There are an extremely large number of fungi that dwell in the world today. Hawksworth (1991) conducted an investigation in which he compared fungal species figures with other groups obtained from ecological regions that had been deeply researched for their biodiversity. These studies led to the conclusion that there were near 1.5 million species of fungi worldwide. Only 70,000 species of fungi are presently known, meaning there is still around 95% of the fungi population that remain undiscovered. 

Organisms that have, for one reason or another, been classified as fungi have now been assigned to three diverse groups:

  • The monophyletic kingdom of fungi, containing a core of 'true' fungi, all seem to have a common ancestor, probably alike to the modern day choanoflagellates (a unicellular protist). The kingdom Fungi comprises of four phyla; Cytridiomycota, Zygomycota, Ascomycota and Basidiomycota. 

  • The kingdom Chromista, also known in some circles as the kingdom Stramenopila AND the Pseudomycota; it includes Oomycota, Hyphochytriomycota and Labyrinthulomycota. All seem related to certain algae groups, but have evolved a typical fungal lifestyle. 

  • An assemblage loosely termed slime moulds, considered as protists, includes the phyla Plasmodiophoromycota, Dictyosteliomycota, Acrasiomycota and Myxomycota. They grow as wall-less protoplasmic stages, often engulfing bacteria and other food particles by phagocytosis. They seemingly are not closely related to each other and their nearest relatives are not yet accurately known.

A number of features have been used in the past to contribute to evolutionary studies of the fungi and fungus like organisms. The following phylogenetic tree is based largely on small subunit ribosomal deoxyribonucleic acid (rDNA) sequence analysis data. It represents the polyphyly of organisms that have at one time or another been confused with fungi. Just click on one of the highlighted fungi shown to find out more evolutionary information on them.

(Redrawn from Alexopoulos, Mims, and Blackwell - Introductory Mycology, 1996)

Organisms Formally Classified as Fungi

Several interesting points can be brought up from the above tree to clarify the identity of the 'true' fungi:

  • An extremely large amount of data, including structural, biochemical, physiological and molecular, has contributed to the knowledge that three groups of organisms traditionally studied by mycologists, the Hyphochytriomycota, Labyrinthulomycota and Oomycota, are so phylogenetically remote from the kingdom fungi that they have been placed in an entirely new kingdom, the kingdom Chromista (or kingdom Stramenopiles). The kingdom, though full of diverse organisms, seems to have a biflagellate condition. Take for instance the Oomycota. This group contains organisms such as the Saprolegniales (water moulds) and Peronosporales, which contains many serious plant pathogens. They resemble fungi in being composed of hyphae and in their absorptive nutrition but differ in fundamental and biochemical features including cellulose-based walls and biochemical features resembling those of plants. It is likely they arose in the Precambrian era from the Xanthophyceae, a group of algae in which the hyphae form is common.

  • The Acrasid and Dictyostelid slime moulds (cellular slime moulds) are amoeboid organisms that phagocytose bacteria and other food particles. They are characterized by discoidal mitochondrial cristae and diverged early in the eukaryotic lineage. The Myxomycota (plasmodial slime moulds) grow as a network of protoplasm that engulfs their food. They have tubular mitochondrial cristae. Both groups differ massively to fungi including their protoplasmic somatic stage and their modes of nutrition (theirs being ingestive rather than absorptive). However, both groups form fungus-like fruiting bodies to release air-borne spores, indicating this is the reason why they were once treated as fungi.

  • Little information is known on the Plasmodiophoromycota, possibly intracellular parasites of fungi, algae and some higher plants. They can form highly persistent thick-walled resting spores, maybe a reason for the confusion with fungi. 

Success of Fungi

It should be finally noted that though many organisms, including various protists and choanoflagellate ancestors of fungi, have adapted a similar lifestyle, none have been as successful as the fungi themselves. This may be due down to some sort of inherent superiority in evolutionary potential of such a lineage. However, it is much more likely it was down to some random chance event in which fungi exploited the niche for absorptive heterotrophs before the other various organisms. They then cemented their position into all the other possible niches before the others had chance to settle, thus excluding them - excluding them, it seems, for many millions upon millions of years.