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In this article we will discuss about:- 1. Asexual (Non-Sexual) Reproduction 2. Sexual Reproduction 3. Parasexual Reproduction.
Asexual (Non-Sexual) Reproduction:
Asexual reproduction is being used here in a broader sense including ‘vegetative’ or somatic reproduction also, in which new individuals are formed from unspecialized, detached portions of the thallus. In ‘asexual’ reproduction, specialized reproductive bodies are formed without involving nuclear fusions. Since biologically there is no difference in the vegetative and asexual reproduction, both being non-sexual, they are put together under asexual reproduction.
The various types of asexual reproduction are as follows:
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a. Fragmentation:
A small bit of the broken hypha establishes a new colony. Fragmentation occurs in nature and is employed in the laboratory to keep the fungus growing by transferring small portions of hyphae to new culture tubes.
b. Budding:
A small soft portion of the cell wall bulges out and a daughter nucleus migrates into it. The bud is pinched off by constriction at the point of origin of the bud. Sometimes, budding is so quick that a chain of cells is formed due to non-detachment of the daughter cells.
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c. Fission:
It is characteristic of bacteria. In fungi, this occurs only in fission yeasts. The cell divides in transverse plane into two cells.
d. Chlamydospores:
These are thick-walled, resistant spores which are formed to tide over adverse environment. These are formed by formation of a thick wall around the cell. Chlamydospores are not detached from the hyphae. When rest of the hyphae die, these remain viable.
e. Arthrospores (= oidia):
The cells of the hyphae at the distal end round off and separate in basipetal succession, i.e., from apex towards the base of hyphae. On germination, the arthrospores give rise to new fungus colony.
f. Sporangiospores and Conidia:
Spores formed internally, inside a sac-like structure, called sporangia, are called sporangiospores. Conidia are formed externally on the tip or sides of specialized hyphae, called conidiophores.
The sporangiospores may be flagellate or non-flagellate. The flagellate spores, which may be uni-, or biflagellate, are called zoospores and are formed mostly by aquatic or amphibious fungi. The non-flagellate spores are called aplanospores and are produced by terrestrial fungi.
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The sporangia are usually borne on specialized hyhpae called sporangiophores, which in some fungi are branched in such a characteristic way that the genera are identified by their sporangiophores, e.g., downy mildews. Similarly, conidia are borne on conidiophores, which too may be characteristically branched and useful in identification of the genera. The conidiophores may lie externally, or inside flask-shaped or globular structures called pycnidium, or flat, disk shaped structure called acervulus.
The flagella are appendages, which are of two types, viz., whiplash and tinsel. The whiplash flagella are much thinner at the tip. The tinsel type flagella, which are found only in members of kingdom Straminopila, have large number of small hair-like out growths, called mastigonemes or flimmers on their entire length.
The flagellum of fungi has a typical 9 + 2 structure similar to flagella and cilia of other eukaryotic organisms. (The bacterial flagella lack the 9 + 2 structure and are made of 8 rows of flagellin (protein) molecules twisted around each other like a rope).
In the 9 + 2 structure, the flagellum is composed of 9 peripheral pairs of fibrils surrounding two central fibrils (hence the name 9 + 2). The two central fibrils are attached to the blepharoplast lying inside the motile cell. The membrane surrounding the 9 + 2 fibrils is continuous with the plasmamembrane of the cell. Sometimes, a thread like rhizoplast connects the blepharoplast to the nucleus.
Sexual Reproduction:
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It involves plasmogamy, i.e., fusion between two sexual cells, and karyogamy, i.e., fusion of nuclei. It results in the formation of a diploid nucleus, which immediately or later undergoes meiosis to form 4 haploid nuclei.
Fungi achieve plasmogamy by a variety of methods, viz.:
i. Gametogamy,
ii. Gametangiogamy,
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iii. Spermatization and
iv. Somatogamy.
i. Gametogamy:
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Fusion (or copulation) between gametes is called gametogamy. Gametes are naked, i.e., wall less, sexual cells which copulate to form a zygote. If the two gametes are similar in size, they are called isogametes and their copulation isogamy. Copulation between two dissimilar gametes, one smaller (male) and the other bigger (female) is called anisogamy. The fusion between a motile male gamete and a non-motile female gamete (oosphere or egg) lying in the oogonium, is called heterogamy.
ii. Gametangiogamy:
Fusion between gametangia is called gametangiogamy. When the gametangia are similar in shape and size, these are called isogametangia and designated as (+) and (-) gametangia, rather than male and female. When the gametangia are different in shape and size, then these are called heterogametangia. The male is usually smaller and club-shaped, while the female is bigger and globular.
Fusion between two similar gametangia results in a zygote, which is called zygospore. The zygote formed by the fusion between morphologically distinct gametangia is called an oospore, and the process oogamy. The plasomogamy between them is brought about by gametangial copulation, or gametangial contact.
Gametangial copulation is of two types. In one, (e.g., Mucor and yeast) the entire gametangia fue, the intervening wall disappears and their contents come to lie in the common cell formed by their fusion. In the other type (e.g., Rhizophidium) the contents of the male gametangium migrate into the female gametangium through a pore or a fertilization tube. The male gametangium is left empty.
Gametangial Contact- The antheridium, and occasionally also the oogonium, is not differentiated into definite protoplasts or gametes, but are represented only by their nuclei. The male nuclei, not the cytoplasm, migrate into the oogonium through a pore dissolved at the point of contact or through a fertilization tube formed by the antheridium, e.g., Pythium, Phytophthora, Albugo.
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iii. Spermatization:
This occurs in Asco-, and Basidiomycota. Spermatia (sing, spermatium), which are minute male gametes, are formed like conidia, on spermatiophores. The spermatiophores may be formed exogenously or inside a spermogonium, e.g., Puccinia. The spermatium, when comes in contact with the female gametangium through, insect, water, sugary exudates etc., releases the male nucleus into the female gametangium through a pore.
iv. Somatogamy:
In some higher Asco- and Basidiomycota sex organs are not formed and somatic cells act as gametangia. Their fusion is governed by rules of homothallism and heterothallism. Thus, somatogamy may occur between cells of the same hyhpa (in a homothallic fungus) or between cells of different thalli (in a heterothallic fungus).
Anastomosis, which is fusion of hyphae, is frequent in higher fungi. Cells of monokaryotic hyhpae of Asco- and Basidiomycota fuse (anastomose) resulting in plasmogomy and establishment of a binucleate cell. From this cell, the dikaryotic mycelium is formed.
Parasexual Reproduction:
The anamorphic (=imperfect) fungi lack sexual reproduction involving karyogamy and meiosis. But the purpose of sexual reproduction, which is genetic recombination, is achieved through a novel method, called parasexual reproduction, discovered in 1952 by Pontecorvo and Roper in Aspergillus nidulans.
