last 70 years. Other polyploid species, if examined in sufficient detail, may be similarly grossly polyphyletic. Furthermore, recurrent formation of T. mirus and T. miscellus also has occurred on a broader geographic scale. Both species have been reported from Flagstaff, AZ (Brown and Schaak, 1972), and T. miscellus has been reported from Gardiner, MT, and Sheridan, WY (M. Ownbey, unpublished notes cited in Roose and Gottlieb, 1976; Sheridan site confirmed by P.S.S. and D.E.S. in 1997; T. miscellus not observed in Gardiner in 1997).
Although such polyphyly calls into question the meaning of the term “polyploid species,” the biological implications of recurrent polyploidizations from the same diploid progenitor species are indeed intriguing. Such multiple formations may play a significant role in shaping the genetic structure of polyploid species, as they are currently recognized. The concept of recurrent formations forces us to consider polyploid species not as genetically uniform, as previous models of polyploid formation imply, but as genetically variable. In fact, multiple formations may represent a significant source of genetic diversity in polyploid species, as a polyploid species may comprise multiple, genetically different lineages. Finally, crossing between individuals of separate origin will break down the distinctions among lineages and may produce novel genotypes through recombination.
The long-term evolutionary significance of recurrent polyploid formations is unclear; however, a host of specific questions can be addressed. For example, do plants of different origins have distinct evolutionary potentials? Does recurrent formation lead to different locally adapted genotypes? How extensive is gene flow between populations of independent origin, and to what extent does gene flow contribute to the genetic diversity of populations? How frequently are new genotypes produced through recombination?
Another possible source of genetic novelty in polyploids is genome rearrangements. Evidence for chromosomal changes has been obtained through a number of techniques, including genome in situ hybridization (GISH), analysis of restriction fragment length polymorphism (RFLP) loci, and chromosome mapping. Among the earliest studies reporting widespread genomic changes in tetraploids relative to their diploid progenitors is an analysis of tobacco genome structure using GISH (reviewed in Leitch and Bennett, 1997). Tobacco (Nicotiana tabacum) is an allotetraploid whose parents are Nicotiana sylvestris and a T-genome diploid from section Tomentosae (Leitch and Bennett, 1997). GISH clearly revealed numerous chromosomal rearrangements. In fact, nine intergenomic transloca-