Reproduction in Fungi: Vegetative, Asexual and Sexual Methods

In this article we will discuss about the reproduction in fungi (1) Vegetative reproduction (2) Asexual reproduction (3) Sexual reproduction

1. Vegetative reproduction:

The most common method of vegetative reproduction is fragmentation. The hypha breaks up into small fragments accidentally or otherwise. Each fragment develops into a new individual. In the laboratory the ‘hyphal tip method’ is commonly used for inoculation of saprophytic fungus.

In addition to above-mentioned common method of vegetative reproduction the fungi reproduced vegetatively by other means, such as fission, budding, sclerotia, rhizomorphs, etc. In fission, the cell constricts in the centre and divides into two giving rise to new individuals.

The budding is commonly found in Saccharomyces. The buds arise from the protoplasm of the parent cells and ultimately become new individuals.

The sclerotia are resistant and perennating bodies. They survive for many years. Each sclerotium is cushion-like structure of compact mycelium. They give rise to new mycelia on the approach of favourable conditions.

As mentioned under the modified mycelium, the rope-like rhizomorphs are also resistant to unfavourable conditions and give rise to new mycelia even after several years on the approach of favourable conditions.

2. Asexual reproduction:

The asexual reproduction takes place by means of spores. Each spore may develop into a new individual. The spores may be produced asexually or sexually and thus named (a) asexual spores and (b) sexual spores. Under asexual reproduction, only asexual spores will be considered.

Asexual spores:

They are innumerable and produced on the diplont mycelium in Phycomycetes and Ascomycetes. In Basidio- mycetes they are produced on the diplont mycelium. The spores are of diverse type and borne upon special structures called the sporophores. These spores are produced asexually and called the asexual spores. Usually the spores are uninucleate and nonmotile but multinucleate and motile spores are also found.

The fungus producing more than one type of spores is called the pleomorphic or polymorphic. The spores produced inside the sporangia are termed the endogenous spores and the spores developing exogenously on the terminal ends of sporophores are called the exogenous spores.

Endogenous spores:

The endogenous spores are produced within the special spore producing cell the sporangium. The sporangia may be terminal or intercalary in their position. The sporophores which bear the sporangia on their apices are called the sporangiophores. They may be branched or unbranched.

The spores produced inside the sporangia are called the endospores or endogenous spores produced inside the sporangia are called the endospores or endogenous spores. They may be motile or non-motile. The motile spores are called the zoospores and the non-motile aplanospores. The zoospores are produced inside the zoosporangia. The protoplasm of the sporangium divides into uninucleate or multinucleate protoplasmic bits and each bit metamorphoses into a spore.

The endogenously produced zoospores are uni or biflagellate. Each spore is without any cell wall, uninucleate and vacuolate. They can move with the help of their flagella. They are usually kidney-shaped or reniform and the flagella are inserted posteriorly or laterally on them. Such zoospores have been recorded from Albugo, Pythium, Phytophthora and many other lower fungi.

The aplanospores are non-motile, without flagella and formed inside the sporangia. They may be uni or multinucleate (e. g., Mucor, Rhizopus). These spores lack vacuoles and possess two layered cell walls. The outer thick layer is epispore or exospore which may be ornamented in many cases. The inner thin layer is endospore.

Exogenous spores:

The spores producing externally or exogenously are either called the exogenous spores or conidia. They are produced externally on the branched or unbranched conidiophores. The condiophores may be septate or aseptate. The conidia borne upon the terminal apices of the conidiophores or the ends of the branches of the condiophores.

The conidia may be produced singly on each sterigma or in chains. The conidial chains may be basipetal to acropetal in succession. The conidia are diverse in their shape and size. They may be unicellular or multicellular, uninucleate or multinucleate. Different genera may be recognized only by the presence of various shaped and various coloured conidia. The conidia of Fungi Imperfecti are multicellular and variously shaped, whereas the conidia of Aspergillus and Penicillium are smoky green coloured and the fungi are called ‘the blue-green molds’.

In other type of exospores, the sporophores develop in groups and form the specialized structure called the pustules, pycnia, aecidia, acervuli, and sporodochia. The pycnia are flask-shaped producing pycniospores in them. The acervuli are saucer- shaped widely open bodies having developed conidia in them on small conidiophores.

In mushrooms the sporophores are compactly arranged and form an umbrella-like fructification. The terminal expanded portion bears gills. In each gill there are hundreds of sporophores called the basidia bearing basidiospores. The sporophores (basidia) are arranged in hymenia.

