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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 first-generation 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.

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.

Cadmium (Cd) tolerance seems to be a constitutive species-level trait in Arabidopsis halleri. In order to identify genes potentially implicated in Cd tolerance, a backcross (BC1) segregating population was produced from crosses between A. halleri ssp. halleri and its closest non-tolerant relative A. lyrata ssp. petraea. The most sensitive and tolerant genotypes of the BC1 were analysed on a transcriptome-wide scale by cDNA-amplified fragment length polymorphism (AFLP). A hundred and thirty-four genes expressed more in the root of tolerant genotypes than in sensitive genotypes were identified. Most of the identified genes showed no regulation in their expression when exposed to Cd in a hydroponic culture medium and belonged to diverse functional classes, including reactive oxygen species (ROS) detoxification, cellular repair, metal sequestration, water transport, signal transduction, transcription regulation, and protein degradation, which are discussed.

The population structure of the pseudo-metallophyte herb, Arabidopsis halleri, was studied using Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP) on chloroplast DNA (cpDNA). The history of metallicolous (M) populations showing increased zinc tolerance was investigated. Eight primer-enzyme combinations out of 72 tested were applied to a total of 625 individuals from 28 widespread populations, 14 of them being M. Eleven distinct chlorotypes were found: five were common to nonmetallicolous (NM) and M populations, whereas six were only observed in one edaphic type (five in NM and one in M). No difference in chlorotype diversity between edaphic types was detected. Computed on the basis of chlorotype frequencies, the level of population differentiation was high but remained the same when taking into account levels of molecular divergence between chlorotypes. Isolation by distance was largely responsible for population differentiation. Geographically isolated groups of M populations were more genetically related to their closest NM populations than to each other. Our results suggest that M populations have been founded separately from distinct NM populations without suffering founding events and that the evolution towards increased tolerance observed in the distinct M population groups occurred independently.

Arabidopsis halleri, a close wild relative of A. thaliana, is a clonal, insect-pollinated herb tolerant to heavy metals (Zn, Pd, Cd) and a hyperaccumulator of Zn and Cd. It is of particular interest in the study of evolutionary processes and phytoremediation. However, little is known about its population gene flow patterns and the structure of its genetic diversity. We used five microsatellite loci to investigate the genetic structure at a fine spatial scale (10 cm to 500 m) in a metallicolous population of A. halleri. We also studied the contributions made by clonal propagation and sexual reproduction (seed and pollen dispersal) to the genetic patterns. Clonal diversity was high (D(G) > 0.9). Clonal spread occurs only at short distances (< 1 m). Both clonal spread and limited dispersal, associated with sexual reproduction, contribute to the significant spatial genetic structure revealed by spatial autocorrelation analysis. The shape of the autocorrelogram suggests that seed dispersal is restricted and pollen flow extensive, which may be related to intense activity by insect pollinators. Clonal spread was more extensive in the lowly polluted zone than in the highly polluted zone. This cannot be interpreted as a strategy for promoting the propagation of adapted genotypes under the harshest ecological constraints (highest heavy metal concentrations). The higher fine-scale spatial genetic structure found in the lowly polluted zone can be ascribed to plant densities that were lower than in the highly polluted zone. No evidence of genetic divergence due to spatial heavy metal heterogeneity was found between lowly and highly polluted zones.

The zinc hyperaccumulator plant Arabidopsis halleri is able to naturally accumulate 100-fold higher leaf zinc concentrations when compared with non-accumulator species such as the closely related A. lyrata and A. thaliana, without showing toxicity symptoms. A novel member of the cation diffusion facilitator (CDF) protein family, an A. halleri metal tolerance protein 1 (MTP1), and the homologous A. thaliana Zn transporter (ZAT)/AtMTP1 metal-specifically complement the zinc hypersensitivity of a Saccharomyces cerevisiae zrc1 cot1 mutant strain. A fusion of the AhMTP1 protein to green fluorescent protein (GFP) localizes to the vacuolar membrane of A. thaliana protoplasts. When compared with A. lyrata and A. thaliana, the total MTP1 transcript levels are substantially higher in the leaves and upregulated upon exposure to high zinc concentrations in the roots of A. halleri. The high MTP1 transcript levels in A. halleri can be primarily attributed to two genetically unlinked genomic AhMTP1 gene copies. The two corresponding loci co-segregate with zinc tolerance in the back-cross 1 generation of a cross between the zinc-tolerant species A. halleri and the zinc-sensitive species A. lyrata. In contrast, a third MTP1 gene in the genome of A. halleri generates only minor amounts of MTP1 transcripts and does not co-segregate with zinc tolerance. Our data suggests that zinc tolerance in A. halleri involves an expanded copy number of an ancestral MTP1 gene, encoding functional proteins that mediate the detoxification of zinc in the cell vacuole. At the transcript level, MTP1 gene copies of A. halleri are regulated differentially and in response to changes in zinc supply.

•  The ability of metallicolous and nonmetallicolous populations of Arabidopsis halleri to accumulate zinc (Zn), cadmium (Cd) and lead (Pb) is compared here in order to explore the extent and variability of this trait in wild A. halleri plants. •  Aerial plant parts and the soil around the harvested plants were collected and analysed for metal concentrations or total and extractable metal concentrations, respectively, for 20 metallicolous and 13 nonmetallicolous populations. •  Results show that metallicolous and nonmetallicolous populations have the same ability to accumulate Zn and Cd but that neither population type is able to accumulate Pb. Between populations within type, an homogenous accumulating response is observed for Zn, whereas the ability to accumulate Cd is variable. •  Zn and Cd accumulation to very high concentrations is a constitutive property of the species. The Zn and Cd hyperaccumulator trait of A. halleri from contaminated sites was confirmed. Interestingly, nonmetallicolous plants are Zn and Cd hyperaccumulators. The possibility of using A. halleri in phytoremediation is discussed.

