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

Deleterious phenotypes in wild Arabidopsis arenosa populations are common and linked to runs of homozygosity

In this study, we aimed to systematically assess the frequency at which potentially deleterious phenotypes appear in natural populations of the outcrossing model plant Arabidopsis arenosa, and to establish their underlying genetics. For this purpose, we collected seeds from wild A. arenosa populations and screened over 2,500 plants for unusual phenotypes in the greenhouse. We repeatedly found plants with obvious phenotypic defects, such as small stature and necrotic or chlorotic leaves, among first-generation progeny of wild A. arenosa plants. Such abnormal plants were present in about 10% of maternal sibships, with multiple plants with similar phenotypes in each of these sibships, pointing to a genetic basis of the observed defects. A combination of transcriptome profiling, linkage mapping and genome-wide runs of homozygosity patterns using a newly assembled reference genome indicated a range of underlying genetic architectures associated with phenotypic abnormalities. This included evidence for homozygosity of certain genomic regions, consistent with alleles that are identical by descent being responsible for these defects. Our observations suggest that deleterious alleles with different genetic architectures are segregating at appreciable frequencies in wild A. arenosa populations.

Genomic Patterns of Local Adaptation under Gene Flow in Arabidopsis lyrata

Short-scale local adaptation is a complex process involving selection, migration, and drift. The expected effects on the genome are well grounded in theory but examining these on an empirical level has proven difficult, as it requires information about local selection, demographic history, and recombination rate variation. Here, we use locally adapted and phenotypically differentiated Arabidopsis lyrata populations from two altitudinal gradients in Norway to test these expectations at the whole-genome level. Demography modeling indicates that populations within the gradients diverged <2 kya and that the sites are connected by gene flow. The gene flow estimates are, however, highly asymmetric with migration from high to low altitudes being several times more frequent than vice versa. To detect signatures of selection for local adaptation, we estimate patterns of lineage-specific differentiation among these populations. Theory predicts that gene flow leads to concentration of adaptive loci in areas of low recombination; a pattern we observe in both lowland-alpine comparisons. Although most selected loci display patterns of conditional neutrality, we found indications of genetic trade-offs, with one locus particularly showing high differentiation and signs of selection in both populations. Our results further suggest that resistance to solar radiation is an important adaptation to alpine environments, while vegetative growth and bacterial defense are indicated as selected traits in the lowland habitats. These results provide insights into genetic architectures and evolutionary processes driving local adaptation under gene flow. We also contribute to understanding of traits and biological processes underlying alpine adaptation in northern latitudes.

Natural variation in tolerance to sub-zero temperatures among populations of Arabidopsis lyrata ssp. petraea

BACKGROUND

Temperature is one of the most important abiotic factors limiting plant growth and productivity. Many plants exhibit cold acclimation to prepare for the likelihood of freezing as temperatures decrease towards 0 °C. The physiological mechanisms associated with enabling increased tolerance to sub-zero temperatures vary between species and genotypes. Geographically and climatically diverse populations of Arabidopsis lyrata ssp. petraea were examined for their ability to survive, maintain functional photosynthetic parameters and cellular electrolyte leakage integrity after being exposed to sub-zero temperatures. The duration of cold acclimation prior to sub-zero temperatures was also manipulated (2 and 14 days).

RESULTS

We found that there was significant natural variation in tolerances to sub-zero temperatures among populations of A. petraea. The origin of the population affected the acclimation response and survival after exposure to sub-zero temperatures. Cold acclimation of plants prior to sub-zero temperatures affected the maximum quantum efficiency of photosystem II (PSII) (Fv/Fm) in that plants that were cold acclimated for longer periods had higher values of Fv/Fm as a result of sub-zero temperatures. The inner immature leaves were better able to recover Fv/Fm from sub-zero temperatures than mature outer leaves. The Irish population (Leitrim) acclimated faster, in terms of survival and electrolyte leakage than the Norwegian population (Helin).

