Classification of Fungi

Traditionally, fungi have been split into four classes - Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes - of which Phycomycetes is really an umbrella term covering the Oomycetes, Chytridiomycetes and Zygomycetes, all considered 'lower fungi' whose features hint at an algal ancestry. A more detailed scheme was proposed in 1979 by Constantine J. Alexopoulos and Charles W. Mims in their book 'Introductory Mycology', splitting fungi into three divisions - Gymnomycota, Mastigomycota and Amastigomycota - and, within those, 8 subdivisions, 11 classes, 1 form-class and 3 form-subclasses (with molecular methods later moving the myxomycetes and oomycetes out of true Fungi and into the separate Kingdom Chromista). The salient features of five of these classes are: Oomycetes - coenocytic mycelium; cell wall of glucan and cellulose (not chitin); a zoospore bearing one whiplash and one tinsel flagellum; oogamous sexual reproduction (e.g. Albugo). Zygomycetes - mostly saprophytic on decaying plant/animal matter, some parasitic (e.g. Entomophthora on the housefly); includes the bread moulds (Mucor, Rhizopus) and dung-loving 'coprophilous' fungi (e.g. Pilobolus); branched, coenocytic mycelium; asexual reproduction by spores formed inside sporangia; sexual reproduction by fusion of gametangia, producing a thick-walled zygospore that stays dormant for long periods before undergoing meiosis and producing new spores. Ascomycetes - a very broad group spanning yeasts, powdery mildews, cup fungi and morels; mostly saprophytic, with a few parasites (e.g. the powdery mildew Erysiphe); asexual reproduction by fission, budding, oidia, conidia or chlamydospores; sexual reproduction by fusion of two compatible nuclei, where plasmogamy is followed only after a delay by karyogamy, so a dikaryotic phase persists for several generations; a special ascogenous hypha forms a hook-shaped 'crozier', whose penultimate cell's two nuclei fuse to form the young ascus; the resulting diploid nucleus undergoes meiosis (four nuclei) then a further mitotic division (eight nuclei), organised into 8 ascospores held inside a bag-like ascus - the reason this group is nicknamed 'Sac fungi'; groups of asci are enclosed by sterile hyphae into a fruiting body called an ascocarp, of four types - cleistothecium (completely closed), perithecium (flask-shaped with an ostiole/pore), apothecium (open, cup-shaped) and pseudothecium. Basidiomycetes - include puff balls, toadstools, bird's-nest fungi, bracket fungi, stinkhorns, rusts and smuts; mostly terrestrial, though a few live in fresh water or marine habitats; a well-developed, branched mycelium with simple septa; asexual reproduction by conidia, oidia or budding; sexual reproduction lacking dedicated sex organs, achieved instead by somatogamy or spermatisation leading to plasmogamy, with karyogamy delayed (again giving a long dikaryotic phase) until it finally occurs inside the basidium, immediately followed by meiosis; the four resulting nuclei become basidiospores borne exogenously on stalk-like sterigmata outside a club-shaped basidium - the reason this group is nicknamed 'Club fungi' - and the whole fruiting body is called a basidiocarp; the mycelium is septate with a distinctive bracket-shaped 'dolipore' septum, occurs in three types (primary/monokaryotic, secondary/dikaryotic, tertiary), and dikaryotic hyphal cells maintain their two-nucl …