The following article will guide you about how do Paramecium reproduces asexually by transverse binary fission and also undergoes several types of nuclear reorganization.
Transverse Binary Fission:
During favourable conditions, the paramecium reproduces by transverse binary fission. Before division, the animal stops feeding and the oral groove and buccal structures begin to disappear. The micronucleus divides by mitosis into two daughter micronuclei which move to the opposite ends of the cell. The macronucleus elongates and divides transversely by amitosis.
Another cytopharynx is budded off and two new contractile vacuoles appear. One near anterior end and another near the posterior end while the two original vacuoles retain the original position. In the meantime, a constriction-furrow appears near the middle of the body and deepens till the cytoplasm divides into two and the two daughter paramecia are almost of equal size, each containing a set of organelles. They grow to normal size and repeat the binary fission. The entire process takes about two hours and may occur one to four times per day.
Conjugation:
Conjugation is defined as the temporary union of two individuals which mutually exchange micro-nuclear material. It is unique type of a sexual process in which two organisms separate soon after exchange of nuclear material. Sonneborn (1947), on the basis of mating behaviour of Paramecium, has reported that each species of Paramecium exists in a number of varieties or syngens.
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Further, within each syngen there are a number of mating types; usually two. The mating types remain morphologically identical but they exhibit physiological differences. In P. aurelia, there are 14 syngens and 28 mating types, while in P. candatum, there are 16 syngens and 32 mating types. Observations have been made that usually paramecia neither conjugate with members of their own mating type nor with other varieties, but only with the second mating type of their own variety.
Process of Conjugation:
During conjugation, two individuals or pre-conjugants of two different mating types belonging to the same syngen come in contact ventrally and unite along their oral grooves. Both stop feeding and the buccal structures disappear. Their cilia produce a substance on the surface of the body which causes adhesion of the two individuals.
The pellicle of ectoplasm degenerated from the attached surface and a protoplasmic bridge is formed between the two so called conjugants. The two individuals continue to swim united. The macronucleus begins to disintegrate and becomes loose in texture. It later breaks up into fragments and is finally absorbed in the cytoplasm.
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The micronucleus of each conjugant divides twice, one of these being reduction division. Thus four haploid daughter micronuclei are formed. Three of these from each individual degenerate and disappear and the remaining one of each conjugant divides mitotically into two unequal pro-nuclei or gametic nuclei forming a large stationary female pro-nucleus and smaller, active migratory male pro-nucleus.
The migratory pro-nucleus of each conjugant crosses over the protoplasmic bridge and fuses with the stationary pro-nucleus of other conjugant to form the diploid conjugation nucleus or synkaryon. This complete fission of two nuclei may be termed as amphimixis. The conjugants after mating from 12 to 48 hours separate and are now called as ex-conjugants.
The zygote nucleus of each ex-conjugant now divides three times in rapid succession to produce eight nuclei of which 4 enlarge to become macronuclei and the other four become micronuclei. However, three of the micronuclei disintegrate and disappear while the remaining micronucleus divides into two.
In the meantime the ex-conjugants divide by binary fission into two daughter paramecia each containing two macronuclei and one micronucleus. The micronucleus of each daughter individual again divides along with the division of the individual so that each daughter individual now contains one macronucleus and one micronucleus. Thus from one individual four daughter paramecia are formed each having newly formed vital and active macronucleus and micronucleus.
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Factors Influencing Conjugation:
The factors inducing conjugation vary from species to species but some of them are given below:
1. Conjugation occurs usually under unfavourable living conditions; starvation or shortage of food and particular bacterial diet or certain chemicals are said to induce the process of conjugation in certain species of Paramecium.
2. Conjugation occurs after about 300 asexual generations of binary fission, or it alternates with binary fission at long intervals to rejuvenate the dying clone, i.e., it occurs in the individuals which must have passed through desirable number of asexual generations, said to be the period of immaturity, and then they become sexually mature of conjugate.
3. Conjugation occurs when there is a change in the physiological condition of paramecia, then it occurs between such individuals which are somewhat smaller in size (210 microns long) and they are at a stage which may be regarded as a period of unhealthy old age; the paramecia of this condition will die if not allowed to conjugate.
4. Sudden darkness in light conditions and low temperatures are said to induce the process of conjugation in some species.
