This relates to lycophyte and fern taxa that usually reproduce sexually, but which can be persuaded to generate sporophytes without sexual fusion. There is no single recipe for this, indeed the triggers may be completely dissimilar even within the ferns. Lycopodium (Freeberg, 1957) and many ferns (Farlow, 1874) can be induced to form sporophytes without fertilization if they are deprived of the water so essential to the operation of the process. High levels of illumination can promote it (e.g., Lang, 1898), and some ferns have been reported to become apogamous when exposed to high concentrations of sugars (Whittier and Steeves, 1960; but see also Menendez et al., 2006). In many others apogamy is favored when nutrients and sucrose concentrations are reduced (e.g., heterosporous ferns, Mahlberg and Baldwin, 1975; Osmunda regalis and Pteris ensi-formis, Fernandez et al., 1999; Pityrogramma calomelanos, Martin et al., 2006). Sugars do not promote apogamy in Equisetum, but the plant growth regulators kinetin and benzyladenine do have an effect (Ooya, 1974; Kuriyama et al., 1990). Kinetin and benzyladenine promote apogamy in Pityrogramma calomelanos (Martin et al., 2006) and ethylene and other plant growth regulators promote apogamy in some ferns (e.g., Whittier, 1966; Elmore and Whittier, 1975). Exogenous and endogenous gibberellins may have a positive or negative role, depending on the species or genus (Jimenez et al., 2001).
Just how these conditions and additives work to promote apogamy is presently unknown, but it is exciting that we are starting to obtain molecular and biochemical tools that should allow us to develop this understanding. For example, several genes have been cloned that regulate hormone responses in the "C" fern (Ceratopteris richardii) (see Banks, 1999, for a review). This semi-aquatic fern has emerged as a productive model system for studying developmental processes (Hickock et al., 1987; Nakazato et al., 2006) and it is exciting that apogamy has recently been induced in this fern (Cordle et al., 2007). As the latter authors suggest, this provides a tractable experimental system for understanding the gene network that controls the switching from one generation to another. We are also at last able to examine plant growth regulator (PGR) effects by looking at the endogenous levels of these molecules. Until recently the only clues we could get about the role of PGRs was through application of exogenous substances (either the PGRs themselves, or inhibitors of them). Now we can measure endogenous hormone levels and the techniques used to study apogamy in Dryopteris affinis ssp. affinis could provide useful information about the process of facultative apogamy (Menendez et al., 2006).
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