ADVERTISEMENTS:
In this article we will discuss about:- 1. Subject-Matter of Paramecium 2. Reproduction and Mode of Action of Paramecium 3. Origin of Paramecium.
Subject-Matter of Paramecium:
Sonneborn in 1943 found that Paramecium aurelia strains could be classified into “Killer” and “Sensitive” types. The killer strain secreted a toxic substance “paramecin” in the growing medium and this toxin was responsible for the destruction of sensitive strains.
The killer strain carries in its cytoplasm particles known as “kappa”; there may be up to 1600 kappa particles in a single animal. In general, about 400 kappa particles per cell are required to be able to produce the toxin for the killer effect.
ADVERTISEMENTS:
Normally, conjugation in Paramecium leads to the exchange of haploid nuclei and there is no cytoplasmic exchange between the conjugation partners. When the killer and sensitive animals are brought together, there would be killer sensitive matings.
In such experiments, care should be taken to transfer only the killer animals, without any amount of their surrounding medium, into the culture of sensitive ones; this is essential because the toxin paramecin is present in the culture medium and therefore, would kill the sensitive animals. Therefore, the progeny from such matings will be of two types: killer and sensitive (Fig. 19.1).
But when conjugation is prolonged, cytoplasmic exchange may also take place, in addition to the exchange of haploid nuclei, and some kappa particles may thus be transferred into the cytoplasm of the sensitive animals (Fig. 19.2). After such matings, the sensitive animals involved in the mating would acquire the killer trait. Thus the killer character exhibits cytoplasmic inheritance pattern.
Reproduction and Mode of Action of Paramecium:
ADVERTISEMENTS:
Multiplication of the kappa particles is controlled by a dominant nuclear gene K of the Paramecium. Autogamy in heterozygous (Kk) animals produces KK and kk segregants in 1 : 1 ratio. The recessive (kk) individuals are unable to support the multiplication of kappa particles.
Therefore, the kk progeny obtained (after autogamy) in killer (KK) and sensitive (kk) matings (Fig. 19.2) become sensitive after 9-15 divisions. The K gene does not produce the kappa particles; it only enables the multiplication of kappa particles if they are already present in such an (Kk) animals.
Therefore, individuals of KK genotype remain sensitive if they are without kappa particles in their cytoplasm. But when kappa particle is introduced in the cytoplasm of KK individuals, they become killers since they sustain these particles in their cytoplasm by supporting their reproduction.
Sonneborn identified the following five types of killers Paramecia based on the different modes of killing:
1. Vacuolizer : these kill the sensitive animals by vacuolizing them.
2. Humper : these animals induce the sensitive animals to form humps.
3. Spinner : this type of killer makes the sensitive to rotate.
4. Paralyser : such killers paralyse the sensitive animals.
ADVERTISEMENTS:
5. Rapid lysis : these kill the sensitive animals by inducing their lysis rapidly.
There is one more type called the mate killer; this kills its sensitive mate only during conjugation. Kappa particles contain proteinaceous “R-bodies” which are responsible for the toxic action of kappa particles on sensitive strains.
Origin of Paramecium:
Apart from kappa particles, several other particles have also been found in the cytoplasm of Paramecium Aurelia; some of these particles are alpha, gamma, delta, lambda, mu and pi etc. Collectively, they are called P particles. Kappa particles range from 1 to 5 µm in length and contain DNA.
Paramecium can live without kappa particles. The cytochrome pigments utilized by kappa for respiration are different from those of the host animal, but they are similar to those present in some bacteria. Electron microscopic studies have shown that the kappa particles are a bacterium called Caedobacter taeniospiralis.
ADVERTISEMENTS:
It has been suggested that kappa particle originated independent of Paramecium in the distant past. But sometime during the long evolutionary time span, these particles established a symbiotic relationship with the Paramecium, i.e., they became endosymbionts. Kappa particles appear either as ‘bright’ or ‘non bright under the light microscope. Preer and Coworkers in 1971 isolated a virus from the bright kappa particles.
This virus was suggested to be a temperate bacteriophage. Occasionally, the temperate bacteriophage multiplies vegetatively and lyses the bacterium (kappa particle). The bacterial host products formed due to viral growth may produce the substance having killer activity.
Thus it may be said that one cytoplasmic agent exists within the other cytoplasmic agent (the phage within the bacterium which resides within Paramecium).
