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In this article we will discuss about:- 1. Introduction to Fungi 2. Meaning of Fungi 3. Characteristics 4. Classification.
Introduction to Fungi:
The beautiful and delicate umbrella-shaped mushrooms (the fungus flowers) have been a source of attraction to the nature lovers since ages. The Greek naturalists of 300 B.C. found them to be highly delicious. However, soon some of them were found to be deadly poisonous. To distinguish them from the delicious mushrooms, the poisonous ones were called toadstools.
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In fact, the word toadstool is a distortion of the original German word todestuhl, which means death chair. Mycophagists (people enjoying eating mushroom) started collecting mushrooms, and recorded characters that differentiated the edible from the poisonous. The poisonous property was so effective that when a most reliable and sure poison was needed, as when killing a king, the poisonous mushrooms were relied upon.
The Roman emperor Claudius Caesar was murdered be his wife in 54 A.D. by mixing a decoction of the mushroom Amanita phalloides with his food. A bad thing indeed! Thus, the study of fungi, i.e. mycology, started with the study of mushrooms. The word mycology means study of mushrooms (mykes = mushroom; logy = study).
The scientific study of fungi started only in the 17th century when microscopes became available. Antonio Micheli (1729) made an extensive study of fungi and discovered that spores were the reproductive structures of fungi. The association of fungi with several plant diseases, especially the wheat smut, was known since long, but it was only in 1861 that a fungus was accepted as the cause, rather than the product of disease.
In 1845, the potato crop in Europe, in general, and in Ireland, in particular, was devastated by the disease late blight of potato. Potato was the staple food of Ireland and this disease caused such a disaster that about one million (ten lacs) people died of starvation and over 1.5 million fled to other countries.
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Commissions were set up to go into the cause of the disease. It was de Bary who discovered that a fungus, now called Phytophthora infestans, was the cause of the disease. With this started plant pathology, which now deals with diseases of plants, caused not only by fungi but also by bacteria, viruses, nematodes and environmental factors?
Since ancient times, man has loved the kick from a drink, without bothering to know the secret. Louis Pasteur, a great French chemist, discovered that a chemical (enzyme) produced by yeast cells, converted sugar into alcohol, which gave the kick.
Since then began the study of chemical activities of living organisms, which is now one of the most fascinating branches of science, called biochemistry. Fungi now are known to produce an array of metabolites which are put to numerous uses.
Penicillin was discovered in 1928 by Alexander Fleming of Britain from the fungus Penicillium notatum, which in 1940s, emerged as the wonder drug for the treatment of bacterial diseases. It gave another important place to fungi in biological sciences, as producers of antibiotics.
Then in Japan, some intelligent farmers observed that the rice plants that grew unusually tall, gave no or poor yield. The disease was named bakane or foolish disease of rice. The tallness, associated with paleness, was caused by a new type of growth hormone, called gibberellins, produced by the fungus Gibberellia fujikuri (= Fusarium moniliforme), which caused the bakane disease. Earlier, the plant hormone Indole Acetic Acid was isolated from Rhizopus niger, before it was detected in plants.
Biochemical genetics, which later developed into the charming and all pervasive molecular biology, started with Neurospora crassa, a fungus. It was especially suited for genetical studies. Neurospora dethroned Drosophila from the kingdom of genetics, but soon yielded to bacteria, which, in turn, yielded to viruses.
Nevertheless, in 1952, Aspergillus nidulans emerged on the scene when Pontecorvo and Roper at Glasgow discovered a novel type of sexual reproduction, the parasex, in this fungus. Fungi continue to be important tools of study in genetics; the most important being Saccharomyces cereviseae, called the Bakers’ and Brewers’ yeast.
Pasteur said, “Without fungi even death will be incomplete”. The dead cellulosic vegetation is decomposed into carbon and minerals by fungi. Thus, the fungi help to maintain the carbon and mineral cycles in nature. However, some of them are a nuisance, growing on everything from jam to leather, and poison or destroy them.
