1997; Ranker and Houston, 2002). The distance antheridiogen is dispersed from a source in nature is not known but experiments using Polystichum acrostichoides with natural steam-sterilized soil showed that it can be found up to 7.5 cm from a source (Greer and McCarthy, 1997), which is only about one third of the distance Voeller and Weinberg (1969) found in the laboratory.
When collecting gametophyte populations from nature and looking carefully at the different individuals, the basal (earliest) cells of some prothalli were very long and thin (Figure 5.8) and nearly lacked chlorophyll (Schneller, 1988). They were interpreted as having germinated in the dark under the influence of antheridiogen. The first few thread-like cells grow in the dark towards the soil surface. Then, when the top of the cell row reaches the surface it begins to form normal sized green cells as found in prothalli growing in the light. My observations led to the assumption that the dark germinated protonemata show a positive phototactic reaction and will form regular gametophytes upon reaching the surface, if they are not too deeply buried in the soil. On untreated soil collected in a natural habitat of ferns (near Zurich) and brought into an adequate box in the laboratory, spores of Dryopteris dilatata, Dryopteris filix-mas,
and Athyrium filix-femina started to germinate and many gametophytes developed. When the fastest growing gametophytes became female (or hermaphrodite) a few hundred fresh spores of Athyrium filix-femina were packed together with soil into a packet made of small fine-pored nylon netting. The packet was then buried ca. 2 cm below the soil surface with the gametophyte populations on the surface. After 4 weeks the packet was excavated and its content was investigated under a microscope. Many spores had germinated in the dark and usually formed one antheridium with viable spermatozoids (Figure 5.9).
When colonizing a new site in nature, the first prothallus/prothalli will, upon reaching the meristic stage, start to produce antheridiogen that will induce antheridia in the smaller (younger) prothalli, and also induce dark germination. However, this is only possible when two or more spores are present at the new site. Ferns like Athyrium filix-femina first develop into larger females and smaller males. When losing the meristic prothalli or when sporophytes are formed the antheridiogen source is exhausted, and older male prothalli will develop into functional females or in other species into hermaphrodites. In A. filix-femina when the archegonia are ripe, the antheridia of the same individual will be empty. Thus hermaphroditic gametophytes will be functionally female. Other species such as Asplenium ruta-muraria and Dryopteris filix-mas develop hermaphroditic gametophytes in mature cultures. When the antheridiogen source is removed the remaining male prothalli develop into hermaphrodites. If only one spore of an outbreeding species such as A. filix-femina reaches a new site, it will be unlikely to establish a new population because of genetic factors (i.e., inbreeding depression) (Schneller, 1979). To establish a new successful population, such species need at least two spores, which very likely will have originated from different sporophytes.
Chiou and Farrar (1997) have demonstrated that epiphytic fern species of the family Polypodiaceae possess an antheridiogen system, which suggests that dark germination may play a role in competition with bryophytes and higher plants.
5.8 Biological and evolutionary implications of the antheridiogen system
Self-fertilization is possible in functional, bisexual fern gametophytes. In such gametophytes, intragametophytic selfing would lead to completely homozy-gous sporophytes, thus ultimately reducing population genetic variability and making genetic recombination unlikely. Thus, the most obvious evolutionary implication of antheridiogen systems is that they promote outcrossing and reduce inbreeding.
Many investigations have shown that polyploids are more likely to self-fertilize than diploids (Klekowski, 1979; Masuyama and Watano, 1990; Watano and Masuyama, 1991; Schneller and Holderegger, 1996a; Vogel et al., 1999a; Soltis and Soltis, 2000; Chiou et al., 2002; Chiou, 2003). Polyploids may be better buffered against inbreeding and environmental changes due to heterosis and the presence of homoeologous genes. Antheridiogen activity is observed, however, in many tetraploid species (e.g., Dryopteris filix-mas, Cystopteris tennesseensis, and many others; Soltis and Soltis 1992), which are able to self fertilize successfully (Haufler and Ranker, 1985; Schneller et al., 1990). The antheridiogen systems in polyploids may simply represent the retention of ancestral traits inherited from their diploid progenitors.
