Important types of reproduction in paramecium are as follows:
(1) Binary fission:
It occurs always transverse i.e., perpendicular to the long axis of body. Prior to it, the animal stops feeding and becomes spindle-shaped.
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Micronucleus elongates and begins to form an intra nuclear spindle in preparation of mitosis. The nuclear membrane remains intact throughout the process.
As the prophase stage of mitosis begins, 36 to 150 (according to race) duplicated chromosomes appear in the nucleus.
In metaphase, the chromosomes get arranged at the equator of the nucleus to form metaphase plate.
In anaphase the micronucleus further elongates and the daughter chromosomes or chromatids move towards opposite poles. In telophase, the micronucleus further streches and breaks into two daughter nuclei.
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Simultaneously, the macronucleus divides amitotically by elongation and constriction. Two oral grooves now begin to form, one in the anterior half and the other in the posterior half.
Two original contractile vacuoles remain intact, one in each half of the dividing parent individual. Two new contractile vacuole form later and two new buccal structures also appear.
In the mean time, a constriction furrow appears near the middle of body. It deepens and ultimately the cytoplasm is completely divided resulting into two daughters Paramecium. The anterior Paramecium is called proter and the posterior, one opisthe.
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They grow to full size and divide again by fission. Binary fission takes about half an hour. In favourable environment when food is plenty, Paramecium multiplies rapidly by successive fission, once, twice or even thrice per day, producing a series of genetically similar descendants called clone.
After some time, rate of fission declines and may eventually stop, indicating ageing. Presumably a clone can recover from ageing i.e., it rejuvenates, by reproducing sexually.
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(2) Conjugation in Paramecium:
Conjugation is frequently referred to as sexual reproduction, but it is simply a temporary union of two individuals of same species for exchanging a part of their micronuclear material. It occurs frequently between binary fission for r ejuvenation.
Process of conjugation:
Two individuals or pre-conjugates, from two different mating types (but same species) fuse together ventrally through their oral region.
They stop feeding and their buccal structures start to 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 conjugants.
The union immediately induces a series of nuclear changes in both conjugants. The vegetative macronucleus breaks and is absorbed in the endoplasm.
The micronucleus of each conjugant’s first grows in size and then divides by meiosis. Of the resulting four haploid nuclei, 3 degenerate or become pycnotic and the remaining one produce 2 unequal pronuclei or gamete nuclei by mitotic division.
The smaller one is the active migratory gamete nucleus and the bigger one is the passive stationary gamete nucleus.
Factors and conditions of conjugation:
Conjugation is a very complex physiological process.
Following factors and conditions have been studied for conjugation to take place, but they vary with the species:
(i) Starvation or a particular bacterial diet is 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.
In P. caudatum conjugation usually starts early in morning and is continued till afternoon.
(iii) Conjugants are usually smaller in size (210 jj long) than the normal individuals (300-350 u long).
(iv) Maupas maintains that individuals must have passed through a desirable number of asexual generations (period of immaturity) before they become sexually mature and conjugate.
(v) Conjugants should be of different mating type.
(vi) Agglutination favours conjugation. It is the interaction between the mating type proteins which are localized in cilia of conjugants.
Significance of Conjugation:
The significance of conjugation has been much discussed but exactly it still remains uncertain. The following effects are attributed to the process:
(a) Rejuvenation:
By reproducing asexually for several generations, the Paramecium loses its vigour and enters into a period of depressed physiological efficiency and a stage of senescence.
To avoid this ageing Paramecium resorts to conjugation to rejuvenate and revive the lost vigour for asexual reproduction.
However Woodruff and Jennings do not support the view that conjugation helps in rejuvenation.
(b) Nuclear reorganization:
During conjugation the nuclear apparatus is reorganized and a part of genetic material, the conjuants exchange between them Macro-nucleus is newly formed and it brings renewed vigour and vitality to accelerate the metabolic activities.
(c) Hereditary variation:
The exchange of nuclear material between conjugants provides the scope for parameicum to have genetic variations.
(3) Autogamy:
W.F. Diller (1936) described a process of nuclear reorganization in P. aurelia, that takes place within a single individual. He called it autogamy or self-conjugation.
Process of autogamy:
During autogamy in P.aurelia the 2 diploid micronuclei divide by meiosis to form eight haploid daughter nuclei.
Seven of them disintegrate, while the remaining single haploid micronucleus undergoes a mitotic division forming 2 gamete nuclei.
Mean- while, the macronucleous grows, disintegrates and is absorbed in cytoplasm. The two gamete nuclei enter a protoplasmic cone temporarily formed near cell mouth and then fuse together to form a completely homozygous diploid zygote nucleus or synkaryon.
This divides twice to yield 4 nuclei, 2 of which become macro-nuclei and 2 micronuclei. The cell body and micronuclei then divide to form two daughter individuals, each with a new macronucleus and 2 micronuclei. Autogamy also rejuvenates Paramecium. Genetically autogamy always results in homozygosity.
(4) Cytogamy:
In 1940, R. Wichterman reported a sexual process in P. Caudatum without exchange of nuclear material which is termed as cytogamy like conjugation, in this process two small paramecia (200 plong) temporarily fuse by their oral surface.
The early nuclear divisions are also similar to those of conjugation, but there is no exchange of nuclear material between the individuals (Cytogamonts), But two haploid gamete nuclei in each individual are said to fuse to form a synkaryon, as in autogamy. The process is completed in about 13 hours.
(5) Endomixis:
Endomixis is an interesting phenomenon involving a total internal nuclear reorganization within a single individual in a culture of a pedigreed race of Paramecium.
It takes place in the absence of conjugation. Woodruff and Erdmann in 1914 first of all reported endomixis in P. aurelia, occurring periodically at regular intervals of about 30 days.
Process of Endomixis:
The vegetative macronucleus degenerates and disappears while the micronuclei divide thrice mitotically, produce 8 daughter nuclei of which 6 degenerate.
At this stage Paramecium also divides, each daughter receiving one micro nucleus. This micronucleous divides twice forming 4 nuclei, 2 of which become macronuclei and 2 micronuclei, in each individual.
The micronuclei again divide with the binary fission of Paramecium into two daughters, each getting one macronucleus and 2 micronuclei.
Thus 4 daughters are produced from a single parent bringing about an intracellular nuclear reorganization and readjustment between the cytoplasm and the nuclear apparatus in each individual.