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Formation of Sexual Structures

Zoosporic Fungi (formerly Chytridiomycota)

The zoosporic fungi are a polyphyletic (highly variable) and ancient group of fungi, rarely studied because we lack appreciation of their significance in economic or ecological arenas. This is not to say that they are unimportant. Our understanding of their sexual reproduction is speculative, based on a few studies. In brief, formation of sexual spores may follow one of a number of processes. In Rhizophydium (Rhizophidiales), fusion of the sporangia of, presumably opposite, mating types results in the transfer of the contents of one to the other. The larger sporangium then forms a resting spore, which on germination releases zoospores. The nuclear status and arrangement during the process remain unclear.

In Chytriomyces (Chytridiales), the fusion of rhizoids of different sporangia is followed by the formation of a separate thick-walled resting spore. Again, the nuclear state and movement of nuclei remains unclear. LINK

Allomyces sex stages
Allomyces sex stages.

The best studied system in the zoosporic fungi is that of Allomyces macrogynus. The fungus is unusual for zoosporic fungi in that it has filamentous growth and development, and a clear biphasic lifecycle. Gametophores form terminally on a haploid thallus with plus gametangia located below minus gametangia (see image). At maturity, zoospores are released into the solution surrounding the thallus. The minus gametes release a sesquiterpene called sirenin, which attracts plus zoospores. Plus gametes release a sesquiterpene called parisin, which attracts minus zoospores. The plus and minus zoospores fuse, ultimately forming a diploid zygote. The diploid thallus then passes through growth, forming a resting sporangium.

zoospore release
Allomyces: zoospore release.

Following meiosis, a haploid zoospore is released.

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Zygomycota and Glomeromycota

Again, our understanding of sexual reproduction in these divisions is based on a few species. The Zygomycota (sensu lato) are paraphyletic, and the details below may not apply widely.

In the Mucorales, some fungi are self-fertile (homothallic), while others require the presence of an opposite mating type (heterothallic) to form zygospores. It appears, however, that induction of zygospores in both homothallic and heterothallic fungi follows a similar path. The induction involves production of hormones which trigger each of the stages of the typical lifecycle. LINK

For simplicity, this discussion will concentrate on heterothallic fungi. β carotene in the interface between the strains when the hyphae of plus and minus mating strains meet. Both strains produce the compound, and then metabolise it to 4-dihydrotrisporol. In the plus strain, this is metabolised to 4 dihydrotrisporic acid. In the minus strain, it is metabolised to trisporol. On diffusion to the opposite mating type, both form trisporic acid.

Note that the compound produced by one mating strain, can only be converted to trisporic acid, by the other. Zygophores are induced by trisporic acid, the different mating strains require each other because they cannot metabolise to trisporic acid the precursor each produces, and therefore require the metabolic capacity of the opposite mating strain for zygospore formation.

The Glomeromycota are largely asexual. Formation of sexual structures has been documented in only one species, and remains to be found in any other group. Because of the paucity of information, we will not consider this division further.

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Ascomycota

Ascomycetous haploid yeast cells express mating type pheromones which ensure conjugation between opposite mating strains. Two mating types are found, a and α. On fusion, the cell immediately undergoes karyogamy. The diploid cell resembles the haploid cell, though is more resistant to environmental stress, and metabolises more slowly. The genes involved in sexual reproduction have been well studied in Saccharomyces cerevisiae, and the precise location, induction, repression and activity are well understood.

Interestingly, silent genes for interconversion from one mating type to the other are present in the haploid cell. If silent copies are transposed to active locations by mechanisms controlled from within the cell, then the opposite mating type can be expressed by a cell.

Sexual reproduction in the Euascomycetes (sensu lato) is basically similar across all groups. Most information has been derived from studies of Neurospora crassa, the basis of this discussion. Following germination of a conidium, a haploid mycelium develops. Macroconidia and microconidia are released, and both can function in sexual reproduction. All cultures also form protoperithecia and trichogynes (specialised hyphae). Trichogynes of one mating type respond specifically to hormones secreted by the opposite mating type, by directing growth towards the conidia. The nucleus on the "male" enters the trichogyne and associates with a "female" nucleus. Both pass through synchronous mitotic divisions in the ultimate cells of the dikaryotic hyphae of ascogonium. Karyogamy takes place in the hook shaped cell called the crozier, followed by the proliferation of the now dikaryotic hyphae, forming new croziers and ultimately, asci.

Ascomycota have several features in common. All have two mating types, and the mating type genetic locus controls production of and response to hormones, among many things. In contrast to yeast, filamentous Ascomycota appear to gain mitochondrial DNA from the perithecial parent, have a limited diploid existence and a differentiated fruiting body.

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Basidiomycota

Among the Basidiomycota, opposite mating types have been found to release hormones, which are constitutive. In Tremella, the yeast-like haploid stage forms a conjugation tube. The hormones also appear to stop budding, in several yeasts. Study of the physiology of mating in many Basidiomycota is complicated by the separation of the formation of the dikaryon and karyogamy. While hormones may determine sexual compatibility of haploid hyphae, environmental factors are likely to be much more important determinants of the timing of karyogamy and meiosis. LINK

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Conclusion

The study of fungal hormones indicates a number of common principles. Fungi identify an appropriate partner by means of diffusible molecules, I call hormones. These compounds are secreted by at least one partner. Response to the hormone is regulated by mating type. The hormones tend to be peptides. The membrane receptors are coupled to G proteins, initiating regulatory proteins which bind to DNA. The life cycles are similar in those cases which have been studied and offer researchers the opportunity to determine the barriers to interactions between species.

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References

Moore D & Novak Frazer LA 2002 Essential Fungal Genetics. Springer.

Gow NAR & Gadd GM 1995 The Growing Fungus, Chapman Hall.

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Advantages of Asexual Reproduction

A significant number of Deuteromycota and most members of the Glomeromycota appear to have lost or never evolved the capacity to form sexual stages. These fungi form asexual reproductive units via blastic or thallic development. Sexual reproduction is suggested to provide a mechanism to remove unwanted mutations during the meiotic process. If asexual fungi accumulate mutations, then how can asexually reproducing organisms survive and why do they continue?

Mutations can be observed in fungi grown on agar. Cultures commonly "sector", indicating that the expression of at least at one locus has changed. These changes indicate loss of or changes to nuclear material, or possibly unstable nuclear combinations.

The "nuclear cleansing" of sexual reproduction can be observed during the parasexual cycle when chromosomes are lost. LINK Thus it might be argued that all of the advantages of sex take place in asexual fungi, but without the disadvantages of sex.

In general terms, a fungus that is well adapted to a particular habitat will be disadvantaged by changes in genome induced by sexual recombination. If nutrients are abundant the fungus can continue. The major disadvantage to each genetic combination would be associated with changing nutrient status.

Thus we might predict that each fungus has a sophisticated response to its complex environment. Multiple gene combinations are necessary just to exist in one changing environment. These combinations would change with sex reducing the fitness at least initially, but only in a few areas of interaction. Each fungus is probably adapted to a range of environments, with greater success at different times of the interaction. The usual discussion surrounding sex is probably immaterial. Fungi produce huge numbers of reproductive units, thus enabling widespread dispersal. It may be that the greater difficulty for a fungus is finding the food in the first place. Asexual reproduction is possible sooner than sex, requires fewer resources and therefore is more efficient. By utilising asexual reproduction, fungi increase chances of dispersing, with the reduction in mutations associated with sex irrelevant because so many propagules are dispersed, and selection is immediate because the genome is haploid. Mutations are important to organisms with few offspring, but when huge numbers are produced?

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