Basidiomycetes Life cycle, Characteristics, Significance, Mycelium

The Basidiomycetes division together with Ascomycota form the subkingdom Dikarya which is also known as the “higher fungi” within the kingdom Fungi. Basidiomycetes contain the following groups; mushrooms, puffballs, stinkhorns, bracket fungi, other polypores, jelly fungi, boletes, chanterelles, earth stars, smuts, bunts, rusts, mirror yeasts, and also a pathogenic yeast called Cryptococcus.

Basidiomycetes si consider the largest group of fungi because they contain 1100 genera which are consisting of 30,000 species and listed under the sub-division Basidiomycotina. 

Basidiomycetes are harmful as well as useful. Their attack foods and ornamental plants, cause many different diseases including seedling diseases, wood rots, root and stem rots, seed diseases (smuts), and rusts, on the other hand, it used as humans foods.

Basidiomycetes characteristics

• These are filamentous fungi made up of hyphae only except for basidiomycota-yeast.
• They are reproduced sexually with the formation of club-shaped end cells known as basidia which usually carry external meiospores (usually four).
• These specific spores are termed as basidiospores.
• Some of the Basidiomycota are asexual reproducers.
• Most of the Basidiomycetes are saprophytes, they are responsible for the decay of litter, wood, or dung. 
• Basidiomycetes confined to only living host plants in nature.
• It contains a well-developed, branched, and septate mycelium which is differentiated into primary, secondary, or tertiary.
• The cell wall is made up of chitin and glucans with 1,3 linked and 1,6 linked B- D – glucosyl units.
• Some of them are found in symbiont form with mycorrhizae in trees, while others are harmful parasites damaging a wide range of woody and herbaceous plants.
• The somatic stage is made up of a well-developed, septate, filamentous mycelium which transfers particularly into two stages, such as Primary mycelium and Secondary or dikaryotic mycelium.
• The Primary mycelium developed by the germination of a basidiospore and carries a single haploid (n) nucleus in each cell. It shows neither sex organs nor any basidia and basidiospores. It is short-lived.
• The Secondary or dikaryotic mycelium forms the primary food absorbing phase and made of cells each comprising 2 haploid nuclei (n+n). It is long-lived and performs a leading role in the life cycle.
• In the Homobasidiomycetidae Secondary or dikaryotic mycelium may continue to grow for years producing fructifications every year by the interweaving of hyphae. The fructifications bear basidia and basidiospores.
• While in the Heterobasidiomycetidae Secondary or dikaryotic mycelium develops teleutospores or brand spores that germinate to create basidia bearing basidiospores.
• In Basidiomycetes, the septal pore is very complex (except for rusts and smuts). It is dolipore parenthesome type.
• This life cycle does not contain any motile cells.
• Most Basidiomycetes formed clamp connection.
• Most of the genera contain Dolipore septa.
• In life cycle the asexual reproduction with the formation of spore paly an important role. Asexual spores are absent in Homobasidiomycetidae, while the Heterobasidiomycetidae produce them in the dikaryotic mycelium. The latter produces uredospore’s and aeciospores in the rusts.
• The Basidiomycetes do not contain sex organs. Sexual reproduction is accomplished by plasmogamy and karyogamy and Karyogamy is directly followed by meiosis.
• The main reproductive organ of Basidiomycetes is Basidium. Here both karyogamy and meiosis take place.
• The basidium contains four basidiospores exogenously, the number may vary based on the types of species.
• These basidiospores are germinated to form the primary mycelium.
• During sexual reproduction, the dikaryotic cell is developed by spermatization, somatogamy, clamp- connection, or Buller phenomenon.

Basidiomycetes examples

Mushroom

Agaricus compestris is a common mushroom with edible basidiocarp. Agaricus brunnescens (=A. bisporus) is cultivated. Primary monokaryotic mycelia give rise to long lived secondary dikaryotic mycelium through somatogamy. Secondary mycelium or spawn produces aerial basidiocarps or basidiocarp has a fleshy stalk or stipe and umbrella-shapped cap or pileus.

A membranous annulus occurs on the upper part of stipe. The under-surface of pileus is flat and has a number of radiating rows of vertical plates or gills. The two sides of each gill bear numberous basidia and paraphyses. Each basidium develops four sterigmata with a basidiospore at the top of each. Basidiospores of mushroom are brownish. On germination, each basidiospore forms a primary mycelium.

