In this article we will discuss about the vegetative body and reproduction of chlamydomonas.
Vegetative Body of Chlamydomonas:
The plant body is unicellular and the cells are either spherical, ellipsoidal or pyriform (pear-shaped). The protoplast is surrounded by a definite layer of cellulose wall, and in some cases an additional external gelatinous sheath may be present Each cell is provided with two flagella, which are inserted at its anterior end and are very close to each other. In some cases, the cell wall is provided with a distinct papilla at the anterior end.
The major portion of the protoplast is usually occupied by a single, massive, cup-shaped chloroplast. There is a single nucleus which is, in most cases, embedded in a colourless cytoplasm filling the cup. Sometimes the chloroplast may be star-shaped or laminate in form. Usually, a single pyrenoid is present in each chloroplast, but in some cases two or more pyrenoids may be present, or they may be totally absent.
Typically, there are two contractile vacuoles at the anterior end of the protoplast occupying the bases of the flagella, but the number and position of these vacuoles may vary in different species. There is a photo-receptive organ, the ‘eyespot’, which lies at the anterior end and is supposed to be concerned in directing the movement of the flagella.
Reproduction in Chlamydomonas:
Chlamydomonas reproduces by three methods:
(a) Isogamy:
During asexual reproduction, the protoplast of each vegetative cell divides longitudinally into two protoplasts, which again divide and re-divide forming 4 or 8 daughter protoplasts. Each daughter protoplast then secretes a wall around it and develops a pair of flagella at its anterior end. The cell wall of the parent cell ruptures or undergoes gelatinization and thereby the daughter cells are liberated.
Very rarely the protoplast of a vegetative cell contracts rounds up and forms an aplanospore after secreting a wall of its own.
When chlamydomonas grows on damp soil, the daughter cell formed by the successive divisions of the protoplast do not develop the flagella but remain embedded in a common gelatinous matrix formed by the gelatinization of the cell wall of the parent cell. This is usually followed by division and re-division of the daughter cell by the subsequent gelatinization of their walls forming colonies consisting of hundreds of cells embedded in a common matrix.
This is known as the ‘palmella stage’ resembling the genu Palmella, a green alga forming amorphous colonies. lf the colonies be flooded with water, each daughter cell may develop its flagella and may swim away from the colony. Sometimes these daughter cells may develop into resting spores, called akinetes. Most species of chlamydomonas reproduce sexually.
(b) Anisogamy:
The method of formation of gametes is similar to the formation of the daughter cells from a vegetative protoplast. But in this case, the number of daughter cells may be 8, 16 or 32, and they are usually naked, biflagellate protoplasts; when walled, the protoplast of each gamete leaves its original wall before union. Union may take place between the gametes derived from the same cell (homothallic) or between gamete derived from different cells (heterothallic) Where fusing gametes are morphologically alike (isogametes), the process is called isogamy.
During the process, two isogametes meet each other and they are entangled with their flagella. Then the gametes fuse end to end or side to side and a quadriflagellate structure is formed. Then the cytoplasm and nuclei fuse to form a zygote. Isogamy is found in Chlamydomonas moewusii.
Anisogamy is evident in Chlamydomonas braunii, where the smaller gamete or microgamete is more active than the macrogamete—the larger gamete. The two gametes unite by their anterior side. Then their flagella disappear. The fusion of cytoplasm along with nucleus takes place and zygote is formed.
(c) Oogamy:
Chlamydomonas coccifera shows oogamous type of sexual reproduction. In this case, the female gamete or macro- gamete is larger. During its formation, the vegetative cell enlarges and its flagella disappear. The male gametes or spermatoziods are smaller and elongated. Each of them bear two flagella. The male gamete fuses with the female gamete to form zygote.
The zygote after its formation, it comes to rest and secretes a wall around it. The wall of the zygote is thick and shows surface reticulations, or may bear spiny projections. Before germination the diploid nucleus of the zygote undergoes reduction division, typically forming four haploid nuclei.
By cleavage process uninucleate protoplasts are formed following reduction division and each protoplast is metamorphosed into a biflagellate daughter cell. These four cells are finally liberated by the rupture of the zygote wall.
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