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Classification of Fungi

Classification of Fungi

The classification of fungi is still in a state of flux. A stable and ideal system of classification is yet to be proposed. Many systems have been proposed from time to time by mycologists. With the advancement of knowledge newer systems are proposed while the older ones are discarded. The classification of fungi is mostly based on the features noted below:

  • Nature of somatic phase whether unicellular or a mycellium, if latter septate or aseptate.
  • Kinds of asexual spores (mitospores)- sporangiospores or conidia; if former motile or non-motile; number, form and arrangement of flagella in the motile spores.
  • Kinds of sporangia.
  • Nature of the life cycle-whether asexual, haplontic, diplontic, haplo-diplontitc or haploid dikaryotic.
  • Presence or absence of the perfect or sexual stage.

It was probably Bauhin who for the first time tried to classify fungi in his book Pinax Theatri Botanici (1623) wherein he described 100 species of fungi. Later, fungi were divided into six classes by Tournefort (1994) in his book Elements de Botanique while Carolus Linnaeus (1753) placed all fungi in his 24th class Cryptogamia in his famous book Species plantarum. The first detailed classification of fungi, however, was proposed by Elias Magnus Fries in his three volume book Systema Mycologicum published during 1821-1832.

He divided fungi into four classes: (i) Coniomycetes (ii) Hyphomycetes (iii) Gastromycetes and (iv) Hymenomycetes. Saccardo (1882) included Bacteria and slime molds in fungi in his famous book Sylloge Fungorum and divided fungi into six classes: (i) Schizomycetes (ii) Myxomycetes (iii) Phycomycetes (iv)Ascomycetes (v) Basidiomycetes and (vi) Deuteromycetes.

Gwynne-Vaughan and Barnes Classification 

Gwynne-Vaughan and Barnes (1926) rejected the contention of Saccardo and excluded bacteria and slime molds from fungi. Slime molds were treated as Forms resembling Fungi. They recognised only four classes of fungi on the basis of structure of mycelium and characteristics of spores. These four classes are:

  1. Phycomycetes- Mycelium aseptate and multinucleate, sexual spores are Oospores and Zygospores.
  2. Ascomycetes- Mycelium fairly branched and septate; Septa simple; Characteristic Spores are endogenously produced ascospores.
  3. Basidiomycetes- Mycelium branched and with Dolipores, Characteristic spores are exogenously produced basidiospores,
  4. Deuteromycetes- Mycelium branched and septate, sexual spotes absent.

Gaumann and Dodge Classification

Gaumann and Dodge (1928), who also did not include slime molds in Fungi, proposed a system of classification based on the life-cycle and development of fructification. According to them, Fungi were divided into the following four classes :

  1. Archimycetes- Thallus naked.
  2. Phycomycetes- Thallus with definite cell wall.
    1. In both the above classes zygote is the diploid stage and meiosis occurs at its germination.
  3. Ascomycetes- Thallus more highly developed, plasmogamy results in a distinct mycelium; Karyogamy delayed; meiosis takes place in a distinct structure leading to the formation of sporangial structures called asci producting endogenously the ascospores.
  4. Basidiomycetes- Thallus more highly developed consisting of dikaryotic mycelium in the somatic phase; meiosis shifted to a much later stage in a structure called conidiophores or basidia exogenously producing basidiospores.

Bessey’s Classification

Bessey (1950) agreed with De Bary (1887) and placed the slime molds under the name Mycetozoa. His system of classification was largely based on the reproductive characters. True fungi were divided as follows:

  1. Lower Fungi-

    Fruit bodies not formed. Only one class Phycomyceteae.

  2. Higher Fungi-

    Fruit bodies are formed, divided to three classes:

  3. Ascomyceteae-

    Fruit body asci; ascospores endogenously formed.

  4. Basidiomyceteae-

    Fruit body basidia; basidiospores formed exogenously.

  5. Deuteromycetae-

    Sexual stage absent.

The practice of dividing Fungi into four classes continued till 1958 when Sparrow (1958) created nine classes emphasising the importance of flagellation of the zoospores in the classification. He considered that flagellation should be treated as a good taxonomic and phylogenetic crietrion. These nine classes are:

Martin’s Classification System 

Again in 1961 Martin proposed a system of classification based on the septation of mycelium and spore characters. The classification proposed by him is as follows:

classification of fungi

It would be clear from the above that modern mycologists are unanimous on the exclusion of bacteria from the fungi on the basis of lack of a true nucleus in the bacterial cell. There is a difference of opinion as regards the slime molds. Many mycologists favour their inclusion among the fungi. The Committee on the International Rules of Botanical Nomenclature has made the following recommendations :

  1. The names of divisions of fungi should end in-mycota.
  2. The sub-divisions should end in-mycotina.
  3. The names of classes should have a suffix-mycetes.
  4. The sub-classes should end with a suffix-mycetideae.
  5. The name of orders should end in-ales.
  6. The families should end with a suffix-aceae.

Division Mycota- The mycologists place division Mycota in the kingdom Plantae. This division includes non-green (achlorophyllous) nucleated thallophytes (fungi) which are characterised by the following features:

  1. Obtain nutrition either by living as saprophytes or as parasites. Lacking chlorophyll, the mode of nutrition is thus heterotrophic and absorptive except slime molds in which it is phagotrophic.
  2. Somatic phase may be filamentous, unicellular or a plasmodium.
  3. Presence of a true nucleus with a nuclear membrane and nucleoli in the cell.
  4. Typically the cell wall is chitinised but some Oomycetes have a cellulose cell wall.
  5. Reserve food accumlates in the form of glycogen and not starch. In addition there are oil drops.
  6. Reproduce both by asexual and sexual methods.