5. Conjugation does not take place during night or darkness; it starts in early morning and continues till afternoon.
6. A proteinous substance in the cilia of mating type individuals is said to induce conjugation.
Significance of Conjugation:
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1. Conjugation serves to rejuvenate and reorganize the individual. After repeated generations of binary fission, the Paramecium loses vitality and becomes physiologically deficient. It ceases to multiply, reduces in size and may ultimately die. However, Woodruff and Jennings do not accept this and they have been able to maintain paramecia for about 36 years without conjugation.
2. There is nuclear reorganization during conjugation. The macronucleus which is responsible for the metabolic activities of the individual loses its potentiality and a new macronucleus with vigour and vitality is added.
3. Though there is no distinction between male and female individuals but the individuals of different mating types only fuse. However, the male or migratory pro-nucleus fuses with the female pro-nucleus.
4. The periodic occurrence of conjugation ensures inherited variation and brings about the blending of two lines of ancestry as in bisexual reproduction of higher animals.
Aberrant Behaviour in Reproduction:
Paramecium shows certain variations in its nuclear behaviour during fission and conjugation, these deviations and endomixis, autogamy, cytogamy and hemixis. In the first three processes genetical recombination is effected and a new macronucleus is formed from the micronucleus.
i. Endomixis:
Endomixis (Gr., endon, within + mixis, mingling) was described in P. aurelia by Woodruff and Erdmann. This process is said to occur periodically in a single individual. The macronucleus degenerates and the micronuclei divide twice to form eight micronuclei. Six of these degenerate.
The animal now divides by binary fission to produce two individuals each with one micronucleus. The micronucleus of each divides twice to form 4 micronuclei. Two of them enlarge to form macronuclei. Each individual again divides and so does the micronucleus so that from each two daughter individuals are formed with one macronucleus and 2 micronuclei.
The process, like conjugation, brings about the replacement of macronucleus with the material from micronucleus and thus brings about the rejuvenation of vitality of the race.
ii. Autogamy:
Autogamy or self-fertilization was described by Diller in Paramecium aurelia which resembles the conjugation but occurs in the same individual. P. aurelia has one macronucleus and two micronuclei. During the process, the two micronuclei divide twice, one of them being meiosis to form 8 micronuclei. Six of these degenerate and disappear.
The macronucleus grows into a mass which breaks up into pieces and is finally absorbed in the cytoplasm. The two pro-nuclei (micronuclei) enter a protoplasmic cone formed near the cell mouth. The two pro-nuclei fuse to form synkaryon which divides twice to form four micronuclei. Two of these enlarge and become macronuclei.
Now the individual divides by binary fission to produce two daughter individuals and simultaneously the two micronuclei also divide. Thus each individual has one macronucleus and two micronuclei. The process is completed in two days.
Autogamy is as important for P. aurelia as conjugation in P. caudatum. It brings about rejuvenation of the race as the new and vitiated macronucleus is formed by the micronucleus. Moreover, two micronuclei are fused. It is a kind of self-fertilisation.
iii. Cytogamy:
Wichterman (1939) has reported another sexual process in Paramecium caudatum, which he termed cytogamy. In cytogamy there is no nuclear exchange. In this process two individuals come together by their ventral surfaces, but the pellicle of the two individuals divides thrice to form eight micronuclei, six of which disintegrate in each individual. The two remaining micronuclei fuse to form a synkaryon in each cell. The animals now separate.
Cytogamy differs from autogamy in that there are two animals in contact with each other, but it resembles autogamy and conjugation in the fusion of two pro-nuclei. Cytogamy differs from conjugation in that there is no nuclear exchange between the two animals which come together.
iv. Hemixis:
Hemixis described by Diller in P. aurelia is mainly the process of macro-nuclear fragmentation and division without any micro-nuclear activity.
Diller classified hemixis into 4 types:
a. Type ‘A’:
Is characterized by the division of macronucleus into two or more parts.
b. Type ‘B’:
Is characterized by the extrusion of one to 20 or more chromatin balls from the macronucleus into the cytoplasm.
c. Type ‘C’:
Is known by the simultaneous splitting of the macronucleus into 2 or more major portions and the extrusion of macro-nuclear balls into cytoplasm.
d. Type ‘D’:
Represents pathogenic condition in which the macronucleus undergoes fragmentation into chromatin balls which finally disappear. Micronuclei also disappear.