REPRODUCTION IN PARAMECIUM

In case of paramecium, the process of reproduction is of the following types:-

1. Asexual method
2. Sexual method

 

ASEXUAL METHOD:-

In case of asexual method, reproduction in paramecia reproduces by transverse binary fission. This occurs in favorable conditions, i.e., under good condition of food and water etc. The division of the body is transverse i.e., at right angle of the longitudinal axis of the body.

 

• Firstly, the paramecium stops the feeding and its oral groove disappears.
• The micronucleus divides by the complicated process of mitosis, during nuclear division. The nuclear membrane remain intact, the two daughter nuclei towards the opposite ends of the cell.

• 36     o 50 chromosomes appears during mitosis (depending upon the race).
• Macronucleus divides a mitotically by simple elongation of the nucleus and constricted off in the middle.
• In the same time when macro nucleus divides, a transverse constriction forms around the middle of the body.
• The furrow continues to deeper and deeper ultimately dividing the cytoplasm into two halves or daughter paramecia.
• The anterior part called porter and posterior opisthe, before the separation of each half, new parts develops i.e., necessary for complete paramecium.
• Each daughter receives one contractile vacuole form the parent and second one is formed. Oral groove develops in each and buccal structures again constituted. These two daughter paramecia are equal in size with complete organelles. These grow to full size before dividing again.
• The process is completed 20 to 120 minutes depending upon condition of food and temperature. It may occur 2 to 3 times a day in Paramecium candatum and 5 times in paramecium aurelia.

Sexual reproduction takes place by the following methods

CONJUGATION:-

This process is given by the HERTAIG and MAUPAS (1889).

Conjugation is the simple temporary union of two individuals of one and the same species for the purpose of exchanging the part of nuclear material.

Two individual comes in contact ventrally by their oral grooves and attached till the end of process. Pellicle and endoplasm degenerates and form protoplasmic bridge between the two individuals.

Each individual called conjugate or gametocytes. The conjugating pair swim actively. Then a sequence of complicated nuclear change takes place in each conjugate in the following manner-

• Macronucleus breaks up into fragments and lastly absorbed by the cytoplasm.
• The micronucleus of each divides twice, forming four (4) Haploid micronuclei. One division is reduction division.
• Out of four, three degenerates in each conjugation and disappears. Remaining one micronucleus divide unequally forming two pronuclei or gametic nuclei (Genetically identical).
• Out of two, one is smaller is called active male pronucleus and the larger is called stationary female pronucleus in each conjugate.
• The male pronucleus passes from one conjugate to the other through protoplasmic bridge into the female pronucleus.
• Now the fusion of male and female pronucleus takes place forming a diploid zygote nucleus or Synkaryon or amphinucleus. The fusion of two nuclei from two different individuals is called Amphimixis.
• The two paramecia now separate after 12-48 hours and are now called as ex-conjugates. In each ex-conjugate, zygote nucleus divides 3 times forming 8 nuclei.
• Of these four become macronuclei and four become micronuclei.
• Three micronuclei disintegrate and disappears, one remaining micronucleus divides with the binary fission of each ex-conjugates.
• Thus, two daughter paramecia are obtained having two macronuclei and one micronucleus.
• The daughter paramecia again divide, with division forms two individuals. Each containing one macronucleus and one micronucleus.
• Thus, each conjugate produces four daughter individuals.

AUTOGAMY OR AUTOMIXIS :-

Autogamy resembling conjugation, Diller 1936 reported this type of reproduction in Paramecium aurelia. Autogamy takes place in a single paramecium.

 Diller called the autogamy as self-fertilization.

•  Two micronuclei of the same organism fuse together forming a Synkaryon.
• During the process in P. aurelia, the macronucleus grows into an irregular mass and breaks into pieces and lastly disappears.
• The two micro -nuclei divide twice, once meiotically to form 8 haploid nuclei.
• Out of 8, 7 nuclei degenerate and one divide to form 2 sister gamete nuclei. These two are homologous to the stationery and migratory pronuclei.
• These enter a protoplasmic cone, temporarily formed near cytostome and then fuse together to form a zygote nucleus or Synkaryon.
• Synkaryon divide twice two form four nuclei, two of which macronuclei and two become micronuclei.
• The Cell body divides and 2 micronuclei also divides to form 4 nuclei, 2 macronuclei as such.
• Paramecium divides into two daughter individuals   having one macronucleus and two micronuclei.
• The process is completed In about two days involving a readjustment   of the nucleo-cytoplasmic relationship. 

 

CYTOGAMY :-

Cytogamy recently reported by WICHTERMAN in 1940 in P. caudatum. Cytogmy is a process without nuclear exchange. The process resembles conjugation in that two small paramecia (200 µ long) fuse together by their oral surfaces.

• The early nuclear division are similar to the conjugation but there are no nuclear exchange between the two individuals.
• The two gametic nuclei in each parent are said to be fused to form a synkaryon as in autogamy. This process is completed in about 13 hours.
•  The genetical consequence of cytogamy is always homozygocity.

 

ENDOMIXIS :-

 Endomixis is an interesting phenomenon involving total internal nuclear reorganization within a single individual.

WOODRUFT and ERDMANN 1914 reported in bimicronucleate species P. aurelia. It is occurring periodically at regular interval of about 30 days in P. caudatum and 16 days in P. aurelia. It occurs as follows-

 

• Micronucleus disintegrates and disappears.
• Two micronuclei divide twice forming 8 daughter nuclei.
• Out of 8, 6 degenerates.
• The paramecium divides, each individual getting one micronucleus, which divide twice forming 4 nuclei.
• Two nuclei become macronucleus and two micronuclei.
•  Paramecium with 2 macro and 2 micro nuclei divides and at the same time 2 micronuclei divides.
• Two daughter cells obtained, each with one macro nucleus and two micro nuclei. In the end, four daughter cells are formed from single individual.

 

HEMIXIS :-

This process is pointed out by DILLER 1936. It is a simple method of nuclear exchange different from conjugation, binary fission and autogamy in single individual.

During these changes termed Hemixis by DILLER.

• The micronuclei do not participate.
• But the macronucleus throws out 2 to many equal or unequal fragments of chromatin balls (DNA) to be absorbed in the cytoplasm.
• Some of the macronuclei, they behave normally at the cell division.
• The significance of hemixis is not well understood.

 

ENCYSTMENT

 Encystment occurs in green species (P. bursaria). It has not been seen except Michelson’s on P. caudatum. The cysts are said to be looks like sand grains.