The origins of Arabidopsis suecica (Brassicaceae) as indicated by nuclear rDNA sequences

Direct sequencing of nuclear rDNA internal transcribed spacer sequences (ITS) from Arabidopsis suecica showed that at least two ITS types were present in individuals of this allopolypoid species. Cloning PCR-amplified ITS sequences and subsequent sequencing confirms that A. suecica contains two distinct types of ITS sequences, one from A. thaliana and the other from either Cardaminopsis arenosa or C. neglecta. Based on these data and on ecology, A. thaliana and C. arenosa are the putative parents of A. suecica. These results support those of other studies employing cpDNA restriction sites, cytology, and isoelectric focusing of Rubisco. No recombination or evidence of concerted evolution of the ITS was observed, suggesting either that homogenization of rDNA repeats through concerted evolution is more efficient within rather than among chromosomes, or that, because of the recent origin of A. suecica, insufficient time has elapsed to allow ITS sequences to be homogenized. Both of these phenomena may be responsible for the observed pattern of ITS variation in A. suecica.