Asexual and sexual reproduction in Paramecium

Asexual reproduction:

  • Paramecium reproduces asexually by transverse binary fission and is found during favorable conditions.

  • Binary fission in Paramecium completes in the following steps:
    1. Paramecium stops feeding.
    2. Oral groove disappears.
    3. The micronucleus becomes spindle shaped and divides mitotically and forms two daughter micronuclei.
    4. The macronucleus changes its shape and through a slit it divides amitotically into two.
    5. The endoplasm is divided into two in the middle of the cell transversely.
    6. The anterior portion is known as proter and the posterior portion is known as opishte.
    7. The proter and opisthe portions are just like that of their parents in size, shape and structure.
    8. The whole process completes within twenty minutes and in 24 hours it divides two or three times.
    9. All such individuals which are formed from a single parent is known as clone.

Sexual reproduction:

  • This type of reproduction is done through conjugation.
  • Conjugation is temporary union between two individuals during which exchange of genetic material takes place.

  • It occurs during unfavorable condition and completes in the following steps:
    1. Paramecium stops its movement and feeding.
    2. Two Parameca meet in the portion of their oral grooves and the pellicle and ectoplasm degenerates.
    3. A new endoplasmic bridge is formed between their buccal apparatus.
    4. These two united Paramecia are known as conjugants.
    5. The macronucleus disintegrates and the micronucleus increases in size and divided mitotically into two parts.
    6. The nucleus of each Paramecium divides again and forms four haploid nuclei.
    7. Out of the four haploid nuclei, three disappear and one persists.
    8. From this single nucleus two nuclei are formed mitotically and distributed into two portions. One is known as stationary or female pro-nucleus and the other is known as migratory nucleus or male pro-nucleus.
    9. The male pro-nucleus passes into another Paramecium through the endoplasmic bridge and fuses with the female pro-nucleus.
    10. Two synkaryons (a cell nucleus formed by the fusion of two pre-existing nuclei) are formed in each Paramecium. They are diploid in nature just like that of their parent cells.
    11. In this way, the heredity materials of both of the Paramecia are transferred to each other.
    12. After the above changes the two Paramecia are separated known as ex-conjugants.
    13. The synkaryon of ex-conjugants divides three times repeatedly and produces eight nuclei. Among them the big four are called macronuclei and the small four are called micronuclei. Out of four micronuclei, three disappear.
    14. Later one micronucleus which was left divides into two. So in each dividend Paramecium one micro and two macronuclei are present.
    15. The micronuclei of two daughter Paramecia divides into two and later the daughter Paramecium also divides mitotically into two.
    16. In this way, eight Paramecia are formed after conjugation of conjugants.

Importance of conjugation:

  • After continuous binary fissions the micronucleus of Paramecium becomes very weak. So conjugation helps in the rejuvenation of it.
  • The mutual transfer of hereditary materials brings the new daughter nuclei, new vigor and vitality.
  • Due to the repeated divisions, the macronucleus also becomes very weak. So by this process it is also rejuvenated, and this helps to store energy and strength in Paramecium.

Asexual and sexual reproduction in Paramecium