The division Mycota includes two sub-divisions, Myxomycotina and Eumycotina.

I. Sub-division Myxomycotina- This sub-division includes the slime molds which possess the following distinctive features: –

    • The thallus is a non-green, multinucleate mass of protoplasm called the plasmodium.
    • The plasmodium lacks a definite cell wall, and thus is amoeboid in shape.
    • It is free living, diploid and holocarpic.
    • The propagative units (spores) are differentiated by meiosis.

All the free-living slime molds are placed in the class Myxomycetes.

II. Sub-division Eumycotina- All the other fungi except the slime molds are included in this sub-division. It is a heterogeneous group comprising about 80,000 known species. They are distinguished as below:-

  • With the exception of a few unicellular forms, the somatic phase is typically a filamentous structure called the mycelium.
  • The unit of structure of the thallus or mycelium is not a cell but a hypha.
  • The cell or hyphal wall is usually composed of fungus cellulose (chitin) or a mixture of cellulose and chitin or rarely of cellulose.
  • There is a definite nucleus with a nuclear membrane and nucleoli in the cell.
  • Excepting a few, all are eucarpic.
  • The septa between the cells when present have each a central pore.
  • The growth in length in the filamentous forms is apical.
  • Reproduce both by asexual and sexual methods.
  • Asexual reproduction or sporulation takes place by mitospores which may be motile or non-motile.
  • There is gradual and progressive simplification and ultimate elimination of the sexual apparatus from the lower to the higher forms.

The older mycologists divide the sub-division Eumycotina, which includes the true fungi, into four classes, namely, Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes. This scheme of classification of true fungi is still in vogue in many countries. The modern mycologists, however, consider that the class Phycomycetes, which includes all the lower fungi, is not a natural group of closely related forms. Alexopoulos (1962), therefore, took a logical step and split the lower fungi included in the class Phycomycetes into six classes.

An outline scheme of classification of Eumycotina (true fungi) followed in this edition of the text, is set out below. It is customary to divide the true fungi into two main groups, the lower and the higher Fungi.

A. The Lower Fungi (Phycomycetes)

They have comparatively a simple thallus which in some is unicellular and in other filamentous (muycelium). The septa usually remain suppressed in the actively growing vegetative mycelium. They reproduce asexually by sporangiospores (generally motile, rarely non-motile), rarely by conidia. The early mycologists placed the lower fungi in a single class Phycomycetes which is split in this text into the following six classes on the basis of presence or absence of motile cells in the life cycle and the number, form and position of flagella on motile cells.

Motile Cells Present
  1. Class Chytridiomycetes-

    Included in this class are the lower fungi in which motile cells have a single flagellum of whiplash type inserted at the posterior end (opisthocont). The members of this class are called the chytrids.

  2. Class Hyphochytridiomycetes- The motile cells (zoospores) possess a single flagellum of tinsel type which is inserted at the anterior end.
  3. Class Plasmodiophoromycetes-

    The motile cells (zoospores) are biflagellate. Both the flagella are of whiplash type but one of these is longer than the other. The longer one has a sharply pointer end and the shorter one has a blunt end. The club root organisms are the examples of this class.

  4. Class Oomycetes-

    The motile cells are biflagellate. The two flagella are usually of nearly equal length. One of these points forwards and the other trails behind. The former is of tinsel type and latter of whiplash type, The common representatives of this class are the water molds, downy mildews and blights.

Motile Cells Absent
  1. Class Zygomycetes-

    Motile cells are absent. Asexual reproduction takes place by sporangiospores which are encapsulated and wind disseminated. The common examples are the pin molds and the bread molds.

  2. Class Trichomycetes-

    The motile cells are lacking. The asexual spores are produced exogenously at the tips of special hyphae (conidiophores) and are called the conidia.

B. The Higher Fungi

The somatic phase consists mostly of a mycelium which is usually septate. The septa have each a central pore and thus are incomplete. No motile cells are produced in the life cycle. The higher fungi comprise the following three classes :

  1. Class Ascomycetes-

    The characteristic spores of the sexual or perfect stage are endogenous in origin and are called the ascospores. They are produced within special, sac-like structures called the asci. Plasmogamy takes place by the fusion of gametangia or somatic hyphae. Somatic phase is usually a septate mycelium which multiplies asexually by conidia, rarely it is unicellular (as in the yeasts) and is propagated by budding or rarely by fission.

  2. Class Basidiomycetes-

    The characteristic spores of the sexual or perfect stage are exogenous in origin and are called the basidiospores. They are borne externally usually on club-shaped structures called basidia. The somatic phase consists of a simple or complex mycelium with or without clamp connections. Sexual organs are lacking. Plasmogamy, however, takes place by hyphal fusion.

  3. Form-class Deuteromycetes or Fungi Imperfecti-

    Included in this class are the higher fungi in which the sexual or perfect stage is unknown. The somatic phase consists of a septate mycelium which multiplies asexually by condia. The conidia are produced on conidiophores which may occur isolated or may be aggregated to form complex structures such as synnema, pycnidia and acervuli.

The classification of Fungi outlined above has been followed in this text. However, for the benefit of the students a brief account of the classification proposed by Alexpoulos and Mims (1979) is given here. They place the fungi including the slime molds in the Kingdom Mycetae of the superkingdom Eukaryota which, in addition, includes four other kingdoms. They divide the Kingdom Mycetae into three divisions namely (i) Gymnomycota (ii) Mastigomycota, and (iii) Amastigomycota. The division is sub-divided into sub-divisions, classes, sub-classes, and orders.

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