The hallucinogenic fungi cause great damage by making us perceive the sound and sight which do not exist. LSD (d-lysergic acid diethylamide) produced from ergot, Calviceps purpurea, is used in the treatment of schizophrenia and other mental disorders. But it is also misused for taking a trip!
Meaning of Fungi:
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It is difficult to define fungi keeping in view their diversity because of which they are distributed in three kingdoms, Fungi, Protozoa and Straminopila. It was wise and pragmatic to call fungi as organisms studied by mycologists. Nevertheless, fungi have been bound by a definition and it is as follows- Fungi are eukaryotic, heterotrophic, developing branching filaments (or, are more rarely single-called), and reproduce by spores.
When it is mentioned in the wider polyphyletic sense, the word fungi is neither italicized (not underlined) nor capitalized to differentiate it from fungi belonging to the kingdom Fungi. Some authors use Eumycota, as the kingdom name, which has the advantage of avoiding confusion with ‘fungi’. The kingdom name as Fungi, however, is preferred as majority of the fungi belong here.
Characteristics of Fungi:
The main characteristics of the fungi can be described under the following heads:
Nutrition – Heterotrophic and absorptive; ingestion of food rare.
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Thallus – Unicellular, filamentous (mycelial) or occasionally plasmodial.
Cell wall – Made of chitin – cellulosic in Oomycota.
Nuclei – Characteristically small, near the limit of resolution of the light microscope.
Life cycle – Simple to complex.
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Sexuality – Asexual, sexual and parasexual.
Habitat – Ubiquitous, saprobes, parasites or symbionts.
Distribution – Cosmopolitan.
We shall examine these characters in some detail:
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1. Nutrition:
Fungi are heterotrophic organisms which derive their nutrition from external sources. They have absorptive or holophytic type of nutrition. The substrate is dissolved and then absorbed. For this, the fungi produce extracellular enzymes which degrade the insoluble substrate into smaller fragments and finally into soluble units which are then absorbed by the hyphae. They live as parasites, saprobes or symbionts.
As parasites, fungi grow on plants, animals and human beings causing diseases by their presence. Fungal parasitism varies from facultative parasitism to obligate parasitism. The facultative parasites can grow saprobically in the absence of the host while obligate parasites (downy mildews, powdery mildews and rusts) grow only on their specific hosts.
All the classes of fungi include important plant pathogens. Fungi, in general, are not important parasites of human beings.
When fungi grow on dead organic matter, they are called saprobes. Here also, the saprobism may vary from facultative saprobism to obligate saprobism. They have poor or no parasitic ability, respectively. Some fungi thrive on a piece of dead leaf in the soil but they cannot grow even little in the soil as such. This is because strong saprobes produce antibiotics and fungi-static substances in the soil, which don’t allow the growth of other fungi-especially the parasites.
This is called antibiosis or mycostasis. There is competition everywhere, and it occurs in the soil also. A teaspoonful of soil contains millions of organisms and thus they have an acute house problem. There is struggle for existence; one prevents the growth of the other. Like ours, theirs is also a busy, vexed, quarrelsome world. But there, survival is of the fittest only!
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Saprobic fungi are indispensible for the maintenance of the carbon and mineral cycles in nature and the decomposition of cellulose and lignin present in the dead vegetation. Thus, they clear the ‘garbage’ of nature.
Fungi enter into permanent association with other organisms in which both the partners benefit. This is called symbiosis. Lichens and Mycorrhizas are such symbiotic associations of fungi. In lichens, they are partners of algae and cyanobacteria, while in mycorrhizas (=fungus roots), the fungi enter into permanent association with plant roots.
Fungi belonging to kingdom Protozoa (slime moulds) have myxamoebae or plasmodia as the somatic phase, and feed on bacteria by ingestion (phagocytosis) with the help of pseudopodia.
The thallus may be plasmodial, unicellular or filamentous, but the filamentous form is typical of fungi.
