Conventional alternation of generations

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The phenomena of apogamy and apospory, although easily induced and in some taxa the norm, have little impact on alternation of generations in the vast majority of lycophytes and ferns. The life cycle shown in Figure 2.2 shows the structures more conventionally concerned with alternation of generations in bracken fern, and which we can use as the basis of a consideration of the process in non-apogamous lycophytes and ferns. The reason for choosing bracken is that we know more about this fern than any other.

There are two main reasons for the plethora of papers on this fern. One is that it contains toxins and carcinogens (e.g., Siman et al., 1999, 2000; Schmidt et al., 2005), which adds to the problems caused by the other reason, which is that it is a vigorous and invasive weed (see Smith and Taylor, 2000, for review). The "it" to which I refer is the genus, within which there are now four species generally recognized (see Marrs and Watt, 2006, for review, and Thomson et al., in press) but taxonomic distinctions are not linked to any substantial differences in life cycle processes in this genus. Small differences, however, between taxa may have far-reaching consequences.

One example of this concerns the production of spores - the event that could be considered as the start of the alternation process from sporophyte to game-tophyte. There has been keen interest in this process and much progress made over the thirteen years since my last review (Sheffield, 1994). One explanation for this is that it has become clear that some fern spores, including both northern and southern hemisphere bracken, contain compounds that could be injurious to human and animal health (Siman and Sheffield, 2006) and the other is that there are important genetic, taxonomic and ecological consequences of spore formation and dispersal (see Chapter 4). Spores are one of the vehicles that affect gene flow - the movement of genes from one location to another. By measuring genes or their products in established plants (see Chapter 4) we can infer events that produced them. Although outcrossing is very clearly the norm for bracken (not intragametophytic selfing) (Wolf et al., 1988, 1990; Kor-pelainen, 1995), allozyme studies indicate very disparate levels of gene flow between Laurasian bracken populations. Levels of genetic exchange between populations of the bracken taxon in Britain are sufficiently high that "aquilinum" should be regarded as a single panmictic entity (Wolf et al., 1991; Thomson et al., in press). However, in contrast, allozymes of geographically close Scandinavian bracken populations (presumably at least predominantly "pinetorum") indicate low levels of gene flow. This is thought to result from the very limited spore production in the stands sampled (Korpelainen, 1995; Thomson et al., in press).

2.5 Sporogenesis

What do we know about the factors that control spore production? In her seminal paper, Conway (1957) suggested that spore production in bracken is "undoubtedly" influenced by the age of the plant, the developmental stage of the frond, seasonal weather conditions, and environmental factors. The latter certainly include shade, and differences in spore production between shaded and unshaded fronds were noticed over a century ago (see Daniels, 1986). Bracken fronds partly covered by a canopy have a correspondingly lower spore output (Schwabe, 1951) and the same is true for some other ferns (e.g., Polystichum acros-tichoides and Cyathea caracasana, Greer and McCarthy, 2000; Arens and Baracaldo, 2000). Steeves and Wetmore (1953) studied (green) spore production in Osmunda cinnamomea plants in heavy woodland and open areas and found the same for this species, and the only reports of increased photosynthetically available radiation (PAR) inhibiting spore production in ferns is that relating to an in vitro study on O. cinnamomea, where high PAR inhibited sporophyll differentiation (Harvey and Caponetti, 1972). It therefore seems likely that the PAR levels used in the latter study were higher than plants are likely to experience in the natural environment, as all subsequent studies indicate promotion of fertility by increased light.

Genotype can have a strong influence on sporogenesis, so it is critical that observations and experiments are conducted on genetically identical or matched samples. One such experiment used clones - two pieces taken from rhizomes of Polypodium vulgare and then cultivated in natural and glasshouse conditions of controlled temperature and light (Siman and Sheffield, 2002). The population of clones raised indoors, with higher temperatures and PAR than that experienced by the outdoor clones, behaved very differently. The plants grown outside produced the usual singe pulse of fertile fronds each season; those grown inside generated fertile fronds continuously. Intriguingly, these plants also produced new fronds in distinct pulses approximately every 3 months, rather than incrementally, as if some element of the normal year-long reproductive cycle had been accelerated (Figure 2.7). Although the mechanism at work was not determined, this study showed clearly that the normal year-long process of fern spore production and release can be interrupted by experimental manipulation of environmental factors.

