Reproduction in Algae (3 Types of Reproduction)

In this article we will discuss about 3 types of reproduction in algae:- 1. Vegetative reproduction in algae, 2. Asexual reproduction in algae and 2. Sexual reproduction in algae

• Reproduction is the biological process by which new organisms are produced from their parents.
• Reproduction in algae takes place by vegetative, asexual and sexual methods.

1. Vegetative Reproduction in Algae

• A portion of plant body becomes separated off and gives rise to a new individual is called vegetative reproduction
• Types of vegetative reproduction in algae: Fragmentation, Fission, Adventitious branches, Tubers, Amylum stars, Budding and Hormogonia

Types of Vegetative Reproduction in Algae

• Fragmentation: Fragmentation is common in filamentous forms. In this process, filament breaks into fragments and each fragment give rise to a new filamentous thallus. The common examples are Ulothrix, Spirogyra, Oedogonium, Zygnema, Oscillatoria, Nostoc etc.
• Fission: This process is common in desmids, diatoms, and other unicellular algae. The cell divides into two by mitotic division and then separation occurs through septum formation.
• Adventitious branches: Certain special structures (propagule or protonema) of thalli are formed which help in formation of new thalli when detached from parent thallus. E.g. Chara (Protonema), Bryopsis, Sphacelaria (propagule)
• Bulbils : They are spherical or globular bodies which are found on lower nodes or rhizoids of Chara. These bulbils when detach from parent plant can give rise to new thalli.
• Amylum stars: In Chara, star shaped bodies filled with amylum starch are formed that give rise to new individual after detaching from the parent plant.
• Budding: In some algae like Protosiphon, budding takes place which results in new individuals.
• Hormogonia: In some cyanobacteria fragments develop and actively moving that may give rise to new thalli. They can be developed at the place of heterocyst in the filaments. E.g. Nostoc

2. Asexual Reproduction in Algae

• Asexual reproduction takes place by spores. Fertilization and fusion of nuclei does not take place.
• Different types of asexual spores: Zoospores, Aplanospores, Tetraspores, Monospores, Autospores, Heterocysts, Endospores, Exospores, Akinetes, Nannocytes, Hypnospores and Cysts

Types of Asexual Spores in Algae

• Zoospores: These are motile, flagellated spores which are formed in zoosporangium or directly from the vegetative cells. The zoospores may be bi, quadric or multiflagellate. E.g., Chlamydomonas (biflagellate), Ulothrix, Cladophora (quadriflagellate), Vaucheria, Oedogonium (multiflagellate). In Pediastrum, the zoospores do not germinate or divide but orientate themselves in a single plane and become opposed to form a colony just like the parent cell. This feature is not met in any other algae. The zoospores of Vaucheria are known as synzoospores as they are multinucleate and multifagellate.
• Aplanospores: These are non flagellated thin walled asexual spores that are formed in majority of aquatic algae by the failure of flagella formation due to some unfavourable conditions. E.g. Cladophora, Ulothrix, Vaucheria
• Tetraspores: Tetraspores are non motile asexual spores that are formed in some members of Rhodophyceae and Phaeophyceae. Tetraspores are formed in tetrads in the tetrasporangia. E.g. Polysiphonia
• Monospores: Single spore formed in the sporangia is called monospore. Monospores are found as asexual mode of reproduction in some members of Rhodophyceae. E.g. Batrachospermum
• Autospores: These are actually aplanospores which appear identical to the parent cell. Hence referred as autospores. E.g. Chlorococcus, Chlorella, Scenedesmus, etc.
• Heterocysts: According to some phycologists, heterocysts are sometimes able to reproduce asexually. However, their exact function is still in question. These structures are found in blue green algae and depending upon the position in thallus they may be terminal or interstitial. E.g. Nostoc, Anabaena etc.
• Endospores: Endospores are developed within the sporangium. After formation of endospores, the sporangium opens and liberate the spores from it. E.g. Dermocarpa clavate, diatoms and blue green algae
• Exospores: The sporangium gets burst at the apex and is exposed to the external environment and spherical spores are formed. All spores get liberated one by one. E.g. Chamaesiphon, Stichosiphon.
• Akinetes: They are thick walled spores formed from vegetative cells of thallus. They survive under unfavourable conditions and can give rise to new individual on occurrence of favourable conditions. E.g., Anabaena, Nostoc, Gloeotrichia, Ulothrix
• Nannocytes: In the members of chroococcales, the cell content divide repeatedly to produce numerous very small spores. The name nannocytes to these very small spores was given by Geitler. E.g., Macrocystis, Gloeocapsa.
• Hypnospores: Hypnospores are thick walled, non flagellated spores with plenty of food reserves. They are produced under unfavourable conditions by some green algae. They germinate into new plants with return of favourable environmental conditions. E.g. Chlamydomonas, Protosiphon. In Chlamydomonas nivalis the walls of hypnospores become red due to the presence of pigment Haematochrome.
• Cysts: Cysts are resting spores of many algae. During dry season the protoplast divides into many small units. Each daughter protoplast secretes a wall around it to form a cyst. When favourable season comes, the cysts germinate into new thalli. E.g. Protosiphon, Vaucheria, Botrydium, Acetabularia

3. Sexual Reproduction in Algae

• Sexual reproduction takes place by fusion of gametes of different sexuality.
• There are the types of sexual reproduction in algae. They are: Autogamy, Hologamy, Isogamy, Anisogamy and Oogamy

Types of Sexual Reproduction in algae

• Autogamy: In this process the fusing gametes are developed from the same mother cell and after fusion they form zygote. For the above, algae developed through autogamy does not show the introduction of any new characteristic. E.g. Diatom (Amphora normani).
• Hologamy: In some unicellular member the vegetative cells of different strains (+ and -) behave as gametes and after fusion they form zygote. It is an ineffi­cient process considering the point of multiplication, but new genetic combi­nations are developed by this process E.g., Chlamydomonas.
• Isogamous: When fusing gametes are morphologically and physiologically similar then the sexual reproduction is called as isogamous. E.g., Chlamydomonas, Ulothrix, Zygnema, Spirogyra.
• Anisogamous: In anisogamous sexual reproduction fusing gametes are morphologically as well as physiologically different. The gametes are produced in different gametangia. The microgametes are male gametes while macrogametes are female gametes. e.g., Chlamydomonas.
• Oogamous: Oogamy is the most advanced type of sexual reproduction in which microgamete or male gamete fuses with a large female gamete or egg. Male gametes are produced in antheridium while female gamete or egg is produced within a structure called as oogonium. During fertilization male gamete reaches the oogonium to fertilize the egg and a diploid zygote is formed. e.g., Chlamydomonas.