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Division Basidiomycota

Asexual Spores

The usual asexual spores formed by these fungi are arthrospores, sometimes called oidia. Few species form blastic conidia. In some cases, such as with some rust fungi, up to 4 different types of asexual spore are formed during the life cycle.

Oidia can have two functions. They may germinate to form mycelium or they may function in the mating process. Asexual spores are usually haploid and the hyphae that form following germination are also haploid.

Classes

Taxonomy of the division is in a state of flux. Originally, the fungi were divided on the basis of the morphology of the basidium culminating in the taxonomy of Talbot in 1968, in which three classes were recognised: Holobasidiomycetes, Phragmobasidiomycetes and Teliomycetes. Microscopic and molecular information is now clarifying the relationships within this group, and much work remains. Using available information, Walker proposes the following:

Class 1: Ustomycetes

Known as the smuts and related fungi. Wall adjacent to the pore slightly swollen. Pore usually very small, with banded occlusion and surrounded by lipid bodies. Basidiomata not formed.

Class 2: Unnamed

Known to form stilboid and pycnidioid gasteroid basidiomata, usually less than 5mm high. The septa are simple. Basidiospores statismosporic.

Class 3: Urediniomycetes

Known as the rusts and related fungi. The fungi have well-differentiated probasidia, often persistent. Septa simple, slightly tapered, often with pore occluded with dense material.

Class 4: Unnamed

Exobasidial plant parasites. Have erumpent basidiomata. Simple septa of uniform width with occluded pore. Yeast phase common.

Class 5: Unnamed

Known as the bunts and related fungi. Have septal dolipores lacking parenthisomes.

Class 6: Unnamed

Known as the jelly fungi. Basidiomata often gelatinous to leathery, phragmobasidia, basidiospores ballistosporic, septa with dolipores and either vesicular or imperforate parenthisomes.

Class 7: Unnamed

Known as the holobasidial jelly fungi. Basidiomata not erumpent when parasitic, septa simple with dolipores, imperforate parenthisome, undivided basidia have stout sterigma, basidia may germinate by repetition, conidia or germ tube in some taxa.

Class 8: Euholobasidiomycetes

Known as the true basidiomycetes, include mushrooms, etc, have dolipores with imperforate parenthisomes. Basidia a single, undivided unit, have fine sterigma. Basidiospores germinate by germ tube.


Chantarelle.
Image from E and R Kearney

Stereum.
Image from E and R Kearney

Euholobasidiomycete.
Image from E and R Kearney

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Cultural Conditions

The division contains a huge diversity of fungi. Some can be cultured on simple substrates. However, many require specific cultural conditions and have the capacity to digest and utilise complex organic compounds such as lignin and wax. Others, such as the rust fungi, are obligate biotrophs of specific plants and have a relationship only with specific races of a single plant species. Many of the fungi grow slowly in culture and take a relatively long time to form fruiting structures.


Amanita.
Image from E and R Kearney

The conditions under which these fungi are grown also influences the secondary metabolites produced. For instance, toxin production may change from negligible to extreme with changing N concentration in the medium or temperature of growth. Generalisations cannot be made about the cultural conditions for growth.

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Ecology


Pathogens of forest trees.
Image from E and R Kearney

Most Basidiomycota are found in terrestrial habitats. A few are marine. Within the division are important saprophytes, plant pathogens, animal pathogens, edible and ectomycorrhizal fungi, most of the fungi that form large, conspicuous fruiting bodies and many of the yeast-like fungi. Among the saprophytes are the degradative fungi involved in breakdown of wood and litter in forests and timber products, most edible fungi, many halucinogenic and toxigenic fungi. Plant pathogens include the rusts and smuts, pathogens of forest trees and soil pathogens of crop plants. Animal pathogens are less important, though Filobasidiella (Cryptococcus) neoformans has become important in immunocompromised people (due to organ transplants or HIV). A huge number of Euholobasidiomycetes form ectomycorrhiza. Not only do they improve mineral uptake of the host, they are also responsible to tight cycling of minerals in undisturbed ecosystems due to their capacity to extract organic N and P from the litter. A relatively small number of fungi are eaten directly. Of these A. bisporus, the mushroom, is by far the most economically important.

The fungi are well known for their secondary metabolites. These compounds include odours, colours, flavours and toxins. The odours may be unpleasant as in the case of the stinkhorns. Odours can be used by animals to locate the fungal structure. Unfortunately, though odours are used to assist identification, cultural conditions influence which compounds areproduced.