CONCLUSION

The ability to survive, recover photosynthetic processes and cellular electrolyte leakage after exposure to sub-zero temperatures is highly dependent on the duration of cold acclimation.

Local adaptation and ecological differentiation under selection, migration, and drift in Arabidopsis lyrata

How the balance between selection, migration, and drift influences the evolution of local adaptation has been under intense theoretical scrutiny. Yet, empirical studies that relate estimates of local adaptation to quantification of gene flow and effective population sizes have been rare. Here, we conducted a reciprocal transplant trial, a common garden trial, and a whole-genome-based demography analysis to examine these effects among Arabidopsis lyrata populations from two altitudinal gradients in Norway. Demography simulations indicated that populations within the two gradients are connected by gene flow (0.1 < 4Ne m < 11) and have small effective population sizes (Ne <  6000), suggesting that both migration and drift can counteract local selection. However, the three-year field experiments showed evidence of local adaptation at the level of hierarchical multiyear fitness, attesting to the strength of differential selection. In the lowland habitat, local superiority was associated with greater fecundity, while viability accounted for fitness differences in the alpine habitat. We also demonstrate that flowering time differentiation has contributed to adaptive divergence between these locally adapted populations. Our results show that despite the estimated potential of gene flow and drift to hinder differentiation, selection among these A. lyrata populations has resulted in local adaptation.

Integration of genetic and seed fitness data to the conservation of isolated subpopulations of the Mediterranean plant Malcolmia littorea

Plant autoecology and population genetics provide a perspective on the likelihood of natural regeneration, which is critical when designing conservation strategies for endangered species. The threatened coastal plant Malcolmia littorea (Brassicaceae) was sampled across its European distribution and studied for genetic diversity and seed fitness, with the aim of providing information for the conservation of isolated and declining populations. Nine microsatellite markers (five chloroplast and four nuclear) were analysed to assess population genetic diversity and structure and to conduct a spatial analysis using the software DIVA-GIS. Germination percentages and rates were assessed by incubating the seeds under eight constant temperatures (0-27 °C). The genetic diversity was found to be similar among subpopulations (chloroplast H = 0.04-0.17; nuclear H_o  = 0.20-0.37), with no correlation between subpopulation diversity and the area of occupancy (AOO). The subpopulations were found to be clustered in three genetic groups, and three of them were identified as conservation priorities due to their unique genetic composition. The germination study revealed a significant influence of the maternal environment and AOO on seed germination, with the smaller subpopulations showing lower germination percentages (P < 0.05). These results highlight the importance of obtaining information on isolated subpopulations through different experimental approaches (e.g. seed germination plus population genetics) to enable planning of effective conservation actions. For M. littorea, seed collection for both in situ and ex situ conservation should take into account the local adaptation of the subpopulation and the genetic structure of the species.

Genome-Wide Analysis of Colonization History and Concomitant Selection in Arabidopsis lyrata

The high climatic variability in the past hundred thousand years has affected the demographic and adaptive processes in many species, especially in boreal and temperate regions undergoing glacial cycles. This has also influenced the patterns of genome-wide nucleotide variation, but the details of these effects are largely unknown. Here we study the patterns of genome-wide variation to infer colonization history and patterns of selection of the perennial herb species Arabidopsis lyrata, in locally adapted populations from different parts of its distribution range (Germany, UK, Norway, Sweden, and USA) representing different environmental conditions. Using site frequency spectra based demographic modeling, we found strong reduction in the effective population size of the species in general within the past 100,000 years, with more pronounced effects in the colonizing populations. We further found that the northwestern European A. lyrata populations (UK and Scandinavian) are more closely related to each other than with the Central European populations, and coalescent based population split modeling suggests that western European and Scandinavian populations became isolated relatively recently after the glacial retreat. We also highlighted loci showing evidence for local selection associated with the Scandinavian colonization. The results presented here give new insights into postglacial Scandinavian colonization history and its genome-wide effects.

