In this article we will discuss about the vegetative, asexual and sexual methods of reproduction in algae.
(A) Vegetative Reproduction:
Vegetative reproduction in algae takes place by the following methods:
Fragmentation is the most common vegetative method of reproduction. The filamentous thallus breaks into fragments, and each fragment is capable of forming new thallus. Fragmentation can take place due to mechanical pressure, insect bite etc. The common examples are Ulothrix, Spirogyra, Oedogoniwn, Zygnema, Oscillatoria etc.
Fission is common in desmids, diatoms and other unicellular algae. The cell divides mitotically into two the cells are separated by septum formation.
Tubers are spherical or globular bodies formed on lower nodes and rhizoids in Cliara. Tubers are formed due to storage of food. On detachment from parent plant, these develop into new plants.
(iv) Adventitious branches:
Adventitious branches like protonema develop on rhizoids of Chara. On detachment they form new thalli. Similar adventitious structures are formed on thalli of Dictyota and Fucus.
In blue green algae like Nostoc, Cylindrospermum, the main filament breaks into small fragments of varying length called hormogonia. The hormogonia may be formed at the place of heterocyst in the filaments.
In Protosiphon budding takes place due to proliferation of vesicles. The buds detach to make new thalli.
(B) Asexual Reproduction:
Asexual reproduction takes place with the help of some spores and structures. Fertilization and fusion of nuclei does not take place. The reproduction takes place only by protoplasm of the cell.
Different methods of asexual reproduction are:
The zoospores are flagellated asexual structures. The zoospores are formed in reproductive body the zoosporangium.
The zoospores can be biflagellate e.g., Chlamydomonas, biflagellate and quadriflagellate e.g., Ulothrix, Cladophora, multi-flagellate e.g., Oedogoniwn. Zoospores move in water before they germinate to make new plants. Zoospores are normally formed under favourable conditions. In Vaucheria, a compound zoospore called synzoospore is formed.
Aplanospores are formed under unfavorable conditions. Aplanospores are non-motile structures, in which protoplasm gets surrounded by thin cell wall. The aplanospores on release form new plants, e.g., Ulothrix.
The akinetes are formed under unfavorable conditions as method of perennation. The akinetes are thick walled, non-motile structures like aplanospores. Akinetes, on release, form new thalli. e.g., Anabaena.
Hypnospores are thick walled structures. These are formed during unfavorable conditions. Under prolonged unfavorable conditions, the protoplasm of hypnospores divides to make cysts. The cysts are capable of forming new thallus. e.g., Chlamydomonas nivalis.
(v) Tetra spores:
Tetra spores are non-motile spores formed in some members of Rhodophyceae and Phaeophyceae. In Polysiphonia, tetra spores are formed in tetra sporangia by reduction division on special tetrasporophytic plants.
(vi) Auto spores:
The auto spores are aplanospores like structures. These are similar to the parent cell. In Chlorella, Scenedesmus, auto spores acquire all characteristics of parent cells before their discharge from sporangium.
(C) Sexual Reproduction:
Sexual reproduction takes place by fusion of gametes of different sexuality. The gametes are formed in gametangia by simple mitotic division or by reduction division. The haploid gametes fertilize to make diploid zygote.
Depending upon morphological and physiological characteristics of gametes, sexual reproduction can be of the following types:
In isogamous reproduction the fusing gametes are morphologically similar. These gametes are physiologically different due to different hormones. The gametes are represented by (-) and (+) strains to show morphological isogamy but physiological anisogamy e.g., Chlamydomonas, Ulothrix, Spirogyra and Zygnema.
In anisogamy the fusing gametes are morphologically as well as physiologically different. These are formed in different gametangia. The microgametes or male gametes are smaller, active and formed in large number. The macrogametes or female gametes are larger, less active and formed in relatively smaller number e.g., Chlamydomonas.
It is the most advanced type of sexual reproduction. The male gametes or microgametes are formed in antheridia. The female gamete is large, usually one and formed in female structure oogonium. During fertilization the male gametes reach oogonium to fertilize egg and a diploid zygote is formed, e.g., Chlamydomonas.
In hologamy the unicellular thallus of opposite strains (-) and (+) behaves as gametes directly. The thalli fuse to make diploid zygote e.g., Chlamydomonas.
In autogamy two gametes of same mother cell fuse to form diploid zygote. Since both gametes are formed by same cell there is no genetic recombination e.g., diatoms.