Are Lycopods With High Chromosome Numbers Ancient Polyploids?

Abstract

It has been established that the number of isozymes (different forms of an enzyme encoded by different gene loci) is highly conserved in diploid angiosperms and gymnosperms. In contrast, allopolyploid angiosperms display an increase in isozyme number due to the addition of divergent genomes. Lycopods (Microphyllophyta) are an ancient lineage of vascular plants having very high chromosome numbers. It has been maintained that lycopods acquired these high chromosome numbers through repeated episodes of polyploidy. Despite high chromosome numbers, however, lycopod species having the lowest chromosome numbers within genera possess the number of isozymes typical of diploid seed plants for all enzymes examined except triosephosphate isomerase. There is, therefore, no genetic evidence from enzyme electrophoresis for polyploidy in these plants. These results are comparable to findings for other homosporous pteridophytes including the ferns (Pteridophyta) and horsetails (Arthrophyta). Alternative hypotheses for widespread genetic diploidy in homosporous pteridophytes are 1) repeated cycles of allopolyploidy followed by gene silencing; 2) repeated cycles of autopolyploidy, which would result in duplicated, but not divergent genes for isozymes; 3) initiation of these lineages with relatively high chromosome numbers.

Assessing the Utility of Isozyme Number for Determining Ploidal Level

“…For example, putatively archaic woody angiosperms with high chromosome numbers are duplicated for a large number of enzymes (Soltis and Soltis unpublished) possibly suggesting that many of the loci duplicated through chromosomal doubling are not silenced. In contrast, presumably ancient polyploid (paleopolyploid) species of Equisetum , Lycopodium (Soltis and Soltis 1988), Mabrya (Elisens and Crawford 1988), Galvezia (Elisens and Crawford unpublished), and several homosporous fern genera (reviewed in Haufler and Soltis 1986) all are characterized by "diploid" isozyme patterns. To account for the "diploid" isozyme patterns observed for many ferns and fern allies with high chromosome numbers, , Soltis and Soltis (1988), and Haufier and Soltis (1988) question the hypothesized paleopolyploid origin of these taxa.…”

“…In contrast, presumably ancient polyploid (paleopolyploid) species of Equisetum , Lycopodium (Soltis and Soltis 1988), Mabrya (Elisens and Crawford 1988), Galvezia (Elisens and Crawford unpublished), and several homosporous fern genera (reviewed in Haufler and Soltis 1986) all are characterized by "diploid" isozyme patterns. To account for the "diploid" isozyme patterns observed for many ferns and fern allies with high chromosome numbers, , Soltis and Soltis (1988), and Haufier and Soltis (1988) question the hypothesized paleopolyploid origin of these taxa. Elisens and Crawford (1988), in contrast, suggest that silencing ofloci duplicated through polyploidy is the most logical explanation for the "diploid" isozyme patterns in Galvezia and M abrya, and that silencing of duplicated loci may not be an uncommon phenomenon in paleopolyploids.…”