Reproduction in fungi

Reproduction in fungi

The formation of new individuals having all the characteristics typical of a species is called reproduction. The fungi reproduce by means of  parasexual reproduction and asexual or sexual.
Reproduction in fungi is takes place by both method sexual reproduction in fungi or asexual reproduction in fungi.

Also read- Thallus organization in fungi

Asexual reproduction is sometimes called  vegetative or somatic and it does not involve union of nuclei, sex cells or sex organs. The union of two nuclei characterizes sexual reproduction.

ASEXUAL REPRODUCTION

Asexual reproduction in fungi is more important for the propagation of species. Asexual reproduction does not involve union of sex cells (gametes) or sex organs (gametangia) or nuclei.  In fungi the following are the common methods of asexual reproduction.

1. Fragmentation of mycelium
2. Fission of unicellular thalli
3. Budding
4. Production of asexual spore

Source Wikipedia

1. Fragmentation of mycelium

The body of fungus is breakdown in several small parts when suitable condition are provided each part grow in a new individuals.

2. Fission of unicellular thalli

It is also known as transverse cell division. Reproduction by the method of fission is rare in fungi. Fission is simple splitting of cells into two daughter cells by constriction and the formation of a cell wall. It is observed in Schizosaccharomyces spp 

Also read - Structure of fungi 

3. Budding in fungi

The production of a small outgrowth (bud)  from a parent cell is called budding. As the bud is formed, the nucleus of the parent cell divides and one daughter nucleus migrates into the bud. The bud increases in size, while still attached to the parent cell and eventually breaks off and forms a new individual. It is  common in yeasts.(Saccharomyces sp.)

4. Production of asexual spores

Reproduction by the assembly of spores is extremely common in many fungi.

SPORES

The term 'spore' is applied to any small propagative, Survival unit or reproductive unit, which separates from a hypha or sporogenous cell and should grow independently into a replacement individual. Spores may be unicellular or multicellular. Multicellular spores are mostly have transverse septa and in some genera like Alternaria a spore will have both longitudinal  and transverse septa. Each cell of a multicellular spore could also be uninucleate, binucleate or multinucleate counting on the fungal species. The spores may be in different shapes and sizes.

Also read- Characterstics of fungi

They may be oval or ovate, spherical, obovate, pyriform, obpyriform, ellipsoid, cylindrical, oblong, allantoid, filiform or selecoid, fusion or falcate. The spores may be with or without simple or branched appendages. The spores may be motile or nonmotile. Motile spore are called planospores  and non-motile spores are called aplanospores. Spores could also be thin or thick walled, hyaline or coloured, smooth or with ornamented walls. The following types of ornamentations are found on the walls.

Asexual spores

The spores produced asexual means are:

a. Sporangiospores

b. Conidia

c. Chlamydospores

a. Sporangiospores

Sporangiospores may be motile (planospores) or non￾motile spores (aplanospores). Sporangiospores in simple fungi  are usually motile and are called zoospores. Sporangiospores are produced in lower fungi, which inhabit moist or aquatic terrestrial substrates. These spores are formed in sac-like structure or globose called sporangium. In the zygomycetes and particularly within the Mucorales, the non-motile asexual spores called aplanospores are contained in globose sporangia surrounding a central core or columella. Sporangia also are known during which there's no columella, or where the spores (aplanospores) are arranged during a row inside a cylindrical sac termed a Merosporangium (e.g. Syncephalastrum spp. Mucorales). These aplanospores may be uni or multinucleate and are unicellular, generally smooth￾walled, globose or ellipsoid in shape. 

When aplanospores mature, they'll be surrounded by mucilage and rain splash or insects usually disperse such spores. The sporangiospores for sporangium may vary from several thousands to just one . Sporangiola are dispersed as a unit. e.g. Choanephora sp. and Blakeslee sp. in Choanephoraceae of Mucorales. In holocarpic thalli, the whole thallus (without differentiation of a sporophore) becomes a sporangium. Its contents cleave into variety of segments which round off and become zoospores. In eucarpic thalli, a neighborhood of the thallus, or special branches from thallus, function as or produce sporangia. In terrestrial and plant parasitic sorts of lower fungi, the sporangium may function as spore and no zoospores are formed. In others zoospores are formed within the sporangium itself or the inner wall of the sporangium may grow out into a brief or long tube which swells to make a vesicle. The contents of the sporangium enter a vesicle and therefore the zoospores are differentiated. E.g. Pythium aphanidermatum. 

