T he wild crucifer Arabidopsis thaliana has become an important model system because it allows genetics to combine with molecular biology (i.e. it has a short generation time, a small genome and is easily transformed) Genetic variation is required for the functional analysis of the Arabidopsis genome Laboratory-induced mutants Currently, the functional analysis of Arabidopsis genes and the dissection of complex traits are based largely on the phenotypic characterization of mutants selected by forward and reverse genetics in a few laboratory 'wild-type' genotypes. The inbred strains generally used are Landsberg erecta (Ler), Columbia (Col) and Wassilewskija (Ws), originally collected from the wild by the pioneers of Arabidopsis research, Friedrich Laibach and colleagues 3 . These forward and reverse genetic approaches using classical (physical or chemical) and insertional (biological) mutagens have proven their usefulness 1 . The possibility of identifying genotypes with an insertion in a gene of known nucleotide sequence, independently of the presence of a phenotype, has led to large-scale projects for disrupting most of the Arabidopsis genes Naturally occurring variants: new times for an old resource As an alternative to generating laboratory-induced mutants, another source of genetic variation can be found among and within naturally occurring populations of Arabidopsis Exploitation of the genetic variation among accessions has been limited because of its mostly quantitative (continuous) nature, in contrast with the commonly studied mutants, which provide qualitative (discrete) variation. This dichotomy is defined basically by the number of loci and the environmental effect underlying the variation under study, which determine the tools used for its analysis. Only in the past decade, with the advent of efficient molecular marker technologies and specific statistical methods, has the map position and the effects of quantitative trait loci (QTL) been establishe
