Fungi main page

Introduction

Fungi have traditionally been classified with the plants. However, ultrastructural and chemical research (now confirmed using molecular methods) has demonstrated without question that they should be placed in a separate Kingdom of life, and that they are actually more closely related to animals than plants. It has also been established that a number of groups traditionally considered as fungi do not belong to the Kingdom Fungi. These include the Myxomycota (slime moulds) and Oomycota (including Phytophthora which causes disease including the potato blight resulting in the Irish famines of the 19th century).

Fungi are one of the most diverse organism groups on Earth, outnumbered only by the insects. Biodiversity research in the 1990s led to estimates that the total number of species probably numbers between 1 and 1.5 million, of which we have basic information of less than 100000. The small size and ephemeral nature of many species makes their detection and identification problematic.

Fungi underpin many ecosystem functions, and are the principal group of organisms that decay plant and animal remains and therefore recycle carbon. Many have symbiotic relationships with other organisms. These include mycorrhizal species that play critical roles in water and nutrient transport in plant roots, and lichenized species that form mutual complexes with algae and cyanobacteria. They are used as food by a wide range of animals, including of course humans. They produce an immense range of chemicals (secondary metabolites, enzymes etc.) that may be deleterious (toxic) or beneficial. Among others, pharmaceutical industries, food manufacturers, household product producers and clothes manufacturers all exploit fungi and fungal products in their production processes.

Fungal identification is problematic for a number of reasons. The sheer number of species is daunting even to the expert, morphological structures required for identification may not be produced in culture, the species themselves may be poorly differentiated, and literature for identification is scattered, outdated and jargon-ridden. For most groups of fungi, morphological and cultural examination are still the techniques of choice. Molecular phylogenetics systems that identify fungi using sequence data (primarily rDNA ITS sequences) are being developed, and are already successful for some groups of fungi. It will be some years before a universal molecular identification system can be developed, due to the number of species for which sequences are not available and uncertainties about species definition using sequence data.

 Classification of the Kingdom Fungi

The true fungi contain species in four phyla (major divisions), the Ascomycota, Basidiomycota, Chytridiomycota and Zygomycota. Many true fungi produce both mitotic and meiotic spores, often from distinct fruiting structures which appear at different times and are associated with different symptoms. Structures producing mitotic spores (or conidia) are referred to as anamorphs, and those producing meiotic spores ascospores, basidiospores etc.) as teleomorphs. For historical reasons, the different spore-bearing forms have frequently been given separate names. As in many cases environmental fungi are most prominent as anamorphs, they are commonly referred to using their anamorph names. In some instances the sexual stage is not known: the fungus may have a purely mitotic life cycle, or the teleomorph may simply not have been recognized. Linkages between anamorphs and teleomorphs can be established using various methods, but many have not been identified. Technically, when the sexual morph is known the fungus as a whole should be referred to by the teleomorph name, but in applied mycology circles the anamorph name is frequently used even though the meotic phase is known.

Fungi (with the exception of yeasts) are filamentous organisms with their vegetative tissues composed of tube-like hyphae, usually with cross-walls (except in the Zygomycota) with pores through which water, nutrients etc. can pass. Hyphae are often branched and anastomozed, and collectively form a network known as the mycelium. This is often difficult to detect as it may be immersed within substrata, but becomes easier to observe in culture. Yeasts are fungi also, and collectively form a biological strategy group rather than a homogenous group in taxonomic terms. Environmentally significant species may have affinities with both ascomycetous and basidiomycetous fungi.