Similarity in G matrix structure among natural populations of Arabidopsis lyrata

Understanding the stability of the G matrix in natural populations is fundamental for predicting evolutionary trajectories; yet, the extent of its spatial variation and how this impacts responses to selection remain open questions. With a nested paternal half-sib crossing design and plants grown in a field experiment, we examined differences in the genetic architecture of flowering time, floral display, and plant size among four Scandinavian populations of Arabidopsis lyrata. Using a multivariate Bayesian framework, we compared the size, shape, and orientation of G matrices and assessed their potential to facilitate or constrain trait evolution. Flowering time, floral display and rosette size varied among populations and significant additive genetic variation within populations indicated potential to evolve in response to selection. Yet, some characters, including flowering start and number of flowers, may not evolve independently because of genetic correlations. Using a multivariate framework, we found few differences in the genetic architecture of traits among populations. G matrices varied mostly in size rather than shape or orientation. Differences in multivariate responses to selection predicted from differences in G were small, suggesting overall matrix similarity and shared constraints to trait evolution among populations.

What's the meaning of local? Using molecular markers to define seed transfer zones for ecological restoration in Norway

According to the Norwegian Diversity Act, practitioners of restoration in Norway are instructed to use seed mixtures of local provenance. However, there are no guidelines for how local seed should be selected. In this study, we use genetic variation in a set of alpine species (Agrostis mertensii, Avenella flexuosa, Carex bigelowii, Festuca ovina, Poa alpina and Scorzoneroides autumnalis) to define seed transfer zones to reduce confusion about the definition of ‘local seeds’. The species selected for the study are common in all parts of Norway and suitable for commercial seed production. The sampling covered the entire alpine region (7–20 populations per species, 3–15 individuals per population). We characterised genetic diversity using amplified fragment length polymorphisms. We identified different spatial genetic diversity structures in the species, most likely related to differences in reproductive strategies, phylogeographic factors and geographic distribution. Based on results from all species, we suggest four general seed transfer zones for alpine Norway. This is likely more conservative than needed for all species, given that no species show more than two genetic groups. Even so, the approach is practical as four seed mixtures will serve the need for restoration of vegetation in alpine regions in Norway.

Inbreeding Affects Gene Expression Differently in Two Self-Incompatible Arabidopsis lyrata Populations with Similar Levels of Inbreeding Depression

Knowledge of which genes and pathways are affected by inbreeding may help understanding the genetic basis of inbreeding depression, the potential for purging (selection against deleterious recessive alleles), and the transition from outcrossing to selfing. Arabidopsis lyrata is a predominantly self-incompatible perennial plant, closely related to the selfing model species A. thaliana. To examine how inbreeding affects gene expression, we compared the transcriptome of experimentally selfed and outcrossed A. lyrata originating from two Scandinavian populations that express similar inbreeding depression for fitness (∂ ≈ 0.80). The number of genes significantly differentially expressed between selfed and outcrossed individuals were 2.5 times higher in the Norwegian population (≈ 500 genes) than in the Swedish population (≈ 200 genes). In both populations, a majority of genes were upregulated on selfing (≈ 80%). Functional annotation analysis of the differentially expressed genes showed that selfed offspring were characterized by 1) upregulation of stress-related genes in both populations and 2) upregulation of photosynthesis-related genes in Sweden but downregulation in Norway. Moreover, we found that reproduction- and pollination-related genes were affected by inbreeding only in Norway. We conclude that inbreeding causes both general and population-specific effects. The observed common effects suggest that inbreeding generally upregulates rather than downregulates gene expression and affects genes associated with stress response and general metabolic activity. Population differences in the number of affected genes and in effects on the expression of photosynthesis-related genes show that the genetic basis of inbreeding depression can differ between populations with very similar levels of inbreeding depression.

