Genetic architecture of a plant adaptive trait: QTL mapping of intraspecific variation for tolerance to metal pollution in Arabidopsis halleri.

Anthropogenic activities are among the main drivers of global change and result in drastic habitat modifications, which represent strong evolutionary challenges for biological species that can either migrate, adapt, or disappear. In this context, understanding the genetics of adaptive traits is a prerequisite to enable long-term maintenance of populations under strong environmental constraints. To examine these processes, a QTL approach was developed here using the pseudometallophyte Arabidopsis halleri, which displays among-population adaptive divergence for tolerance to metallic pollution in soils. An F2 progeny was obtained by crossing individuals from metallicolous and non-metallicolous populations from Italian Alps, where intense metallurgic activities have created strong landscape heterogeneity. Then, we combined genome de novo assembly and genome resequencing of parental genotypes to obtain single-nucleotide polymorphism markers and achieve high-throughput genotyping of the progeny. QTL analysis was performed using growth parameters and photosynthetic yield to assess zinc tolerance levels. One major QTL was identified for photosynthetic yield. It explained about 27% of the phenotypic variance. Functional annotation of the QTL and gene expression analyses highlighted putative candidate genes. Our study represents a successful approach combining evolutionary genetics and advanced molecular tools, helping to better understand how a species can face new selective pressures of anthropogenic origin.

Evolutionary dynamics of quantitative variation in an adaptive trait at the regional scale: The case of zinc hyperaccumulation in Arabidopsis halleri.

Metal hyperaccumulation in plants is an ecological trait whose biological significance remains debated, in particular because the selective pressures that govern its evolutionary dynamics are complex. One of the possible causes of quantitative variation in hyperaccumulation may be local adaptation to metalliferous soils. Here, we explored the population genetic structure of Arabidopsis halleri at fourteen metalliferous and nonmetalliferous sampling sites in southern Poland. The results were integrated with a quantitative assessment of variation in zinc hyperaccumulation to trace local adaptation. We identified a clear hierarchical structure with two distinct genetic groups at the upper level of clustering. Interestingly, these groups corresponded to different geographic subregions, rather than to ecological types (i.e., metallicolous vs. nonmetallicolous). Also, approximate Bayesian computation analyses suggested that the current distribution of A. halleri in southern Poland could be relictual as a result of habitat fragmentation caused by climatic shifts during the Holocene, rather than due to recent colonization of industrially polluted sites. In addition, we find evidence that some nonmetallicolous lowland populations may have actually derived from metallicolous populations. Meanwhile, the distribution of quantitative variation in zinc hyperaccumulation did separate metallicolous and nonmetallicolous accessions, indicating more recent adaptive evolution and diversifying selection between metalliferous and nonmetalliferous habitats. This suggests that zinc hyperaccumulation evolves both ways-towards higher levels at nonmetalliferous sites and lower levels at metalliferous sites. Our results open a new perspective on possible evolutionary relationships between A. halleri edaphic types that may inspire future genetic studies of quantitative variation in metal hyperaccumulation.

Can zinc pollution promote adaptive evolution in plants? Insights from a one-generation selection experiment.

Human activities generate environmental stresses that can lead plant populations to become extinct. Population survival would require the evolution of adaptive responses that increase tolerance to these stresses. Thus, in pseudometallophyte species that have colonized anthropogenic metalliferous habitats, the evolution of increased metal tolerance is expected in metallicolous populations. However, the mechanisms by which metal tolerance evolves remain unclear. In this study, parent populations were created from non-metallicolous families of Noccaea caerulescens. They were cultivated for one generation in mesocosms and under various levels of zinc (Zn) contamination to assess whether Zn in soil represents a selective pressure. Individual plant fitness estimates were used to create descendant populations, which were cultivated in controlled conditions with moderate Zn contamination to test for adaptive evolution in functional traits. The number of families showing high fitness estimates in mesocosms was progressively reduced with increasing Zn levels in soil, suggesting increasing selection for metal tolerance. In the next generation, adaptive evolution was suggested for some physiological and ecological traits in descendants of the most exposed populations, together with a significant decrease of Zn hyperaccumulation. Our results confirm experimentally that Zn alone can be a significant evolutionary pressure promoting adaptive divergence among populations.

