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Section of an Ascomycete Fungus showing ascospores

{M I Walker/NHPA}

The information stored in a computer needs to be readily accessible. To ensure this, individual inputs are given file names and grouped with other similar files in a named folder. Folders are themselves grouped under named categories, one example being the different users of the computer. This hierarchical pigeonholing of data into discrete bundles is analogous to the methods of classification into groups (taxa) used by taxonomists seeking to categorise species of living organism. Just as different computer users have different systems of grouping their stored information, so taxonomists differ in the way that they classify organisms. It is only since the latter part of the 20th century that the fungi, the group that include mushrooms, have been placed in their own discrete folder, often referred to as the fifth kingdom (see here).

The problem for mycologists is how best to place the world’s 100,000 named species (equivalent to 100,000 files) into groups (folders, etc.) that reflect the similarity and evolutionary relationship of the species within each group. Given that the scientific names (and as a result the common names; see here) of species reflect the groups into which they are placed, the classification system has important knock-on effects.

Until relatively recently all major groups of the fungal kingdom were separated by features such as the shape of the fruitbody and colour of the spores, or by details visible with an ordinary microscope. The latter includes the nature of the spore-producing tissue, the size, shape and ornamentation of the spores and details of the hyphal threads. Such classification systems invariably resulted in assemblages of fungi that had only one thing in common: they did not fit anywhere else. Most systems were far from perfect. The use of the electron microscope, chemical analysis and, above all, genetic analysis including the sequencing of DNA has resulted in a major regrouping of many fungal species. One result of this reclassification has been a change to the scientific names of some fungi, including species of mushroom and toadstool. In addition, some species formerly included within the fungal kingdom have been moved to other kingdoms.

Most mycologists now recognise five main groups (phyla) within the fungal kingdom. These are Basidiomycota, Ascomycota, Glomeromycota, Zygomycota and Chytdridiomycota.

Members of the Basidiomycota produce sexual spores on the surface of special club-shaped cells known as basidia. The agarics, a historical term for mushrooms and toadstools with a mushroom-shaped fruitbody, belong in the Basidiomycota, in a class known as the basidiomycetes. Older schemes divided the agarics into some five orders, largely based on spore colour. Current thinking is to have just three orders: one order includes the boletes, such as the edible cep (Boletus edulis), with tubes in place of gills; a second embraces the crumblecaps (Russula spp.) and milkcaps (Lactarius spp.), typical of woodland habitats; and a third includes all other agarics.

The basidiomycetes also include puffballs and their relatives, bracket and crust-forming fungi, club and coral fungi, and most of the larger jelly fungi. Two other classes of Basidiomycota include the rusts, smuts and other plant parasites; in total, around 500 British species of microfungi which, unlike the basidiomycetes, do not produce large fruitbodies.

Candlesnuff fungus – Xylaria hypoxylon

{Stephen Dalton/NHPA}

Members of the Ascomycota, usually referred to as the ascomycetes, produce sexual spores inside elongated sac-shaped cells known as asci. This group includes morels and truffles which, along with some cup-fungi such as orange-peel fungus (Aleuria aurantia), have relatively large fruitbodies and are usually included in books about mushrooms. The ascomycetes also embrace thousands of species of cup fungi with miniature fruitbodies only a few millimetre across, most of which grow on rotting plant material. Those in a group known as the flask fungi also have tiny fruitbodies, but in a few species these are clumped together in a larger mass of black tissue; examples include King Alfred’s Candlesnuff fungus – cakes (Daldinia concentrica) and the aptly-named candlesnuff fungus (Xylaria hypoxylon). Xylaria hypoxylon Another very large group of ascomycete fungi have entered into symbiotic relationships with primitive algae or bacteria in organisms known as lichens.

Other ascomycotous microfungi include species that cause plant galls, such as witches’ broom on birch and leaf curl on peach. The group includes the commercially important singled-celled fungi known as yeasts. These frequent sugar-rich habitats such as ripe fruit and are essential aids for the making of leavened bread and alcohol. Powdery mildews on plants and a range of moulds are also included here. Many of these rarely, if ever, produce sexual spores, but spread by asexual means. Among the moulds, many of which contaminate foodstuffs including bread and jam, is one called Penicillium chrysogenum, the original species from which penicillin was isolated (see here). In Britain there are at least 5,500 named ascomycetes, including over 1,800 cup fungi and about the same number of lichenised species.

The other three phyla of the kingdom Fungi may be of little interest to the average mushroom hunter, but include some very important fungi. Although there are fewer than 20 named British species in the phylum Glomeromycota, they include those that are probably some of the most ubiquitous of all fungi, forming mycorrhizal (fungus root) relationships with a huge range of non-woody plants. Their significance is discussed on page 55. Included in the Zygomycota are common moulds such as species of Mucor and fungi that parasitise insects. One of these kills house flies and leaves the moribund insects stuck to window panes, each surrounded by a halo of spores. The final group, Chytridiomycota, has spores that are mobile in liquid. Not surprisingly, many are aquatic or live in damp soil.

Several other groups that were previously considered to be fungi have recently been placed in the kingdom Chromista, although their classification remains a contentious issue. Species in the kingdom Chromista are largely aquatic or require damp conditions. They include the diatoms and some species of seaweed. The most fungal-like organisms now placed with the Chromista include species of Pythium which cause ‘damping off among overcrowded seedlings. Species of Phytophthora, one of which causes potato blight, have been moved into the kingdom Chromista.

The slime moulds or Myxomycota have also been ejected from the fungal kingdom and now reside in the kingdom Protozoa. Although bearing fungal-like fruitbodies, these organisms are not composed of thread-like hyphae and include a mobile, amoeboid stage as part of their life cycle. In a number of common species this stage is readily visible as sick-like patches of white or yellow, on grass or rotting wood, usually after periods of heavy rainfall.