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diploidization (Seoighe and Wolfe, 1998). Vertebrates are another example of diploidized ancient polyploids; it is believed that vertebrates are degenerate polyploids owing to two polyploid events before the radiation of fish and mammals (Postlethwait et al., 1998). Similar examples come from plants; for example, both Glycine (soybean) (Shoemaker et al., 1996) and Brassica species (Bohuon et al., 1996; Cavell et al., 1998) seem to be degenerate polyploids. Based on this information, one can conclude that diploidization after polyploidy is evolutionarily common.

For maize, it should be possible to garner insights into the processes of rearrangement and diploidization from extant patterns of chromosomal duplication. Mapping studies have documented regions of chromosomal duplication in maize (Table 1). (It is important to note that Table 1 includes only those chromosomes that were explicitly defined as duplicated by the authors; Table 1 does not include all of the chromosome pairs on which markers are known to crosshybridize.) As Table 1 demonstrates, there is some disagreement among studies about chromosomal duplications, for two reasons. First, different studies use different data, leading to different conclusions. Second, and perhaps more importantly, researchers rarely denote their criteria for defining chromosomal duplications, and thus criteria likely differ among studies. Ultimately, chromosomal duplications should be defined by objective statistical criteria.

Nonetheless, there is a consensus about some chromosomal pairs. For example, it is now well established that portions of chromosome 1 are duplicated on chromosomes 5 and 9 (Table 1). The evolutionary implication for these pairings is that the process of diploidization rearranged one copy of chromosome 1. (Alternatively, chromosome 1 could be an amalgamation of regions from different parental chromosomes.) Chromosome

TABLE 1. Duplicated chromosomes in maize and the studies that identified them

Duplicated chromosomes

References

1–5

Helentjaris et al., 1988; Wilson et al., 1999; Gale and Devos, 1998

1–9

Helentjaris et al., 1988; Wilson et al., 1999; Gale and Devos, 1998

2–4

Helentjaris et al., 1988

2–7

Helentjaris et al., 1988; Wilson et al., 1999; Gale and Devos, 1998

2–10

Helentjaris et al., 1988; Ahn and Tankley, 1993; Wilson et al., 1999; Gale and Devos, 1998

3–8

Helentjaris et al., 1988; Ahn and Tankley, 1993; Wilson et al., 1999; Gale and Devos, 1998

3–10

Gale and Devos, 1998

4–5

Wilson et al., 1999; Gale and Devos, 1998

6–8

Helentjaris et al., 1988; Wilson et al., 1999; Gale and Devos, 1998

6–9

Wilson et al., 1999; Gale and Devos, 1998



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