Inter-simple sequence repeat data reveals high genetic diversity in wild populations of the narrowly distributed endemic Lilium regale in the Minjiang River Valley of China

Lilium regale E.H. Wilson is endemic to a narrow geographic area in the Minjiang River valley in southwestern China, and is considered an important germplasm for breeding commercially valuable lily varieties, due to its vigorous growth, resistance to diseases and tolerance for low moisture. We analyzed the genetic diversity of eight populations of L. regale sampled across the entire natural distribution range of the species using Inter-Simple Sequence Repeat markers. The genetic diversity (expected heterozygosity= 0.3356) was higher than those reported for other narrowly distributed endemic plants. The levels of inbreeding (Fst = 0.1897) were low, and most of the genetic variability was found to be within (80.91%) than amongpopulations (19.09%). An indirect estimate of historical levels of gene flow (Nm =1.0678) indicated high levels of gene flow among populations. The eight analyzed populations clustered into three genetically distinct groups. Based on these results, we recommend conservation of large populations representing these three genetically distinct groups.

Contrasting patterns of genetic structuring in natural populations of Arabidopsis lyrata Subsp. petraea across different regions in northern Europe

Level and partitioning of genetic diversity is expected to vary between contrasting habitats, reflecting differences in strength of ecological and evolutionary processes. Therefore, it is necessary to consider processes acting on different time scales when trying to explain diversity patterns in different parts of species' distributions. To explore how historical and contemporary factors jointly may influence patterns of genetic diversity and population differentiation, we compared genetic composition in the perennial herb Arabidopsis lyrata ssp. petraea from the northernmost parts of its distribution range on Iceland to that previously documented in Scandinavia. Leaf tissue and soil were sampled from ten Icelandic populations of A. lyrata. Seedlings were grown from soil samples, and tissue from above-ground and seed bank individuals were genotyped with 21 microsatellite markers. Seed bank density in Icelandic populations was low but not significantly different from that observed in Norwegian populations. While within-population genetic diversity was relatively high on Iceland (H(E) = 0.35), among-population differentiation was low (F(ST) = 0.10) compared to Norwegian and Swedish populations. Population differentiation was positively associated with geographical distance in both Iceland and Scandinavia, but the strength of this relationship varied between regions. Although topography and a larger distribution range may explain the higher differentiation between mountainous Norwegian relative to lowland populations in Sweden, these factors cannot explain the lower differentiation in Icelandic compared to Swedish populations. We propose that low genetic differentiation among Icelandic populations is not caused by differences in connectivity, but is rather due to large historical effective population sizes. Thus, rather than contemporary processes, historical factors such as survival of Icelandic lineages in northern refugia during the last glacial period may have contributed to the observed pattern.

Strong inbreeding depression in two Scandinavian populations of the self-incompatible perennial herb Arabidopsis lyrata

Inbreeding depression is a key factor influencing mating system evolution in plants, but current understanding of its relationship with selfing rate is limited by a sampling bias with few estimates for self-incompatible species. We quantified inbreeding depression (δ) over two growing seasons in two populations of the self-incompatible perennial herb Arabidopsis lyrata ssp. petraea in Scandinavia. Inbreeding depression was strong and of similar magnitude in both populations. Inbreeding depression for overall fitness across two seasons (the product of number of seeds, offspring viability, and offspring biomass) was 81% and 78% in the two populations. Chlorophyll deficiency accounted for 81% of seedling mortality in the selfing treatment, and was not observed among offspring resulting from outcrossing. The strong reduction in both early viability and late quantitative traits suggests that inbreeding depression is due to deleterious alleles of both large and small effect, and that both populations experience strong selection against the loss of self-incompatibility. A review of available estimates suggested that inbreeding depression tends to be stronger in self-incompatible than in self-compatible highly outcrossing species, implying that undersampling of self-incompatible taxa may bias estimates of the relationship between mating system and inbreeding depression.