Mushroom is rich in minerals, vitamins and fibres (1.1%). Its other components are water (92.6%), protein (1.5%), cholesterol free fat (0.5%) and carbohydrate (0.4%). Besides Agaricus, other edible mushrooms are Volvariella volvacea (Paddy Straw Mushroom) and Lentinus edodes (shitake Mushroom). Honey Mushroom (Armillariella = Armillaria mellea) is edible but is a root parasite on many tress. Its mycelium is luminous. Psilocybe mexicana (Sacred Mushroom) is hallucinating like L.S.D.

Toadstool

A Poisonous mushroom is called toadstool. It often possesses white basidiospores e.g., amanita polloides/A. caesarea (Death Cap/Caesar’s Mushroom).

Bracket or Shelf Fungi

The basidiocarps are like brackets or shelves appearing on tree trunks or logs (lignocolous = epixylic), e.g., Polyporus, Fomes, Ganoderma.

Puffballs

They are edible in the young state but send puffs of spores on ripening e.g., lycoperdon, Calvatia. The latter has anticancer properties.

Smuts

Smuts are pathogenic basidiomycetes which possess thick-walled black-coloured resting spores called chlamydospores, teleutospores or smut spores. Smuts are two types, loose and covered In loose smuts the spores are exposed from the beginning, e.g., loose smut of Wheat (Ustilago tritici).

In covered smuts, the spores remain covered till before liberation, e.g., Corn smut (Ustilago maydis, grains become very large gall-like), bunt of Wheat (tilletia tritici), whip smut of Sugarcane (U.scitaminae).

Rusts

The pathogenic basidiomycetes basidiomycetes produce rusty pustules, e.g., Puccinia graminis tritici (black rust of Wheat). Puccinia completes its life cycle on two hosts, Wheat and Barberry, Such a parasite is called heteroecious.

Stinkhorn.

Phallus impudicus (Dead Man’s Finger). Stinking odour is due to spore mass which is attractive to files.

Significance of Basidiomycetes

1. There are several Basidiomycetes that cause diseases in cereals and other economically important plants which may lead to great trouble in production that may bring hunger in certain parts of the world. Among pathogenic Basidiomycetes, Rusts and smuts are important.
2. Nowadays Mushrooms are edible become and popular food for their flavor. Now, different Mushrooms are cultivated all over the world such as Agaricus, Pleurotus, and Volvoriella.
3. Some Mushrooms deadly poisonous such as Amanita spp. Few of them also produce hellucinogenic chemical substances. i.e. Psilocybe spp. 
4. The polypores cause tremendous injury to forest trees as parasites and to timber as saprophytes.
5. Mycelia of basidiomycetous fungi play a significant role in the decomposition of organic matter and recycling of nutrients. It is because they bear the ability to produce various extracellular enzymes which break down complex chemicals like cellulose and lignin.

Mycelium of Basidiomycetes

• The mycelium of Basidiomycetes is well-developed, filamentous, and made up of branched, septate hyphae usually developing in a fan-shaped manner.
• The cell wall is chitinous in nature and plasma membrane is present within the cell wall.
• The cytoplasm includes all the organelles which are present in a normal cell, except the chloroplasts.
• The septum, as in the Ascomycetes, begins as an annular outgrowth on the interior of the tubular wall. It develops inwards like a tight shelf decreasing diameter of the pore.
• The hyphae ramify in the substratum and consume food. The mycelium usually is a weft of interlacing and anastomosing hyphae.
• In some genera, the mycelial hyphae move parallel to one another and become bundled together to develop distinct and prominent thick cords of macroscopic size. These are called the rhizomorphs.
• The rhizomorph is surrounded by a sheath (cortex) and acts as a unit. 
• The mycelium in various species varies in color and may be white, yellow or orange. It is usually perennial.
• The mycelium of Basidiomycetes moves through 3 different stages such as the primary, the secondary, and the tertiary stage before the fungus completes its life cycle.

