6. Fungi and decomposition

Fungi that do not form supportive or parasitic relationships with plants and animals feed on the debris of life. These are saprotrophs that decompose dead roots, leaves, flowers, fruits, seeds, twigs, branches, upright tree trunks, and fallen logs. Wood is decomposed by a combination of white rot and brown rot basidiomycetes that produce mushrooms. Other fungi grow on animal faeces and decompose the tissues of dead invertebrates and vertebrates. Fungi also clear up our mess, breaking down every natural product used in a lifetime of consumerism and destroying synthetic materials made by industry. ‘Fungi and decomposition’ describes the saprotrophic fungi and considers their significance in the global carbon cycle.

4. Fungal mutualisms

‘Fungal mutualisms’ considers symbiotic relationships in which the fungus and its partner benefit from their biological interaction. Examples of mutualisms with insects include fungi that trap scale insects, fungi cultivated by ambrosia beetles, and leaf-cutter ants and termites that grow mushroom gardens. These highly developed relationships involve substantial structural, biochemical, and behavioural adaptations in the fungi and insects. Fungi in mycorrhizal associations with plants operate as accessory root systems for plants, whereas fungi called endophytes house themselves inside plant tissues without any connection to an external mycelium. Lichens—composite organisms produced by a fungus and a single-celled alga or cyanobacterium—are the best-known mutualisms involving fungi.

8. Edible mushrooms and fungal biotechnology

‘Edible mushrooms and fungal biotechnology’ turns to the ways fungi are used to improve our lives. The pleasure of eating wild mushrooms, tempered with awareness of the poisonous nature of a few species, is an ancient experience born from our prehistory as hunter-gatherers in forest ecosystems. Cultivation of edible mushrooms is one of the oldest biotechnological uses of fungi, although brewing and baking with yeast go back further. In modern times, fungi are used to produce antibiotics, cyclosporin, and other medicines. Fungi are a source of industrial enzymes and acids, and the use of fungi in biofuel production is one of the most exciting ventures in modern biotechnology.

7. Fungi in animal health and disease

‘Fungi in animal health and disease’ considers the fungi that populate the healthy human microbiome and the nature of superficial and systemic fungal infections in humans and other animals. Most of our associations with fungi are harmless and some of them support our well-being. Fungal diseases or ‘mycoses’ can develop, however, when our immune defences are weakened. The resulting opportunistic infections can be difficult to treat. Infections can also develop when the skin is damaged by a severe burn or if fungi contaminate tissues exposed during surgery. Fungal spores carry allergens and are a significant cause of asthma. A small number of mushroom species produce hallucinogens and others contain poisons.

1. What is a fungus?

‘What is a fungus?’ explores the evolutionary origins of fungi and how they differ from plants and animals. The fungi are united by three principal characteristics: they are eukaryotes, which feed by absorption, and reproduce by forming spores. Fungi are more closely related to animals than they are to plants; fungi and animals both belong to the Opisthokonta taxonomic supergroup. The emergence of the fungi as a distinctive group of organisms is estimated to have happened between 760 million years ago and one billion years ago. The structure, growth, and physiology of the fungal cell are also described.

3. Fungal genetics and life cycles

All of the structures produced by fungi, from simple budding yeast cells to long-lived bracket mushrooms, are encoded in genes. The genome of the fungus is a blueprint for its organization and operation. ‘Fungal genetics and life cycles’ explores the sequencing of genomes and progress in understanding the molecular mechanisms that link gene expression to fungal growth and development. Genetic experiments on yeast have led to major advances in our understanding of cell division and cancer. Filamentous fungi are also important research subjects for geneticists. The life cycles of mushrooms are complex and some species have thousands of mating types, or sexes.

2. Fungal diversity

‘Fungal diversity’ considers three species that illustrate the spectacular range of structural complexity found among fungi: Olpidium brassicae, Spirodactylon aureum, and Sphaerobolus stellatus. There are more than 70,000 species of fungi described by mycologists and over 90% of them are classified within Phylum Basidiomycota (basidiomycetes) and Phylum Ascomycota (ascomycetes). Half of the basidiomycetes produce mushrooms; the others include rusts and smuts that cause plant disease. The ascomycetes include the yeast Saccharomyces cerevisiae, fungi with beautiful cup-shaped fruit bodies, truffles, and morels. The other major groups of fungi are less well known and include species whose cells swim in water.