Classification and evolution of fungi

Classification of fungi

Until the 1980s, the major groups of fungi and the relationships between them were based on light microscopic examinations of comparative morphology and the developmental patterns of the sexual reproductive structures.

Examples of traditional taxonomic features include the presence or absence of septa in hyphae, fine details of the type, formation and release mechanisms of spores, or aspects of biology or ecology of fungi.

Major criteria used in the classification and phylogeny of fungi

1. Morphology

2. Anatomical Characters

3. Nutrition and Physiology

4. Chemistry of Low-Molecular-Weight Compounds

5. Carbohydrates and Cell Wall Composition

6. Molecular Methods

• Basic shape of the fungal thallus. • Form, colour and size of the asexual or sexual spore producing structures.

1.Morphology

2. Anatomical Characters

• The arrangement of hyphae comprising the tissues of spore-producing structures

• The arrangement of asci or basidia and also the sterile structures like paraphysis and cystidia within the hymenium.

• Analysis of hyphal structures.

• Histochemical localization of certain compounds by using fluorescent probes and dyes.

They are of great practical importance in yeasts because of their limited morphological features and absence of sexual sporulation in some.

Features of importance are:

• Whether growth is limited to aerobic/anaerobic conditions or fermentation occurs.

• Whether it can utilize nitrate as a nitrogen source.

• Which sugars and glycosides it can metabolize?

• What is the pattern of susceptibility to various antifungal compounds?

3. Nutrition and Physiology

The term chemotaxonomy is generally applied to the utilization of low molecular weight compounds in classification and identification

This approach is important to understand lichen taxonomy as they are known to produce a wide variety of secondary metabolites, such as pigments, colorless compounds, and some products in the form of crystals

This approach relies on procedures such as Gas Chromatography (GC), Mass Spectrometry (GCMS), High Performance Liquid Chromatography (HPLC) and Nuclear Magnetic Resonance (NMR).

4.Chemistry of Low-Molecular-Weight Compounds

• Fungal walls have a complex structure with specific polysaccharides present in different groups for example the walls of Oomycota contain cellulose, whereas in most other fungi chitin is present.

• Estimates of the amount chitin have been significantly employed in yeast taxonomy.

• Glucose and mannose are the main carbohydrates found on hydrolysis of the walls of all yeasts. However, presence or absence of certain sugars like fucose, galactose, rhamnose and xylose, though in smaller amounts, may also help in classifying yeasts.

5.Carbohydrates and Cell Wall Composition

• These methods are based either directly on the DNA sequence characters or on the properties of their protein products, especially enzymes.

• This can provide information on the direction of evolution leading to the development of a phylogenetic tree.

6. Molecular Methods

Some earlier systems of classification 1. System proposed by HCI Gwynne-Vaughan & B Barnes (1937).

2. System proposed by EA Bessey (1950).

3. System proposed by GM Smith (1955)

GM Smith an American botanist (1885-1959), proposed to include all fungi into seven classes belonging to two divisions.

Fungi

Lower Fungi (Phycomycetes) Higher Fungi

Group 1 Uniflagellatae Class 1 Chytridiomycetes Class 2 Hyphochytridiomycetes Class 3 Plasmodiophoromycetes

Group 2 Biflagellatae Class 1 Oomycetes

Group 3 Aplanatae Class 1 Zygomycetes Class 2 Trichomycetes

Class 1 Ascomycetes Subclass 1 Hemiascomycetidae Subclass 2 EuascomycetidaeSubclass 3 Loculoascomycetidae Subclass 4 Laboulbeniomycetidae

Class 2 Basidiomycetes Subclass 1 Heterobasidiomycetidae Subclass 2 Homobasidiomycetidae

4. Outline of the classification proposed by Lilian E Hawker (1966).

5. System proposed by Greta B Stevenson (1970).

Division Mycota Class 1 Chytridiomycetes Class 2 Oomycetes Class 3 Zygomycetes Class 4 Ascomycetes Class 5 Basidiomycetes Class 6 Deuteromycetes

6. System proposed by GC Ainsworth (1973).

7. System proposed by Constantine John Alexopoulos and Charles W Mims (1979).

Modern systems of classification

Fungal taxonomy is evolving at an unprecedented speed at present due mainly to the contributions of molecular phylogeny. These classification systems are based on evolutionary relationships and are known as phylogenetic classifications.

Outline of classification system proposed by CJ Alexopoulos, CW Mims and M Blackwell (1996)

Outline of classification system adopted by J Webster, and Roland WS Weber in their book, Introduction to Fungi (2007) This classification scheme is based on the classification proposed by McLaughlin et al (2001). Mainly this scheme also is based on molecular phylogeny and is broadly same as proposed by Alexopoulos et al. (1996).

The phylogenetic relationships of Fungi and fungus-like organisms studied by mycologists with other groups of Eukaryota.The analysis is based on comparisons of18S rDNA sequences.

Fungi have been assigned to the plant kingdom because of their mode of nutrition, non-motile nature, presence of cell-wall, and reproduction by spores

Today, biologists clearly agree that fungi are not plants and therefore, separate these two groups into their own kingdoms. However, recent molecular phylogenies also suggest that fungi are more closely related to animals than to plants or protists.

Biochemical evidence supporting the hypothesis of a fungi-animalia clade• The protein from multiple protein data sequences of fungi show homology to those of animals. E.g. multiple protein data sequences from members of the Dictyosteliomycetes

• Elongation factors found in the fungal ribosomes also provide evidence for this thesis E.g. Elongation factor 3 (EF-3), EF-1 alpha• Fungi also display similarities to animals in the biosynthesis of polyunsaturated fatty acids. E.g. Gamma linolenic acid• Fungi also show similaritie to animals in their cytochrome systems.

Therefore, the conclusion came that these features may be a biochemical parameter for phylogenetic affinities between fungi and animals

In both fungi and animals, plastids are absent, while plants possess plastids with two envelopes, usually containing starch

Microfibrils and matrix polysaccharides are another piece of evidence supporting thehypothesis of a closer resemblance between fungi and animals.

Morphological and structural cellular evidences

Both have non-discoidal plate-like mitochondrial cristae, while plants have mostly tubular mitochondrial cristae.

The fungal cell wall is mostly made up chitin, a unbranched polymer of B-14, 4 linked N-acetyglychosamine units, which is distinct from higher plants. Chitin is found in protozoa and higher animals

Unlike plants, but like animals, in the fungal mitochondrial code, ‘UGA codes for tryptophan,not chain termination”

Fungi lack chloroplasts, as do animals. Fungi do not produce chlorophyll pigments like green plants do.

Therefore, a brief summary of the modes of nutrition is as follows: photosynthesis in plants, ingestion in animals, and absorption in fungi

There are two types of bonding in the glycoproteins of fungi that are shared withglycoproteins of animal origin. They are O-glycosidic linkage and N linkage

The cellulose of fungi is also slightly different from that found in the cell walls of green plants.(X-ray diffraction images shows fungal cellulose is less crystalline than plant cellulose)

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