Demographic history of the trace metal hyperaccumulator Noccaea caerulescens (J. Presl and C. Presl) F. K. Mey. in Western Europe.

Noccaea caerulescens (Brassicaceae) is a major pseudometallophyte model for the investigation of the genetics and evolution of metal hyperaccumulation in plants. We studied the population genetics and demographic history of this species to advance the understanding of among-population differences in metal hyperaccumulation and tolerance abilities. Sampling of seven to 30 plants was carried out in 62 sites in Western Europe. Genotyping was carried out using a combination of new chloroplast and nuclear neutral markers. A strong genetic structure was detected, allowing the definition of three genetic subunits. Subunits showed a good geographic coherence. Accordingly, distant metallicolous populations generally belonged to distinct subunits. Approximate Bayesian computation analysis of demographic scenarios among subunits further supported a primary isolation of populations from the southern Massif Central prior to last glacial maximum, whereas northern populations may have derived during postglacial recolonization events. Estimated divergence times among subunits were rather recent in comparison with the species history, but certainly before the establishment of anthropogenic metalliferous sites. Our results suggest that the large-scale genetic structure of N. caerulescens populations pre-existed to the local adaptation to metalliferous sites. The population structure of quantitative variation for metal-related adaptive traits must have established independently in isolated gene pools. However, features of the most divergent genetic unit (e.g. extreme levels of Cd accumulation observed in previous studies) question the putative relationships between adaptive evolution of metal-related traits and subunits isolation. Finally, admixture signals among distant metallicolous populations suggest a putative role of human activities in facilitating long-distance genetic exchanges.

Potential preadaptation to anthropogenic pollution: evidence from a common quantitative trait locus for zinc and cadmium tolerance in metallicolous and nonmetallicolous accessions of Arabidopsis halleri.

As a drastic environmental change, metal pollution may promote the rapid evolution of genetic adaptations contributing to metal tolerance. In Arabidopsis halleri, genetic bases of zinc (Zn) and cadmium (Cd) tolerance have been uncovered only in a metallicolous accession, although tolerance is species-wide. The genetic determinants of Zn and Cd tolerance in a nonmetallicolous accession were thus investigated for the first time. The genetic architecture of tolerance was investigated in a nonmetallicolous population (SK2) by using first backcross progeny obtained from crosses between SK2 and Arabidopsis lyrata petraea, a nonmetallophyte species. Only one significant and common quantitative trait locus (QTL) region was identified explaining 22.6% and 31.2% of the phenotypic variation for Zn and Cd tolerance, respectively. This QTL co-localized with HEAVY METAL ATPASE 4 (AhHMA4), which was previously validated as a determinant of Zn and Cd tolerance in a metallicolous accession. Triplication and high expression of HMA4 were confirmed in SK2. In contrast, gene duplication and high expression of METAL TOLERANT PROTEIN 1A (MTP1A), which was previously associated with Zn tolerance in a metallicolous accession, were not observed in SK2. Overall, the results support the role of HMA4 in tolerance capacities of A. halleri that may have pre-existed in nonmetallicolous populations before colonization of metal-polluted habitats. Preadaptation to metal-contaminated sites is thus discussed.

The five AhMTP1 zinc transporters undergo different evolutionary fates towards adaptive evolution to zinc tolerance in Arabidopsis halleri.

