Mycology, Biology tutorial

Introduction to Mycology:

The word 'mycology' is derived from the Greek word 'mykes' meaning mushroom. Thus mycology is basically the study of fungi. The capability of fungi to occupy animal and plant tissue was noticed in the early 19th century however the first documented animal infection by any fungus was made by Bassi, who in the year 1835 studied the muscardine disease of silkworm and confirmed the that the infection was caused due to a fungus Beauveria bassiana. In the year 1910 Raymond Sabouraud published his book Les Teignes that was an inclusive study of dermatophytic fungi. He is as well regarded as the father of medical mycology.

Significance of fungi: Fungi live in nearly each and every niche in the environment and humans are exposed to such organisms in different fields of life.

Beneficial Effects of Fungi:

1) Decomposition - carbon and nutrient recycling.

2) Biosynthetic factories - the fermentation property is employed for the industrial production of the fats, alcohols, citric, oxalic and Gluconic acids.

3) Significant sources of antibiotics like Penicillin.

4) Model organisms for the biochemical and genetic studies. Example: Neurospora crassa.

5) Saccharomyces cerviciae is widely employed in recombinant DNA technology that comprises the Hepatitis B Vaccine.

6) Several fungi are edible (like mushrooms).

7) Yeasts give nutritional supplements like vitamins and cofactors.

8) Penicillium is employed to flavor Roquefort and Camembert cheeses.

9) Ergot generated by the Claviceps purpurea includes medically significant alkaloids which assist in inducing uterine contractions, controlling the bleeding and in the treatment of migraine.

10) Fungi (such as Leptolegnia caudate and Aphanomyces laevis) are employed to trap mosquito larvae in the paddy fields and therefore help in malaria control.

Harmful Effects of Fungi:

1) The demolition of lumber, food, paper and cloth.

2) Animal and human diseases, comprising allergies.

3) Toxins generated by poisonous mushrooms and in food (such as Mycetism and Mycotoxicosis).

4) Plant diseases.

5) Spoilage of agriculture produces such as vegetables and cereals in the godown.

6) Harm the products like magnetic tapes and disks, marble statues, glass lenses, bones and wax.

General properties of fungi:

1) They are generally eukaryotic cells having membrane bound cell organelles comprising nuclei, Golgi apparatus, mitochondria, endoplasmic reticulum, Lysosomes and so on. They as well represent mitosis.

2) Encompass ergosterols in their membranes and have 80S ribosomes.

3) Encompass a rigid cell wall and are thus non-motile, a feature which separates them from animals. All the fungi have cell wall made up of chitin.

4) Are chemoheterotrophs (need organic compounds for both carbon and energy sources) and fungi be deficient in chlorophyll and are thus not autotrophic.

5) Fungi are osmiotrophic; they get their nutrients through absorption.

6) They get nutrients as saprophytes (that is, live off of decaying matter) or as parasites (that is, live off of living matter).

7) All the fungi need oxygen and water and there are no necessitating anaerobes.

8) Usually reproduce asexually and/or sexually by means of producing spores.

9) They grow either reproductively through budding or non-reproductively through hyphal tip elongation.

10) Food storage is usually in the form of lipids and glycogen.

Classification of fungi:

Fungi were primarily categorized with plants and were a subject of interest for botanists; therefore the influence of botany can be seen on their categorization. In the year 1969 R.H Whittaker categorized all living organisms into five kingdoms namely Monera, Protista, Fungi, Plantae and Animalia.

Usually the categorization continues in this fashion:

Kingdom - Subkingdom - Phyla/phylum - Subphyla - Class - Order - Family - Genus- Species

Based on Sexual reproduction:

1) Zygomycetes: that produces via the production of zygospores.

2) Ascomycetes: that produces endogenous spores termed as ascospores in cells termed as asci.

3) Basidiomycetes: that produces exogenous spores termed as basidiospores in cells termed as basidia.

