METHODS OF REPRODUCTION IN ALGAE

Algae reproduce by three different methods of reproduction – Vegetative, asexual and sexual reproduction.

1. VEGETATIVE REPRODUCTION:- In this method the new individuals are formed directly from the vegetative parts without any change in the protoplast. It may be of the following types –

(a) Simple cell division and Fission:- In unicellular forms of algae, the individual cell divides by simple cell division and forms two daughter cells (Synechococcus). In fission the cell divides by formation of deep constriction on the sides of the cell as seen in Diatoms.

(b) Fragmentation:- In filamentous algae, the plant body i.e., the filament breaks up into one or more fragments, each of which develops into a new individual plant. Fragmentation may occur by – accidental, by formation of special structures called separation discs or by some mechanical forces e.g., Spirogyra.

(c) Splitting of Colonial form:- In this type, the mature colonies split up into one or more small parts, each of which gives rise to new daughter colonies, e.g., Dictyosphaerium, Aphanothece, etc.

2. ASEXUAL REPRODUCTION:- Asexual reproduction takes place by the formation of various kinds of motile and non-motile spores. These are as follows –

(a) Zoospores:- These are flagellate, motile and naked spores which may be formed either within a specialized structure called zoosporangium or from the vegetative cell directly. The zoospores may be bi-flagellate (two flagella), quadric-flagellate (four flagella) or multi-flagellate (many flagella). Each zoospore swims in water, comes to rest, drops its flagella and germinates into new individual.

          (b) Aplanospores:- These are non-flagellate, non-motile spores formed inside a special spore producing structure called aplanosporangium. Each aplanospore is surrounded by a definite wall which is distinct from a parent cell wall. Each aplanospore germinates and gives rise to new individual plant in response to suitable environmental condition.

(c) Hypnospores:- Very thick-walled aplanospores with abundant food reserves are called hypnospores. A hypnospore germinates after a resting period to produce a new individual plant.

(d) Akinetes:- These are thick-walled and elongated spores produced during dry condition from the vegetative cells of some filamentous algae, e.g., Oedogonium, Nostoc, etc. Each akinete under favourable condition germinates into new individual plant.

(e) Autospores:- These are non-motile resting spores resembling the parent cell in shape and structure. Autospore formation occurs in many members of Chlorococcales, e.g., Chlorella.

(f) Hormospores:- These are actually fragments i.e., short portion of trichomes called hormogonia (hormogones). Such hormogones when develop at the tips of trichome become different  shaped cells with much thicker walls and become multicellular spore like structure called hormospores. Hormospores tide over unfavourable conditions, and when favourable condition exists they germinate to produce new individual plant, e.g., Westiella, Nostoc, etc.

(g) Endospores:- These are non-flagellate, thin-walled spores formed internally i.e., inside the cell as a result of division of the entire protoplast of the vegetative cell of some unicellular algae, e.g., Dermocarpa.

(h) Exospores:- These are also thin-walled, non-flagellate spores which are cut off externally from the protoplast of the vegetative cell of some unicellular algae, e.g., Chamaesiphon.

(i) Heterocysts:- These are specialized, larger and thickened, double walled vegetative cells of some filamentous algae. In some algae heterocysts germinate producing new individual.

3. SEXUAL REPRODUCTION:- Sexual reproduction takes place by the union of sexual reproductive units called gametes. Three types of sexual union can be observed in algae. These are –

1. Isogamy:- In this type, two morphologically identical gametes, i.e., gametes of equal size and form, fuse to form a zygote. The fusing gametes may be flagellate (motile) or non-flagellate (non-motile) and are called isogametes, e.g. Ulothrix.

2. Anisogamy:- In this type, two morphologically non identical gametes, i.e., gametes of unequal size and form, called anisogametes or heterogametes fuse together to form a zygote, e.g., Chlamydomonas. Of the two non-identical gametes, the smaller gametes are called micro-gametes and the larger gametes are called macrogametes.

          3. Oogamy:- In this type fusion takes place between two heterogametes. Of the two mating type, one is smaller, active and flagellate (motile), and is known as antherozoid or sperm or male gamete. The other one is larger, passive and non-flagellate (non-motile) and is known as ovum or oosphere or egg or female gamete. As a result of fusion, zygote is formed which directly develops into new individual plant.

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