Gene duplication is a major mechanism facilitating adaptation to changing environments. From recent genomic analyses, the acquisition of zinc hypertolerance and hyperaccumulation characters discriminating Arabidopsis halleri from its zinc sensitive/non-accumulator closest relatives Arabidopsis lyrata and Arabidopsis thaliana was proposed to rely on duplication of genes controlling zinc transport or zinc tolerance. Metal Tolerance Protein 1 (MTP1) is one of these genes. It encodes a Zn(2+)/H(+) antiporter involved in cytoplasmic zinc detoxification and thus in zinc tolerance. MTP1 was proposed to be triplicated in A. halleri, while it is present in single copy in A. thaliana and A. lyrata. Two of the three AhMTP1 paralogues were shown to co-segregate with zinc tolerance in a BC1 progeny from a cross between A. halleri and A. lyrata. In this work, the MTP1 family was characterized at both the genomic and functional levels in A. halleri. Five MTP1 paralogues were found to be present in A. halleri, AhMTP1-A1, -A2, -B, -C, and -D. Interestingly, one of the two newly identified AhMTP1 paralogues was not fixed at least in one A. halleri population. All MTP1s were expressed, but transcript accumulation of the paralogues co-segregating with zinc tolerance in the A. halleri X A. lyrata BC1 progeny was markedly higher than that of the other paralogues. All MTP1s displayed the ability to functionally complement a Saccharomyces cerevisiae zinc hypersensitive mutant. However, the paralogue showing the least complementation of the yeast mutant phenotype was one of the paralogues co-segregating with zinc tolerance. From our results, the hypothesis that pentaplication of MTP1 could be a major basis of the zinc tolerance character in A. halleri is strongly counter-balanced by the fact that members of the MTP1 family are likely to experience different evolutionary fates, some of which not concurring to increase zinc tolerance.

Nuclear and chloroplast DNA phylogeography reveals vicariance among European populations of the model species for the study of metal tolerance, Arabidopsis halleri (Brassicaceae).

Arabidopsis halleri is a pseudometallophyte involved in numerous molecular studies of the adaptation to anthropogenic metal stress. In order to test the representativeness of genetic accessions commonly used in these studies, we investigated the A. halleri population genetic structure in Europe. Microsatellite and nucleotide polymorphisms from the nuclear and chloroplast genomes, respectively, were used to genotype 65 populations scattered over Europe. The large-scale population structure was characterized by a significant phylogeographic signal between two major genetic units. The localization of the phylogeographic break was assumed to result from vicariance between large populations isolated in southern and central Europe, on either side of ice sheets covering the Alps during the Quaternary ice ages. Genetic isolation was shown to be maintained in western Europe by the high summits of the Alps, whereas admixture was detected in the Carpathians. Considering the phylogeographic literature, our results suggest a distinct phylogeographic pattern for European species occurring in both mountain and lowland habitats. Considering the evolution of metal adaptation in A. halleri, it appears that recent adaptations to anthropogenic metal stress that have occurred within either phylogeographic unit should be regarded as independent events that potentially have involved the evolution of a variety of genetic mechanisms.

Sublethal effects of metal toxicity and the measure of plant fitness in ecotoxicological experiments.

Anthropogenic pollution is a major driver of global environmental change. To be properly addressed, the study of the impact of pollutants must consider both lethal effects and sublethal effects on individual fitness. However, measuring fitness remains challenging. In plants, the total number of seeds produced, i.e. the seed set, is traditionally considered, but is not readily accessible. Instead, performance traits related to survival, e.g., vegetative biomass and reproductive success, can be measured, but their correlation with seed set has rarely been investigated. To develop accurate estimates of seed set, relationships among 15 vegetative and reproductive traits were analyzed. For this purpose, Noccaea caerulescens (Brassicaceae), a model plant to study local adaptation to metal-contaminated environments, was used. To investigate putative variation in trait relationships, sampling included several accessions cultivated in contrasting experimental conditions. To test their applicability, selected estimates were used in the first generation of a Laboratory Natural Selection (LNS) experiment exposing experimentally plants to zinc soil pollution. Principal component analyses revealed statistical independence between vegetative and reproductive traits. Traits showing the strongest positive correlation with seed set were the number of non-aborted silicles, and the product of this number and mean silicle length. They thus appeared the most appropriate to document sublethal or fitness effects of environmental contaminants in plant ecotoxicological studies. The relevance of both estimates was confirmed by using them to assess the fitness of parental plants of the first generation of an LNS experiment: the same families consistently displayed the highest or the lowest performance values in two independent experimental metal-exposed populations. Thus, both these fitness estimates could be used to determine the expected number of offspring and the composition of successive generations in further LNS experiments investigating the impact of multi-generational exposure of a plant species to environmental pollution.