Adaptation at range margins: common garden trials and the performance of Arabidopsis lyrata across its northwestern European range

Widely distributed species, such as the perennial plant Arabidopsis lyrata, face a range of environmental conditions across space, creating selective pressures for local evolutionary adaptation. The species' fragmented distribution may reduce gene flow, which could either reduce or increase adaptive potential. The substantial variation in phenotypic traits observed across this species' northwestern European range may reflect a combination of plastic responses to environmental conditions, evolutionary adaptation and nonadaptive genetic differentiation. We conducted multi-site common garden experiments to study differences in plant performance in core and marginal areas. Plants from eight source populations representing the species' full geographic and altitudinal range in northwestern Europe were planted out in Iceland, Sweden, Scotland and Wales. We found evidence of both strong plastic responses and apparently adaptive differentiation in performance. Most evidence for local adaptation was found at range margins, with the strongest effects on reproductive output. Both biotic and abiotic factors affected performance, especially at range margins. Performance of most plants was best in the Scottish and Swedish common garden sites, in the core of the species' distribution. Despite adaptations at range margins, the performance of the species declines at distributional limits, with extreme southern populations looking particularly vulnerable.

Mining of EST-SSR markers of Musa and their transferability studies among the members of order the Zingiberales

Expressed sequence tags (ESTs) databases of 11 Musa complementary DNA libraries were retrieved from National Center of Biotechnology Information and used for mining simple sequence repeats (SSRs). Out of 21,056 unique ESTs, SSR regions were found only in 5,158 ESTs. Among these SSR containing ESTs, the occurrence of trinucleotide repeats are the most abundant followed by mono-, di-, tetra-, hexa-, and pentanucleotides. Moreover, this study showed that the rate of class II SSRs (<20 nucleotides) was higher than the class I SSRs (<20 nucleotides), and proportion of class I and II SSRs as abundant for tri-repeats. As a representative sample, primers were synthesized for 24 ESTs, carrying >12 nucleotides of SSR region, and tested among the various genomic group of Musa accessions. The result showed that 88 % of primers were functional primers, and 43 % are showing polymorphism among the Musa accessions. Transferability studies of Musa EST-SSRs among the genera of the order Zingiberales exhibited 100 and 58 % transferability in Musaceae and Zingiberaceae, respectively. The sequence comparison of SSR regions among the different Musa accessions confirmed that polymorphism is mainly due to the variation in repeat length. High percentage of cross-species, cross-genera, and cross-family transferability also suggested that these Musa EST-SSR markers will be a valuable resource for the comparative mapping by developing COS markers, in evolutionary studies and in improvement of the members of Zingiberaceae and Musaceae.

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 130,000 generations between North American and European subspecies to 39,000 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 Karhumäki population. Local adaptation among Northern and central European populations has emerged during the last 39,000 generations. Populations that are reproductively isolated by prezygotic mechanisms have been separated for a longer time period of ∼70,000 generations but still have shared nucleotide polymorphism.

CONCLUSIONS

In A. lyrata, reproductively isolated populations started to diverge ∼70,000 generations ago and more closely related, locally adapted populations have been separate lineages for ∼39,000 generations. However, based on the posterior distribution of divergence times, the processes leading to reproductive isolation and local adaptation are likely to temporally coincide.

Additive and non-additive effects of simulated leaf and inflorescence damage on survival, growth and reproduction of the perennial herb Arabidopsis lyrata

Herbivores may damage both leaves and reproductive structures, and although such combined damage may affect plant fitness non-additively, this has received little attention. We conducted a 2-year field experiment with a factorial design to examine the effects of simulated leaf (0, 12.5, 25, or 50% of leaf area removed) and inflorescence damage (0 vs. 50% of inflorescences removed) on survival, growth and reproduction in the perennial herb Arabidopsis lyrata. Leaf and inflorescence damage negatively and independently reduced flower, fruit and seed production in the year of damage; leaf damage also reduced rosette size by the end of the first season and flower production in the second year. Leaf damage alone reduced the proportion of flowers forming a fruit and fruit production per plant the second year, but when combined with inflorescence damage no such effect was observed (significant leaf × inflorescence damage interaction). Damage to leaves (sources) caused a greater reduction in future reproduction than did simultaneous damage to leaves and inflorescences (sinks). This demonstrates that a full understanding of the effects of herbivore damage on plant fitness requires that consequences of damage to vegetative and reproductive structures are evaluated over more than 1 year and that non-additive effects are considered.

