Types of Reproduction in Algae – Class 11 | Chapter – 3 | Short Notes Series PDF
Types of Reproduction in Algae: Reproduction in algae takes place by vegetative, asexual and sexual methods. Vegetative mode of reproduction is encountered by primitive algae whereas in higher forms both asexual and sexual reproductions are common.
1-Vegetative Reproduction
2-Asexual Reproduction
3-Sexual Reproduction
Types of Reproduction in Algae
Vegetative Reproduction
The vegetative reproduction in algae takes place by following methods:
(i) 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.
(ii) 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.
(iii) Adventitious branches- Protonema develops in certain algae like Chara and give rise to new thalli when detached from parent thallus. These adventitious branches develop mainly on the rhizoids. Other examples include Dictyota and Fucus.
(iv) Tubers- Tubers are spherical or globular bodies which are found on lower nodes or rhizoids of Chara. These tubers when detach from parent plant can give rise to new thalli.
(v) Amylum stars- In Chara, star shaped bodies filled with amylum stars are formed that give rise to new individual after detaching from the parent plant.
(vi) Budding- In some algae like Protosiphon, budding takes place which results in new individuals.
(vii) Hormogonia- In some cyanobacteria like Nostoc, Cylindrospermum hormogonia develop that may give rise to new thalli. These hormogonia are of varying lenths and may develop at the place of heterocysts in the thallus. These
hormogones are produced by breakage of filament into two or more cells.
Asexual Reproduction
In a large number of algae asexual reproductions takes place with the help of different kind of spores and other structures. Basically, spores are meant for asexual reproduction and each spore can grow into a new thallus. Spores are one cellwd structure and are produced internally in the case of algae. They are produced within the vegetative cell (Chlamydomonas) or in a specialized structure called sporangia. They may be motile or non-motile. Motile spores are called zoospores and non-motile as aplanospores.
Different types of asexual spores and structures are as follows:
(i) Akinetes- In filamentous forms, certain vegetative cells become thick walled elongated structures called as akinetes. Akinetes are perrenating bodies that can survive under unfavourable conditions and can give rise to new individual on occurrence of favourable conditions. e.g., Anabaena
(ii) Hypnospores- Hypnospores are thick walled, non flagellated spores with plenty of food reserves. They are perrenating structures. Hypnospores 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 due to which snow becomes red.
(iii) Zoospores- These are flagellated asexual spores which are formed in zoosporangium or directly from the vegetative cells. The zoospores may be bi, quadric or multiflagellate. The multiflagellate zoospores are of again two typesflagella arranged on entire length of body or arranged in a ring surrounding a beak like projection. 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.
(iv) 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.
Sexual Reproduction
Sexual reproduction is found in advanced algae as compared to less advanced forms where vegetative and asexual methods are main modes of reproduction. Sexual reproduction takes place by fusion of gametes of different sexuality. There is a wide range of variation in the nature of gametes and the mode of sexual reproduction. Any vegetative cell of thallus may produce gametes and thus behave as gametangium or a specialized gametangium may be developed.
The gametangia may be morphologically similar (isogametangia) or dissimilar (heterogametangia). The gametes are produced in the gametangia by simple mitotic division or by reduction division. The haploid gametes fuse to make diploid zygote that give rise to the thallus. Depending upon the morphological and physiological characteristics of gametes, sexual reproduction can be of the following types-
(i) Isogamous- When fusing gametes are morphologically similar and physiologically different (+ and -) then the sexual reproduction is called as isogamous. E.g., Chlamydomonas, Ulothrix, Zygnema, Spirogyra.
(ii) 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.
(iii) 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.
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