Exploring the pattern of phenotypic and genetic polymorphism in the arsenic hyperaccumulator Pteris vittata L. (Chinese brake fern)

In plants, hyperaccumulation of metals and metalloids is considered to be a positively selected trait for defense against herbivory and parasitism. However, few investigations have been performed on the patterns of molecular polymorphism of genes putatively related to the hyperaccumulation phenotype. We explored the genetic polymorphism of 32 accessions across China of the arsenic (As)-hyperaccumulator fern Pteris vittata. Nineteen loci, 9 of which code for proteins putatively involved in As hyperaccumulation, were sequenced. A relatively high polymorphism was detected in genes putatively related to As accumulation. No evidence for positive selection was present at any of the investigated loci. Mantel's test performed on genetic distances versus geographical, soil As concentration, translocation and bioaccumulation factor distances showed a partial geographical clustering of populations but no correlation between genetic distances and edaphic or hyperaccumulation parameters. Our study suggest that constitutive As hyperaccumulation in P. vittata, though variable between accessions, did not produce detectable signatures on protein coding sequences of gene putatively related to As hyperaccumulation. These data may be in agreement with an ancient origin of the hyperaccumulation phenotype and, at least for the protein sequence, a relaxation of selective pressure on genes implied in this trait.