Types of Basidiomycetes mycelium

1. Primary mycelium: It develops from the basidiospore. It composed of uninucleate cells and hence they are termed as homokarion, although, the first stage of the primary mycelium may be multinucleate but then septa formation divides it into uninucleate cells.
2. Secondary mycelium: It develops from the fusion of two uninucleate cells of primary mycelium; the process of confertion of primary mycelium into secondary mycelium called as dicariotization or dipodization. The basidia formed from binucleate cells of secondary mycelium.
3. Tertiary mycelium: The secondary mycelium in some higher basidiomycetes organized into complex tissues to develop sporophores or basidiocarps, this stage known as tertiary mycelium.

Clamp Connections

• When the dikaryotic cell (A) is beginning to divide a pouch-like outgrowth occurs from its wall (B), it arises midway between the two nuclei of the dikaryon. The two closely associated nuclei of the cell now divide simultaneously. This is known as conjugate division.
• One of the four daughter nuclei, generally the lower one of the upper pair, passes into the pouch (C). A septum appears at the base of the pouch (D). As a result, the pouch is cut off from the main cell. It may now be called a clamp cell.
• The clamp cell extends and forms a hook-like structure. The tip of the clamp now bends and gets attached to the lateral wall (E) of the parent cell and function as a bridge. This is known as clamp connection.
• An extra septum is attached vertically under the bridge usually at about the level of the upper end of the clamp connection (E). It separates the mother cell into two daughter cells. The terminal daughter cell contains two nuclei.
• All of these are a sister nucleus of the mother dikaryon. The lower or basal daughter cell contain one nucleus. The fourth nucleus lies in the clamp connection. The nucleus of the clamp connection now migrates into the basal daughter cell. The latter also becomes binucleate (F).
• The clamp connection thus simply functions as a bypass. It ensures that the sister nuclei formed by the conjugate division of the dikaryon separate into two newly formed daughter cells. The clamp connections are usually formed on the terminal cells of the hyphae of the secondary mycelium.
• They generally persist on the old dikaryotic hyphae. The presence of hook like clamp connections is a safe criterion for distinguishing a secondary or dikaryotic mycelium from the primary or monokaryotic mycelium.
• The clamp connections by some mycologists are considered homologous to the hooks of the ascogenous hyphae of the Ascomycetes.
• This view is supported by the general occurrence of a clamp connection at the base of a basidium. Moore and Mclear (1962) studied the fine structure of septa of the dikaryotic hyphae of Basidiomycetes.
• They found that the septum which is a cross wall flares sharply and broadly at the centre of the hypha to form a barrel- shaped structure with open ends.
• This is the actual pore surrounded by a swollen rim which is a part of the annular septum. This type of septum is termed a dolipore septum. The septum and the swelling are covered by the cell membrane.
• The opening on either side of the dolipore septum is guarded by a curved or crescent-shaped double membrane pore cap which in section looks like a parenthesis (round bracket).
• The septal pore-cap is thus given the name parenthesome. The parenthesome is similar to the endoplasmic reticulum. The lower parenthesome is seen to be interrupted by gaps in the form of pores. The upper parenthesome may be continuous.
• The dolipore parenthesome septum complex is unique to the Basidiomycetes. It maintains cytoplasmic continuity but prohibits nuclear migration.
• The secondary mycelium in the fruit bodies of the higher Basidiomycetes becomes organised into specialised tissues. It is called the tertiary mycelium. The fructifications are thus formed of the tertiary mycelium.
• The cells of the tertiary mycelium are also binucleate. The distribution of the dolipore parenthe-some septum complex in the Basidiomycetes is widespread.
• It is characteristic of the primary, secondary, and generative tertiary mycelium of the Homo-basidiomycetes as well as of the basidiocarpic mycelium of the Heterobasidiomycetes. The major exceptions are the rusts and smuts.
• The formation of basidia, the dikaryotic mycelium, the dolipore septum with the parenthesomes guarding the pore on both sides and clamp connections are the four diagnostic features of this class.

What is Diploidisation or Dikaryotisatton?

The conversation process of primary mycelium to secondary or dikaryotic mycelium is known as diploidisation or dikaryotisation. In the first step of this process is began with the establishment of a dikaryon in the fusion cell.

The repeated divisions of clamp connections coil the dikaryotised and as a result, a secondary mycelium is formed in which ‘each cell possesses a dikaryon (two neclei).

