Kingdom Fungi: body, nutrition and reproduction

Fungi are a distinct kingdom of heterotrophs with great variety in form and habitat: mould on moist bread and rotting fruit, mushrooms and toadstools, and the white spots on mustard leaves caused by a parasitic fungus.

Yeast, a unicellular fungus, is used to make bread and beer; Puccinia causes wheat rust; Penicillium is a source of antibiotics.

Fungi occur in air, water, soil and on plants and animals, and prefer warm, humid places; this is why food is kept in the refrigerator, to slow bacterial and fungal spoilage.

Except for unicellular yeasts, fungi are filamentous. Their bodies are made of long thread-like hyphae, and the network of hyphae is the mycelium.

Some hyphae are continuous tubes with many nuclei in the cytoplasm (coenocytic hyphae); others are divided by cross walls called septae. Fungal cell walls are made of chitin and polysaccharides.

Most fungi are saprophytes, absorbing soluble organic matter from dead substrates; some are parasites on living plants and animals; some are symbionts, with algae as lichens and with plant roots as mycorrhiza.

Vegetative reproduction is by fragmentation, fission and budding. Asexual spores are conidia, sporangiospores or zoospores; sexual spores are oospores, ascospores or basidiospores, all formed in fruiting bodies.

The sexual cycle has three steps: plasmogamy (fusion of protoplasms of two gametes, motile or not), karyogamy (fusion of two nuclei) and meiosis in the zygote, which gives haploid spores.

In sexual reproduction two haploid hyphae of compatible mating types fuse. In some fungi this gives diploid (2n) cells at once; in ascomycetes and basidiomycetes a dikaryotic stage (n + n, two nuclei per cell) comes in between, called the dikaryon, in the dikaryophase.

The parental nuclei later fuse to make the cells diploid, and meiosis in the fruiting bodies produces haploid spores. Fungi are split into classes by the form of the mycelium, how spores are formed, and the fruiting bodies.