5S Ribosomal DNA of Distantly Related Quercus Species: Molecular Organization and Taxonomic Application

The genus Quercus (oak) is a widespread and economically important genus of tree plants. The intrageneric taxonomy of this group remains controversial due to widely distributed interspecific hybridization and convergent similarity of morphological traits. Further progress in this problem requires application of molecular methods. Taking into account that the comparison of 5S rDNA was successfully used in the molecular taxonomy of plants, we cloned and sequenced this genomic region for representatives of three taxo-nomically distant Quercus species: Q. acutissima (sect. Cerris) and Q. glauca (sect. Cyclobalanopsis) from East Asia and Q. texana (sect. Lobatae) from North America. We also identified 5S rDNA in the genome sequences available in the Genbank database for North American species Q. lobata (sect. Quercus) and for the related genus Castanea. It was found that the 5S rDNA repeated units demonstrate a high level of intragenomic sequence similarity in representatives of genus Quercus. The potential external elements of the 5S rDNA promoter differ from those in other families of dicotyledonous plants. The results of the comparative sequence analysis of the 5S rDNA intergenic spacer region support the existing taxonomy of the genus and indicate the isolated position of section Cyclobalanopsis, which can be considered as a separate subgenus.