Paramecium
Q.10. Describe conjugation in paramecium and discuss its significance. (2007, 08, 17, 18)
Related Questions -
Q. Describe the process of conjugation in Paramecium. (2010)
Q. Write short note on Conjugation in Paramecium. (2019)
Ans. Paramecium undergoes a sexual phenomena which is called conjugation. It is 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.
Mating Types: -
Each species of Paramecium exists in a number of varieties or syngens and within each syngen there are a number of mating types. These mating types are morphologically identical but show physiological differences. In P.caudatum there are 16 syngens each with 2 mating types. P.aurelia is with 14 syngens and 28 mating types. Ordinarily conjugation will not occur between the individuals of the same mating types. An individual of a mating type will conjugate with the individual of another mating type both belonging to the same syngen. Mating type specificity seems to be expressed in chemical charachteristics of the surface membrane allowing contact in conjugation. These have no connection with sex as both are hermaphrodite.
Process of Conjugation: -
In conjugation two individuals or preconjugants from two different mating types of one syngen 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 like the siamese twins, the conjugating pair continues to swim actively and a sequence of completed nuclear changes takes place in each animal.
The vegetative macronucleus simply breaks up into fragments, which are later absorbed by the cytoplasm. The diploid micronucleus of each conjugant first grows in size and then divides by meiosis. Thus four haploid daughter micronucleus are produced of which three degenerate and disappear, while the remaining one divides by mitosis forming two unequal gamete nuclei. The smaller one is the active migratory gamete nucleus and the larger 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.
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 eight nuclei of which four enlarge to become macronuclei and other four become micronuclei. Three micronuclei disintegrate and disappear while the remaining micronucleus divides with the binary fission of the exconjugants. Thus from each exconjugant two daughter Paramecia are obtained each containing two macronuclei and one micronucleus. 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.
Factors and Conditions of Conjugation: -
Conjugation is a very complex process -
(i) 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 species.
(ii) A certain range of light and temperature differing with species is said to be essential for conjugation to occur.
(iii) In P.caudatum conjugation usually starts early in the morning and is continued till after the afternoon.
(iv) The conjugating individuals are usually smaller in size than the normal individuals.
(v) A definite state of nutrition is indispensible sine starved or overfed individuals generally will not conjugate.
(vi) 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 diffrentiation exists in paramecium.
Significance of Conjugation: -
(i) 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, reduce in size, degenarates in organization and eventually dies off. To avoid this senile decay of race, conjugation is resorted to and the process seems to rejuvenate and revive the lost vigour for asexual reproduction.
(ii) 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.
(iii) Hereditary Variations: -
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.
Q.11. Write short note on Reproduction Processes in paramecium. (2013)
Ans. Reproduction Processes in paramecium: - Paramecium multiplies very quickly by transverse binary fission. But this is interrupted at intervals by the so called sexual process- conjugation. Several types of nuclear reorganisations such as endomixis, hemixis, autogamy and cytogamy have also been reported.
1.Binary Fission: -
During favourable conditions, Paramecium multiplies by transverse binary fission. The animal stops feeding and the micronucleus divides into two by an elongated type of mitosis. The two daughter micronuclei move apart towards the opposite ends. Meanwhile the macronucleus elongates and gets constricted into two amitotically. A transverse constriction appears in the middle of body, which deepens gradually dividing it into two equal valves. The oral groove of the parent is retained by one half and the mouth and cytopharynx by other. These, later on, regenerate the essential parts. The contractile vacuole in each half also divides. These now separate and start their free existence. The entire process is completed within 1/2 to 2 hours.
2. Conjugation: -
Conjugation is temporary pairing of two individuals of the same species but from different mating types for the exchange of their nuclear material. It occurs after repeated binary fission and is essential for rejuvenation and continuity of race.
Paramecia ready to pair are sticky and smaller in size. The individuals of two different strains pair with their oral surfaces together. The pellicle and ectoplasm in the region of union degenerate and a protoplasmic continuity is established between the two. These are called conjugants. The following nuclear changes occur in each conjugant simultaneously:
(A) Macronuclear changes: -
Soon after pairing the macronucleus degenerates into fragments and is absorbed in the cytoplasm.
(B) Micronuclear behaviour: -
(a) Simultaneously, the micronucleus undergoes two pregametic divisions, of which first is reduction. As a result four daughter micronuclei are formed each with haploid number of chromosomes.
(b) Three of the four daughter micronuclei degenerate in each conjugant.
(c) The remaining micronucleus divides unequally producing a small active migratory male pronucleus and large and passive stationary pronucleus potentially female. These are comparable to the nuclei of gametes of higher animals.
(d) The migratory male pronucleus of the two conjugants are exchanged so that the male pronucleus of one passes into the other and fuses with the female pronucleus forming the zygote nucleus or synkaryon.
(e) The conjugants now separate and are called exoconjugants.
(f) The synkaryon in each conjugant divides thrice and eight nuclei are formed. Four of them enlarge and form macronuclei, while the remaining four are known as micronuclei.
(g) Three of the four micronuclei disintegrate.
(h) The single micronucleus in each exoconjugant divides twice and each division is accompanied with the division of body. As a result four daughter paramecia are formed from each exoconjugant each with one micro and one macronucleus.