Genetic consequences of seed banks in the perennial herb Arabidopsis lyrata subsp. petraea (Brassicaceae)

PREMISE OF THE STUDY

Seed banks may increase the effective population size (N(e)) of plants as a result of elevated coalescence times for alleles residing in the populations. This has been empirically demonstrated in populations of the annual Arabidopsis thaliana, whereas comparable data for perennial species are currently lacking. We studied the contribution of seed banks to effective sizes of natural populations of the self-incompatible, perennial Arabidopsis lyrata subsp. petraea, a close relative of A. thaliana. •

METHODS

Fourteen populations of A. lyrata collected throughout the Norwegian distribution range were analyzed using microsatellite markers. •

KEY RESULTS

The genetic composition of seed-bank and aboveground cohorts was found to be highly similar, with little genetic differentiation between cohorts in most populations. However, the proportion of private alleles was higher in aboveground than in seed-bank cohorts. The presence of seed banks significantly increased total N(e), but the contribution from seed banks to overall N(e) were lower than the contribution from aboveground cohorts in most populations. Estimated historical N(e) values, reflecting the effective sizes of populations throughout the history of the species, were considerably higher than estimates of contemporary N(e), reflecting number of reproducing individuals within the past few generations. •

CONCLUSIONS

The results show that the seed bank contributes to total N(e) in the perennial herb A. lyrata. However, the contribution is similar to or lower than that of the above-ground fraction of the population and markedly weaker than that previously documented in the annual A. thaliana.

The relative importance of factors determining genetic drift: mating system, spatial genetic structure, habitat and census size in Arabidopsis lyrata

• The mating system, dispersal and census size are predicted to determine the magnitude of genetic drift, but little is known about their relative importance in nature. • We estimated the contributions of several population-level features to genetic drift in 18 populations of Arabidopsis lyrata. The factors were outcrossing rate, within-population spatial genetic structure, census size and substrate type. The expected heterozygosity (H(E)) at 10 microsatellite loci was taken to reflect the effective population size (N(e)) and the strength of genetic drift. • The mating system explained most of the variation in H(E) (60%), followed by substrate (10%), genetic structure (9%) and census size (6%). The most outcrossing population had a +0.32 higher predicted H(E) than the most selfing population; the estimated N(e) of selfing populations was less than half that of outcrossing populations. Rocky outcrops supported populations with a +0.14 higher H(E) than did sandy substrates. The most structured population had a +0.24 higher H(E) than the least structured population, and the largest population had a +0.18 higher H(E) than the smallest population. • This study illustrates the importance of outcrossing, genetic structure and the physical environment--together with census size--in maintaining H(E), and suggests that multiple population-level characteristics influence N(e) and the action of genetic drift.

Investigating the effects of topography and clonality on genetic structuring within a large Norwegian population of Arabidopsis lyrata

BACKGROUND AND AIMS

The gene flow through pollen or seeds governs the extent of spatial genetic structure in plant populations. Another factor that can contribute to this pattern is clonal growth. The perennial species Arabidopsis lyrata ssp. petraea (Brassicaceae) is a self-incompatible, clonal species found in disjunctive populations in central and northern Europe.

METHODS

Fourteen microsatellite markers were employed to study the level of kinship and clonality in a high-altitude mountain valley at Spiterstulen, Norway. The population has a continuous distribution along the banks of the River Visa for about 1.5 km. A total of 17 (10 m x 10 m) squares were laid out in a north-south transect following the river on both sides.

KEY RESULTS

It is shown that clonal growth is far more common than previously shown in this species, although the overall size of the genets is small (mean diameter = 6.4 cm). Across the whole population there is no indication of isolation by distance, and spatial genetic structure is only visible on fine spatial scales. In addition, no effect of the river on the spatial distribution of genotypes was found.

CONCLUSIONS

Unexpectedly, the data show that populations of small perennials like A. lyrata can behave like panmictic units across relatively large areas at local sites, as opposed to earlier findings in central Europe.

