, Order: Volvocales
Genus: Chlamydomonas algae كلاميدوموناس
Chlamydomonas has 150 species. It is the commonest unicellular Volvocales. It is one of the simplest unicellular widely distributed algae. It is found in standing water of ponds, pools, ditches and on moist soil. It often grows in abundance in water rich in ammonium compounds. The turbid green colour of the water of .he stagnant ponds is due to the presence of thousands of these rants. Some of the species occur in very unusual places. inlamydomonas ehrenbergi occurs in saline water. Some of its species are found in sea. A bright red pigment is often abundant in certain species, especially in The resting stages. Such plants make the pools red.
Structure of Chlamydomonas
Its vegetative stage has very simple structure. Its size is about 0.02 mm. Each cell is spherical, ellipsoidal, sub-cylindrical or pyriform. Their anterior end is more or less pointed. It has following structure: 1. Cell wall: There is a thin cell wall on the outside. It occasionally
possesses an outer thin mucilage sheath. This sheath projects into a beak-like process at the anterior end.
Fig: cell structure of ChlamYdomonas
- Flagella: The two flagella situated anteriorly near the pointed. They either project through one aperture in the wall or through two separate canals. These flagella arise form two basal granules called blepharoplast.
- Contractile vacuoles: Each cell typically possesses two contractile vacuoles at the base of flagella. They have respiratory and excretory functions.
- Eye spot: An orange-red pigment spot or eye-spot (stigma) is present near the origin of flagella. This eye spot is covered by a minute lens in the thickness of the cell wall. It supposed to function as an extremely primitive eye. This is sensitive to light. Therefore, it directs the movement of the individual. In C. pluristigtna, there are two or three eye-spots. Eye spots are very sensitive to intensity of light.
- Chloroplast: There is a large cup-shaped chloroplast towards the broader end. The chloroplast varies in shape. In C reticulate, g is reticulate. In C. aplina, it is small discoid. The plants are autotrophic. They prepare their own food by photosynthesis. by taking carbon dioxide from the water.
- Pyrenoid: Pyrenoid is embedded in the chloroplast. Pyrenoid consists of a central protein body surrounded by numerous minute starch grains. The number of pyrenoids is variable in different species. There may be two to any or no pyrenoid at all.
- Nucleus: All species are uninucleate. A single nucleus lies in the cytoplasm, filling the cup of the chloroplast.
Both asexual and sexual reproductions occur in Chlamydomonas. Asexual reproduction
Asexual reproduction takes place by the following methods:
I. Zoospore formation: Zoospores are formed when the
conditions are favourable. During the formation of zoospores the cell becomes quiescent (nonmotile). Its flagella are retracted or discarded. The contractile vacuoles disappear. Its protoplast divides longitudinally into two. It is followed by a simultaneous division of each daughter protoplast and sometimes by a third series of division. Each division of the protoplast is preceded by the division of the nucleus into many parts. Thus each cell produces 2-16 pieces. Each piece secretes a wall around it and forms two flagella. Contractile vacuole and pigment spots also soon appear. In this way 2-16 swarmspores or zoospores are formed within the parent cell. The zoospores or swarmspores are liberated by gelatinization or by the rupturing of the patent cell wall. Each of them develops into a new Chlamyclomonas plant. This process of division is repeated every 24 hours. Thus by the end of week, from one parent plant about 2,000,000 individuals are formed.
- Aplanospore formation: Aplanospores are formed under conditions of drought. The zoospores instead of being liberated found up and develop into aplanospores. Aplanospores germinate directly or divide to produce zoospores on the approach of favourable conditions.
- Palmella stage: This stage is formed under less favourable, but not very dry conditions. In this condition, the ponds are gradually drying up. So the plants are growing on damp soil. The daughter cells are produced by the division of the parent cells. These cells do not develop flagella. They are embedded in the mucilage formed by the gelatinization of the parent cell wall. The daughtei’cells divide further into four. Their cell walls also become mucilaginous. Thus a large number of small non-motile structures are formed. They are embedded in jelly-like substance. In this way, an amorphous colony is formed. It has hundreds or thousands of cells. All its cells are embedded in a common gelatinous matrix. This stage is known as the palmella stage. It ;s named so because the older phycologists thought it to be a species of an alga named Palmella. This is however, a temporary phase. Soon all small daughter cells develop flagella and become motile.
Sometimes, cells of the Palmella stage develop into thick-walled hypnospores. These are red coloured due to the presence of a red pigment called the haematochrome.
The sexual reproduction may be isogamy to anisogamy and oogamy. At the time of sexual reproduction, the protoplast of a cell divides into 16,32 or even 64 biflagellate gametes. The gametes may be naked called gymnogametes. Or they may be enclosed in a cell wall called calyptogametes. The walls of calyptogametes may be left behind as the gametes fuse in pair or they may be retained.
I. Isogamy: In this case, the fusing pairs of gametes are naked and equal in size. It occurs in Creinhardi and C.Iongistigma.
- Anisogamy: In this case, fusing pairs are similar in shape but different in size. The female cell produces four larger macrogametes. The male cell produces eight smaller microgametes. It occurs in C.monocia, C. Braunii etc.
- &natty: In this case, the fusing gametes are different both in size and shape Ooaamy occurs in Ccorcifera and C.00gonium. rhe frmale call functions as oogonium. It produces a large macrogamete or egg. The male cell functions as antherozoid. It produces 8, 16 or 32 small spherical biflagellate microgametes or sperms. The egg is not completely non-motile. It is surrounded by a number of motile male gametes or sperms.FertilizationThe flagella disappear during fertilization. In some species, flagella may persist. Therefore, quadriflagellate zygote may remain motile for more than fifteen days. Then it secretes a wall. Sexual process is chemically regulated. Fusion occurs between the gametes of different cell types.Germination of zygospore
- The resting zygospore or zygote secretes thick wall. It becomes red as the chlorophyll is masked by haematoohrome. Its starch is converted into fats.
- Before germination the protoplast of the zygote becomes green. Its nucleus divides twice followed by the division of the protoplast into four parts.
- Each uninucleate piece of protoplast is metamorphosed into a biflagellate zoospore. Thus four zoospores are formed.
- They are liberated in water by the rupturing of the zygote wall. Meiosis takes place during the formation of zoospores from the zygote. They zygote is diploid. But the normal vegetative cells are haploid.
Origin of differentiation of sex in Chlamydomonas
Chlamydomonas shows origin and differentiation of sex. The zoospores and gametes are morphologically similar. They differ only in size and behaviour. This difference occurs due to greater number of divisions of the parent cells during gamete formation. The small individuals thus produced lose the power of independent existence. Thus the zoospore takes the course of fusion. It is the first step towards the establishment of the sexual process in algae. Differentiation in the size and mobility of the gametes produce further evolution in sex i-e, Isogamy, anisogamy and oogamy.