Biochemical composition of Phleum pratense pollen grains: A review.

The Poaceae family is composed of 12,000 plant species. Some of these species produce highly allergenic anemophilous pollen grains (PGs). Phleum pratense pollen grains (PPPGs) emerged as a model for studies related to grass allergy. The biochemical composition of allergenic PGs has not yet been fully described despite potential health effects of PG constituents other than allergenic proteins. This review brings together the information available in literature aiming at creating a comprehensive picture of the current knowledge about the chemical composition of allergenic PGs from timothy grass. PPPGs have an average diameter between 30-35 µm and the mass of a single PG was reported between 11 and 26 ng. The pollen cytoplasm is filled with two types of pollen cytoplasmic granules (PCGs): the starch granules and the polysaccharide particles (p-particles). Starch granules have a size between 0.6-2.5 µm with an average diameter of 1.1 µm (estimated number of 1000 granules per PG) while p-particles have a size ranging around 0.3 to 0.4 µm (estimated number between 61,000-230,000 p-particles per PG). The rupture of PG induces the release of PCGs and the dispersion of allergens in the inhalable fraction of atmospheric aerosol. PPPGs are composed of sporopollenin, sugars, polysaccharides, starch, glycoproteins (including allergens), amino-acids, lipids, flavonoids (including isorhamnetin), various elements (the more abundant being Si, Mg and Ca), phenolic compounds, phytoprostanoids, carotenoids (pigments) metals and adsorbed pollutants. PPPG contains about a hundred different proteins with molecular masses ranging from 10 to 94 kDa, with isoelectric points from 3.5-10.6. Among these proteins, allergens are classified in eleven groups from 1 to 13 with allergens from groups 1 and 5 being the major contributors to Phl p pollen allergy. Major allergen Phl p 5 was quantified in PPPGs by several studies with concentration ranging from 2.7 and 3.5 µg.mg-1 in unpolluted environment. Values for other allergens are scarce in literature; only one quantitative assessment exists for allergen groups Phl p 1, 2 and 4. The extractible lipid fraction of PPPGs is estimated between 1.7-2.2% of the total PG mass. The main chemical families of lipids reported in PPPGs are: alkanes, alkenes, alcohols, saturated and unsaturated fatty acids, di- and tri-hydroxylated fatty acids, aldehydes and sterols. Several lipid compounds with potential adjuvant effects on allergy have been specifically quantified in PPPGs: E2-like prostaglandin (PGE2), B4-like leukotriene (LTB4), unsaturated fatty acids (linoleic and linolenic acids and their hydroxylated derivatives), adenosine, vitamins and phenolic compounds. Some other biochemical characteristics such as NAD(P)H oxidase, protease activity and pollen microbiome were described in the literature. The bioaccessibility in physiological conditions has not been described for most biochemicals transported by allergenic PPPGs. There is also a considerable lack of knowledge about the potential health effects of pollen constituents other than allergens. The variability of pollen composition remains also largely unknown despite its importance for plant reproduction and allergy in an environment characterized by chemical pollution, climate change and loss of biodiversity.

When population genetics serves genomics: putting adaptation back in a spatial and historical context.

