Vaucheria ppt.pptx

Vaucheria ppt.pptx

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  Sem. I, PaperI Xanthophyta VAUCHERIA Associate Prof. Dr. S. A. Gaikwad Dept. of Botany, V. M. Sangola.
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  Unit 5 :Xanthophyta Division Xanthophyta- General Characters 1. Div. Xanthophyta -1.Xanthophyceae 1. The cells are eukaryotic in nature. 2. The cell wall is often absent, but when present it contains pectic compounds. Occasionally cellulose is also present. 3. The motile forms bear usually two flagella but rarely one. They are unequal and inserted at the anterior end. The flagella are of two types. The larger one is tinsel and the shorter one is whiplash type. 4. The plastids are yellow-green in colour. The photosynthetic pigments are chlorophyll a, chlorophyll e, β-carotene and xanthophylls. The chief xanthophyll is diadinoxanthin. The other xanthophylls are violaxanthin, neoxanthin, flavoxanthin and flavacin. The carotenoids are normally present in excess amount than chlorophyll. Phycobilins are absent. The pyrenoids are absent or rarely present. The chromatophores are discoid in shape and are numerous in each cell. 5. The reserve food is oil, lipid and lucosin. 6. Plant reproduces commonly by vegetative and asexual means. Sexual reproduction though rare, may be isogamous, anisogamous or oogamous.
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  VAUCHERIA *Systematic position /classification Division: Xanthophyta Class: Xanthophyceae Order: Siphonales Family: Vaucheriaceae Genus: Vaucheria
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  Occurrence • The genusVaucheria (named after J. P. Vaucher) is represented by 54 species, out of which 9 species are found in India. • The majority of species are terrestrial and aquatic. • The terrestrial species grow in winter in the form of green and thick layer on damp moist soil. • The aquatic species found mostly in fresh water. A few species grow in brackish water and some are marine.
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  Structure of Thallus/ Filament • The haploid thallus, i.e., the plant body of Vaucheria, is unicellular, filamentous, coenocytic and siphonaceous or tubular. • The filaments are profusely branched and yellowish-green in color. • The thallus is multinucleate and aseptate (transverse or cross-walls) showing a coenocytic condition. • Septa may form during injury or on development of sex organ. • In terrestrial species the plant body remains attached to the soil surface with much branched thread like structure, the rhizoid or branched holdfast called the hapteron. In floating members, the rhizoids are either absent or not well developed.
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  Cell structure The thallusstructure is differentiated into cell wall and protoplasm. The cell wall of thallus is thin, weak and non-elastic. The cell wall is made of two layers, the outer layer is pectic and the inner layer is made up of cellulose. Inner to the cell wall there is thick layer of protoplasm. A very large central vacuole filled with cell sap runs from one end of the filament to another forming a continuous canal or siphon.
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  Cell structure In peripheralpart of protoplasm are present a large number of small oval or disc shaped chloroplasts. The cytoplasm is present between the vacuole and the cell wall. The cytoplasm contains numerous circular or disc-like chloroplasts, nuclei, and chromatophores. Pyrenoids are usually absent in chloroplasts. Many nuclei are scattered in the cytoplasm near the vacuole. The cytoplasm also contains other membrane bound cell organelle such as mitochondria, small vesicles and food is stored in the form of oil.
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  Reproduction Reproduction in Vaucheriatakes place by vegetative, asexual and sexual methods.
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  a. Vegetative Reproduction The vegetative reproduction in Vaucheria takes place by fragmentation.  It takes place due to accidental breakage of the vegetative filament into small fragments by mechanical injury or insect bites.  Septa are formed in the injured region of the filament. The broken fragments, which are detached from the thallus, develop into new Vaucheria plants.
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  b. Asexual Reproduction Theasexual reproduction in Vaucheria takes place by the formation of various types of asexual spores, such as zoospores, aplanospores, and akinetes. 1. By zoospores  Zoospores are formed singly inside the zoosporangium, developed at the apical region of the filament.  The entire protoplasm of the zoosporangium contracts to form oval zoospore. Opposite to each nucleus two flagella are produced making zoospore a multi-flagellate structure.
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  Structure of Zoospore: Thezoospores are large, yellowish-green, ovoid, multinucleate and multiflagellate structures, having small central vacuole and many chromatophores situated between the vacuole and peripheral nuclei. The paired flagella are unequal in length. Fritsch (1948) regarded this kind of zoospore as compound zoospore or synzoospore as a number of biflagellate zoospores have failed to separate from one another.
