Reproduction and Life History
Pteridium aquilinum is a genetically diploid, sexual, homosporous fern producing haploid spores that are dispersed by air and gravity. The spores germinate to produce small, potentially bisexual gametophytes. In spite of the potential for intragametophytic self-fertilization, genetic data suggest that bracken sporophytes in nature mainly are the result of outcrossing between gametophytes (Wolf et al., 1988, 1991). Klekowski (1972) presented data showing that this outcrossing was not due to the presence of a genetic incompatibility system, but that genetic load (the presence of deleterious alleles in natural populations that minimize the number of successful self-fertilizations) might play a role in enforcing outcrossing.
In vitro studies also have shown that bracken gametophytes have an antheridiogen system (Döpp, 1950); that is, the first gametophytes developing at a site become functionally female and produce hormonal substances (antheridiogens) that cause subsequently developing gametophytes nearby to become functionally male. In fact Döpp’s initial report on the existence of antheridiogens involved the study of cultured gametophytes of Pteridium aquilinum, and the species is still used a standard for assaying the production of antheridiogen A in ferns.
The studies of Wolf et al. (1991) on plants in Britain also have shown that the high dispersability of bracken spores leads to high rates of gene flow and consequently potentially very large metapopulations.
In nature, many bracken plants rarely produce sporangia. In such populations, sexual reproduction does not occur regularly. Vegetative reproduction is thus very important in bracken and is accomplished by fragmentation of the long-creeping, branched rhizomes. Some rhizome branches are leafless and elongate extensively as a mechanism to increase the overall size and spread of the clone. Bracken is one of the largest organisms in the world. Sheffield et al. (1989b) used isozyme data to document a single clone of Pteridium aquilinum in Great Britain whose rhizome apparently covered an area 390 m in length.
Apospory on aberrant fronds in nature was demonstrated by Farlow (1889) and confirmed by Whittier (1966a). Whittier (1966b) also demonstrated in vitro that normal haploid gametophytes and diploid gametophytes resulting from apospory could be induced to become apogamous.
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