Sexual and Asexual reproduction in Fungi || Fungus Reproduction

∆  Asexual Reproduction

In fungi asexual reproduction is a more common method than sexual reproduction. It is usually repeated several times in a season. It takes place by the formation of special reproductive cells called spores.

The formation of spores in fungi is called sporulation. Each spore develops into a

new mycelium. These spores are produced as a result of mitosis in the parent cell and hence they are also called mitospores.

The spores vary in colour, shape and size, number, arrangement on hyphae and in the way in which they are borne. They may be hayline, green, yellow, orange, red, brown to black in colour and are minute to large in size. In shape they vary from globose to oval, oblong, needle-shaped to helical. Thus an infinite variety of spores can be observed in fungi and you will find them very fascinating under the microscope.

Usually the spores are unicellular. They may be uninucleate or multinucleate. In some fungi like Alternaria and Curvularia they are multicellular.

The mitospores produced in the fungi are of three types :- 

1. Sporangiospores

2. Zoospores

3. Conidiophores

~ Sporangiospores

The sporangiospores are produced inside a sac-like structure called sporangium. The hypha bearing a sporangium is called sporangiophore. They are characteristically branched. The sporangiospores may be motile or non-motile. The non-motile sporangiospores are called aplanospores. These are characteristic of terrestrial species like Mucor and Rhizopus.

~ Zoospores 

In aquatic fungi like Pythium of the Division Oomycota motile biflagellate sporangiospores are produced. These are called zoospores and the sporangium bearing them is called zoosporangium.

A zoospore is a motile spore lacking a cell wall. After a swarming period it secretes a wall and germinates to form a germ tube. In contrast to zoospores, the aplanospores have a definite spore wall and are dispersed by wind and insects.

Figure: Parts of Zoospores

~ Conidiophores

The conidia are non-motile, deciduous mitospores formed externally as single separate cells. They develop either directly on the mycelium or on morphologically differentiated hyphae called conidiophores. The

conidiophores may be simple or branched, septate or aseptate. The conidia are produced singly 

e.g., Phytophthora or in chains at the tips of the conidiophores e.g.

Aspergillus or at the tips of their branches e.g., Penicillium.

Often the conidiophores arise singly

and are scattered in the mycelium. Sometimes they arise in specialised structures called fruiting

bodies. According to their appearance they are termed as synnema, sporodochia, acervuli (saucer-shaped), pycnidia (flask-shaped, globular) or pustules.

Figure: Conidiophores 

∆ Sexual Reproduction 

Sexual reproduction involves the fusion of two compatible sex cells or gametes of opposite strains. Fungal sex organs are called gametangia. They may be equal in size. In many higher ascomycetes morphologically different gametangia are formed. The male gametangia are Called antheridia and the female ones ascogonia.

The fungus may be homothallic, that is, the hsing gametes come from the same Mycelium or may be heterothallic, that is, the fusing gametes come from different strains of mycelia.

In Fungi, Sexual reproduction involves the following three phases:-

1. Plasmogamy

2. Karyogamy and

3. Meiosis

These three processes occur in a regular -sequence and at a specific time, duriug the sexual stage of each species.

Plasmogamy

It is the union of protoplasts of reproductive hyphae or cells, one from the male and the other from the female to bring about the nuclei of the two parents close together as a pair. However, the two

nuclei do not fuse with each other. Such a cell is called a dikaryon. The dikaryotic condition is unique to fungi and may continue for several generations as the two nuclei (dikaryon) divide simultaneously during cell division. These are passed on to the daughter hypha.

Karyogamy

The fusion of the two nuclei which takes place in the next phase is called karyogamy. It may immediately follow plasmogamy as in lower fungi, or it may be delayed for a long time as in higher fungi.

Meiosis

Karyogamy which eventually occurs in all sexually reproducing fungi is sooner or later followed by meiosis producing four genetically different spores.