Plasmodium is a big (up to 1mm in diameter) naked mass of multinucleated cytoplasm that moves and feeds by pseudopodia. It may develop inside plant cells as obligate parasites, as in Plasmodiophoromycota, or have an independent existence, as in Myxomycota. Plasmodium can absorb soluble nutrients or feed on encysted myxamoebae and bacteria. Besides Plasmodium, small, uninucleate amoebae represent the somatic phase in Acrasiomycota, which move and feed on bacteria with the help of pseudopodia.
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These are called myxamoebae to differentiate them from other amoebae. The myxamoebae are less than 10 µm in diameter and can be seen only under a phase contrast microscope. The aggregation of myxamoebae results in a pseudoplasmodium, in which the individual amoebae do not fuse but lead a communal life, acting in coordination. Acrasiomycota are, therefore, called ‘cellular’ or ‘communal’ slime moulds.
a. Unicellular Fungi:
These are represented by yeasts and cellular slime moulds mentioned above. These multiply by budding or fission. Yeasts are found in Ascomycota, Basidiomycota and Zygomycota.
b. Filamentous Fungi:
The hyphae (sing. hypha) collectively form the mycelium (pl. mycelia). The hyphae grow at the tip. The growth involves cell enlargement, accompanied by cell division and cytoplasm synthesis. The tip of the hyphae (50-100 µm) is the zone of elongation and is filled with protoplasm. The rest of the hypha is vacuolated and is incapable of elongation.
But it helps in growth by synthesizing the cytoplasm, which is continuously transported to the tip. The growing hypha forms branches in acropetal order behind the growing apical region. A soft portion appears on the wall which bulges out as a bud and extends to form a lateral hypha. Secondary and tertiary branches are formed in the same way.
Growth is measured by increase in the radius of the fungal colony growing in a Petri dish, or more accurately by taking the dry weight of the fungus grown in a liquid medium after 5, 10 or 15 days.
The hyphae may be unseptate or septate. The unseptate hyphae have the nuclei scattered in the cytoplasm. This condition is known as coenocytic condition. However, in such hyphae, adventitious septa are formed to delimit reproductive structures or older portions of hyphae.
In the septate hyphae, the septa have perforations through which cytoplasmic strands, containing nuclei, migrate from one cell to the other. The plasmamembrane is continuous through the pore. A characteristic dolipore septum is found in certain basidiomycetes.
Sometimes, the hyphae aggregate to form tissue-like structures called plectenchyma. In cross section, it appears like parenchymatous cells of higher plants. This is called pseudoparenchyma and consists of rounded fungal cells. Less compact structures, consisting of hyphae made of elongated cells are called prosenchyma. These are found mostly in stroma or fruiting bodies of Asco-, and Basidiomycota.
The cell wall is made of chitin and β glucan in members belonging to kingdom Fungi, and of cellulose in kingdom Straminopila (Oomycota). Chitin is a polymer of N-acetyl glucosamine units, which is also found in the exoskeleton of insects. Cellulose is a polymer made of β glucose units and is also found in the cell wall of plants and algae.
The nuclei of fungi are extremely small and lie near the limit of resolution power of the light microscope. This makes the cytological (karyological) studies difficult. Electron microscopic studies have revealed that the nuclear membrane does not disappear but constricts like a dumbbell during nuclear division. This type of division is called karyochoresis. In meiosis, however, the nuclear membrane disappears.
The fungi are mostly haploid organisms, i.e., their nuclei are haploid. The thallus multiplies by asexual method. Eventually, through sexual reproduction, a diploid zygote is formed. The zygote represents the diploid phase, which is terminated by meiosis forming haploid spores. The spores on germination form the haploid somatic phase.
In Asco- and Basidiomycota, plasmogamy is not immediately followed by karyogamy. The two nuclei of opposite strains remain as paired nuclei, called dikaryon, which increase in number by simultaneous (=conjugate) divisions and give rise to dikaryotic hyphae. These are of limited duration in Ascomycota, as only a small portion of the mycelium, viz., and the ascogenous hyphae, become dikaryotic.
