Paramecium: Sexual Reproduction and Asexual Reproduction
Posted on : 26-11-2017 Posted by : Admin

 

REPRODUCTION INTRODUCTION

Paramecium reproduces both asexually and sexually. It also undergoes several kinds of nuclear organizations. It can also multiply during nuclear organizations. Various processes of reproduction in Paramecium are listed below:

 

ASEXUAL REPRODUCTION: BINARY FISSION

Binary fission occurs in transverse fashion in Paramecium. It is a common method of reproduction in Paramecium which occurs during favorable 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 stops feeding before initiating binary fission and then 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-Paramecia are produced.

Daughter from anterior end is called protor and another daughter from posterior end is called opisthe. They are of equal size and contain a complete set of cell organelles as in parents. Oral groove and cytopharynx are newly formed in both the daughters. 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.

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SEXUAL REPRODUCTION: CONJUGATION

In Paramecium, Conjugation is a form of sexual reproduction. It is a temporary union of two individuals of same species for mutual exchange of genetic materials. Continuous multiplication by binary fission is interrupted by conjugation as it is necessary for the survival and rejuvenation of the race.

Conjugation inducing factors: The following are the factors which induce conjugation,

• Unfavorable conditions like some degree of starvation, shortage of food, a particular bacterial food, a certain range of light and temperature and certain chemicals induce conjugation.
• Also conjugation is induced after certain number of asexual binary fissions to rejuvenate the paramecium.

Types of mating systems in paramecium: Each variety of the species of paramecium has two mating types which differ in chemical characteristics of their surface membranes. Conjugation can occur only between different mating types of the same variety or syngen.

Conjugation process: The two Paramecium of opposite mating types come in contact by their ventral surface during swimming. They stick together through their oral groove region. Following this attachment their cilia, Trichocysts, feeding apparatus degenerate. Pellicle and ectoplasm degenerate at the point of contact and a cytoplasmic bridge.  This cytoplasmic bridge is also known as protoplasmic bridge or Conjugation Bridge. These united paramecia are called conjugants.

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The conjugants attach with each other for several hours during which complicated reorganization and exchange of nuclear material occurs as described below,

Changes in macronucleus: The macronucleus breaks up into fragments and these fragments are later absorbed into the cytoplasm.

Changes in micronucleus: Lot of complicated changes and divisions take place in the micronucleus.

• The diploid micronucleus in each conjugant grows in size and then divides by meiosis (in meiotic division a single cell gives four haploid daughter cells)
• As a result of meiotic division four haploid daughter micronuclei are produced. Out of these four micronuclei, three degenerate and disappear.
• The remaining one micronucleus divides by mitosis to form two unequal pronuclei or gamete nuclei.  The smaller gamete nuclei is active in migratory in nature and is called migratory gamete nucleus. The larger gamete is passive and stationary in nature and is called stationary gamete nucleus.
• The migratory gamete nucleus of one conjugant passes through the protoplasmic bridge into other conjugant and fuses with its stationary gamete nucleus. This fusion results in the formation of a single diploid zygote nucleus. This zygote nucleus is also known as synkaryon. And this process of complete fusion of two nuclei from two different individuals to form a zygote nucleus is called as amphimixis.
• After the formation of zygote nucleus, the attached conjugants separate and now they are called ex-conjugants.
• The zygote nucleus in each exconjugants divides thrice by mitosis finally forming eight nuclei.
• Four of these eight nuclei grow in size to become macronuclei. The remaining four small nuclei are called micronuclei.
• Out of the four micronuclei, three degenerate and disappear. And the remaining one micronucleus divides mitotically into two. Simultaneously the exconjugants also divide into two daughter paramecia.
• Each daughter paramecia thus obtained from exconjugants has 2 macronuclei and 1 micronucleus.
• A micronucleus of daughter individuals again divides mitotically with the division of cytoplasm to produce four daughter individuals each with one macronucleus and one micronucleus.

In this way, 8 paramecia are produced as a result of conjugation four from each mating conjugant.

 

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 vigor.

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

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SEXUAL REPRODUCTION: AUTOGAMY

W. F Diller in 1936 described Autogamy in Paramecium aurelia. The nuclear changes occurring in both conjugation and autogamy are much similar but nucleus is not exchanged in autogamy. Moreover autogamy occurs within a single individual. Autogamy occurs in the following way,

• Macronucleus grows in size and breaks open into fragments which are absorbed by the endoplasm
• Two diploid micronuclei divide meiotically and produce eight haploid daughter nuclei. Out of these eight, seven disintegrate. (note that P. aurelia consists of one macronucleus and two micronuclei)
• The remaining one nucleus divides by mitosis and produces two nuclei, now called as gamete nuclei.
• A temporary protoplasmic cone develops near the mouth. The two gamete nuclei enter this cone and fuse together to produce a diploid zygote nucleus or synkaryon. This zygote nucleus contains all genes in homozygous condition.
• Zygote nucleus divides twice by mitosis to produce four nuclei.
• Two of these nuclei grow to become two macronuclei and remaining two become micronuclei.
• Each micronucleus and the body of paramecium now divide and produce two daughter paramecia, each with a new macronucleus and two micronuclei.

Significance of autogamy

1. As a result of autogamy, a new macronucleus is formed which rejuvenates paramecium by increasing its vitality.
2. It is a type of sexual reproduction in which self-fertilization takes place and homozygous offsprings are produced.

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SEXUAL REPRODUCTION: ENDOMIXIS

Woodruff Erdmann reported endomixis in Paramecium aurelia. It is one of the methods of nuclear organization. There is no meiosis and no nuclear fusion in this process.

It occurs in single individual in the following way,

• The macronucleus degenerates and disappears.
• The two micronuclei divide twice by mitosis and produce eight nuclei. Six of these eight degenerate.
• The Paramecium divides into two. Each daughter Paramecium contains one nucleus.
• The nucleus divides again twice by mitosis to produce four nuclei. Two nuclei enlarge and become macronuclei and two become micronuclei
• Each micronucleus and the body of Paramecium divide and produce two daughter Paramecia. Paramecium gets one macronucleus and two micronuclei.
• At the end of endomixis four daughter Paramecia are produced from a single individual.

Significance of endomixis

1. As a result of endomixis a new macronucleus is formed which rejuvenates Paramecium.
2. A single individual produces four daughter individuals.

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SEXUAL REPRODUCTION: CYTOGAMY

R. Wichterman in 1940 reported Cytogamy in Paramecium caudatum. Generally cytogamy is less frequent. In cytogamy two Paramecia form a pair and become attached at their oral surface as in conjugation. Early nuclear divisions are similar to that of conjugation but there is no nuclear exchange between the individuals called as cytogamonts. Two haploid gamete nuclei in each of the individual, fuse to form a synkaryon. The individuals now separate and divide as in conjugation. Also a new macronucleus is formed as in conjugation.

Significance of cytogamy

1. A new macronucleus is formed which rejuvenates Paramecium
2. Self- fertilization occurs in this process also and completely homozygous offsprings are produced

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1. Discuss the steps involved in the binary fission of Paramecium.
   
2. How does Paramecium reproduce. List various processess of reproduction.
   
3. Discuss the steps involved in the Conjugation of Paramecium.
   
4. Give the significance of Conjugation and cytogamy.
5. Discuss the steps involved in the endomixis of Paramecium.
   
6. Give the significance of Conjugation and cytogamy.

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