Population structure and historical biogeography of European Arabidopsis lyrata

Understanding the natural history of model organisms is important for the effective use of their genomic resources. Arabidopsis lyrata has emerged as a useful plant for studying ecological and evolutionary genetics, based on its extensive natural variation, sequenced genome and close relationship to A. thaliana. We studied genetic diversity across the entire range of European Arabidopsis lyrata ssp. petraea, in order to explore how population history has influenced population structure. We sampled multiple populations from each region, using nuclear and chloroplast genome markers, and combined population genetic and phylogeographic approaches. Within-population diversity is substantial for nuclear allozyme markers (mean P=0.610, A(e)=1.580, H(e)=0.277) and significantly partitioned among populations (F(ST)=0.271). The Northern populations have modestly increased inbreeding (F(IS)=0.163 verses F(IS)=0.093), but retain comparable diversity to central European populations. Bottlenecks are common among central and northern Europe populations, indicating recent demographic history as a dominant factor in structuring the European diversity. Although the genetic structure was detected at all geographic scales, two clear differentiated units covering northern and central European areas (F(CT) =0.155) were identified by Bayesian analysis and supported by regional pairwise F(CT) calculations. A highly similar geographic pattern was observed from the distribution of chloroplast haplotypes, with the dominant northern haplotypes absent from central Europe. We conclude A. l. petraea's cold-tolerance and preference for disturbed habitats enabled glacial survival between the alpine and Nordic glaciers in central Europe and an additional cryptic refugium. While German populations are probable peri-glacial leftovers, Eastern Austrian populations have diversity patterns possibly compatible with longer-term survival.

Clonal structure and genetic diversity of three desert phreatophytes

The objective of this paper was to assess clone sizes of three perennial desert plant species with AFLP markers and to relate them to clonal and genetic diversity and to hydroecology. The study was carried out at the southern rim of the Taklamakan Desert, where sexual regeneration is only possible shortly after rare flooding events, resulting in rarely established cohorts with subsequent extensive vertical growth and horizontal clonal spread. In this environment, repeated seedling establishment is excluded. We expected decreasing clonal and genetic diversity with increasing clone size and increasing distance to the groundwater table and a common response pattern among all study species. Maximum sizes of Populus euphratica and Alhagi sparsifolia clones were 121 ha and 6.1 ha, respectively, while Tamarix ramosissima clones reached a maximum size of only 38 m(2). In P. euphratica and A. sparsifolia, clonal diversity declined with increasing clone size and increasing distance to the groundwater table, while genetic diversity remained unaffected. Tamarix ramosissima differed from the other species because of a much smaller clonality. Clone size and clonal diversity were found to be good proxy variables for clone age. Despite the considerable age of the clones, genetic diversity is maintained in the populations.

Population-specific metabolic phenotypes of Arabidopsis lyrata ssp. petraea

Plant populations growing at the margin of their range may exhibit traits that indicate genetic differentiation and adaptation to their local abiotic environment. Here, it was investigated whether geographically separated marginal populations of Arabidopsis lyrata ssp. petraea have distinct metabolic phenotypes within the plant foliage. Seeds of A. petraea were obtained from populations along a latitudinal gradient (49-64 N), namely Germany, Wales, Sweden and Iceland and grown in a controlled cabinet environment. Targeted metabolic profiles and fingerprints were obtained at the same initial developmental stage. The free amino acid compositions were population specific, with fold differences in arginine, aspartic acid, asparagines, glycine, phenylalanine, alanine, threonine, histidine, serine and gamma-aminobutyric acid (GABA) concentrations. Sucrose, mannose and fructose concentrations were also different between populations but polyhydric alcohol concentrations were not. Principal component analysis (PCA) of metabolite fingerprints revealed metabolic phenotypes for each population. It is suggested that glucosinolates were responsible for discriminating populations within the PCA. Metabolite fingerprinting and profiling has proved to be sufficiently sensitive to identify metabolic differences between plant populations. These findings show that there is significant natural variation in metabolism among populations of A. petraea.