evolution of reaction norms in natural populations. Arabidopsis thaliana is an ideal organism for the study of Genotype × Environment interactions (i.e., genetic variation for plasticity), because of the ease with which it can be grown in large numbers and due to the amount of information already available on its genetics, physiology and developmental biology. In this paper, we report on the plasticity, genetic variation and G × E interactions of four populations of A. thaliana in response to three environmental gradients (water, light and nutrients), each characterized by four levels of the controlled parameter. We measured nine traits and obtained their reaction norms. Path analysis was used to study the plasticity of character correlations. We found a tendency for A. thaliana reaction norms to be linear (either flat, i.e. no plasticity, or with a significant slope), in accordance with previous studies. We detected substantial amounts of genetic variation for plasticity in the light and nutrient gradients, but not in the water gradient. Dramatic restructuring of character correlations was induced by changes in environmental conditions, although some paths tended to be stable irrespective of the environment, thereby suggesting some degree of canalization.     

Inflorescences contribute more than rosettes to lifetime carbon gain in Arabidopsis thaliana (Brassicaceae)

A metamorphosis from rosette to inflorescence in many annuals shifts photosynthetic tissue from a two-dimensional array in the soil boundary layer during cool months to a three-dimensional structure in the troposphere as spring progresses. We propose that this shift allows escape from both self-shading and an increasingly stressful boundary layer microclimate, permitting continued increases in growth. As a first step in exploring this hypothesis, we compared the lifetime C gain, water loss, and instantaneous water use efficiency (WUE) of five Arabidopsis thaliana genotypes by measuring gas exchange across the life cycle. On average, the inflorescence contributed 55% (± 5% SE) of lifetime C gain, but only 25% of lifetime water loss. Mean inflorescence WUE was nearly fourfold that of the rosette. The inflorescence continued to fix C after rosette senescence. The percentage inflorescence: total C gain varied among genotypes, from 36% to 93%. Genotypes differed in WUE for both structures. We suggest that local climates may have selected for divergence in these traits. For many annuals and winter annuals, understanding C and water budgets and their evolution must include measures of both rosette and inflorescence gas exchange.     

Correlative controls of senescence and plant death in Arabidopsis thaliana (Brassicaceae)

Like most monocarpic plants, longevity of Arabidopsis thaliana plants is controlled by the reproductive structures; however, they appear to work differently from most dicots studied. Neither male- and female-sterility mutations (ms1-1 and bell1, respectively) nor surgical removal of the stems with inflorescences (bolts) at various stages significantly increased the longevity of individual rosette leaves, yet the mutants and treated plants lived 20-50 d longer, measured by the death of the last rosette and/or the last cauline leaf. A series of growth mutations (clv2-4, clv3-2, det3, vam1 enh, and dark green) also increased plant longevity by 20-30 d but did not delay the overall development of the plants. The mutations prolonged plant life through the production of new leaves and stems with inflorescences (bolts) rather than by extending leaf longevity. In growing stems, the newly-formed leaves may induce senescence in the older leaves; however, removal of the younger leaves did not significantly increase the life of the older leaves on the compressed stems of Arabidopsis. Since plants that produce more bolts also live longer, the reproductive load (dry weight) of the bolts did not seem to drive leaf or whole plant senescence here. The developing reproductive structures caused the death of the plant by preventing regeneration of leaves and bolts, which are green and presumably photosynthetic. They also exerted a correlative control (repression) on the development of additional reproductive structures.     

The efficiency of Arabidopsis thaliana (Brassicaceae) root hairs in phosphorus acquisition

Arabidopsis thaliana root hairs grow longer and denser in response to low-phosphorus availability. In addition, plants with the root hair response acquire more phosphorus than mutants that have root hairs that do not respond to phosphorus limiting conditions. The purpose of this experiment was to determine the efficiency of root hairs in phosphorus acquisition at high- and low-phosphorus availability. Root hair growth, root growth, root respiration, plant phosphorus uptake, and plant phosphorus content of 3-wk-old wild-type Arabidopsis (WS) were compared to two root hair mutants (rhd6 and rhd2) under high (54 mmol/m) and low (0.4 mmol/m) phosphorus availability. A cost-benefit analysis was constructed from the measurements to determine root hair efficiency. Under high-phosphorus availability, root hairs did not have an effect on any of the parameters measured. Under low-phosphorus availability, wild-type Arabidopsis had greater total root surface area, shoot biomass, phosphorus per root length, and specific phosphorus uptake. The cost-benefit analysis shows that under low phosphorus, wild-type roots acquire more phosphorus for every unit of carbon respired or unit of phosphorus invested into the roots than the mutants. We conclude that the response of root hairs to low-phosphorus availability is an efficient strategy for phosphorus acquisition.     

Asexual reproduction in a close relative of Arabidopsis: a genetic investigation of apomixis in Boechera (Brassicaceae)

Understanding apomixis (asexual reproduction through seeds) is of great interest to both plant breeders and evolutionary biologists. The genus Boechera is an excellent system for studying apomixis because of its close relationship to Arabidopsis, the occurrence of apomixis at the diploid level, and its potentially simple inheritance by transmission of a heterochromatic (Het) chromosome. Diploid sexual Boechera stricta and diploid apomictic Boechera divaricarpa (carrying a Het chromosome) were crossed. Flow cytometry, karyotype analysis, genomic in situ hybridization, pollen staining and seed-production measurements were used to analyse the parents and resulting F1, F2 and selected F3 and test-cross (TC) generations. The F1 plant was a low-fertility triploid that produced a swarm of aneuploid and polyploid F2 progeny. Two of the F2 plants were fertile near-tetraploids, and analysis of their F3 and TC progeny revealed that they were sexual and genomically stabilized. The apomictic phenotype was not transmitted by genetic crossing as a single dominant locus on the Het chromosome, suggesting a complex genetic control of apomixis that has implications for future genetic and evolutionary analyses in this group.     

