THE ORIGINS OF ARABIDOPSIS-SUECICA (BRASSICACEAE) AS INDICATED BY NUCLEAR RDNA SEQUENCES

Direct sequencing of nuclear rDNA internal transcribed spacer sequence s (ITS) from Arabidopsis suecica showed that at least two ITS types we re present in individuals of this allopolypoid species. Cloning PCR-am plified ITS sequences and subsequent sequencing confirms that A. sueri ca 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 putat ive parents of A. suecica. These results support those of other studie s employing cpDNA restriction sites, cytology, and isoelectric focusin g of Rubisco. No recombination or evidence of concerted evolution of t he ITS was observed, suggesting either that homogenization of rDNA rep eats 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 homogeniz ed. Both of these phenomena may be responsible for the observed patter n of ITS variation in A. suecica.