The species Arabidopsis halleri, an emerging model for the study of heavy metal tolerance and accumulation in plants, has evolved a high level of constitutive zinc tolerance. Mapping of quantitative trait loci (QTL) was used to investigate the genetic architecture of zinc tolerance in this species. A firstgeneration backcross progeny of A. halleri ssp. halleri from a highly contaminated industrial site and its nontolerant relative A. lyrata ssp. petraea was produced and used for QTL mapping of zinc tolerance. A genetic map covering most of the A. halleri genome was constructed using 85 markers. Among these markers, 65 were anchored in A. thaliana and revealed high synteny with other Arabidopsis genomes. Three QTL of comparable magnitude on three different linkage groups were identified. At all QTL positions zinc tolerance was enhanced by A. halleri alleles, indicating directional selection for higher zinc tolerance in this species. The two-LOD support intervals associated with these QTL cover 24, 4, and 13 cM. The importance of each of these three regions is emphasized by their colocalization with HMA4, MTP1-A, and MTP1-B, respectively, three genes well known to be involved in metal homeostasis and tolerance in plants.M ETAL tolerance in plants has been considered ''an example of more powerful evolution in action than industrial melanism in moths' ' (Antonovics et al. 1971). Therefore it has been the focus of many evolutionary studies, in which it was argued that metal tolerance could evolve rapidly following exposure to heavy metal stress (Wu et al. 1975;Al-Hiyali et al. 1988). Some heavy metals, like zinc and copper, are oligo-nutrients and thus essential in small quantities for normal plant development. To avoid metal toxicity, all plants have evolved basic tolerance mechanisms. Binding by proteins or nonprotein thiol peptides in the cytoplasm and subsequent sequestration in the vacuole are the major component processes in the cellular heavy metal detoxification (Clemens 2001;Krämer 2005). However, at so-called metalliferous sites, heavy metals can occur at highly elevated concentrations in the soil, either through ancient natural processes, as in nickel-rich serpentine soils, or through recent human activities, as in zinc-and cadmium-rich calamine soils surrounding smelters. The total metal content of contaminated sites, depending on the metal, can be up to 10-to 1000-fold higher than that of uncontaminated sites (Bert et al. 2002). At these extreme concentrations, both essential and nonessential heavy metals become toxic (Clemens 2001;Hall 2002) and only a small number of plant species have evolved tolerance to such concentrations. These species have been classified as either absolute (strict or eu-) or facultative (pseudo-) metallophytes, according to their occurrence either on contaminated sites only or on both metalliferous and nonmetalliferous soils (Lambinon and Auquier 1964).The genetic basis of adaptive quantitative traits is still the matter of strong debates among evolutionists. Current que...

Summary• 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.

Summary• This study sought to determine the main genomic regions that control zinc (Zn) hyperaccumulation in Arabidopsis halleri and to examine genotype · environment effects on phenotypic variance. To do so, quantitative trait loci (QTLs) were mapped using an interspecific A. halleri · 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 · 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.