Intragametophytic selfing may be advantageous for pioneer species especially in the early stages of colonization (Lloyd 1974; Crist and Farrar, 1982; Soltis and Soltis, 1990; Lott et al., 2003; Flinn, 2006). Selfing may also be an advantage when the availability of safe sites is low such that the chance of more than one spore arriving at a site is highly unlikely (Crist and Farrar, 1982). This is the case for rock inhabiting species that live in narrow crevices, for example some species of Asplenium and many members of the Pteridaceae. Asplenium ruta-muraria, A. trichomanes, and A. septentrionale are well-investigated examples of this strategy (Schneller and Holderegger, 1996a; Vogel et al, 1999b; Suter et al., 2000). Young populations of A. ruta-muraria, for instance, are composed of genetically identical, completely homozygous individuals, which arose from one (founder) prothallus. When genetically different spores arrive at a site, antheridiogen would promote outcrossing (Schneller and Holderegger, 1996b). When intragametophytic selfing in bisexual prothalli is predominant, one could argue that antheridiogen is not necessary. But Schneller and Hess (1995) observed the presence of antheridiogen in A. ruta-muraria, which predominantly propagates by intragametophytic selfing and may even develop mechanisms of outbreeding depression (Schneller, 1996). Schneller (1996) found that within gametophyte populations many males developed. Schneller and Hess (1995) suggested that the presence of an antherid-iogen system in A. ruta-muraria could be a matter of optimal resource allocation, rather than the promotion of outcrossing per se, allowing female gametophytes to dedicate resources to egg and sporophyte formation while forcing neighboring gametophytes to spend resources on sperm cell production (see also Willson, 1981).
Antheridiogen not only influences gametophytes on the surface of a substrate, but it also mobilizes buried spores. Schraudolf (1985) argued that the hemispheric field of active antheridiogen has a diameter of about 10 cm in Anemia phyllitidis. How the different prothalli react may be related to competition between sexes. Males remain smaller and therefore do not compete much for space and light with female gametophytes.
Vittaria gemmae are special features of prothalli that allow vegetative dispersal of the haploid generation. Interestingly they are sensitive to antheridiogen. This may be a means to promote outbreeding whenever gametophytes formed by the gemmae are growing closely together (Dassler and Farrar, 2001). Emigh and Farrar (1977) suggested that the capability of Vittaria gemmae to form antheridia under the influence of the pheromone is related to sexual reproduction in addition to their role in vegetative reproduction.
Although we have a good understanding of the role of antheridiogens, many questions remain unanswered. For example, are antheridiogen systems as common in tropical ferns (e.g., Korpelainen, 1994; Ranker et al., 1996) as they appear to be in temperate taxa? How widespread are antheridiogens in members of the Aspleniaceae? How big is the active radius of the effect of antheridiogens under natural conditions?
A phenomenon that has not been well researched is the interspecific and/or intergeneric response to the pheromone. Is there some sort of antagonistic and competitive behavior? Can we see special mechanisms in sympatric species that may use the pheromone as a weapon in competition? The short-celled, dark germinated protonema of Athyrium filix-femina under the influence of the antherid-iogen of Dryopteris filix-mas (Schneller, 1988) could be interpreted as a means to reduce the success of Athyrium filix-femina because in many cases the dark germinated prothalli do not reach the surface and therefore are lost.
Fast growing prothalli may also gain an advantage by causing more slowly growing, neighboring prothalli to become male, thus reducing competition from other females (Willson, 1981). The influence of antheridiogen seems to be more important in outbreeding species, because species that are able to self fertilize would not necessarily benefit from an antheridiogen response.
How many similarities or differences in sex determination based on antherid-iogens will we find between different species? Can we see differences within species when growing under different environments? The experiments of Greer and McCarthy (1999) revealed that under more severe conditions, three of four species compensate for this disadvantage by exhibiting greater reproductive effort. What exactly is the influence of different nutritional conditions on the production of antheridiogen and the regulation of sex? How stable is antheridio-gen under natural conditions? The seasonal timing of reproduction in temperate climates, similar to mosses (Greer, 1993; Hock et al., 2004), may have an important influence on the consequences of sex determination and predicting the survival of progeny (Khare, 2006). Kazmierczak (2003) showed that antheridiogen can be used to reveal developmental processes such as antheridial ontogeny. Future molecular investigations are promising in developing methods to learn more about the genetic regulation of sex determination and gain deeper insights into the functions of antheridiogen, for instance its influence on the phytochrome system (Banks, 1999).
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