So light and temperature have a powerful influence on sporogenesis, but the studies reported above cannot separate out the effects of higher PAR and temperature, as both vary considerably between heavy woodland and open areas, outside and inside a glasshouse. A preliminary study involving clones of bracken cultivated in growth chambers reported that each factor independently promotes fertility (Wynn et al., 2000). Rhizomes were excavated in the middle of winter and cultivated in pots subjected to two different temperature regimes, and two levels of PAR. They generated fronds that bore the first stages of spore-forming tissue in less than 9 weeks - again showing that the usual cycle of spore production can be interrupted, as bracken in the UK normally generates spores

Figure 2.7 Mean numbers of fronds per pot produced in two genetically matched cultivated populations of the fern Polypodium vulgare plotted against time. Open circles represent the indoor population, grown in higher temperatures and light than the outdoor population (closed circles), and show three distinct waves of frond recruitment. (From Siman, 2000.)

Figure 2.7 Mean numbers of fronds per pot produced in two genetically matched cultivated populations of the fern Polypodium vulgare plotted against time. Open circles represent the indoor population, grown in higher temperatures and light than the outdoor population (closed circles), and show three distinct waves of frond recruitment. (From Siman, 2000.)

in late summer. Significantly more fertile material was generated by plants in the higher temperatures and light, and there appeared to be some differences between genotypes. The possibility, however, that development of fertile material reflects the nutritional status of the rhizomes, and therefore the environmental conditions at the sites from which they were excavated, could not be ruled out.

Later experiments with the same rhizomes and genotypes clarified this (Wynn, 2002). Rhizomes originated from two contrasting environments in the UK. One was 550 kilometers north of the other, and experiences a considerably harsher climate. Bracken in the more northerly location had never been seen to generate spores, whereas plants from the southerly collection site routinely did so. Plants of two distinct genotypes used in the experiments conducted a year earlier had been maintained in cultivation in potting compost and their responses to environmental variables was measured in comparison with plants raised from newly excavated rhizomes from the source populations. Higher fertility was again noted in plants grown in the highest light and temperatures, and there were clear-cut differences between genotypes that were unrelated to behavior observed in the field. A genotype never observed to spore in Scotland, at the more northerly site, showed significantly greater fertility than a plant that spored profusely in its native site in Manchester, when cultivated in identical conditions. Although the rate of development of sporing material was similar in both plants, this shows that the amount of sporogenous tissue is under genetic control.

These experiments also showed that transplantation/rhizome disturbance is not required to trigger formation of fertile material. Tyson et al. (1995) studied nutrient flow through bracken rhizomes that generated spore-bearing fronds in cultivation. Sheffield (1996) hypothesized that disturbance, stress, and/or interference with sink-source nutrient flow might therefore promote sporing. Wynn et al.'s experiments included rhizomes that were both newly excavated and undisturbed, and that were supplied with both high and low nutrients and water. Newly excavated rhizomes yielded significantly less, not more, reproductive tissue, and the different nutrient and water treatments had no discernable effect on sporangial development. It may be that the differences between the conditions used were insufficient to reveal an effect, as authors have reported significant effects of water availability on fertility in other ferns. The rheophyte Thelypteris angustifolia, for example, shows reduced fertile-frond production in response to dry conditions (Sharpe, 1996). As bracken tolerates flooded conditions poorly, and characteristically grows on low-nutrient soils (Pakeman et al., 1996), the expectation would be that this fern would have a high tolerance to a range of nutrient and water availability, but that copious amounts of water might be too much of a good thing. Further experiments to establish this would be useful in view of current predictions on climate change.

Unlike the vast majority of lycophyte and fern taxa, most of the bracken plants currently growing in temperate regions produce spores rarely. Bracken growing in warmer locations, e.g., the Hawaiian Islands (Sheffield et al., 1995) sporulates profusely - perhaps adding weight to the hypothesis that the genus originated in the tropics (Page, 1990). Given the intolerance of frost in this species and the increase in frost-free days and temperature predicted by most climate change models (e.g., Hulme and Jenkins, 1998) the interaction of PAR (which may fall with increased cloud cover) and water (likely increased rainfall) is likely to be an important influence on future levels of bracken sporogenesis (see Kendall et al., 1995). It seems very likely that environmental conditions currently experienced by bracken plants in temperate regions are not conducive to the completion of this part of the life cycle before winter ensues, but that the plants are capable of becoming fully fertile if conditions improve. In this respect the story echoes that of the Killarney fern, Trichomanes speciosum, in which current genetic variability within temperate sites occupied by gametophyte populations is attributed to spores produced in more favorable environmental conditions (Rumsey et al., 1999). In Europe this species currently disperses almost exclusively via gemmae (see Figure 2.1; Sheffield, 1994 and Chapter 9 for a review) and alternation of generations is undoubtedly extremely rare at present.

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