Reproduction in Fungi | Vegetative, Asexual & Sexual Reproduction
Fungi reproduce by all the three modes, i.e., vegetative, asexual and sexual.
1. Vegetative reproduction:
It occurs by the following methods :
- Fragmentation : The mycelium breaks up into two or more fragments due to mechanical injury, decay or some other reasons. Each fragment grows into independent mycelium.
- Fission : Here, simple splitting of vegetative cells into two daughter cells takes place by simple constriction.
- 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.
The various means of asexual reproduction are as follows:
(a) Zoospore: Many fungi, especially aquatic members produce these types of spores. Zoospore may be uniflagellate, e.g., Synchytrium or biflagellate, e.g., Saprolegnia, Pythium and are naked uninucleate structures formed in zoosporangia. They germinate to give rise to new mycelium. Biflagellate zoospores are of two kinds (e.g., Saprolegnia) pear shaped or pyriform with 2 flagella placed at anterior end (primary zoospore) and kidney shaped or bean shaped, bearing two laterally inserted flagella (secondary zoospore). This phenomenon of having two types of zoospores is called diplanetism.
(b) Sporangiospore: Sporangiospores are thin walled non-motile spores produced endogenously in a sporangium during favourable conditions, which after liberation give rise to new mycelium, e.g., Rhizopus, Mucor.
(c) Conidia: Conidia are non motile, thin walled exogenous spores produced at the tips of erect hyphae called conidiophore. They are arranged in chains upon the conidiophore, e.g., Aspergillus and Penicillium.
(d) Chlamydospore: In some fungi the hyphae under unfavourable conditions, forms thick walled resting resistant spores which later get separated from each other. They may be terminal or intercalary. They may remain viable for several years. On return to favourable conditions they germinate to give rise to new individuals. Thus, chlamydospores are structures for perennation also, e.g., Rhizopus.
(e) Oidia: Non-motile thin walled spores developing under sugar rich conditions in medium. Their budding condition is called torula stage.
3. Sexual reproduction:
It occurs through oospores, ascospores and basidiospores. The various spores are produced in distinct structures called fruiting 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-ascus). 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 compatible mating type or gametes.
Plasmogamy occurs by the following methods :
(a) Planogametic copulation / Gametic fusion : This is the simplest form of sexual reproduction. In this process, fusion of two gametes of opposite sex or strains takes place. One or both of the fusing gametes are motile. It results in the formation of a diploid zygote, e.g., Allomyces.
(b) Gametangial contact: In this process two gametangia come in contact with one another. A fertilization tube is developed to facilitate the migration of entire contents of male gametangium into the female gametangium. Both the gametangia never fuse together losing their identity, e.g., Pythium, Albugo (Oomycetes)
(c) Gametangial copulation : In this process, direct fusion of entire contents of two gametangia is accomplished by dissolution of their common walls resulting in the tormation of a single cell, in which protoplasts of two gametangia fuse, e.g., Mucor, Rhizopus (Zygomycetes)
(d) Spermatization : Some fungi produce many minute, spore like, single-celled structures called spermatia (non motile male gametes) on spermatiophores (hyphae). These structures are transferred through agencies like water, wind and insects to special female receptive hyphae (Basidiomycetes). The contents migrate into receptive structure. Thus, dikaryotic condition is established, e.g., Puccinia.
(e) Somatogamy: This takes place in most of the higher true fungi, where formation of gametes is absent. In such fungi, direct fusion of somatic hyphal cells occur to establish dikaryophase, e.g.. Agaricus.
2. Karyogamy:
Fusion of two nuclei is called karyogamy. In some fungi, the fusion of two haploid cell immediately results in diploid cells (2n). However, in some fungi (e.g. Aganicus, Aspergillus), an intervening dikaryotic stage (n + n ie. two nuclei per cell) occurs, such a condition is called a 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.