4) Deuteromycetes (Fungi imperfecti): fungi which are not known to produce any sexual spores (that is, ascospores or basidiospores). This is a heterogeneous group of fungi where no sexual reproduction has yet been explained.

Based on Morphology:

1) Moulds (Molds): Filamentous fungi Example: Aspergillus sps, Trichophyton rubrum.

2) Yeasts: Single celled cells which buds Example: Cryptococcus Neoformans and Saccharomyces cerviciae.

3) Yeast like: Alike to yeasts however produce pseudohyphae Example: Candida albicans.

4) Dimorphic: Fungi existing in the two different morphological forms at two dissimilar ecological conditions.

They survive as yeasts in tissue and in vitro at 37oC and as moulds in their natural habitat and in vitro at room temperature. Example: Blastomyces dermatidis, Histoplasma capsulatum, Paracoccidiodes brasiliensis and Coccidioides immitis. A few 200 'human pathogens' have been acknowledged from among an anticipated 1.5 million species of fungi.

Morphology of fungi:

Fungi exist in two basic forms; the filamentous (or hyphal) and single celled budding forms (or yeast). However, for the categorization sake they are studied as yeasts, moulds, yeast like and dimorphic fungi. All the fungi encompass typical eukaryotic morphology. They contain rigid cell wall comprised of chitin, which might be layered by mannans, glucan and other polysaccharides in relationship with polypeptides. A few lower fungi have cellulose in their cell wall. Several fungi like Cryptococcus and yeast made up of Histoplasma capsulatum have polysaccharide capsules which help them to evade phagocytosis.

Internal to the cell wall is the plasma membrane which is a typical bi-layered membrane in addition to the presence of the sterols. Fungal membranes have ergosterols in contrast to cholesterol found in the mammalian cells. The cytoplasm comprises of different organelles like mitochondria, Golgi apparatus, ribosomes, endoplasmic reticulum, Lysosomes, microtubules and a membrane enclosed the nucleus. A unique property of nuclear membrane is that it persists all through the metaphase of mitosis unlike in animal and plant cells where it dissolves and re-forms. The nucleus has paired chromosomes.

Moulds:

The thallus of mould is made up of hyphae that are cylindrical tube similar to structures which lengthens by growth at tips. A mass of hyphae is termed as mycelium. It is the hypha which is accountable for the filamentous nature of mould. The hyphae might be branched or unbranched. They might be septate or aseptate. Hyphae generally have cross walls which divide them into many cells. These cross walls, termed as septa contain small pores via which cytoplasm is continuous all through the hyphae. Thus all the hyphal fungi tend to be coenocytic (or multinucleate). With exception of Zygomycetes (or Rhizopus, Mucor), all moulds are septate. Non-septate hyphae are considered to be more primal because when a hyphal strand is damaged the whole strand dies. If a septate hyphal strand is damaged, the pores between adjacent compartments can be plugged, therefore preventing death of the whole hyphal strand.

Mycelium is of three types:

1) Vegetative mycelium is such that penetrates the surface of the medium and absorbs nutrients.

2) Aerial mycelium is such that grow above the agar surface.

3) Fertile mycelium is aerial hyphae which bear reproductive structures like sporangia or conidia.

Yeasts:

Yeasts are the unicellular spherical to ellipsoid cells. They reproduce through budding, which outcome in blastospore (or blastoconidia) formation. In certain cases, as the cells buds the buds fail to disengage and elongate therefore forming a chain of elongated hyphae similar filament termed as pseudohyphae. This property is observed in the Candia albicans. The similar species as well encompass the capability to produce true hypha that is seen as germ tube. The difference among the two is that there is a constriction in psueudohyphae at the point of budding, whereas the germ tube consists of no constriction.

Reproduction in fungi:

Fungi reproduce by means of vegetative, asexual and sexual means.

Vegetative and asexual process of reproduction which don't involve the fusion of nuclei or sex cells or sex organs are, though, clubbed by numerous mycologists into asexual methods of reproduction. Therefore, they recognize just two methods, sexual and asexual.

