Conjugation In Paramecium
Reproduction in paramecium occurs by binary fission and conjugation. Due to repeated binary fissions, there is a decrease in the vigour ad vitality of Paramecium. To rejuvenate the lost vigour, conjugation takes place.
Conjugation: Temporary union between two paramecia of different clones for the exchange of nuclear material and multiplication is called conjugation.
During t he process of conjugation two senile paramecia from two different mating types come together and closely appose one another. They attach at the region of cytopharynx by their ventral surfaces. The individual participating in this act are called the conjugants. The pellicle at the site of the union disintegrates and the cytoplasm of the two conjugants merges with each other.
The macronuclei of both the conjugants disintegrate slowly. The micronucleus of each conjugant divides meiotically to produce four haploid nuclei. Of the four haploid nuclei formed three from each conjugant disintegrate so that one nucleus remains in each. The remaining nucleus in each conjugant divides unequally to form a small migratory pronucleus and a large stationary pronucleus. Now the migratory pronucleus of one conjugant passes into the other conjugant and fuses with stationary pronucleus of that conjugant.
Thus a diploid synkaryon or conjugation nucleus is formed in each conjugant. The two conjugants now separate. Each separated individual is called exconjugant. The synkaryon of each exconjugant divides thrice by mitosis so that each exconjugant has eight nuclei. Of these eight nuclei four enlarge in size to become macronuclei. The remaining four behave as micronuclei.
Three of the four micronuclei disintegrate so that each exconjugant contains four macronuclei and one micro nucleus. Then each exconjugant along with its micro nucleus divides in such a way that each of the resulting individuals has two macronuclei and one micro nucleus. Again these individuals divide so that the final daughter nucleus has one macro nucleus and one micro nucleus. These last two divisions are mitotic divisions. By the end of conjugation eight daughter paramecia form both the conjugants with rejuvenated vigour are formed.
Significance: The conjugation is a method of nuclear reorganization. It restores the lost vigour and vitality due to repeated binary fissions. Due to the exchange of nuclear material between the members of the different clones, it results in hereditary variations.