Recent advances in molecular biology have opened new perspectives for the study of plant adaptation, especially at the intraspecific level. Nowadays, scientists employing -omic results in multiple scientific fields can be optimistic of their chances of revealing mechanisms involved in adaptive population divergence. However, the investment required by integrative studies greatly reduces the number of experiments that can be performed. In this context, a comprehensive choice of accessions under study is crucial. We maintain this choice could be appreciably enlightened by population genetics because it helps putting adaptive population divergence in a spatial and historical context. As an example, we highlight the usefulness of knowledge about population genetic structure in the integrative study of metal tolerance in Arabidopsis halleri.

Quantitative trait loci analysis of mineral element concentrations in an Arabidopsis halleri x Arabidopsis lyrata petraea F2 progeny grown on cadmium-contaminated soil.

SUMMARY: This study describes the quantitative trait locus (QTL) analysis of cadmium (Cd), zinc (Zn), iron (Fe), potassium (K), magnesium (Mg) and calcium (Ca) accumulation in the pseudometallophyte Arabidopsis halleri under conditions of Cd excess using an interspecific A. halleri x Arabidopsis lyrata F(2) population. *Our data provide evidence for the implication of one major QTL in Cd hyperaccumulation in A. halleri, and suggests that Cd tolerance and accumulation are not independent in A. halleri. Moreover, the major loci responsible for Zn hyperaccumulation in the absence of Cd appear to be the same when Cd is present at high concentrations. *More than twofold higher Fe concentrations were measured in A. halleri shoots than in A. lyrata, suggesting a different regulation of Fe accumulation in the hyperaccumulator. *With the exception of Ca, the accumulation of Cd was significantly correlated with the accumulation of all elements measured in the F(2) progeny, suggesting pleiotropic gene action. However, QTL analysis identified pleiotropic QTLs only for Cd, Zn and Fe. Mg accumulation was negatively correlated with Cd accumulation, as well as with dry shoot biomass, suggesting that it might indicate cellular damage.

Variability of zinc tolerance among and within populations of the pseudometallophyte species Arabidopsis halleri and possible role of directional selection.

We estimated the level of quantitative polymorphism for zinc (Zn) tolerance in neighboring metallicolous and nonmetallicolous populations of Arabidopsis halleri and tested the hypothesis that divergent selection has shaped this polymorphism. A short-term hydroponic test was used to capture the quantitative polymorphism present between edaphic types, among and within populations. We measured six morphological and physiological traits on shoots and roots to estimate the response of A. halleri to Zn. In order to assess the adaptive value of Zn tolerance polymorphism, we compared differentiation of quantitative traits with that of molecular markers. Zinc tolerance of metallicolous populations was, on average, higher than that of nonmetallicolous populations according to the morphological and physiological traits measured. Phenotypic variability within edaphic types was very high and mainly explained by polymorphism among individuals within populations. Genetic differentiation for photosystem II yield of leaves (a measure of photosynthetic efficiency) was greater than the differentiation for microsatellite and thus, probably shaped by divergent selection. Overall, these results suggest that, in the sampled populations, Zn tolerance has been increased in metallicolous populations through selection on standing genetic variation within local nonmetallicolous ancestral populations.

Genetic architecture of zinc hyperaccumulation in Arabidopsis halleri: the essential role of QTL x environment interactions.

SUMMARY: This study sought to determine the main genomic regions that control zinc (Zn) hyperaccumulation in Arabidopsis halleri and to examine genotype x environment effects on phenotypic variance. To do so, quantitative trait loci (QTLs) were mapped using an interspecific A. halleri x Arabidopsis lyrata petraea F(2) population. *The F(2) progeny as well as representatives of the parental populations were cultivated on soils at two different Zn concentrations. A linkage map was constructed using 70 markers. *In both low and high pollution treatments, zinc hyperaccumulation showed high broad-sense heritability (81.9 and 74.7%, respectively). Five significant QTLs were detected: two QTLs specific to the low pollution treatment (chromosomes 1 and 4), and three QTLs identified at both treatments (chromosomes 3, 6 and 7). These QTLs explained 50.1 and 36.5% of the phenotypic variance in low and high pollution treatments, respectively. Two QTLs identified at both treatments (chromosomes 3 and 6) showed significant QTL x environment interactions. *The QTL on chromosome 3 largely colocalized with a major QTL previously identified for Zn and cadmium (Cd) tolerance. This suggests that Zn tolerance and hyperaccumulation share, at least partially, a common genetic basis and may have simultaneously evolved on heavy metal-contaminated soils.

