2.5 The Kingdom Fungi

As we have seen, around the middle of the 20th century the three major kingdoms of eukaryotes were finally recognised. One of the crucial character differences was the mode of nutrition:

  • Animals engulf.
  • Plants photosynthesise.
  • Fungi absorb externally-digested nutrients.

To these can be added many other differences. For example: in their cell membranes animals use cholesterol, fungi use ergosterol; in their cell walls, plants use cellulose (a glucose polymer), fungi use chitin (a glucosamine polymer); recent genomic surveys show that plant genomes lack gene sequences that are crucial in animal development, and vice-versa, and fungal genomes have none of the sequences that are important in controlling multicellular development in animals or plants. This latter point implies that animals, plants and fungi separated at a unicellular grade of organisation.

The fungal kingdom is now recognised as one of the oldest and largest clades of living organisms. Kingdom Fungi is a monophyletic group that diverged from a common ancestor with the animals about 800 to 900 million years ago. In the most recent phylogenetic classification scheme, the true fungi (or Eumycota), which make up this monophyletic clade called Kingdom Fungi comprises the seven phyla (the taxon ‘phylum’ has been borrowed from animal taxonomy):

  • Chytridiomycota, (706 species in 105 genera);
  • Blastocladiomycota, (179 species in 14 genera);
  • Neocallimastigomycota, (20 species in 6 genera);
  • Microsporidia, (1300+ species in 170 genera);
  • Glomeromycota, (169 species in 12 genera);
  • Ascomycota, (64163 species in 6355 genera);
  • Basidiomycota, (31515 species in 1589 genera).

The last two phyla being combined in the subkingdom Dikarya (Hibbett et al. 2007) and there are four subphyla that were traditionally placed in the phylum Zygomycota (1065 species in 168 genera). These taxa will be described in more detail in Chapter 3 and we say something about Zygomycota below.

For the moment remember that when fungi were still classified in the Plant Kingdom (subkingdom Cryptogamia, Division Fungi, subdivision Eumycotina) they were separated into four classes:

  • Phycomycetes,
  • Ascomycetes,
  • Basidiomycetes,
  • Deuteromycetes (the latter also known as Fungi Imperfecti because they lacked a sexual cycle).

You may still encounter these traditional names for groups of fungi, but if they are used today, you must appreciate that they can only be used informally. Many organisms included in these groups (particularly among the phycomycetes and the slime moulds) are no longer considered to the true fungi, even though mycologists might study them. This applies to many of the water moulds, like the Oomycota (which include the plant pathogen Phytophthora), and Hyphochytriomycota, all of which have been removed from the fungi, and are now classified with brown algae and diatoms in the Kingdom Chromista. Similarly, the Amoebidales, which are parasites or commensals of living arthropods and previously considered to be trichomycete fungi within the Zygomycota are now considered to be protozoan animals. None of the slime moulds are now considered to belong to Kingdom Fungi and their relationship to other organisms, especially animals, is still in dispute.

Molecular analyses have led to dramatic changes in our understanding of relationships of fungi placed in the traditionally phyla Chytridiomycota and Zygomycota. The Chytridiomycota is retained in the 2007 scheme, but in a much more restricted sense. For one thing, one of its traditional orders, the Blastocladiales, has been raised to phylum status as the Blastocladiomycota. Similarly, the group of anaerobic rumen chytrids previously known as order Neocallimastigales has also been recognised as a distinct phylum, the Neocallimastigomycota.

In contrast, the phylum Zygomycota is not accepted in the most recent classification because of remaining doubts about relationships between the groups that have traditionally been placed in this phylum. The consequences of this decision are the recognition of the phylum Glomeromycota and of four subphyla incertae sedis: Mucoromycotina, Kickxellomycotina, Zoopagomycotina and Entomophthoromycotina. As more work is done and the position clarifies, the name Zygomycota may be reinstated to encompass some of the taxa once represented in the traditional group. At the time of writing Zygomycota has not been given a proper diagnosis and can only be used informally. We continue to use it in Chapter 3 (and elsewhere in this book) as a convenient ‘container’ for the four subphyla mentioned above and, more importantly, because you are bound to encounter it in other, older, books.

Kingdom Fungi has also gained a few recruits on the basis of recent molecular phylogenetic analysis, notably Pneumocystis, the Microsporidia, and Hyaloraphidium. Pneumocystis carinii is a pathogen causing pneumonia in mammals, including humans (the human pathogen is called P. jirovecii) with weakened immune systems (pneumocystis pneumonia or PCP) is the most common opportunistic infection in people with HIV and has been a major killer of people infected with HIV). Pneumocystis was initially described as a trypanosome, but evidence from sequence analyses of several genes places it in the Taphrinomycotina in the Ascomycota.

The Microsporidia are obligate intracellular parasites of animals. They are extremely reduced organisms, without mitochondria. Most infect insects, but they are also responsible for common diseases of crustaceans and fish, and have been found in most other animal groups, including humans (probably transmitted through contaminated food and/or water). They were thought to be a unique phylum of protozoa for many years. Recent molecular studies show that these organisms are related to the Zygomycota.

Hyaloraphidium curvatum, an organism previously classified as a colourless green alga is now recognised as a fungus on the basis of molecular sequence data, which show it is a member of the Monoblepharidales in the Chytridiomycota.

Updated December 16, 2016