However, in Basidiomycota, these extend through the major portion of the life cycle. The monokaryotic mycelium is of short duration and its function is only to establish the dikaryotic mycelium, as soon as possible. Thus, the dikaryophase, which is of short duration in Ascomycota, is extensive in the life cycles of Basidiomycota.
Seven basic types of life cycles are found in fungi, depending on the extent of haploid, diploid or dikaryotic phases of the mycelium.
These are:
1. Asexual cycle – Found in all Anamorhpic (=imperfect) fungi,
2. Haploid cycle – Mucor, Rhizopus etc.
3. Haploid cycle with restricted dikaryophase – Eurotium, Talaromyces,
4. Haploid-dikaryotic cycle – Ustilago maydis,
5. Dikaryotic cycle – Ustilago tritici, Puccinia graminis,
6. Haploid-diploid cycle – Saccharomyces cereviseae,
7. Diploid cycle – Pythium, Phytophthora, Albugo.
6. Sexuality (= Reproduction):
Fungi multiply by asexual and sexual methods. The asexual methods (fragmentation, budding, fission, chlamydospores, oidia, zoospores, aplanospores, conidia, etc.) serve to multiply and disseminate the fungus to far off places. But the function of sexual reproduction is different. By karyogamy and meiosis it brings about genetic recombination leading to genetic variability.
It plays great role in the process of evolution and origin of new organisms. In addition to asexual and sexual methods, there is a parasexual reproduction in which the events of sexual reproduction occur in somatic cells, and the genetic recombination occurs by an unusual method, called mitotic crossing over.
7. Habitat:
Fungi are ubiquitous, i.e., they exist everywhere – in water, soil and air. They are found in hot deserts and on high mountains. They grow on rocks, living or dead plant and animal bodies, jams and jellies, leather, paper and plastics. The wide distribution of fungi is because of their potential to utilize any kind of substrate under any kind of environment.
Anything that can be decomposed to yield energy, will find some fungi to colonize it and, therefore, it is difficult to enumerate their numerous habitats. There are cellulolytic fungi (that degrade cellulose), cutinolytic fungi (that degrade cutin), keratinophilic fungi (that degrade keratin in skin, nail, hairs etc.), coprophilous fungi (that grow on dung), and lignicolous fungi (that degrade lignin).
Fungi are cosmopolitan in distribution. Some fungi are more widespread than plants. This is especially true for some lichenized fungi, ascomycetes, myxomycetes, polypores, opportunistic fungi and soil fungi. Another important observation is that saprobic fungi are more widely distributed than the parasitic ones.
Finally, how many fungi are there? According to Kirk et al., the total number of fungi recorded is 80,000 species. Among these, 960 species belong to kingdom Protozoa, 900 to Straminopila and rest to kingdom Fungi.
Classification of Fungi:
Ainsworth’s classification (1966, 1973), which was followed earlier, is no more acceptable in view of the new information on phylogenetic relationships of fungi obtained by small subunit (18S) ribosomal DNA gene comparisons. Also, the recognition of domain as the taxon above kingdom, and creation of more kingdoms in Eukarya (Chromista, Protozoa and Straminopila) made it necessary to have a new look on the status and classification of fungi.
This was done by consensus arrived at by international efforts of fungal taxonomists and adopted in The Ainsworth and Bisby’s Dictionary of The Fungi (8th edition 1995), edited by Hawksworth et al., and (9th edition 2001), edited by Kirk et al. The classification, given by Hawksworth et al. in 1995, and partly modified by Kirk et al. in 2001, is now universally accepted.
Hawksworth and Kirk’s Classification (1995, 2001) – The fungi are placed in 3 kingdoms, Fungi, Protozoa and Straminipila under various phyla, classes, orders, families, genera and species. It is to be noted that the taxon division is no more valid and has been replaced by phylum, a term used earlier only by zoologists. Another departure is that now all taxa, kingdom to species, are italicized in print and underlined when hand-written.