The fungi tend to be long-lived and over their lifecycle produce huge numbers of spores. However, dilution techniques, such as from soil, intending to isolate all fungi usually fail to isolate basidiomycota. This is because their spores, if present, require specific triggers to break dormancy and because they are slow to respond to triggers. Also many of these fungi survive periods of stress as hyphae rather than spores.

Some of the fungi also form rhizomorphs enabling access to fresh sources of organic nutrients. The wood rot fungi, in particular, tend to form rhizomorphs, or mycelial cords, as organic nutrient concentrations decline. When concentrations increase, hyphae fan out, penetrating the substrate more thoroughly.

Overall, the fungi tend to be slow growing producers of a wide range of secondary metabolites, utilising a variety of complex substrates and habitats.

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Hyphal Characteristics

Hyphal walls have many layers and the cells may contain one, two or many nuclei. If one, the nuceus may be haploid or diploid. Mixtures of haploid and diploid nuclei in cells have also been recorded. The hypha arising from germination of a spore commonly contains a single haploid nucleus and is a monokaryon. This hypha will continue to grow until it meets a hypha of a compatible mating type. These mycelia fuse, maybe at several points, and nuclei from one mating type pass into the second mycelium. Nuclei of the migrating type multiply independently of mitosis and nuclei migrate through the mycelium until the mycelium is binucleate (and dikaryotic). The hyphae within the cell may fuse in karyogamy with the resulting mycelium becoming diploid, though most commonly, the mycelium remains dikaryotic.

A few fungi have a multinucleate state. More than 2 nuclei are present in each cell from germination of spores. It is assumed that the nuclei are haploid replicas of one genotype.

Changes in nuclear state are accompanied by formation of clamp connections in many species of Euholobasidiomycetes. It is assumed that the clamp enables multiplication of both nuclei in the cell. However, some species lack clamps and in others formation of clamps seems to be influenced by environmental conditions.

While a single cell is rather small, the entire mycelium may be quite large. Genets of the plant pathogen Armillaria ostoyae have been found throughout 15 ha of a single forest suggesting that the fungus is the largest organism ever documented. While the entire mycelium may be spatially separated into units, because basidiomycota anastomose readily, it is likely that the entire mycelium will have cytoplasmic continuity from one end to the other. Further, basidiomycota commonly form rhizomorphs or mycelial cords. The aggregations aid connections between spatially separated zones of nutrient concentrations.

Pores

Any fungus that contains a dolipore septum is definitely a member of the Basidiomycota. However, other pore types are seen in members of the division (see classes above).

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Sexual Reproduction

The characteristic of this division is the formation of basidiospores from basidia. Within the division we find considerable variation in the structure of the basidium. Holobasidia are single cells from which arise sterigma. The remaining fungi have basidia that are either septate or isolated teliospores that germinate to form a septate germ tube on which basidiospores emerge. Basidia may be vertically or horizontally septate, with a sterigma attached to each.

Typically, the diploid nucleus goes through meiosis within the basidium and the resulting haploid spores emerge through the sterigma and are held on the tips. Sometimes more than one nucleus passes into a spore and sometimes mitotic division results in the spore being multinucleate.

In most Euholobasidiomycetes, the basidiospore is held on the sterigma obliquely and is forcibly discharged from the sterigma (see Ingold & Hudson, 1993).

Spore are released from structures that are used to characterise the fungi. These are the mushroom, brackets, earth stars, stinkhorns, puffballs, rusts, smuts, bunts, jelly fungi and coral fungi. The fruiting bodies usually collapse following spore release. One group, the bracket fungi, may form perennating fruit bodies and these can release huge numbers (up to 1000,000,000) of spores each year.

Homothallism is rare in this division. Where sexual compatibility has been investigated, most have either single factor (plus/minus) or multifactor (multiple alleles at two or more loci) heterothallism.

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Further Reading

Alexopoulos, Mimms & Blackwell 1996 Introductory Mycology (4th edition), Ch 16/22.

Carlile & Watkinson, 1994 The Fungi, pp 52 - 62.

Ingold & Hudson 1993 The Biology of Fungi Ch 5.

Kendrick B. 1992 The Fifth Kingdom (2nd edition) Ch 5.

Walker, 1996 Fungi of Australia, Vol 1a, pp 1 - 170.

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