Polyploidy and Breeding Systems in Homosporous Pteridophyta: A Reevaluation

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Abstract
Despite extremely high chromosome numbers, homosporous Pteridophyta are genetically diploid. There is no genetic evidence from enzyme electrophoresis to support the claim that these organisms are highly polyploid. Because these data suggest that homosporous ferns are diploid organisms without large numbers of duplicated genes, there also is no genetic evidence that these plants maintain heterozygosity through homoeologous chromosome pairing. Instead, homosporous ferns appear to maintain genetic variation in the same manner as do other diploid organisms, through various levels of outcrossing. Although homosporous ferns may have bisexual gametophytes, there is no evidence that these gametophytes characteristically self-fertilize. In contrast, electrophoretic data indicate that intergametophytic matings occur frequently in homosporous Pteridophyta. Although a few species exhibit high levels of intragametophytic selfing, for most species analyzed, nearly all matings appear to be intergametophytic. Like seed plants, homosporous ferns exhibit a diversity of breeding systems, from inbreeding through mixed to outcrossing. Gene-flow estimates indicate that high levels of interpopulational gene flow may occur in species of homosporous ferns. In contrast to earlier investigations of homosporous ferns, which emphasized that dramatic differences in polyploidy, breeding system, selfing rates, and gene flow existed between these plants and all heterosporous plants, available data indicate striking overall similarities. However, if homosporous ferns are truly diploid organisms initiated with high chromosome numbers, fundamental differences in genome organization could exist between these plants and heterosporous plants. These plants may possess, for example, fewer active genes per chromosome, lower levels of gene linkage, and large amounts of inactive DNA.

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