Methods of Diploidisation

1. Hyphal Fusions: In this method, the vegetative cells and two adjacent hyphae of the main mycelia of opposing sexual strains (A) fuse.
2. Conjugation of basidiospores (B): In this method, the basidiospore of two opposite strains meet and conjugate. Next, the newly formed binucleate basidiospore germinates to develop a secondary mycelium.
3. Diploidisation can take place by the fusion of a germinating oidium of one strain with a cell of the primary mycelium of the opposite strain (C). The binucleate cell formed in this way by elongation and division by clamp connections develops into a secondary mycelium.
4. It can take place if the germinating basidiospore and a haploid cell of the basidium (D) fuse. For example, U. violacea
5. It can take place if two haploid cells of opposite strains of the basidium are fuse  (E). For example, U. carbo.
6. It can take place if the basidia produced by the germination of two smut spores (F) are fuse.

Note: Basidia are always produced from the binucleate cells of the secondary mycelium.

Basidiomycetes life cycle or Reproduction

Basidiomycetes reproduced by both sexual and sexual methods.

Asexual Reproduction

The asexual reproduction of Basidiomycetes is accomplished by either budding or asexual spore formation. During the budding procedure, an outgrowth or bud occurs in the parent cell. Then it gets separated from the parent cell by the formation of transverse septum and give rise to a new daughter cell.

The formation of an asexual spore in Basidiomycetes occurs at the end of a specialized structure known as conidiophores. The septae of concluding cells mature fully defined, dividing a random estimate of nuclei into single cells. The cell walls then harden into a protective coat. The preserved spores break off and are distributed.

Sexual Reproduction

Basidiomycetes reproduce sexually by the formation of basidiospores. Sometimes basidiomycetes are termed as club fungi, because of their microscopic club-shaped basidia (sing., basidium). 

Basidia are comparable in function to the asci of ascomycetes. Each basidium is an enlarged hyphal cell that undergoes meiosis to form four basidiospores. Each fungus can produce millions of basidiospores, and each basidiospore develops a new primary mycelium.

Life cycle

1. During the growth, a hypha of a primary mycelium meets a compatible monokaryotic hypha, typically of a different mating type, the two hyphae fuse (plasmogamy). As in the ascomycetes, the two haploid nuclei remain separate within each cell. In this way, a secondary mycelium with dikaryotic hyphae is born, in which each cell carries two haploid nuclei. The n + n hyphae of the secondary mycelium grow rapidly and widely.
2. On returning of favorable environmental condition, the hyphae form compact masses, termed buttons, along the mycelium. 
3. Each button grows into a fruiting body that we know as a mushroom. A mushroom, which made of a stalk and a cap, is more formally referred to as a basidiocarp.
4. Each basidiocarp consists of intertwined, matted hyphae. The lower surface of the cap usually consists of many thin, perpendicular plates called gills that radiate from the stalk to the edge of the cap.
5. Karyogamy occurs within the growing basidia on the gills of the mushroom. The haploid nuclei fuse in the dikaryotic cells, forming diploid zygote nuclei. These are the only diploid cells that form during a basidiomycete’s life history.
6. Meiosis then takes place, forming four haploid nuclei with different genotypes. These
7. nuclei move to the outer edge of the basidium. 
8. Fingerlike extensions of the basidium develop, into which the nuclei and some cytoplasm move; each of these extensions become a basidiospore. 
9. A septum forms that separates the basidiospore from the rest of the basidium by a delicate stalk that breaks when the basidiospore is forcibly discharged.
10. Each basidiospore can germinate and give rise to a primary mycelium.

References

1. Biology by Solomon, Eldra, Berg, Linda, Martin, Diana W.
2. Abdelghany, Tarek & El-Sheikh, and. (2018). Mycology. https://www.researchgate.net/publication/326207059_Mycology
3. https://www.sparknotes.com/biology/microorganisms/fungi/section1/
4. https://www.biologydiscussion.com/fungi/basidiomycetes-meaning-features-and-significance-fungi/54815
5. https://en.wikipedia.org/wiki/Basidiomycota
6. https://www.intechopen.com/books/an-introduction-to-mushroom/wild-medicinal-mushrooms-potential-applications-in-phytomedicine-and-functional-foods
7. http://bioweb.uwlax.edu/bio203/f2012/rudolph_paig/reproduction.htm
8. https://www.biologydiscussion.com/fungi/life-cycle-of-basidiomycetes-with-diagram-club-fungi/63604#Reproduction_in_Basidiomycetes
9. https://www.embibe.com/study/examples-on-basidiomycetes-concept