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one would expect for diploid plants (Gottlieb, 1982; Crawford, 1983). Analyses of enzyme expression indicate that multiple enzymes are indeed expressed in putatively paleopolyploid angiosperm families, such as those listed in Table 3 (Soltis and Soltis, 1990); issues of the regulation of duplicated genes are discussed by Wendel (1999). Second, some copies of these multiple genes might be expected to be silenced, particularly in the more ancient families (see Gene Silencing below). Third, reorganization of the original polyploid genome might have led to a novel genomic arrangement and perhaps to novel phenotypes. Finally, given that all members of a family have chromosome numbers that are multiples of a single lower number, it appears that, after polyploidization, diversification continued at the new polyploid level, with subsequent episodes of polyploidy superimposed on this initial polyploid level. This pattern of divergent speciation at the polyploid level contradicts the view of polyploids as evolutionary dead-ends.

Homosporous Pteridophytes

Homosporous pteridophytes are those ferns (including Psilotum and Tmesipteris; Manhart, 1994; Wolf, 1997; Soltis et al., 1999b), lycophytes, and Equisetum with a homosporous life cycle; all of these groups are the descendants of ancient plant lineages that extend back to the Devonian Period (Kenrick and Crane, 1997). The mean gametic chromosome number for homosporous pteridophytes is n = 57; for angiosperms, it is n = 16 (Klekowski and Baker, 1966). Despite their high chromosome numbers, however, homosporous pteridophytes exhibit diploid gene expression at isozyme loci (Haufler and Soltis, 1986; Soltis, 1986; D. Soltis and Soltis, 1988; P. Soltis and Soltis, 1988). At least two possible explanations can explain this paradox of high chromosome numbers and genetic diploidy. First, these plants are ancient polyploids that have undergone extensive gene silencing to produce genetic diploids, and second, they may have achieved high chromosome numbers through another mechanism, such as chromosomal fission.

Gene Silencing

Genes duplicated through polyploidy have several possible fates: retention of both copies as functional genes, acquisition of new function by one copy, and gene silencing (Wendel, 2000). Several models of genome evolution, in which a polyploid genome gradually will undergo gene silencing and return to a diploid condition, have been presented (Ohno, 1970; Haufler, 1987). Unfortunately, little empirical evidence is available to support or to refute these models.



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