Vegetative Reproduction Methods and Types of Asexual and Sexual Spores in Algae
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Vegetative reproduction in algae can occur through fragmentation, the formation of adventitious branches, bulbils, or hormogonia. Algae also reproduce asexually through spores like zoospores, aplanospores, tetraspores, akinetes, exospores, and endospores. Sexual reproduction involves the fusion of gametes, which can be isogamous (similar gametes), anisogamous (different sized gametes), or oogamous (egg and sperm). Zygotes form after gamete fusion and develop into new algae.
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Vegetative Reproduction Methods and Types of Asexual and Sexual Spores in Algae
- 1. Vegetative Reproduction Vegetative part of the thallus develops into new individual. Fission Fragmentation Hormogonia By Adventitious Branches Bulbils Called binary fission as found in Chlamydomonas, Synechococcus diatoms etc. In this method the vegetative cell divides mitotically into two daughter cells, The fragmentation may be accidental or by the formation of separation discs or by some other mechanical force or injury. It is found in Spirogyra, Ulothrix, Oedo- gonium, Zygnema, Cylindospermum The trichomes of blue- green algae break up within the sheath into many-celled segments called hormogonia. They remain delimited by the formation of heterocysts,. Hormogonia are com- monly found in Nostoc, Oscillatoria, Cylindosporium etc. which, when detached from the plant body, develop into new individuals (e.g., Fucus, Dictyota). Protonema- like adventitious branches are formed from the internodes of Chara, stolons of Cladophora glomareta etc. Tuber-like outgrowths are deve- loped due to storage of food at the tip of rhizoids and on the lower nodes of Chara, called bulbils (Fig. 3.16C). After detachment from the plant body, bulbils grow into new plants.
- 2. Asexual spores Zoospores • Motile naked spores provided with two, four or many flagella and called as bi-, quadri- or multiflagellate zoospores, respectively. • Biflagellate zoospores - Chlamydomonas, Ulothrix Ectocarpus etc., • Quadriflagellate zoospores – Ulothrix • Multiflagellate zoospores - Oedogonium • Multinucleate and multiflagellate zoospores as found in Vaucheria are called synzoospores
- 3. Aplanospores: Aplanospores are non- motile spores. unfavourable conditions, especially in drought Ulothrix Microspora. The aplanospores may also be formed in certain algae of semiaquatic habitat. Identical to the parent cell, they are referred to as Scenedesmus, Chlorella etc. Aplanospores with thickened wall and abundant food reserve are known as hypnospores Pediastram, Sphaerella etc.). They are formed to overcome prolonged period of desiccation.
- 4. Tetraspores: Diploid plants of some algae Polysiphonia, produce a special type of haploid aplanospores, called tetraspores, formed within tetrasporangium. The diploid nucleus of a tetrasporangium divides meiotically to form four haploid nuclei which — with little amount of protoplasm — are deve- loped into four tetraspores. After liberation the tetraspores germinate to form male and female gametophytes. Akinetes: The vegetative cells of certain filamentous algae develop into elongated thick-walled spore-like structures with abundant food reserves, called akinetes Gloeotrichia,
- 5. e. Exospores: In some algae, spores are regularly cut off at the exposed distal end of the protoplast in basipetal succession, called exospores. These spores aggregate in groups and develop new colonies, Chamaesiphon f. Endospores: These are small spores formed by the divisions of the mother protoplast. They are also called conidia or gonidia. They are set free after the dissolution of mother wail. Without taking rest, the spores germinate directly and develop into a new plant, e.g., Dermocarpa Akinetes: The vegetative cells of certain filamentous algae develop into elongated thick-walled spore-like structures with abundant food reserves, called akinetes Gloeotrichia
- 6. (non-motile in Rhodo- phyceae) male gamete (sperm or antherozoides) with a large non-motile female gamete (egg or ovum). Male gametes develop within antheridium, whereas the female gamete within the oogonium, e.g., Oedogonium, Vaucheria, Chara, Laminaria, Sargassum, Polysiphonia, Batrachospermum etc. Sexual Reproduction: All algae except the members of the class Cyanophyceae reproduce sexually. During sexual reproduction gametes fuse to form zygote. The new genetic set up can develop by the fusion of gametes coming from the different parents. Developed from the same mother cell and after fusion they form zygote. No new character e.g., Diatom (Amphora normani). Unicellular member the vegetative cells of different strains (+ and -) behave as gametes y form zygote new genetic combinations are developed by this process, e.g., Chlamydomonas. Between two gametes which are morpho- logically and physiologically similar form zygote. The gametes are called isogametes. Usually they are flagellate, e.g., Chlamydomonas eugametos, Ulothrix etc. Morphologically and phy- siologically different. The smaller and more active microgamete (male), whereas the larger and less active one is the macrogamete (female), e.g., Chlamydomonas braunii. Deviating from the typical anisogamy, when the uniting gametes show morphological similarity with physiological difference, it is called physiological anisogamy. e.g., Zygnema, Spirogyra etc. Autogamy: Hologamy Isogamy Anisogamy Oogamy