Further insights into the phylogeny of Arabidopsis (Brassicaceae) from nuclear Atmyb2 flanking sequence

Arabidopsis thaliana is the preeminent plant model organism. However, significant advances in evolution and ecology are being made by expanding the scope of research beyond this single species into the broader genus Arabidopsis. Surprisingly, few studies have rigorously investigated phylogenetic relationships between the nine Arabidopsis species, and this study evaluates both these and hypotheses related to two instances of intra-generic hybridization. DNA sequences from the 5' flanking region of the nuclear Atmyb2 gene from 12 of the 14 Arabidopsis taxa were used to reconstruct the generic phylogeny. The strict consensus tree was highly concordant with previous studies, identifying lineages corresponding to widespread species but exhibiting a large basal polytomy. Our data indicates that the paternal parent of the allopolyploid A. suecica is A. neglecta rather than A. arenosa s.l., although the need for a detailed phylogeographical study of these three species is noted. Finally, our data provided additional phylogenetic evidence of hybridization between Arabidopsis lyrata s.l. and A. halleri s.l. Taken together, the well-defined lineages within the genus and the potential for hybridization between them highlight Arabidopsis as a promising group for comparative and experimental studies of hybridization.     

Phylogeography of the invasive weed Hypochaeris radicata (Asteraceae): from Moroccan origin to worldwide introduced populations

In an attempt to delineate the area of origin and migratory expansion of the highly successful invasive weedy species Hypochaeris radicata, we analysed amplified fragment length polymorphisms from samples taken from 44 populations. Population sampling focused on the central and western Mediterranean area, but also included sites from Northern Spain, Western and Central Europe, Southeast Asia and South America. The six primer combinations applied to 213 individuals generated a total of 517 fragments of which 513 (99.2%) were polymorphic. The neighbour-joining tree presented five clusters and these divisions were supported by the results of Bayesian analyses: plants in the Moroccan, Betic Sierras (Southern Spain), and central Mediterranean clusters are all heterocarpic. The north and central Spanish, southwestern Sierra Morena, and Central European, Asian and South American cluster contain both heterocarpic (southwestern Sierra Morena) and homocarpic populations (all other populations). The Doñana cluster includes two homocarpic populations. Analyses of fragment parameters indicate that the oldest populations of H. radicata are located in Morocco and that the species expanded from this area in the Late Quaternary via at least three migratory routes, the earliest of which seems to have been to the southwestern Iberian Peninsula, with subsequent colonizations to the central Mediterranean area and the Betic Sierras. Homocarpic populations originated in the southwestern Iberian Peninsula and subsequently spread across north and central Spain, Central Europe and worldwide, where they became a highly successful weed.     

Density and reproductive success in wild populations of Diplotaxis erucoides (Brassicaceae)

Summary One possible consequence of low population density, particularly in self-incompatible plants, is reproductive failure. I surveyed seed set per flower in two populations of the self-incompatible annual Diplotaxis erucoides (Brassicaceae) in Jerusalem, Israel. Widely spaced plants had lower fruit set and fewer seeds per filled silique than did plants growing close to conspecific neighbors. Such density-dependent reproductive success could help explain the maintanence of spatial patchiness in plant populations, and could also have implications for population dynamics of rare species.     

Polypeptide antibiotic 26a from Bacillus subtilis. III. Physicochemical and biological in vitro properties.

Antibiotic 26a, a weakly basic (pK1 3.85 and pK2 7.1) polypeptide compound, has been recovered from the fermentation fluids of bacillus subtilis cultures as hydrochloride salt easily soluble in water and dimethylsulphoxide, sparingly soluble in lower alcohols and insoluble in several organic solvents. At low concentrations 26a was effective against gram-positive bacteria, mainly micrococci and corynebacteria, moderately active against mycobacteria, and inactive against gram-negative bacteria, yeasts and moulds even at 300 microgram/ml concentration. From the viewpoint of elemental analysis, electrometric titration, optical rotation, UV, IR and NMR spectra, amino acid composition, molecular weight and biological observations, 26a can be considered as an antibiotic, if not identical, then closely related to bacitracin family polypeptides.     

Palatability and tolerance to simulated herbivory in native and introduced populations of Alliaria petiolata (Brassicaceae)

The European herb garlic mustard (Alliaria petiolata) is a serious invader of North American deciduous forests. One explanation for its success could be that in the absence of specialized herbivores, selection has favored less defended but more vigorous genotypes. This idea was addressed by comparing offspring from several native and introduced Alliaria populations with respect to their palatability to insect herbivores and their tolerance to simulated herbivory. Feeding rates of a specialist weevil from the native range were significantly greater on American plants, suggesting a loss of resistance in the introduced range. In contrast, there was significant population variation but no continent effect in the feeding rates of a generalist caterpillar. After simulated herbivory, A. petiolata showed a substantial regrowth capacity that involved changes in plant growth, architecture, and allocation. Removal of 75% leaf area or of all bolting stems reduced plant fitness to 81% and 58%, respectively, of the fitness of controls. There was no indication of a difference in tolerance between native and introduced Alliaria populations or of a trade-off between tolerance and resistance.