No abstract

The goals of this study were to test the hypothesis that plasticity of life history traits is greater among species with broad ecological distributions than among species with narrow ecological distributions and to determine whether homosporous filicalean fern gametophytes exhibit adaptively meaningful responses to soil substrates. Sporophyte populations of Asplenium platyneuron and Polystichum acrostichoides have broader ecological distributions and were therefore expected to exhibit a greater amount of gametophytic plasticity than either Asplenium rhizophyllum or Diplazium pycnocarpon. Changes in reproductive effort (i.e., gametangia mm-2) were investigated across the four following soil treatments, which represent major edaphic environments on which sporophyte populations of these species occur: stream bank, Pinus strobus canopy, oak forest ridge top, and soil from a lime-bearing rock outcrop. Our results failed to support the "ecological breadth" hypothesis. Three of the four species-A. rhizophyllum, D. pycnocarpon, and P. acrostichoides-exhibited similar amounts and directions of plasticity, with reproductive effort greatest in treatments that were unfavorable to growth; A. platyneuron exhibited the least amount of plasticity. Nevertheless, species-specific patterns in gametophytic size and reproductive effort reflected the distributions of their sporophytic counterparts. Plasticity of size-related reproductive effort may be common among homosporous filicalean fern gametophytes because of their short life spans and limited capacity for vegetative competition.

Patterns of growth and reproduction were documented in a natural population of Polystichum acrostichoides in southeastern Ohio during the 1994 and 1995 growing seasons. The proportion of biomass allocated to fronds increased with plant biomass, indicating fronds are an increasingly dominant component of the body of P. acrostichoides. Regression analysis indicated a minimum size threshold exists at which this species first becomes reproductive. Both reproductive status and frequency of reproduction were positively associated with greater plant biomass and above-ground growth rates. A cost of reproduction to growth was apparent; above-ground growth rates increased during non-reproductive years among individuals that reproduced in only 1994. Minor increases in reproductive effort were associated with increasing plant biomass; ranging from approximately 0.01% to 2.11%. Nevertheless, reproductive effort may be plastic in R acrostichoides; the frequency of reproduction correlated negatively with cation concentrations and positively with phosphorous concentrations, and reproductive effort increased with decreasing canopy cover. Together, these observations suggest reproduction in P. acrostichoides only occurs when resources are sufficient to offset it*s cost to future growth; a life history that may optimize the advantages of early reproduction and life-time fecundity in a species whose colonizing phases (i.e., gametophyte and juvenile sporophyte) have high risks of mortality.

No abstract

In addition to genetic load, the induction of maleness in leptosporangiate gametophytes by the pheromone antheridiogen may facilitate rates of outcrossing similar to those found in angiosperms. The antheridiogens that have been chemically identified are similar to gibberellins and probably evolved from this common plant hormone. The purposes of this study were to determine the functions of endogenous gibberellins in morphological development and gender expression in leptosporangiate fern gametophytes and to elucidate how antheridiogens may have evolved from gibberellin precursors. We grew gametophytes of Osmunda regalis and Athyrium filix-femina on nutrient agar enriched with APOGEE, which blocks gibberellin synthesis. Osmunda regalis is a member of Osmundaceae, the only family in Osmundales, the sister group of all remaining extant leptosporangiate ferns. This family possesses a male-first gender in isolation and lacks any known antheridiogen systems. In contrast, A. filix-femina is a member of a derived family, Woodsiaceae, which possesses antheridiogen systems. Disruption of gibberellin synthesis retarded notch development and expression of both maleness and femaleness (i.e. gender status) in both species. On the basis of these results, we offer a simple model of gametophyte evolution driven by gender-based fitness gain curves and the influence of exogenous gibberellins.