Characteristics and Structure of Fungal groups, Biology tutorial


Fungi are everywhere and very diverse with characteristics and structure and are therefore categorized into some classes. The classes of interest are Chytridiomycetes, Abscomycetes, Oomycetes and Basidiomycetes.

Class Chytridiomycetes:

The smallest fungi ever studied are the chytrids of the family Chytridiaceae and order Chytridiales. These are the fungi which produce motile spores termed as zoospores (or planogametes). The zoospores encompass a single posterior whip-lash resembling flagellum. The coenocytic thallus is build up of simple hyphae and the thallus is much developed. The conjugating zoospores soon develop or convert into a resting sporangium.

Habitat: They are generally aquatic however some are found in soil, whereas a good number are parasitic in the tissue of angiosperms. They can be collected and cultivated simply by using baits such as pollen grain, fruits, boiled grass blades, egg albumen, cellophane, nectars of flowers, insect excuvia, petals, dead insects and so on. Colonies are frequently inside or outside the bait.

Somatic structures of chytrids:

In each and every primitive chytrids, the somatic structures are unicellular and holocarpic in fungi by holocarpic somatic structures, the similar somatic structure serves up for both vegetable and reproductive purposes. Such fungi don't encompass respectable mycelium. In slightly higher ones, there are some rhizoids for anchorage and nutrition. The sporogenous portion of the thallus is separated from the rest through a septum.

On the evolutionary trend of chytrids, the somatic structures range from the simple holocarpic to eucarpic. It as well ranges from simple cell to the simple rhizoids, to complex rhyziods and wide rhizomycellium. The mode of reproduction ranges from ansogamous, isogamous, gamentagial copulation to somatogamy as in N. ramose A few higher chytrids encompass branches of rhizomycellium and there are several with well developed hyphae having pseudo-septa.

Asexual reproduction in chytrids:

This is by means of the sporangia. The sporangium is primarily filled with densely fluid in form of protoplasm followed through cleavage. The protoplasm becomes separated and surrounds the nucleus.

The zoospores can be discharged in a number of manners:

1) Whenever the chytrid is inoperculate, the zoosporangium breaks.

2) When it is operculate, the operculum opens and the chytrids flee from the operculum.  The zoospores therefore liberated would swim for a slight while and encyst, lose their flagellum and secretes a cell wall round themselves. After a quantity of time, the zoospores geminate as their ancestors.

Sexual reproduction in chytrids:

Sexual reproduction in chytrids is more complicated and can take any of the given forms:

1) Planogametic copulation: The two swimming gametes conjugate and form a zygote. If the two swimming gametes are equivalent in size, the copulation is stated to be isogamous. Quite often the gametes might be unequal therefore termed as planogametes. The conjugation of two unequal gametes leads to the formation of the motile gamete that later looses the flagella and becomes resting. It is then termed as anisogamous. The non-motile gamete is generally the female, bigger and sedentary whereas the motile gamete is the male. The non-motile female gametes are termed as oogonia and the males are the antheridia.

2) Gamentagial copulation: The body of the gamentagial of two gametes fuses to make a zygote. The whole protoplasm of 1 gamentaguim fuses by the other. The two fuses altogether after which cleavage takes place and spores are formed.

Class Oomycetes:

Illustrations comprise Saprolegnia ferax and S. parasitica.

Fungi in this class are differentiated through the presence of two flagella that is, they are biflagellate. They encompass motile cells having two literally inserted flagella, one tinsel and anteriorly directed; the other whiplash and posteriorly directed. In the main zoospore, one flagellum is tinsel whereas the other is whip-lash resembling at their anterior part whereas the secondary zoospore is kidney-shaped having two flagella.

There are four various orders in the class Oomycetes that are: (a) Saproleginiales (b) Legenidiales (c) Leptomitales and (d) Peronosporales.

The first and fourth orders are significant. Usually, Oomycetes were called water moulds until in the year 1925 when Harvey isolated some Saprolegiales from the soil, therefore the modification of the statement therefore the word is no more applicable in the lower fungi.

Fungi in the order Saprolegniales are found in the water and have as well been from moist or dry soil. Majority are saprophytic whereas a few are parasitic. Saprolegnial parasites are known to be cause of diseases of fishes and its egg thus capable of influencing adult fish and fingerlings, egg hatcheries, crabs, lobsters and a number of aquatic animals. Illustrations of such comprise: Achlya spp and Saprolegnia spp.

The other genus Phanomyces that have some parasitic species that cause some parasitic diseases of serious economic losses such as  legumes, peas, sugar beads, rice, maize especially those which grow in submerged regions. Saprolegniales are broadly distributed globally and are much simple to grow and cultivate.

