The Influence of Soil Topography and Spore-Rain Density on Gender Expression in Gametophyte Populations of the Homosporous Fern Aspidotis densa

Abstract

The gametophytes of homosporous pteridophytes, being free-living and independent of the sporophyte, experience an immediacy with the external environment unique among the gametophytes of vascular plants. As a result, the environment directly influences the demographics of homosporous fem gametophytes, particularly their distribution, development, and mating systems. Because most homosporous fern gametophytes are photosynthetic, require light to germinate, and develop gametangia sequentially, environmental factors that alter the quantity and quality of light to neighboring spores and gametophytes may stimulate asynchronous germination, growth, and sexual maturation. The influence of environmental factors upon the population biology of fern gametophytes may be augmented by the production of antheridiogens. The meristematic gametophytes of some species have been shown to produce antheridiogens or as yet unspecified metabolites that generate effects (induced germination in darkness, precocious maleness, and delayed development of a notch-meristem) in ameristic neighbors (Naf, 1963, 1975; Schedlbauer & Klekowski, 1972; Hamilton, 1989; Haufler & Gastony, 1989; Schneller, et al., 1990). In some species, individuals with notch-meristems are insensitive to antheridiogen (Niif, 1963). In such species, asynchronous growth and development among neighbors is essential for an antheridiogen to influence the gender composition of a population. Microvariation in soil topography may alter the availability of light to spores and their ensuing gametophytes. Spores shed on well-illuminated regions of a heterogeneous soil profile are likely to germinate early compared to less-illuminated neighbors. The resulting gametophytes will mature rapidly, and through the action of an antheridiogen, influence the germination and sexual expression of neighbors in poorly illuminated regions. Because antheridiogens are water soluble and of uncertain stability in the environment (Naf, 1963), and because homosporous fern gametophytes require water for fertilization, the proximity of neighboring gametophytes may be another important determinant of gender composition and mating systems. For example, the probability that colonizing gametophytes are reproductively isolated is associated with a higher probability of selfing, resulting in genetically homozygous progeny without genetic load (Lloyd 1974a, 1974b; Crist & Farrar, 1983; McCauley, et al., 1985; Soltis, et al., 1988; Peck, et al., 1990; Holsinger, 1991; Watano & Masuyama, 1991). Alternatively, in taxa producing dense gametophyte populations, close proximity may facilitate interactions between neighbors, leading to outcrossing mating systems, high levels of genetic load, and heterozygous sporophytes (Lloyd, 1974a, 1974b, 1988; Singh & Roy, 1977; Duckett & Duckett, 1980; Soltis & Soltis, 1986, 1987; Soltis, et al., 1988; Holsinger, 1991). Thus, by decreasing distances between neighbors, dense spore-rain may amplify the influences that asynchronous germination and development have on the sexual expression and mating system of a population and low densities of spore-rain may result in distances too large for interactions among gametophytes (Cousens & Horer, 1970; Tryon & Vitale, 54