Zinc distribution and speciation in Arabidopsis halleri x Arabidopsis lyrata progenies presenting various zinc accumulation capacities.

The purpose of this study was to investigate the relationship between the chemical form and localization of zinc (Zn) in plant leaves and their Zn accumulation capacity. An interspecific cross between Arabidopsis halleri sp. halleri and Arabidopsis lyrata sp. petrea segregating for Zn accumulation was used. Zinc (Zn) speciation and Zn distribution in the leaves of the parent plants and of selected F(1) and F(2) progenies were investigated by spectroscopic and microscopic techniques and chemical analyses. A correlation was observed between the proportion of Zn being in octahedral coordination complexed to organic acids and free in solution (Zn-OAs + Zn(aq)) and Zn content in the leaves. This pool varied between 40% and 80% of total leaf Zn depending on the plant studied. Elemental mapping of the leaves revealed different Zn partitioning between the veins and the leaf tissue. The vein : tissue fluorescence ratio was negatively correlated with Zn accumulation. The higher proportion of Zn-OAs + Zn(aq) and the depletion of the veins in the stronger accumulators are attributed to a higher xylem unloading and vacuolar sequestration in the leaf cells. Elemental distributions in the trichomes were also investigated, and results support the role of carboxyl and/or hydroxyl groups as major Zn ligands in these cells.

Bulk pollen sequencing reveals rapid evolution of segregation distortion in the male germline of Arabidopsis hybrids.

Genes that do not segregate in heterozygotes at Mendelian ratios are a potentially important evolutionary force in natural populations. Although the impacts of segregation distortion are widely appreciated, we have little quantitative understanding about how often these loci arise and fix within lineages. Here, we develop a statistical approach for detecting segregation distorting genes from the comprehensive comparison of whole genome sequence data obtained from bulk gamete versus somatic tissues. Our approach enables estimation of map positions and confidence intervals, and quantification of effect sizes of segregation distorters. We apply our method to the pollen of two interspecific F1 hybrids of Arabidopsis lyrata and A. halleri and we identify three loci across eight chromosomes showing significant evidence of segregation distortion in both pollen samples. Based on this, we estimate that novel segregation distortion elements evolve and achieve high frequencies within lineages at a rate of approximately one per 244,000 years. Furthermore, we estimate that haploid-acting segregation distortion may contribute between 10% and 30% of reduced pollen viability in F1 individuals. Our results indicate haploid acting factors evolve rapidly and dramatically influence segregation in F1 hybrid individuals.

The genetic architecture of a complex trait: Resistance to multiple toxins produced by Bacillus thuringiensis israelensis in the dengue and yellow fever vector, the mosquito Aedes aegypti.

