Q. 4. Give an account of the reproductive processes in Paramecium.
Or
Give an account of asexual and sexual reproduction in Paramecium.
Ans.4. Reproductive 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.
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.
(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.
(C) Significance of conjugation : Conjugation is an important process nuclear reorganisation and nuclear exchange recurring in between the sexual reproduction. To some extent, it resembles the sexual reproduction of higher animals, the end products of this process are eight daughter paramecia formed after the asexual multiplication of the exconjugants. The sexual process ends with the fusion of two gametic nuclei.
Therefore, the process is mainly concerned with the exchange of nuclear material and formation of new macronucleus. It is, therefore, treated as an episode in reproduction which leads to the following important events:
1. Rejuvenation: It has been found that individuals cannot continue to multiply indefinitely by asexual methods. After a definite number of asexual generations the rate of fission declines. The individual starts losing its vigour and physiological efficiency, gives an unhealthy appearance and ultimately dies. The conjugation is the only prospective measure to regain the former vigour, to revive its healthy organisation, and thus avoids the senile decay of race.
2. Nuclear reorganization: The macronucleus is formed of trophochromatin which governs all the physiological activities of the individual. Due to repeated fissions its potentiality to coordinate the life processes ceases and the individual enters a state of depressed physiological activities. This old and decaying macronucleus is replaced by a new one during conjugation. This brings about a renewed vigour and vitality:
3. Heredity variations: The fusion of pronucleus facilitates exchange of nuclear material. This ensures the new combinations of heritable characters from two different individuals and better adaptability to the new conditions of life.
Factors Responsible for Conjugation
1. It does not occur under favourable conditions. Shortage of food and a particular bacterial diet or certain chemicals induce conjugation.
2. A certain range of temperature and light, different for different species is essential for conjugation.
3. The conjugating individuals are usually smaller in size than the normal individuals.
4. A definite state of nutrition is necessary for conjugation.
5. A definite number of binary fissions is necessary for paramecia to conjugate.
6. Pairing conjugants are isogamous and there is no morphological sexual dimorphism into male and female conjugants.
7. Conjugation takes place between individuals of different strains and mating types.
8. Agglutination favours conjugation. It is the interaction of mating type substances localised in cilia.
BSc 1st Year Lower Non-chordates Paramecium Sample Model Practice Question Answer Papers