Kingdom Fungi | Structure, Characteristics & its Classification

This Kingdom contains achlorophyllous, eukaryotic, heterotrophic, spore producing, thalloid organisms. It include diverse organisms which range in structure from unicellular yeasts to highly complex edible mushrooms, non-edible toad stools. They are more abundant in warm and humid areas. Therefore, great diversity show in morphology and habitat.

General characters of Kingdom Fungi

1. They are cosmopolitan and occur in air, water soil and on animals and plants. They are mostly terrestrial. They may grow on tree bark, dung, wood, burnt wood and keratinous material (e.g., hair, horns)and are called corticolous (bark), coprophilous (cow dung), epixylic (wood), xylophilous (burnt wood) and keratinophilous (keratin) respectively.
2. The body is haploid (n) and thalloid, i.e., not differentiated into root, stem and leaves. They are multicellular (except Yeast and synchytrium). The fungal body is made up of thread like elongated tubular structures, called hyphae. These cris-cross with one another to from a network known as mycelium.
3. The hyphae may be aseptate and multinucleate. Such a hypha is termed coenocytic. In most of the fungi, the mycelium is septate. The septum, however, is not complete, but has a pore through which continuity of the cytoplasm of the adjoining cells is maintained. The septum may have simple central pore as in ascomycetes, but in higher fungi (class basidiomycetes), the septum is dolipore septum, in which central pore possesses a barrel shaped inflation. In septate mycelium, individual cell may contain single nucleus (monokaryotic – feature of primary mycelium) or an intermediate phase of two nuclei (dikaryotic – feature of secondary mycelium).
4. The cell wall of hyphae is made up of chitin or fungal cellulose, which is a polysaccharide containing nitrogen compound and it is basically made up of acetylglucosamine. In some fungal, the cell wall is made up of cellulose (e.g., phytophthora, pythium and other oomycetes), Reserve food material is stored in the from of oil and glycogen.
5. Cells have unicisternal golgi bodies.
6. Mitosis in somatic cells is karyochorisis type (mitosis with intranuclear spindle formation.
7. Nutrition is heterotrophic which includes saprophytes, parasites and symbionts.
8. In most of the fungi, there are two distinct phases in the life cycle, the vegetative or assimilative phase and the reproductive phase. In vegetative phase. Fungus is microscopic hidden in the substratratum and is hardly visible to the naked eyes. The fungus enters into reproductive phase after attaining maturity in the vegetative phase. In unicellular yeasts, the same cell performs both assimilative and reproductive functions. Such type of fungal bodies in which entire cell gets transformed into reproductive structure are known as holocarpic. Fungal body is termed eucarpic in which a part of mycelium is used up in the development of reproductive structures.

Reproduction in Fungi

Fungi reproduce by all the three modes, i.e., vegetative, asexual and sexual.

1. Vegetative reproduction

It occurs by the following methods.

1. Fragmentaion: The mycelium breaks up into two or more fragments due to mechanical injury, decay or some reasons. Each fragment grows into independent mycelium.
2. Fission: Here, simple spliting of vegetative cells into two daughter cells takes place by simple constriction.
3. Budding: Some fungi like yeast produce small outgrowths, i.e., buds from their vegetative body. Eventually, the buds are cut off from parent cell and mature to form new individuals.

2. Asexual reproduction

It occurs through spores. These are single celled specialized structures which separate from the organism, get dispersed and germinate to produce new mycelium after falling on suitable substrate. The spores produced during asexual reproduction in fungi are formed by mitotic division and are thus termed, mitospores.

Various types of asexual reproduction are as follows:

• Zoospore
• Sporangiospore
• Conidia
• Chamydospore
• Oidia

3. Sexual reproduction

It occurs through spores oospores, ascospores and basidiospores. The various spores are produced in distinct structures called fruting bodies. The fruiting bodies are ascocarps and basidiocarp which contain asci and basidia respectively. The ascospores are a type of non-motile spores which are produced inside special sacs called asci (singular-asvus. Basidiospores are non-motile which are formed exogenously (i.e. outside the body) on short outgrowths of club-shaped structure called basidium.

Sexual cycle involves three steps:

1. Plasmogamy : There is union of protoplasm between two haploid hyphae of protoplasm between two haploid hyphae of compatible mating type or gametes.

Plasmogamy occurs by the following methods:

• Planogametic copulation / Gametic fusion
• Gametangial contact
• Gametangial copulation
• Spermatization
• Somatogamy

2. Karyogamy: Fusion of two nuclei is called karyogamy. In some fungi, the fusion of two haploid cell immediately results in diploid cells (2). However, in some fungi (e.g. Agaricus, Aspergillus), an intervening dikaryotic stage (n+n i.e. two nuclei per cell) occurs; such a condition is known as dikaryon and the phase is called dikaryophase. Later, the parental nuclei will fuse and the cells become diploid in karyogamy.
3. Meiosis: The fungi form fruiting bodies in which reduction division (meiosis) occurs leading to the formation of haploid spores.