Reproduction in fungi, Biology tutorial

Types of Reproduction:

Whenever the mycelium of a fungus reaches a specific phase of maturity and accumulates reserve food, it begins reproducing. As in algae, reproduction in fungi is of three types:

1) Vegetative

2) Asexual

3) Sexual

Vegetative and asexual process of reproduction which don't involve the fusion of nuclei or sex cells or sex organs are, though, clubbed by numerous mycologists into asexual methods of reproduction. Therefore, they recognize just two methods, sexual and asexual.

Asexual Reproduction:

In fungi, asexual reproduction is a more general process than sexual reproduction. It is generally repeated quite a few times in a season. It occurs by the formation of special reproductive cell termed as spores. The formation of spores in fungi is termed as sporulation. Each and every spore builds up into a new mycelium. These spores are generated as an outcome of mitosis in the parent cell and therefore they are as well termed as mitospores. The spores differ in color, shape and size, number, arrangement on hyphae and in the manner in which they are borne. They might be hyaline, yellow, green, orange, red, brown and black in color and are minute to big in size. In shape they differ from globose to oval, oblong, needle-shaped to helical. Therefore an infinite diversity of spores can be viewed in fungi and you will discover them very fascinating beneath the microscope.

Generally the spores are unicellular. They might be uninucleate or multinucleate. In a few fungi such as Alternaria and Curvularia they are multicellular. The mitospores generated in fungi are of two kinds: sporangiospores and conidia. 

The sporangiospores are generated within a sac-like structure termed as sporangium.  The hypha bearing a sporangium is termed as sporangiophore. They are normally branched. The sporangiospores might be motile or non-motile. The non-motile sporangiospores are termed as aplanospores from Greek, not + planets wanderer + spores seed, spore. These are feature of terrestrial species such as Mucor and Rhizopus. In aquatic fungi such as Pythium of the division Oomycota motile biflagellate sporangiospores are generated. These are termed as zoospores and the sporangium bearing them is termed as zoosporangium. A zoospore is a motile spore deficient of a cell-wall. After a swarming period it produces a wall and germinates to make a germ tube. In contrast to zoospores, the aplanospores encompass a definite spore wall and are dispersed through insects and wind.

The conidia are non-motile, deciduous mitospores made externally as single separate cells. They build up either directly on the mycelium or on morphologically distinguished hyphae termed as conidiophores. The conidiophores might be simple or branched, septate or aseptate. The conidia are generated singly example: Phytophthora or in chains at the tips of the conidiophores example: Aspergillus or at the tips of their branches example: Penicillium.

Often the conidiophores occur singly and are dispersed in the mycelium. At times they arise in specialized structures termed as fruiting bodies. According to their appearance they are known as synnema, acervuli, sporodochia and pustules.

Sexual Reproduction:

The sexual phase in fungi is termed as the perfect state in contrast to the imperfect state that is the asexual stage. Sexual reproduction comprises the fusion of two compatible sex cells or gametes of the opposite strains. Fungal sex organs are termed as gametangia. They might be equivalent in size. In most of the higher ascomycetes morphologically different gametangia are formed. The male garnetangia are termed as antheridia and the female ones ascogonia.

The fungus might be homothallic, that is, the fusing gametes come from the similar mycelium or might be heterothallic, that is, fusing gametes come from various strains of mycelia. 

In fungi, sexual reproduction comprises the given three stages: (a) Plasmogamy, (b) Karyogamy and (c) Meiosis. These three processes take place in a regular series and at a particular time, throughout the sexual phase of each species.

1) Plasmogamy:

This is the union of protoplasts of reproductive hyphae or cells, one from the male and the other from the female to carry about the nuclei of the two parents close altogether as a pair. Though, the two nuclei don't fuse with one other. Such a cell is termed as a dikaryon. The dikaryotic condition is exclusive to fungi and might continue for some generations as the two nuclei (dikaryon) split or divide concurrently throughout cell division. These are passed on to the daughter hypha.   

2) Karyogamy:

The fusion of the two nuclei which occurs in the next stage is termed as karyogamy. It might instantly follow plasmogamy as in lower fungi, or it might be delayed for a long time as in the higher fungi. 

3) Meiosis:

Karyogamy that eventually takes place in all sexually reproducing fungi is sooner or later followed through meiosis producing four genetically dissimilar spores.

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