Paramecium caudatum

Systematic Position

Phylum :        Protozoa

Subphylum:  Ciliophora

Class:            Ciliata

Genus :         Paramecium

Species:       caudatum

Occurrence

Paramecium caudatum is one of the most common species of paramecium having worldwide distribution. It is found in freshwater ponds, pools, ditches, streams, rivers, lakes, reservoirs, etc. It is usually abundant in those waters which contain a great deal of decaying organic matter. It thrives well in ponds or slowly running streams containing aquatic plants.

External Structure

1. Size: Paramecium is a microscopic, elongated organism which is visible to the naked eye as a whitish or greyish spot. Its species vary in length from 80µ to 350µ. P. caudatum, the largest species, measures between 170µ and 290µ. The greatest diameter of the cylindrical body is about two-third of its entire length.
2. Shape: Paramecium is slipper shaped. Its shape is constant and is asymmetrical. Because of its slipper-like shape it is sometimes called the slipper animalcule. The body is elongated blunt and rounded at the anterior end and somewhat pointed at the posterior end. In cross section, it is circular with greatest diameter behind the centre of body. The anterior half of body is slightly twisted. The body, is distinguished into an oral or ventral surface and an aboral or dorsal surface.
3. Oral groove: Ventral surface of body bears a prominent, oblique and shallow depression, called oral groove. It originates from the middle of body and extends to the left side of anterior end. Posteriorly, the oral groove leads into a deeper conical vestibule which in turn communicates with a buccal cavity having a basal mouth or cytostome.
4.  Pellicle: External covering of body is a living, clear, firm and elastic cuticular membrance, the pellicle. When observed under light microscope, the pellicle appears to be a regular series of polygonal (or hexagonal) depressions with their raised rims. A single cilium emerges out from the middle of each polygonal space.
5. Cilia: The entire body surface is covered by numerous, tiny, hair-like fine projections, called cilia. These measure 10-12µ in length one cilium arises from the centre of each polygonal depression (circumciliary space) of- pellicle. There are 10,000 to 14,000 cilia covering the whole-body surface. These motile organelles are arranged in regular longitudinal rows. They remain equally distributed though out body surface. Their length is uniform throughout, except for a few longer cilia at the extreme posterior end of the body, forming a caudal tuft, hence the species name caudatum is given.

Internal Structure:

1. Cytoplasm: - Inside pellicle, the cytoplasm of body is clearly differentiated into two regions.

(a) Ectoplasm: The narrow, peripheral, clear and dense region is called the ectoplasm. It consists of the structure of the infraciliary system and the trichocysts.

(b) Endoplasm: The large, central, granular and semi-fluid region is the endoplasm. It consists of the usual cell components like mitochondria, Golgi bodies, ribosomes, reserve food granules, etc. Prominent endoplasmic inclusions are nuclei, contractile vacuoles and food vacuoles.

2. Infraciliary system: Immediately beneath the pellicle the infraciliary system constituted        by the basal bodies and kinetodesmata is present.

(a) Basal bodies: The base of each cilium is produced into a tube-like structure, called basal body or kinetosome. The basal bodies are self duplicating units and progenitors of new cilia. Each basal body is either a centriole or its derivative.

(b) Kinetodesmata: Associated closely with basal bodies of cilia and lying in the ectoplasm is a system of specialized striated fibrils, called kinetodesmal fibrils. A single fibril or kinetodesmos arises from the kinetosome or basal body of each cilium and runs anteriorly somewhat tapering along the, course.' It joins its counterparts from the posterior kinetosomes, forming a bundle of overlapping longitudinal fibrils, called kinetodesma (pleural, kinetodesmata). The number of fibrils in each kinetodesma remains constant (5) because the individual fibrils do not run anteriorly farther than 5 basal bodies. It has been suggested that fibrils coordinate ciliary beat and movement, but the evidence is very conflicting.

The kinetosomes of a longitudinal row. plus their kinetodesmata constitute a structural unit, called the kinety. A kinety system is apparently characteristic of all ciliates.

