INTRAGENIC RECOMBINATION IN THE ADH LOCUS OF THE WILD PLANT ARABIDOPSIS-THALIANA

Nucleotide variation in the Adh region of the wild plant Arabidopsis t haliana was analyzed in 17 ecotypes sampled worldwide to investigate D NA polymorphism in natural plant populations. The investigated 2.4-kb Adh region was divided into four blocks by intragenic recombinations b etween two parental sequence types that diverged 6.3 million years (My r) ago, if the nucleotide mutation rate mu = 10(-9) is assumed. Within each block, dimorphism of segregating variations was observed with in termediate frequencies, which caused a substantial amount of nucleotid e variation in A. thaliana at the species level. The first recombinati on introduced the divergent variation that resulted in dimorphism in t his plant species similar to 3.3 Myr ago, and three subsequent intrage nic recombinations have occurred sporadically in similar to 1.1-Myr in tervals. It was shown that there was only a limited number (six) of se quence types in this species and that no clear association was observe d between sequence type and geographic origin. Taken together, these r esults suggest that A. thaliana has spread over the world only recentl y. It can be concluded that recombination played an important role in the evolutionary history of A. thaliana, especially through the genera tion of DNA polymorphism in the natural populations of this plant spec ies.