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by models of polyploid formation involving a single origin. (iv) Genome rearrangement may be a common attribute of polyploids, based on evidence from genome in situ hybridization (GISH), restriction fragment length polymorphism (RFLP) analysis, and chromosome mapping. (v) Several groups of plants may be ancient polyploids, with large regions of homologous DNA. These duplicated genes and genomes can undergo divergent evolution and evolve new functions. These genetic and genomic attributes of polyploids may have both biochemical and ecological benefits that contribute to the success of polyploids in nature.

Polyploidy, the presence of more than two genomes per cell, is a significant mode of species formation in plants and was one of the topics closest to the heart of Ledyard Stebbins. In Variation and Evolution in Plants, Stebbins (1950) devoted two chapters to polyploidy and addressed the following issues: the frequency, taxonomic distribution, and geographic distribution of polyploidy; the origins of polyploidy and factors promoting polyploidy; the direct effects of polyploidy; the polyploid complex; the success of polyploids in extreme habitats (including weeds); ancient polyploidy; and the role of polyploidy in the evolution and improvement of crops. He continued to explore these and other themes in his subsequent work, most notably in Chromosomal Evolution in Plants (Stebbins, 1971). In this paper we pay tribute to Ledyard, who was an inspiration and a friend, by exploring some of the questions that he asked about polyploids and by reviewing recent advances in the study of polyploidy.

Estimates of the frequency of polyploid angiosperm species range from ≈30–35% (Stebbins, 1947) to as high as 80% (Masterson, 1994); most estimates are near 50% (Stebbins, 1971; Grant, 1981). Levels of polyploidy may be even higher in pteridophytes, with some estimates of polyploidy in ferns as high as 95% (Grant, 1981). Polyploids often occupy habitats different from those of their diploid parents, and have been proposed to be superior colonizers to diploids. Furthermore, most crop plants are of polyploid origin, as noted by Stebbins (1950). In contrast, although genome doubling has been reported from other major groups of eukaryotes (reviewed in Wendel, 2000), it is not nearly as common in these groups as it is in plants.

The question often has arisen as to why polyploids are so common and so successful, and several possible explanations have been proposed. Stebbins (1950) considered vegetative reproduction and the perennial habit to be important factors promoting the establishment of polyploids, along with an outcrossing mating system to allow for hybridization (between species, subspecies, races, populations, etc.) in the formation of the

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