tions have occurred within the genome of tobacco, that is, translocations between the chromosomes donated by N. sylvestris and the T-genome parent. Most of the chromosomes of tobacco are therefore mosaics, composed of regions of both parental chromosome sets.
In Brassica, there is evidence that such genome rearrangements may occur very soon after the formation of the tetraploid. Song et al. (1995) produced artificial tetraploids resulting from interspecific crosses between Brassica rapa and Brassica nigra and between B. rapa and Brassica oleracea. They compared genome structure in the F5 derivatives of these crosses with their F2 ancestors and found genetic divergence in these few generations, with distances as high as almost 10%. In addition, Song et al. (1995) found evidence of cytoplasmic–nuclear interactions—the maternal genotype had definite control over aspects of the nuclear genome. They concluded that a possible result of polyploid formation is the production of novel genotypes. Furthermore, extensive genetic change can occur in the early generations after polyploid formation and may therefore be important in the formation of a functional polyploid. Chromosome mapping of diploid Brassica and comparison with the map of Arabidopsis thaliana suggest that the diploid species of Brassica (n = 9) may actually be ancient hexaploids (Lagercrantz, 1998, but see Quiros, 1998 for a different interpretation).
Such intergenomic translocations are not limited to tobacco and Brassica. Instead, extensive chromosomal changes have been reported in a number of other polyploids, including maize, oats, and soybeans. Such intergenomic translocations may be mediated by transposable elements (Matzke and Matzke, 1998) and may be an important source of genetic novelty in polyploids (see also Wendel, 2000). Furthermore, cytoplasmic-nuclear interactions may be important in the establishment of a fertile polyploid (reviewed in Leitch and Bennett, 1997).
Estimates of ancient polyploidy generally have relied on chromosome number alone; Stebbins (1950), for example, viewed those plants with a base chromosome number of n = 12 or higher to be polyploid, and others (Goldblatt, 1980; Grant, 1981, 1982) used similar criteria. Based on this criterion, a large number of angiosperm families, most of which trace their roots far back into angiosperm phylogeny, are considered to be the products of ancient polyploid events whose diploid ancestors are now extinct. For example, the Illiciales have n = 14, and both the Lauraceae and Calycanthaceae of Laurales have a base number of n = 12. The lowest