Asexual Reproduction:

In fungi, asexual reproduction is a more general process than sexual reproduction. It is generally repeated quite a few times in a season. It occurs by the formation of special reproductive cell termed as spores. The formation of spores in fungi is termed as sporulation. Each and every spore builds up into a new mycelium. These spores are generated as an outcome of mitosis in the parent cell and therefore they are as well termed as mitospores. The spores differ in color, shape and size, number, arrangement on hyphae and in the manner in which they are borne. They might be hyaline, yellow, green, orange, red, brown and black in color and are minute to big in size. In shape they differ from globose to oval, oblong, needle-shaped to helical. Therefore an infinite diversity of spores can be viewed in fungi and you will discover them very fascinating beneath the microscope.

Generally the spores are unicellular. They might be uninucleate or multinucleate. In a few fungi such as Alternaria and Curvularia they are multicellular. The mitospores generated in fungi are of two kinds: sporangiospores and conidia.

The sporangiospores are generated within a sac-like structure termed as sporangium.  The hypha bearing a sporangium is termed as sporangiophore. They are normally branched. The sporangiospores might be motile or non-motile. The non-motile sporangiospores are termed as aplanospores from Greek, not + planets wanderer + spores seed, spore. These are feature of terrestrial species such as Mucor and Rhizopus. In aquatic fungi such as Pythium of the division Oomycota motile biflagellate sporangiospores are generated. These are termed as zoospores and the sporangium bearing them is termed as zoosporangium. A zoospore is a motile spore deficient of a cell-wall. After a swarming period it produces a wall and germinates to make a germ tube. In contrast to zoospores, the aplanospores encompass a definite spore wall and are dispersed through insects and wind.

The conidia are non-motile, deciduous mitospores made externally as single separate cells. They build up either directly on the mycelium or on morphologically distinguished hyphae termed as conidiophores. The conidiophores might be simple or branched, septate or aseptate. The conidia are generated singly example: Phytophthora or in chains at the tips of the conidiophores example: Aspergillus or at the tips of their branches example: Penicillium.

Often the conidiophores occur singly and are dispersed in the mycelium. At times they arise in specialized structures termed as fruiting bodies. According to their appearance they are known as synnema, acervuli, sporodochia and pustules.

Sexual Reproduction:

The sexual phase in fungi is termed as the perfect state in contrast to the imperfect state that is the asexual stage. Sexual reproduction comprises the fusion of two compatible sex cells or gametes of the opposite strains. Fungal sex organs are termed as gametangia. They might be equivalent in size. In most of the higher ascomycetes morphologically different gametangia are formed. The male garnetangia are termed as antheridia and the female ones ascogonia.

The fungus might be homothallic, that is, the fusing gametes come from the similar mycelium or might be heterothallic, that is, and fusing gametes come from various strains of mycelia.

In fungi, sexual reproduction comprises the given three stages: (a) Plasmogamy, (b) Karyogamy and (c) Meiosis. These three processes take place in a regular series and at a particular time, throughout the sexual phase of each species.

a) Plasmogamy:

This is the union of protoplasts of reproductive hyphae or cells, one from the male and the other from the female to carry about the nuclei of the two parents close altogether as a pair. Though, the two nuclei don't fuse with one other. Such a cell is termed as a dikaryon. The dikaryotic condition is exclusive to fungi and might continue for some generations as the two nuclei (dikaryon) split or divide concurrently throughout cell division. These are passed on to the daughter hypha.  

b) Karyogamy:

The fusion of the two nuclei which occurs in the next stage is termed as karyogamy. It might instantly follow plasmogamy as in lower fungi, or it might be delayed for a long time as in the higher fungi.

c) Meiosis:

Karyogamy that eventually takes place in all sexually reproducing fungi is sooner or later followed through meiosis producing four genetically dissimilar spores.

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