Over half of all plant species are derived through polyploidy. However, despite the prevalence of polyploid speciation little is known about the genetic consequences of polyploidy. Studies of sequences in the Internal Transcribed Spacers (ITS) of ribosomal DNA (rDNA) in tetraploid cotton demonstrated bidirectional evolution of rDNA arrays (Wendel et al. 1995), suggesting that concerted evolution is a rapid process. Rates and patterns of gene and genome evolution in polyploids can best be addressed in polyploids of known parentage and recent origin. The allotetraploids T. mirus and T. miscellus arose in eastern Washington within the past 70 years from parental species that are native to Europe, and are therefore excellent candidates for studies of gene and genome evolution in polyploids. We used DNA sequence variation in the ITS regions of T. mirus, T. miscellus, and their diploid parents to determine whether or not concerted evolution had homogenized the rDNA arrays in the tetraploids during the approximately 70 years since their formation. Two populations of each allotetraploid and its diploid parents were sampled, and each population was represented by four individuals. The ITS sequences of the diploid parents of T. mirus differed at 18 nucleotide positions; those of the parental species of T. miscellus differed at 17 sites. The ITS sequences of individuals of T. mirus are polymorphic at sites that differentiate its parents, combining the nucleotides of its parents. Likewise the ITS sequences of individuals of T. miscellus combine the nucleotides of its diploid parents. Cloned PCR products from each tetraploid match the sequence of the respective parents. These results indicate that the rDNA arrays of the recent tetraploids T. mirus and T. miscellus do not appear to have undergone rapid concerted evolution.

Key words: concerted evolution, ITS polymorphism, polyploidy, Tragopogon