3. Trichocysts: Trichocysts are peculiar rod like or oval organelles present throughout the ectoplasm alternating with basal bodies and oriented at right angles to the body surface. These are very small in size, measuring about 4µ in length. Each trichocyst consists of an elongated shaft and a terminal pointed tip, called the spike or barb, covered by a cap. The matrix of shaft consists of a dense mass of a fibrous protein, called trichinin.

Function of trichocysts is not well known. It is believed that these discharges and anchor the animal to a firm substratum when it feeds upon bacteria. Others believe that these are organelles of defence.

Discharge of trichocysts is triggered by mechanical, chemical or electrical stimulation. It occurs. in a span of a few milliseconds. When fully discharged, the shaft becomes a long cross-striated rod and measures about 40µ in length.

4. Nucleus: Paramecium is heterokaryotic as it possesses two types of nuclei. In P. caudatum, there is a large macronucleus and a small micronucleus. Besides the macronucleus, two micronuclei are present in P. aurelia and many in P. multimicronucleatum.

(a) Macronucleus: The macronuclens is roughly kidney-shaped and with inconspicuous nuclear membrane. It is polyploid and possesses many nucleoli and much more chromatin material (DNA). Macronucleus is the somatic or vegetative nucleus and controls the day-to-day metabolic activities of the cell.

(b) Micronucleus: The micronucleus is present in a depression on the surface of the macronucleus. It is usually spherical, with a nuclear membrane and with diploid number of chromosomes.

5. Contractile apparatus: In Paramecium, there are two contractile vacuoles, occupying somewhat fixed positions in endoplasm. One vacuole lies near each end of body, close to the dorsal surface. Each of them is surrounded by a circlet of 6 to 10 long, narrow, spindle-shaped radial canals extending far into cytoplasm. Each contractile vacuole opens to outside through a permanent pore in pellicle of dorsal side of body. The two contractile vacuoles do not lose their identity when water is expelled.

Each radial canal consists of terminal part, ampulla and injector canal. The radial canal collects liquid from large part of body and pour it into vacuole. When vacuole attain maximum size, it collapses discharging its contents to the outside. It is osmoregulatory and excretory in nature.

6. Food vacuoles: Numerous non-contractile food vacuoles, or gastrioles are present moving with the streaming endoplasm (cyclosis). They differ in shape and size according to the nature of ingested food particles, but mostly they are rounded in form.

7. Oral apparatus: In Paramecium, oral groove leads ventrally and posteriorly as a tubular structure, called vestibule. It leads directly into a wide tubular passage, the buccal cavity. In its turn, it opens into a narrow gullet or cytopharynx through a narrow aperture, the cytostome. The cytopharynx, at its proximal end, forms a food vacuole.

8. Cytopyge: Near posterior end of body, a little behind cytostome a small portion of ectoplasm and pellicle is somewhat weak. Here, at the time of egestion, a minute aperture called cell anus, cytopyge or cytoproct, is visible. It is, however, difficult to say whether it is a permanent opening with tightly closed lips or a temporary opening formed at the time of egestion.

Reproduction:-

Parameium reproduces asexually by transverse binary fission and also undergoes several kinds of nuclear reorganizations, such as conjugation, endomixis, autogamy, etc. Under certain conditions of food and temperature, it undergoes encystment.

1. Transverse binary fission

            During favourable conditions, Paramecium commonly reproduces by transverse or horizontal binary fission. During it paramecium stops feeding and its oral groove and buccal structures begin to disappear. While this is happening, the micronucleus starts dividing by the complicated process of mitosis, into two daughter micronuclei. The daughter micronuclei then separate. Simultaneously, the macronucleus divides amitotically by simply becoming -elongated and constricted in the middle. Two oral grooves now begin to form, one in the anterior half and the other in the posterior half. Two original contractile vacuoles remain, one in each half of the dividing parent individual. Two new contractile vacuoles are later formed. Two new buccal structures also appear. In the meantime, a constriction furrow appears near the middle of body. It deepens and ultimately the cytoplasm is completely divided, resulting into two daughter paramecia. Of the two daughter paramecia, the anterior one is called proter and the posterior, opisthe. These grow to full size and divide again by fission.

P. caudatum divides 2-3 times in. a day by binary fission. The process is completed in about 30 minutes, though separation of daughter paramecia takes about one hour or more. The term clone refers to all the individuals that are produced asexually from one parent paramecium. All the members of a clone are genetically alike.

