|
Items in cart [0] |
Questions? Call 888-624-8373 |
PAPERBACK Free Resources |
|||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||
The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. “identical” populations of T. miscellus also were of separate origin. For T. mirus, five populations with isozyme multilocus genotype 1 (28) and two populations with isozyme genotype 2 (28) were sampled. Each population had a unique RAPD profile (and, in fact, two populations were polymorphic), suggesting that each population may have had a separate origin. Taken with other data, T. mirus may represent a collection of as many as 11 lineages (41). RAPD data for three populations of isozyme genotype 1 (28) of T. miscellus demonstrated that all three were distinct and possibly of separate origin, raising the number of genetically distinct populations of T. miscellus to five (41). The Tragopogon tetraploids represent remarkable cases of recurrent formation on a small geographic scale and in a short period, perhaps the 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reported from Flagstaff, AZ (47), and T. miscellus has been reported from Gardiner, MT, and Sheridan, WY (M. Ownbey, unpublished notes cited in ref.9; Sheridan site confirmed by P.S.S. and D.E.S. in 1997; T. miscellus not observed in Gardiner in 1997).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? Genome Rearrangements in PolyploidsAnother 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 ref.48). Tobacco (Nicotiana tabacum) is an allotetraploid whose parents are Nicotiana sylvestris and a T-genome diploid from section Tomentosae (48). GISH clearly revealed numerous chromosomal rearrangements. In fact, nine intergenomic translocations 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. (49) produced artificial tetraploids resulting from interspecific crosses between Brassica rapa and Brassica nigra Table 3. Angiosperm families with high chromosome numbers, suggested to be of ancient polyploid origin (from ref.1)
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. (49) 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 (ref.50, but see ref.51 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 (52) and may be an important source of genetic novelty in polyploids (see also ref.6). Furthermore, cytoplasmic–nuclear interactions may be important in the establishment of a fertile polyploid (reviewed in ref.48). Ancient Polyploidy and Gene SilencingBasal Angiosperms. Estimates of ancient polyploidy generally have relied on chromosome number alone; Stebbins (1), for example, viewed those plants with a base chromosome number of n = 12 or higher to be polyploid, and others (5,53,54) 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 chromosome number in the Magnoliaceae is n = 19, and the family exhibits a range of numbers that are multiples of this base number. Some early eudicots, such as Trochodendron and Tetracentron (with n = 19) and Platanus (with n = 21), also have high chromosome numbers. Some families of more recent origin [e.g., Salicaceae (willows and poplars), Hippocastanaceae (horse chestnuts and buckeyes), Fraxinus (ashes) and other Oleaceae, and Tilia (linden and basswood)] also are considered ancient polyploids. Some families of possible ancient polyploid origin, along with their chromosome numbers, are listed in Table 3, and the phylogenetic distribution of these families (on portions of the tree of refs.55 and56) is shown in Fig. 2. Stebbins (1,2) also suggested that the ancestral base chromosome number for angiosperms is x = 6, 7, or 8; other, later authors (5,54,57,58) have concurred. Reconstruction of chromosomal evolution
|
|
||||||||||||||||||||||||||||||||||||