Structurally, they have extensive mycelium generally filamentous, coenocytic, no crosswall or septa apart from in regions which limit the reproductive structures from the vegetative structures. Asexual reproduction occurs by either production of zoospores or gemmae. In the process of growth of sporangia, the apical part of the filament is cut off through a septum after the tip has been filled by dense protoplasm.

Primarily, there are long coenocytic hyphae and by reproduction, we have separation of the hyphae. When this takes place, the densely fluid filled cytoplasmic part cleaves and separates areas where there are nuclei surrounded through cytoplasm which is the starting of the sporogenesis. The nuclei present distinguish into zoospores. An opening builds up at the tip of the sporangium and pear-shaped, mainly zoospores escape to the surrounding aqueous atmosphere. They swim for sometime (generally from a minute to over an hour), then withdraw their flagella and encyst. This cyst, after a period of rest (2 - 3 hours, based on the species), geminates to discharge a further bean-shaped secondary zoospore. The secondary zoospore might swim vigorously for some hours before encysting again. This encysted spore now germinates by sending a germ tube which builds up into a hypha, making a new colony.

When conditions are favorable for sexual reproduction, the somatic hyphae give mount to antheridia and oogonia.

Class Zygomycetes:

Illustrations are Mucor racemosus, M. rouxii and Rhizopus stolonifer. 

Mucor: Members of the genus take place plentifully in soil and manure and on fruits, vegetables and starchy foods. General species are M. racemosus and M. rouxii. Morphologically, the mycelia are generally white or gray and are non-septate. Sporangiophores might be branched. The columellae (that is, sterile structures in sporangium) are rounding, cylindrical or oval. Spores are black or brown and are smooth in the appearance. Zygospores are generated when plus and minus strains of the organism are both present. The plus and minus strains are so called as there is no morphological differentiation among the male and the female strains even although there is physiological heterothallism. Zygospores are as well formed in growth on artificial media in the laboratory. No stolons or rhizoids are generated. In the absence of a fermentable carbon source in a medium comprising of yeast extract and peptone supplemented having potassium acetate, Mucor might grow in a yeast-like form rather than a filamentous form.            

Class Ascomycetes:

All fission yeasts fit in to the genus Schizosaccharomyces. They reproduce by transverse division and also by ascospores. The best known species is S. octosporus that has been isolated from the currants and honey. Its cells are globase to cylindrical, uninucleate and haploid.

S. pombe is the fermenting yeast of some of beer (example: African millet beer) isolated from the sugar molasses and from grape juice. S. vertisatilis, isolated from the grape juice grows such as yeast however it can as well form a true mycelium.

In the genus Saccharomyces, there are around fifty species. The best known is S. cerevisiae strains of which are employed in the fermentation of beer and wine and in baking. It is found in the nature on ripe fruits. Grapevine is often made through spontaneous fermentation by yeast growing on the surface of the fruit. S. cerevisiae is thus of great economic significance. Its cells are elliptical, measuring around 6 to 8 by 5µm. They multiply asexually by budding. If a bud is formed on a cell, a raised scar remains. As many as 23 buds scars have been observed on a single cell. Throughout budding, the nucleus divides by constriction and a portion of it enters the bud all along by other organelles. Under suitable conditions, S. cerevisiae forms asci. The cytoplasm of the cell distinguishes into 4 thick-wall spherical spores; however the number of spores can be fewer. The cells forms which asci build up are diploid and the nuclear divisions which proceeded spore formation are meiotic. It might be noted that most of the strains of the yeast are heterothallic, and the ascospores are of two mating kinds. Mating kind is controlled specifically through a single gene that exists in two allelic states a and α, and segregation at reduction division preceding ascospore formation gives mount to two a and two α ascopsores. Fusion generally takes place only among cells of differing mating types through a procedure known as legitimate copulation. Such fusions result in diploid cells that form asci having viable ascospores. Studies on yeast genetics revealed that by means of hybridization (that is, crossing of different yeasts), it is possible to build up strains of yeasts (that is, hybrids) having desirable features from two genetically dissimilar strains.

Class Basidiomycetes:

In the genus, Agaricus, the finest known species is Agaricus compestris, the field mushroom and A. bisporus the cultivated mushroom. The young gill is colored pink due to the cytoplasmic pigment in the spores. The gills afterward turn a purplish brown due to the deposition of dark pigments in the spore wall. Most of the bigger species of Agaricus are edible.

There are around 12,000 species of Basidiomycetes; none was implicated in human diseases till recent times. The perfect phase of Cryptococcus Neoformans was discovered in the year 1975. It is now termed as Filobasidiella neoformans. It is a significant basidiomycetous pathogen of humans, causing cryptococcosis, a generalized (or systemic) mycotic infection comprising the bloodstream and also the lungs, central nervous system and other organs.

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