Introduction to Fungi – Third Edition / مقدمة عن الفطريات – الطبعة الثالثة

About 80 000 to 120 000 species of fungi have been described to date, although the total number of species is estimated at around 1.5 million (Hawksworth, 2001; Kirk et al., 2001).

This would render fungi one of the least-explored biodiversity resources on our planet.

It is notoriously difficult to delimit fungi as a group against other eukaryotes, and debates over the inclusion or exclusion of certain groups have been going on for well over a century.

In recent years, the main arguments have been between taxonomists striving towards a phylogenetic definition based especially on the similarity of relevant DNA sequences, and others who take a biological approach to the subject and regard fungi as organisms sharing all or many key ecological or physiological characteristics, the ‘union of fungi’ (Barr, 1992).

Being interested mainly in the way fungi function in nature and in the laboratory, we take the latter approach and include several groups in this book which are now known to have arisen independently of the monophyletic ‘true fungi’ (Eumycota) and have been placed outside them in recent classification schemes (see Fig. 1.25).

The most important of these ‘pseudofungi’ are the Oomycota (see Chapter 5).

Based on their lifestyle, fungi may be circumscribed by the following set of characteristics (modified from Ainsworth, 1973):

1. Nutrition. Heterotrophic (lacking photosynthesis), feeding by absorption rather than ingestion.

2. Vegetative state. On or in the substratum, typically as a non-motile mycelium of hyphae showing internal protoplasmic streaming. Motile reproductive states may occur.

3. Cell wall. Typically present, usually based on glucans and chitin, rarely on glucans and cellulose (Oomycota).

4. Nuclear status. Eukaryotic, uni- or multinucleate, the thallus being homo- or heterokaryotic, haploid, dikaryotic or diploid, the latter usually of short duration (but exceptions are known from several taxonomic groups).

5. Life cycle. Simple or, more usually, complex.

6. Reproduction. The following reproductive events may occur: sexual (i.e. nuclear fusion and meiosis) and/or parasexual (i.e. involving nuclear fusion followed by gradual de-diploidization) and/or asexual

(i.e. purely mitotic nuclear division).

7. Propagules. These are typically microscopically small spores produced in high numbers. Motile spores are confined to certain

groups.

8. Sporocarps. Microscopic or macroscopic and showing characteristic shapes but only limited tissue differentiation.

9. Habitat. Ubiquitous in terrestrial and freshwater habitats, less so in the marine environment.

10. Ecology. Important ecological roles as saprotrophs, mutualistic symbionts, parasites, or hyperparasites.

11. Distribution. Cosmopolitan.