Lloyd's (1975) and Charlesworth & Charlesworth's (1978) phenotypic selection models for the maintenance of androdioecy predict that males (female-sterile individuals) must have an advantage in fertility (K) of at least two in order to invade a hermaphroditic population, and that their equilibrium frequency (x(eq)=(K - 2)/2(K - 1)) is always less than 0.5. In this paper, we develop a model in which male fertility is frequency-dependent, a situation not investigated in the previous models, to explore the conditions under which a high frequency of males (i.e. more than 50%) could be maintained at equilibrium. We demonstrate that a gametophytic self-incompatibility (GSI) locus linked to a nuclear sex determination locus can favour rare alleles through male function, by causing frequency-dependent selection. Thus, the spread of a female-sterility allele in a hermaphroditic population may be induced. In contrast with the previous models, our model can explain male frequencies greater than 50% in a functionally androdioecious species, as long as there is (i) dominance of female-sterility at the sex locus, and (ii) a few alleles at the self-incompatibility locus, even if the advantage in fertility of male phenotype is lower than two.

Androdioecy is a rare breeding system in which low male frequency is expected in populations because males require a strong increase in their fertility to be maintained by selection. Phillyrea angustifolia L. has previously been reported as possibly functionally androdioecious. However, 1&rcolon;1 sex ratios have been reported and suggest functional dioecy. In this article, we compared both pollen tube growth and siring success of male and hermaphrodite pollen in two single-donor pollination experiments. We verified at both pre- and postzygotic levels that hermaphrodites produce functional pollen. Self-incompatibility was also clearly established. However, pollen from hermaphrodites was less efficient than male pollen. The probability of a pollen tube growing through the style was higher for male than for hermaphrodite pollen donors, and males sired twice as many fruits as hermaphrodites. The twofold male advantage in relative fecundity was mainly because of lower pollen fertility of hermaphrodites and possible cross-incompatibility among hermaphrodites.

The hyperaccumulation of metals by a rare class of plants is a fascinating and little understood phenomenon. No genetic analysis has been possible since no intraspecific variation is known for this character. Here, we report on crosses between the zinc-hyperaccumulating and -tolerant species Arabidopsis halleri and the non-hyperaccumulating, non-tolerant species Arabidopsis petraea. The F2 segregates for both characters and it appears that the two characters are genetically independent. The data for tolerance are consistent with a single major gene for this character (although the number of genes for hyperaccumulation cannot be determined), and is probably not very large.

Recent advances in molecular biology have allowed the development of techniques to contrast spatial differentiation in nuclear and cytoplasmic genes and thus provide important data on relative levels of gene flow by pollen and seed in higher plants. In this paper, we compare the spatial structure of nuclear (allozymes) and cytoplasmic (cpDNA) genes among populations of the gynodioecious Thymus vulgaris in southern France. Based on a combination of three restriction enzymes (CfoI, EcoRV, and PstI), eight chlorotypes (combination of three restriction enzyme patterns revealed by Southern hybridization of Beta vulgaris cpDNA) were identified in the 13 studied populations. One chlorotype was particularly abundant and was detected in nearly all populations. Only one chlorotype was specific to a single population. Up to four different chlorotypes were observed in some populations. An FST of 0.238 (P &lt; 0.002) for cpDNA haplotypes indicates spatial structure of cytoplasmic genes among the studied populations. Similar patterns were found within a single young population (CAB) structured in patches and surrounded by a continuous cover of T. vulgaris where the FST is 0.546 (P &lt; 0.002). No significant correlation between sex and chlorotype nor between cpDNA diversity and female frequency was detected. Allozyme markers showed markedly less spatial structure (FST = 0.021 among populations and 0.019 in the CAB population, P &lt; 0.001). This difference between cpDNA and nuclear allozyme markers suggests that pollen dispersal is more important than seed dispersal both among and within populations.

The nuclear receptor (NR) superfamily comprises, in addition to ligand-activated transcription factors, members for which no ligand has been identified to date. We demonstrate that orphan receptors are randomly distributed in the evolutionary tree and that there is no relationship between the position of a given liganded receptor in the tree and the chemical nature of its ligand. NRs are specific to metazoans, as revealed by a screen of NR-related sequences in early- and non-metazoan organisms. The analysis of the NR gene duplication pattern during the evolution of metazoans shows that the present NR diversity arose from two waves of gene duplications. Strikingly, our results suggest that the ancestral NR was an orphan receptor that acquired ligand-binding ability during subsequent evolution.

A much-debated issue in plant evolutionary biology concerns the maintenance of a high frequency of male sterility in natural populations. For the past decade, a theoretical framework has been provided by the concept of nucleocytoplasmic conflict. Recent molecular studies on cytoplasmic male sterility indicate that novel chimeric genes, resulting from duplications and rearrangements of mitochondrial DNA sequences, are involved In its control. Thus, male sterility, which is phenotypically the loss of the male function, is encoded by a new mitochondrial function at the molecular level. Molecular data are in agreement with theoretical models that consider cytoplasmic male sterility as a stage in the coevolution between nucleus and mitochondria, and not simply as a deleterious mitochondrial mutation., (Copyright © 1994. Published by Elsevier Ltd.)