2. Conjugation

Paramecium undergoes a sexual phenomenon, which is called conjugation. It is frequently referred to as sexual reproduction, but it is simply a temporary union of two individuals of one and the same species for the purpose of exchanging a part of their micronuclear material. This remarkable process in Paramecium occurs frequently between binary fissions and' is necessary for the continued vitality of the species.

1. Process of conjugation: In conjugation, two individuals or preconjugants, from two different mating types, come in contact ventrally and unite by their oral grooves. They stop feeding and their buccal structures disappear. The pellicle and ectoplasm degenerate at the point of contact and a protoplasmic bridge is formed between the two individuals, which are now called the conjugants. While so united, the conjugating pair continues to swim actively and a sequence of complicated nuclear changes takes place in each animal.

The vegetative macronucleus simply breaks up into fragments and disappears.  The diploid micronucleus of each conjugant first grown in size and then divide by meiosis. Thus, 4 haploid daughter micronuclei are produced of which 3 degenerate and disappear in each conjugant, while the remaining one divides by mitosis forming 2 unequal pronuclei or gamete nuclei. The smaller one is the active migratory gamete nucleus and the bigger one is the passive stationary gamete nucleus. The migratory nucleus of one conjugant then passes through the protoplasmic bridge into the other individual and fuses with its stationary nucleus, forming a single diploid zygote nucleus or synkaryon. The complete fusion of two nuclei from two different individuals forming a zygote nucleus is termed amphimixis.

The two pairing paramecia, after a union of about 12 to 48 hours, separate and are now called exconjugants. In each exconjugant, the zygote nucleus divides by mitosis three times in rapid succession producing 8 nuclei, of which 4 enlarge to become macronuclei and other 4 become micronuclei. Three micronuclei disintegrate and disappear, while the remaining micronucleus divides, with binary fission of exconjugant. Thus, from each exconjugant two daughter paramecia are obtained, each containing two macronuclei and one mincronuclei. The micronucleus again divides with the division of each daughter paramecium, forming two individuals each containing one macronucleus and one micronucleus. Thus, each conjugant produces four daughter individuals at the end of conjugation.

2. Factors and conditions of conjugation. Conjugation is very complex physiologically. The factors and conditions governing conjugation are several and these may also vary with the species.

1. Conjugation does not occur under favourable living conditions. Starvation or 

    shortage of food and a particular bacterial diet or certain chemicals are said to

    induce conjugation in some special.

2.  A certain range of, light and temperature is said to be essential, for conjugation.

3. In P. caudatum, conjugation usually starts early in morning and is continued till 

    afternoon.

4. The conjugating individuals are usually smaller in size than the normal individuals.

5. Conjugation never takes place among the members of a "pure line", that is among     the descendants of a single individual. It occurs only between individuals belonging     to two different mating types. Thus, a sort of physiologically sexual differentiation exists in Paramecium.

3. Significance of conjugation: The significances of conjugation are:

(a) Rejuvenation. If binary fission continues repeatedly for several generations, the Paramecium loses its vigour and enters upon a period of depressed physiological efficiency and senescence. The individual ceases to multiply, reduces in size, degenerates in organization and eventually dies off.

b) Nuclear reorganization. During conjugation, the nuclear apparatus is reorganized and a readjustment occurs between it and the cytoplasm, Probably the macronucleus loses its potentialities in performing its manifold metabolic activities. Its replacement by a new macronucleus brings renewed vigour and vitality to accelerate the metabolic activities. '

C) Hereditary variation. During asexual reproduction by fission, the hereditary material of the parent passes unchanged on to the progeny, so that all the descendants of one Paramecium have the same inheritance. The periodic occurrence of conjugation, however, ensures inherited variation. It brings about, the blending of two lines of ancestry just as bisexual reproduction does.

4. Genetic consequences of conjugation: If conjugation takes place between two paramecia, one homozygous for a dominant gene (AA) and the other homozygous for its recessive gene (aa), the first generation would be heterozygous (Aa). If the two conjugants are already heterozygous (Aa), then the resulting progeny would be either homozygous or heterozygous, depending upon which gene gets eliminated at the stage of disintegration of three micronuclei in each conjugation.