Zoospore(Gr.Zoon=animal+spora=seed,spore) 

It is an asexually produced spore, which is motile by means of flagellum or flagella. Zoospore is naked and its covering is merely a hyaloplasm membrane. Normally, zoospores are uninucleate and haploid. Zoospores could also be spherical, oval, pyriform, obpyriform, elongate or reniform in shape. The zoospores are given one or two flagella (sing. flagellum, L. flagellum=whip) for its movement within the surrounding film of water. Flagellum is a hair or tinsel￾like structure that serves to propel a motile cell. 

These flagella may be anterior, posterior or laterally attached to a groove in the body. There are two types of flagella in zoospores. They are whiplash and tinsel types. The whiplash flagellum features a long rigid base composed of all the eleven fibrils and a brief flexible end formed of the 2 central fibrils only. The tinsel flagellum features a rachis, which is roofed on all sides along its centre length with short fibrils. In uniflagellate zoospores the flagellum could also be anterior or posterior. But in biflagellate zoospores one is whiplash and therefore the refore the other is tinsel type and one points forward and the other backward. 

Zoospores undergo the three phases viz., motility, encasement and germination. Their motility lenght depends on available temperature, moisture and presence of  inhibitory or stimulatory  substances within the environment. Later the zoospores become sluggish, spend or cast their flagella (except in cytridiacious fungi and primary zoospores in Saprolegniales where flagella are shed but withdrawn into its body become secrete and spherical  thin wall around itself and become encysted. The encysted zoospores germinate. The functions of zoospores include initiation of latest generation and acting as gametes.

b. Conidiospores

Conidia or Conidiospores are asexual reproductive structures borne on special spore bearing hyphae conidiophores. Conidia or Conidiospores are found in many different groups of fungi, but especially in ascomycotina,Basidiomycotina and Deuteromycotina. In Deuteromycotina conidia are the sole means of reproduction. Conidia may be borne singly or in chains or in cluster. They vary from unicellular (e.g. Colletotrichum), bicellular, microconidia of Fusarium spp. and multicellular (Pestalotiopsis, Cercospora). Amerospores (one celled spore),  Didymospores (two celled spores), multicellular spores are called phragmospores. The multicellular conidia may be divided by the septa in one to three planes. In Alternaria species, conidia are with both longitudinal and transverse septa are called dictyospores.

The shape of the conidium may vary. They may be elliptical, ovoid, globose, cylindrical, branched or spirally coiled or star-shaped (staurospores). The colour of the conidia may be coloured (phaeospore) pink or hyaline (hyalospore), green, or dark. The dark pigments are probably melanins. The colour of the conidia and  conidiophores are important features which are used in classification. In the order Entomophthorales (e.g. Basidiobolus, Pilobolus) agamogenesis is by means or forcibly discharged uninucleate or multinucleate primary conidia. At the time of germination primary conidia develops binucleate or uninucleate secondary conidia. In species of Fusarium one or two-celled microconidia and lots of celled macroconidia are common.

Conidia could also be formed in acropetal (oldest conidium at the bottom and therefore the youngest at the apex) or basipetal (oldest conidium at the apex and youngest at the base) succession. Generally the term 'conidia'is used for any asexual spores aside from sporangia and spores formed directly by hyphal cells. When the spore isn't much differentiated from the cells of the conidiophore in shape the term oidium is usually used for conidia. A distinction between sporangiospores and conidia is that, before germination of sporangiospores a replacement wall, eventually continuous with the germ tube, is laid down within the first spore wall whilst in conidia there's no new wall layer laid down. Conidiophores are also known as sporophores. They are special hyphae bearing conidia.

They may be free, simple or branched. They could also be distinct from one another or may be aggregated to make compound sporophores or fruiting bodies like synnemata, sporodochia, acervuli and pycnidia. They may be given sterigmata or specialized branches on which they bear conidia. Some conidial spores are inflated at the ideas (e.g. Aspergillus).They are generally produced on the surface of the host. The sporogenous a part of the conidiophore is usually apical but could also be laterally placed. The apical zone of differentiation of conidiophore may produce to one conidium or more often, to a succession of conidia chained , false heads. 

c. Chlamydospores

Chlamydospore (Gr. Chlamys = mantle + spora = seed, spore) may be a thick-walled thallic conidium that generally function as a resting spore. Terminal or intercalary segments or mycelium may become full of food reserves and develop thick walls. The walls may be colourless or pigmented with dark melanin pigment. 

These structures are known as chlamydospores. e.g. Fusarium, Mucor racemosus, Saprolegnia.Generally there's no mechanism for detachment and dispersal of chlamydospores. They become separated from each other by the disintegration of intervening hyphae. They are the important asexual survival or organs in soil fungi. When chlamydospores are found in between fungal cells they're called 'intercalary chlamydospores'. 

Key Questions

1. How do fungi reproduce sexually?
2. Which fungi reproduce only asexually?
3. How do threadlike fungi reproduce?
4. How do fungi reproduce sexually?