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  Germination of zoospores: •Before liberation, entire protoplast of the zoosporangium contracts slightly. During liberation a narrow aperture develops at the apex through which the zoospore is liberated. Flagella are formed during liberation. • The zoospores swim in water for 5-15 minutes and germinate without undergoing any significant period of rest. • The zoospores get attached to the substratum, withdraw flagella and secrete thin walls. The chromatophores move outwards and nuclei inwards as in vegetative condition.
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  • The twotube like outgrowths develop in opposite directions. One of the two outgrowths elongates, branches to form colourless lobed holdfast and the other outgrowth forms yellow-green tubular coenocytic filament.
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  2. By Aplanospores The aplanospores are generally formed by terrestrial species. Aquatic species form aplanspores under unfavorable condition of drought.  The aplanospores are non-motile asexual spores formed in special structures called aplanosporangia. Aplanospores are developed singly within aplanosporangium, during unfavorable conditions.  Like zoospores, the aplanospores are developed singly inside the aplanosporangium at the apical side of the aerial hyphae.  During development it also cuts off from rest of the filament by transverse septum. The aplanosporangium is club-shaped, which develops single non-flagellate club-shaped or globose aplanospore.  The aplanospore is liberated by rupturing the apical wall. After liberation the aplanospore germinates into new-filament.
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  3. By Akinetes Akinetes are thick walled structures formed during unfavorable conditions like drought, and low temperature.  The akinetes are formed on the terminal part of lateral branches where protoplasm migrates to the tips followed by cross-wall formation.  These multinucleate, thick walled segments are called akinetes or hypnospores.
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   The akinetesby successive divisions may form numerous thin walled bodies called cysts.  When many akinetes remain attached to the parent thallus, the thallus gives the appearance of another alga Gongrosira. Hence this stage of Vaucheria is called Gongrosira stage.  During favorable Conditions, the akinetes and cysts develop into new thalli.
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  C. Sexual Reproduction:In Vaucheria sexual reproduction is of advanced oogamous type. It takes place by antheridium, the male sex organ and oogonium, the female sex organ. Most of the species are homothallic or monoecious but a few are heterothallic or dioecious.  Structure of Antheridium: The young antheridium is usually green in colour. It contains cytoplasm, nuclei and chloroplasts. The mature antheridia are yellow, may be cylindrical, tubular, straight or strongly curved. The antheridia may be sessile (without stalk) or with stalk. The antheridium grows and becomes high curved structured, its upper part is main antheridium and the lower part is stalk. The nuclei of antheridium get surrounded by cytoplasm and develop into biflagellate, yellow colored, spindle shaped many antherozoids. The antherozoids are
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   Structure ofOogonium: The mature oogonium is a large globular uninucleate structure. The nucleus of oogonium with protoplasm develops into a single egg. The oogonium contains single large, uninucleate egg
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   Fertilization: The oogoniumsecretes a gelatinous drop through a pore near the beak. A large number of liberated antherozoids stick to the drop. Many antherozoids push into the oogonium. The antherozoids strike violently, fall back and push forward again and fall back. Only one antherozoid enters into the oogonium. After its entry the membrane develops at the pore to stop the further entry of antherozoids. The male nucleus increases in size and fuses with the egg nucleus to make diploid zygote.
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  Zygote(Oospore) The zygote secretes3-7 layered wall around it and is now called as oospore. The protoplast accumulates oil droplets. Initially the zygote remains green, but later it becomes red due to breakdown of chlorophyll. It remains within the oogonium and later comes out by decay of oogonial wall. It remains dormant for long period which varies with the duration of unfavourable condition like high temperature, desiccation etc.
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  Germination of Zygote(Oospore): The oospore undergoes a period of rest before germination. During favourable season the oogonial wall disintegrates and the oospore is liberated. The liberated zygote germinates to form new thallus. The nucleus (2n) of zygote undergoes first meiotic division followed by several mitotic divisions thus forming a coenocytic condition. The zygote walls cracks at a point and inner protoplasm elongates, which gradually forms lower rhizoids and an aerial branch like the mother Vaucheria plant. The oospore germinates to make haploid thallus of Vaucheria.
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  Diagrammatic representation oflife cycle of Vaucheria