Characteristics of Microsatellites in Brassica rapa genome and their potential utilization for comparative genomics in Cruciferae

We evaluated the nature of microsatellites in Brassica rapa in order to develop an informative and reliable DNA marker system for Brassica genetic analysis. Microsatellites were isolated by hybridization screening of an unconcentrated small-insert genomic library using tri- and dinucleotide probes. Of 45,000 clones screened, 210 had repeat sequences, in which 228 microsatellites were identified. The most frequent microsatellite motif was (GA)(n) at a frequency of one every 4.8 x 10(5) bp, followed by (CAA)(n) at one every 5.0 x 10(5) bp. The frequency of the tri- and dinucleotide microsatellites throughout the B. rapa genome was estimated to be one every 120 Kb. The number of repeats and the polymorphism information content of the dinucleotide microsatellites were higher than those of the trinucleotide microsatellites. More than 90% of the primer pairs successfully amplified the corresponding microsatellite regions in other Brassica species. Furthermore, a considerable portion of them could be used in other Cruciferous species, 78.5% in Raphanus sativus, 68.6% in Sinapis alba and 39.8% in Arabidopsis thaliana. Based on these results, we concluded that B. rapa microsatellites have a high potential for the development of DNA markers that could contribute to the genetic analysis of Brassica and other Cruciferae.