POLYMORPHISM AT GENES INVOLVED IN SALT TOLERANCE IN ARABIDOPSIS THALIANA (BRASSICACEAE)

Premise of the study: Genes involved in relevant functions for environmental adaptation can be considered primary candidates for their variation having been shaped by natural selection. Detecting recent selective events through their footprint on nucleotide variation constitutes a challenging task in species with a complex demographic history such as Arabidopsis thaliana. We have surveyed nucleotide variation in this species at nine genes involved in salt tolerance. The available genomewide information for this species has allowed us to contrast the levels and patterns of variation detected at the candidate genes with empirical distributions obtained from noncandidate regions. Methods: We sequenced nine genes involved in salt tolerance (similar to 32 kb) in 20 ecotypes of A. thaliana and analyzed polymorphism and divergence at the individual gene and multilocus levels. Key results: Variation at the nine genes studied was characterized by a generalized skew toward polymorphisms with low-frequency variants. Except for genes RCD1 and NHX8, this pattern was similar to that generally detected in the A. thaliana genome and could thus be primarily explained by the species demographic history. The more extreme deviation at the NHX8 gene and its excess of polymorphism relative to divergence points to the recent action of selection on this gene. Conclusions: The analysis of nucleotide polymorphism and divergence at nine genes involved in salt tolerance provided little evidence for the recent action of positive selection. Only the signals detected at NHX8 from both polymorphism and divergence were suggestive of the putative contribution of this gene to local adaptation.