title: Genetic analyses of nickel tolerance in a North American serpentine endemic plant, Caulanthus amplexicaulis var. barbarae (Brassicaceae).
abstract: OBJECTIVE
  The evolution of metal tolerance in plants is an important model for studies of adaptation to environment, population genetics, and speciation. Here, we investigated nickel tolerance in the North American serpentine endemic Caulanthus amplexicaulis var. barbarae in comparison with its nonserpentine sister taxon C. amplexicaulis var. amplexicaulis. We hypothesized that the serpentine endemic would have a heritable growth advantage on nickel-containing substrates.
  
  
  METHODS
  We employed an artificial growth assay to quantify biomass accumulation. Study plants were crossed to create an F(2:3) population that was used to determine the heritability of nickel tolerance and to map quantitative trait loci (QTL). Nickel accumulation in both laboratory populations and native specimens was examined using energy-dispersive x-ray fluorescence (EDXRF).
  
  
  RESULTS
  The serpentine endemic had a dramatic growth advantage at concentrations of nickel >30 µmol/L. Caulanthus amplexicaulis var. barbarae and its nonserpentine sister taxon both accumulated nickel to substantial levels. Nickel tolerance was highly heritable (h(2) = 0.59) and not associated with accumulation. The QTL analyses identified two major loci for nickel tolerance, on linkage group 2 (LG2) and linkage group 9 (LG9).
  
  
  CONCLUSIONS
  In our study, nickel tolerance was determined by two major loci with large effects. At both loci, alleles from the serpentine parent conferred positive effects on nickel tolerance, suggesting that they are adaptive in the natural serpentine environment. The mechanism of nickel tolerance in the serpentine plant was not exclusion of nickel. Nickel tolerance may have an inducible component in C. amplexicaulis var. barbarae.
authors.affiliation: Department of Biology, Texas A&M University, TAMUS 3258, College Station, Texas 77843-3258, USA.
authors.forename: A Millie
authors.initials: AM
authors.lastname: Burrell
authors.forename: Angela K
authors.initials: AK
authors.lastname: Hawkins
authors.forename: Alan E
authors.initials: AE
authors.lastname: Pepper
mesh_terms: D000222:Adaptation, Physiological / Q000235:genetics*; D017632:Asbestos, Serpentine / Q000032:analysis / Q000633:toxicity; D018533:Biomass; D019607:Brassicaceae / Q000187:drug effects* / Q000235:genetics / Q000254:growth & development; D018745:Genome, Plant / Q000235:genetics; D005838:Genotype; D009532:Nickel / Q000032:analysis / Q000633:toxicity*; D010641:Phenotype; D040641:Quantitative Trait Loci / Q000235:genetics; D012987:Soil / Q000737:chemistry; D012989:Soil Pollutants / Q000032:analysis / Q000633:toxicity*
chemical_list: D017632:Asbestos, Serpentine; D012987:Soil; D012989:Soil Pollutants; D009532:Nickel