Recent mapping and chromosome painting studies of polyploid crops indicate that extensive genomic restructuring can occur following polyploidization. For example, different patterns of rDNA evolution are found in crop allopolyploids: interlocus homogenization (gene conversion) of rDNA loci can occur in Nicotiana and Gossypium, but is not reported in Brassica and Triticale. However, while synthetic polyploids of crop plants have been used to demonstrate rapid genome evolution, a naturally occurring diploid-allopolyploid system still needs to be well characterized. Tragopogon provides unique opportunities to investigate recent, recurrent allopolyploidy. Previous molecular studies indicate that T. mirus may have evolved 12 times and T. miscellus may have formed 20 times within the past 70 years. We are evaluating the two allopolyploids, T. mirus and T. miscellus, relative to their three diploid progenitors using molecular cytogenetic and phylogenetic approaches. Fluorescent in situ hybridization (FISH) is being used to identify several loci on individual chromosomes to detect chromosomal evolution. Probes for the 18S-5.8S-26S array, 5S array, and other repeats are being evaluated in diploids and recurrent polyploids for number of loci and activation. Nucleolar dominance is a common feature in many allopolyploids, presumably only when the units have not homogenized and remain clearly related to the diploid progenitors. Thus, the rDNA unit structure data predicts that nucleolar dominance may be a feature of Tragopogon allopolyploids. Also, phylogenetic studies of nrDNA show perfect additivity (no interlocus homogenization) in that all cloned ITS sequences from Tragopogon polyploids give sequences from the progenitor diploids. However, Southern hybridization to the diploids reveals that all units of the 18S units of an array are similar, yet there are clearly population differences amongst the diploids, showing that intralocus gene conversion (homogenizing the entire array) must be occurring. Integrating molecular cytogenetics and phylogenetics is proving a powerful approach in assessing the complex histories of these dynamic polyploid genomes.

Key words: genome evolution, molecular cytogenetics, phylogenetics, polyploidy, rDNA gene conversion, Tragopogon