The bacterial insecticide Bacillus thuringiensis subsp. israelensis (Bti) is an increasingly popular alternative to chemical insecticides for controlling mosquito populations. Because Bti toxicity relies on the action of four main toxins, resistance to Bti is very likely a complex phenotype involving several genes simultaneously. Dissecting the underlying genetic basis thus requires associating a quantitative measure of resistance to genetic variation at many loci in a segregating population. Here, we undertake this task using the dengue and yellow fever vector, the mosquito Aedes aegypti, as a study model. We conducted QTL (Quantitative Trait Locus) and admixture mapping analyses on two controlled crosses and on an artificial admixed population, respectively, all obtained from resistant and susceptible lab strains. We detected 16 QTL regions, among which four QTLs were revealed by different analysis methods. These four robust QTLs explained altogether 29.2% and 62.2% of the total phenotypic variance in the two QTL crosses, respectively. They also all showed a dominant mode of action. In addition, we found six loci showing statistical association with Bti resistance in the admixed population. Five of the supercontigs highlighted in this study contained candidate genes as suggested by their function, or by prior evidence from expression and/or outlier analyses. These genomic regions are thus good starting points for fine mapping of resistance to Bti or functional analyses aiming at identifying the underlying genes and mutations. Moreover, for the purpose of this work, we built the first Ae. aegypti genetic map based on markers associated with genes expressed in larvae. This genetic map harbors 229 SNP markers mapped across the three chromosomes for a total length of 311.9cM. It brought to light several assembly discrepancies with the reference genome, suggesting a high level of genome plasticity in Ae. aegypti.

Using biomarkers in an evolutionary context: lessons from the analysis of biological responses of oligochaete annelids to metal exposure.

Anthropogenic activities may lead to the accumulation of inorganic and organic compounds in topsoils. Biota living in close contact with contaminated soils may experience stress at different levels of biological organization throughout the continuum from molecular to community level. Biological responses observed at the individual or infra-individual level of biological organization led to the development of biomarkers. The development of biomarkers consists often in evidencing biological modifications following a contaminant stress in laboratory conditions, using naïve organisms and it is sometime proposed to use the biological state of individuals from sentinel species collected in the field to evaluate the level of environmental exposure. However, considering the possibility of local adaptation following long-term exposure, organisms response sampled in the field may substantially differ from laboratory specimens. In this review, we discuss this point focusing on the definition and validity of molecular biomarkers of metal pollution using earthworms of the Lumbricidae family.

Merging methods in molecular and ecological genetics to study the adaptation of plants to anthropogenic metal-polluted sites: implications for phytoremediation.

Metallophyte species that occur naturally on metal-enriched soils represent major biological resources for the improvement of phytoremediation, a benign and cost-effective technology that uses plants to clean up anthropogenic metal-polluted soils. Within the last decade, molecular genetic studies carried out on several model organisms (including Arabidopsis halleri) have considerably enhanced our understanding of metal tolerance and hyperaccumulation in plants, but the identification of the genes of interest for phytoremediation purposes remains a challenge. To meet this challenge, we propose to combine '-omics' with molecular ecology methods. Using A. halleri, we confronted molecular genetic results with: (i) within-species polymorphism and large-scale population differentiation for zinc tolerance; (ii) the demographical context (e.g. migration pattern) of the species for zinc tolerance evolution; (iii) the Quantitative Trait Loci (QTL) analysis of the genetic architecture for zinc tolerance; and (iv) the fine-scale dissection of identified QTL regions, to discuss more precisely the nature of the genes potentially involved in the adaptation to zinc-polluted soils.

A broad-scale analysis of population differentiation for Zn tolerance in an emerging model species for tolerance study: Arabidopsis halleri (Brassicaceae).

Although current knowledge about the overall distribution of zinc (Zn) tolerance in Arabidopsis halleri populations is scarce, the species is an emerging model for the study of heavy metal tolerance in plants. We attempted to improve this knowledge by testing the Zn tolerance of scattered European metallicolous (M) and nonmetallicolous (NM) populations of A. h. subsp. halleri and A. h. subsp. ovirensis in hydroponic culture. The occurrence of constitutive tolerance was unconditionally established in A. h. halleri and tolerance was extended to the subspecies ovirensis. M populations were the most tolerant but there was a continuous range of variation in tolerance from NM to M populations. Finally, relatively high levels of tolerance were detected in some NM populations, suggesting that enhanced tolerance could be present at high frequency in populations that have not experienced metal exposure. We used our results to argue the evolutionary dynamics and origin of Zn tolerance in A. halleri.