Intragenic recombination in the Adh locus of the wild plant Arabidopsis thaliana.

Nucleotide variation in the Adh region of the wild plant Arabidopsis thaliana was analyzed in 17 ecotypes sampled worldwide to investigate DNA polymorphism in natural plant populations. The investigated 2.4-kb Adh region was divided into four blocks by intragenic recombinations between two parental sequence types that diverged 6.3 million years (Myr) ago, if the nucleotide mutation rate mu = 10(-9) is assumed. Within each block, dimorphism of segregating variations was observed with intermediate frequencies, which caused a substantial amount of nucleotide variation in A. thaliana at the species level. The first recombination introduced the divergent variation that resulted in dimorphism in this plant species approximately 3.3 Myr ago, and three subsequent intragenic combinations have occurred sporadically in approximately 1.1-Myr intervals. It was shown that there was only a limited number (six) of sequence types in this species and that no clear association was observed between sequence type and geographic origin. Taken together, these results suggest that A. thaliana has spread over the world only recently. It can be concluded that recombination played an important role in the evolutionary history of A. thaliana, especially through the generation of DNA polymorphism in the natural populations of this plant species.