Reproduction in Paramecium

It reproduces asexually by transverse binary fission and sexually by conjugation.

Transverse Binary Fission:

It is a common method of reproduction in Paramecium which occurs during favourable condition. A fully grown Paramecium is divided into two daughter individuals. Division occurs at the right angle to the longitudinal axis of the body. Nuclear division is followed by the cytoplasmic division.

Paramecium stop feeding and its oral groove disappears. Macronucleus divides by amitosis and micronucleus divides by mitosis. After division, they move towards the opposite end. At the same time, the constriction develops at the middle part of the body which divides cytoplasm into two equal parts. In this way, two daughter Paramecium are produced. One form anterior end called protor and another from posterior end called opisthe. They are of equal size and contains a complete set of cell organelles as in parents. Oral groove and cytopharynx are newly formed. One contractile vacuole goes to protor and another to opisthe. Other two are newly formed.

The whole process is completed within 2 hours and may occur one to four times a day.

Conjugation in Paramecium:

It is a temporary union of two individuals for mutual exchange of genetic materials. It occurs during unfavourable conditions. It involves following steps:

• Two paramecium of opposite mating types come in contact with their ventral surface. Pellicle and ectoplasm at the point of contact degenerate to form a cytoplasmic bridge. This united paramecium is called conjugants.
• Macronucleus is disintegrated and finally disappear.
• Micronucleus undergoes two successive divisions one of which is reduction division. So, 4 haploid nuclei are produced in each conjugant.
• Out of four three nuclei disappear in each conjugant.
• The remaining nucleus divides unequally to produce smaller male pronucleus and larger female pronucleus, but they are genetically identical. Male pronucleus is active and migratory whereas female pronucleus is inactive and stationary.
• Male pronucleus of each conjugant cross the cytoplasmic bridge and fused with female pronucleus to form diploid zygote nucleus.
• The conjugants then separates from each other after 12 to 48 hours of the union. They are called exconjugants.
• Zygote nucleus of each exconjugants divides three times to produce eight nuclei.
• Out of eight four become macronuclei whereas other four become micronuclei of which three disappear.
• Micronucleus then divides with the division of exconjugants to produce two daughter individuals, each with two macro and one micronucleus.
• A micronucleus of daughter individuals again divides with the division of cytoplasm to produce four daughter individuals each with one macronucleus and one micronucleus.

In this way, 8 parameciums are produced as a result of conjugation. They are metabolically active and can undergo repeated binary fission.

Significance of Conjugation:

Nuclear Reorganization:

In conjugation new and metabolically active macronucleus is produced by reorganization of micro nuclear materials.

Rejuvenation:

In conjugation old, weak and defective macronucleus is replaced by new one which can control metabolism growth and the environment. Because of this paramecium is rejuvenated i.e it gains the previous vigority.

Genetic variation:

In conjugation, genetic materials are exchanged between Paramecium of opposite mating types. It brings variation in daughter individuals due to genetic recombination.