Microsatellite polymorphism in natural populations of the wild plant Arabidopsis thaliana

Variation in repeat number at 20 microsatellite loci of Arabidopsis thaliana was studied in a worldwide sample of 42 ecotypes to investigate the pattern and level of polymorphism in repetitive sequences in natural plant populations. There is a substantial amount of variation at microsatellite loci despite the selfing nature of this plant species. The average gene diversity was 0.794 and the average number of alleles per locus was 10.6. The distribution of alleles was centered around the mean of repeat number at most loci, but could not be regarded as normal. There was a significantly positive correlation between the number of repeats and the amount of variation. For most loci, the observed number of alleles was between the expected values of the infinite allele and stepwise mutation models. The two models were rejected by the sign test. Linkage disequilibrium was detected in 12.1% of the pairwise comparisons between loci. In phylogenetic tree, there was no association between ecotype and geographic origin. This result is consistent with the recent expansion of A. thaliana throughout the world.