There are standard endings for the various taxa. The phylum ends in – mycota, class in – mycetes, sub-class in – mycetidae, order in – ales and family in – cetes. Genus and species have no fixed endings. The former, i.e. genus, is always written with the first letter in capital, as it is a proper noun and species in small letters as it is adjective, qualifying the noun.
The shift from a homogeneous kingdom Fungi to three kingdoms is a drastic change. But it is neither surprising nor shocking. From the very beginning, plasmodial and amoeboid (cellular) slime moulds, having phagotrophic nutrition, were put separately from true fungi (Eumycota), under Myxomycota. Zoologists put them under Mycetozoa (Protozoa). Hyphochytriomycetes and Oomycetes, because of their cellulosic cell wall, were put along with algae under former kingdom Protista.
Members belonging to kingdom Fungi are characterized by:
(1) Unicellular or filamentous somatic phase,
(2) Cell wall made of chitin and ß glucan,
(3) Absorptive mode of nutrition,
(4) Presence of only whiplash type of flagella,
(5) Mitochondria with flattened cristae, and
(6) Presence of perioxisomes and Golgi bodies.
There are 4 phyla:
a. Chytridiomycota,
b. Zygomycota,
c. Ascomycota and
d. Basidiomycota.
The ‘imperfect’ fungi that reproduce only asexually and earlier placed in Deuteromycotina are no more accepted as a distinct taxonomic category. They are not monophyletic unit but are fungi which have either lost a sexual phase or which are anamorphs (asexual state) of Ascomycota and Basidiomycota. Those which have not been shifted to Asco-, or Basidiomycota are placed under Mitosporic Fungi or Anamorphic Fungi. We shall use the latter term as done by Kirk et al. (2001).
Fungi belonging to Straminopila are characterized by:
(1) Unicellular or filamentous somatic phase,
(2) Absorptive mode of nutrition,
(3) Cell wall made of cellulose,
(4) Presence of both whiplash and tinsel type of flagella,
(5) Tubular mitochondrial cristae, and
(6) Presence of peroxisomes and Golgi bodies.
The characters (1), (2), and (6) are same as in kingdom Fungi and the difference lies in the nature of cell wall and the flagella. In 18S rDNA phylogenetic studies these fungi show closer genetic relationship to brown algae and diatoms than the true fungi belonging to kingdom Fungi.
There are 3 phyla:
1. Hyphpchytriomycota,
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2. Labyrinthulomycota and
3. Oomycota.
Fungi belonging to kingdom Protozoa differ drastically from other fungi in form, function and life cycle. The somatic phase is plasmodial or amoeboid; there is no cell wall in the somatic phase; nutrition is by ingestion (engulfment or phagocytic) except in plant parasites where the Plasmodium obtains nutrition by absorption (e.g., Plasmodiophoromycota); mitochondrial cristae are tubular.
There are 4 phyla:
1. Acrasiomycota,
2. Dictyosteliomycota,
3. Myxomycota and
4. Plasmodiophoromycota.
Heinrich Anton de Bary, a German, was a medical man. In 1855, he took up Botany and made important contributions to Mycology, de Bary’s mycological interests were more biological and physiological than taxonomic. He discovered many things.
The establishment of Phytophthora as cause of the late blight of potato (which ravaged the potato crop in Europe in 1840s) and the discovery of heteroecism stand out as the most important, de Bary is known as ‘The founder of modern Mycology’ and ‘The father of Plant Pathology’.
Pasteur, a Frenchman, was by training a chemist. His interest in fermentation stemmed from his observation that Penicillium glaucum utilized only the d-isomer of tartaric acid from a racemic (dl) mixture. After that Pasteur never looked back and made a tremendous impact on biological thought.
It was he who put forward irrefutable evidence which killed and buried the theory of ‘spontaneous generation’ of organisms. He propounded and established the ‘germ theory of disease’. In 1888, the Institute Pasteur was established in Paris to continue his good work.
