Apospory and Apogamy: Implications for Understanding the Plant Life Cycle

Although apospory and apogamy are commonly regarded as aberrations, it is conceivable that they were frequent phenomena in ancient floras. In living plants apospory and apogamy offer the opportunity to investigate the factors causing change of reproductive phase in situations uncomplicated bysporogenesis or by gametogenesis and fertilization. Work with homosporous plants indicates that stress, either in the form of nutritional deprivation or conversely enhancement, particularly in the form of respirable substrate, is effective in switching gene activation from sporophytic to gametophytic and vice versa. In the bryophytes, notably the mosses, both phases can often be made unstable equally readily in appropriate experimental conditions. In vascular homosporous plants the change from sporophyte to gametophyte is usually more easily induced than the converse. The same conceptual approach can be applied to the heterosporous plants, but here the sporophytic condition seems more stable than the gametophytic. In the flowering plants, although sporophytic growth can sometimes be induced from the female gamete by nutritional means, microspores can be switched into sporophytic growth by a variety of "shock" treatments. I suggest that, both here and in other instances of stress-induced phase change, molecular chaperones may play a mediating role. In normal cycles the isolation of the spore mother cells and the structures enclosing the differentiating female gametes by barriers which occlude symplastic connections can be credibly envisioned as leading to conditions within the sequestered cells which resemble those effective in inducing phase change in vitro. Natural apomictic cycles can probably be resolved in similar causal terms. In ferns the spores of obligate apomicts have volumes up to eight times those of sexual relatives, attributable largely to the greater amount of cytoplasm. This imbalance may lead on germination to precocious activation of the sporophytic genes. Aposporous apomixis in angiosperms, characterized by an uncoupling of meiosis and embryo sac formation, results in an unreduced egg cell in an embryo sac lacking a callosed boundary. This may allow an inflow of nutrients stimulating the parthenogenetic development of the egg.