Dryopteris affinis ssp. affinis needs no prompting to generate sporophytes apogamously, but we know that their induction is stimulated by auxins and gibberellins (Menendez et al., 2006). In common with many other apogamous species, this taxon is widely distributed and very successful. Bommeria pedata, for example, has a considerably more extensive distribution than most of its sexually reproducing congeners (Gastony and Haufler, 1976). It is usually held that the reason for this success is that the apogamous process avoids the need for water in excess of the hydration needs of each plant. Apogamous taxa are certainly successful in drier habitats than their sexually reproducing relatives, but this may well reflect the rapid growth and maturation rate of their game-tophytes in at least equal measure to their lack of requirement of water for
fertilization. Non-apogamous taxa with fast-growing/maturing gametophytes are also very successful (e.g., Pteridium, bracken fern).
The life cycle of obligate apogamous ferns is characterized by two consecutive events. First, during the formation of spores (sporogenesis), there is an avoidance of the reduction in chromosome number that normally attends meiosis (see Raghavan, 1989, for a comprehensive review of this process). This means that the spores (and hence the subsequent gametophytes) have the same chromosome number as the parent plant. Next, as in the facultative process described above, a sporophyte forms without the union of two gametes, directly from the game-tophytic tissue (Figure 2.5). This generates a sporophyte with the same chromosome number as the gametophyte (and of course, the sporophyte that generated the spore). This explains the earlier remark that alternation of generations does not necessarily mean changes in chromosome number, but there are at least two pathways of sporogenesis in obligate apogamous ferns and the reader is referred to Walker (1985) and Gastony and Windham (1989) for details. This production of "clones" rather than genetically variable offspring might seem pointless in plants so adept at asexual reproduction, but it is critical to remember that spores have enormously greater dispersal potential than the more cumbersome bulbils, plantlets, offshoots, and stolons that clone non-apogamous taxa. The important message in the current context is that there is nothing abnormal or aberrant in apogamous ferns, and that alternation of generations for obligate apogamous species allows them to disseminate successful genotypes at enormously greater distances than would be possible using strictly vegetative growth.
It is exciting that two obligate apogamous laboratory strains of the model fern Ceratopteris have now been described (Cordle et al., 2007), as this adds to our suite of methods with which to explore the mechanisms of this process. It is clear that a differentiating cell in a gametophyte can follow one of multiple pathways: vegetative gametophyte, gamete, or vegetative sporophyte (apogamy), depending on the developmental cues to which it responds. Hopefully we now have techniques that will allow us to discover exactly how those cues are received and interpreted, and perhaps also to manipulate responses in other ferns and lycophytes.
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