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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 Soltis et al., 1999a; Soltis et al., 2000) is shown in Fig. 2. Stebbins (1950, 1971) also suggested that the ancestral base chromosome number for angiosperms is x = 6, 7, or 8; other, later authors (Ehrendorfer et al., 1968; Raven, 1975; Grant, 1981, 1982) have concurred. Reconstruction of chromosomal evolution across the angiosperms is partially consistent with Stebbins' hypothesis. Although the high chromosome numbers of the basal angiosperm groups make it difficult to infer base chromosome numbers for those groups of angiosperms and therefore for angiosperms as a whole, our reconstructions show an ancestral number of x = 8 for the eudicots (D.E.S., unpublished data), that is, the large clade that makes up 75% of angiosperm species. Identifying the ancestral number for all angiosperms will require teasing apart the base numbers of the ancient polyploid groups and will require further work.

Most, if not all, angiosperms may have experienced one or more cycles of genome doubling (Wendel, 2000), and these hypotheses of ancient polyploidy have several implications for the genetics, genomics, and evolutionary biology of these plants. First, if they are indeed polyploids, then these plants should exhibit extra copies of their genes above the level that

TABLE 3. Angiosperm families with high chromosome numbers, suggested to be of ancient polyploid origin (Stebbins, 1950)

Basal angiosperms

Chromosome number, n

Family

Illiciaceae

14

Schisandraceae

14

Lauraceae

12

Calycanthaceae

12

Magnoliaceae

19

Eudicot families

Trochodendraceae

19

Platanaceae

21

Cercidiphyllaceae

19

Salicaceae

19

Hippocastanaceae

19



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