3. Sexual reproduction:

A large number of fungi reproduce sexually. However, the members of Fungi Imperfecti, or ‘Deuteromycetes’ lack sexual reproduction.

Usually two phases are found in the life cycle of the plants. These phases are called haploid and diploid phases respectively. The haploid phase possesses the (n) number of chromosomes in the nucleus, whereas this number becomes (2n) in the diploid phase.

The gametes are always haploid (n) and by a sexual fusion they result in diploid (2n) sexual spores, such as zygospores, oospores, etc. To bring haploid (n) phase once again in the life cycle the reduction division (meiosis) takes place and the number of chromosomes becomes half.

The gametes taking part in sexual fusion may be morphologically or physiologically different. Such two gametes taking part in fusion are of opposite sexes or strains, which may be called male and female sex organs or plus and minus strains. When both the sex organs or strains occur on the same mycelicum, the fungus is said to be monoecious or homothallic, and when the male and female sex organs or plus and minus strains occur separately on different mycelia the fungus is said to be dioecious or heterothallic.

The gametes taking part in fusion are usually formed in the cells of sacs called gametangia (singular-gametangium). The morphologically identical male and female gametes are called the isogametes. The morphologically dissimilar male and female gametes are called the heterogametes.

In such cases the male gametes are called the antherozoids and the female ones are the eggs. The fusion of the plasma of the gametes is called the plasmogamy, which is usually followed by the nuclear fusion, i.e., karyogamy. The whole process is called the fertilization.

Sometimes, in some of the fungi, e.g., Phycomycetes and Ascomycetes, the entire contents of the two gametangia fuse with each other, the process is called the gametangial copulation. In the members of Phycomycetes and Ascomycetes the gametangia taking part in gametangial copulation are called the antheridia (singular-antheridium) and the oogonia (singular-oogonium)

In the lower fungi, there is complete fusion of the nuclei of the two different strained gametes in the sexual union, i.e., karyogamy, whereas in the higher fungi, i.e., Ascomycetes and Basidiomycetes, the fusion of the two nuclei of different strains is delayed and the pairs of the nuclei called the ‘dicaryons’ are formed. The mycelium having such pairs of nuclei is called the ‘dicaryotic mycelium’. In the opposite cases where the mycelium possesses single haploid nucleus of either strain in each cell is called the monocaryotic mycelium. The most common methods of sexual reproduction are as follows:

i. Planogametic copulation:

This type of sexual reproduction involves the fusion of two naked gametes one or both of them are motile. The motile gametes are known as planogametes. The most primitive fungi produce insogamous planogametes, e.g., Synchytrium, Plasmodiophorq etc. The anisogamous planogametes are only found in the genus Allomyces of order Blastocladiales. In Monoblepharis (order Monoblepharidales) the unique condition is present here the female gamete is non-motile whereas the male gamete is motile. The male gamete enters the oogonium and fertilizes the egg.

ii. Gametangial contact:

This method of reproduction is found in many lower fungi (class Phycomycetes). In this method two gametangia of opposite sex (oogonium and antheridium) come in contact and one or more gamete nuclei migrate from the male gametangium (antheridium) to the female gametangium (oogonium).

In no case the gametangia actually fuse. The male nuclei in some species enter the female gametangium through a pore developed by the dissolution of the wall of contact (e.g., in Aspergillus, Penicillium, etc.); in other species the male nuclei migrate through a fertilization tube (e.g., Phythium, Albugo, Peronospora, etc.). After the migration of the nuclei the antheridium eventually disintegrates but the oogonium continues its development in various ways.

iii. Gametangial copulation:

In this method of sexual reproduction the fusion of the entire contents of two contacting compatible gametangia takes place (e.g., Mucor, Rhizopus, Entomophthora, etc.)

iv. Spermatization:

The sexual reproduction in Neurospora (Class- Ascomycetes) and other fungi takes place by means of this method. The minute, uninucleate, spore-like male structures are known as spermatia. They are produced in several ways. The spermatia are carried by outer agencies to the receptive hyphae (trichogynes) of female gametangia, to which they become attached. A pore develops at the wall of contact and the contents of spermatium pass into the female gametangium through the receptive hypha.

v. Somatogamy:

The sex organs are not produced. The somatic cells take part in sexual fusion, e.g., Morchella, many higher fungi.