KAPPA PARTICLES IN PARAMECIUM

KAPPA PARTICLES IN PARAMECIUM

Sonneborn in 1938 discovered certain strains in paramecium which showed a killer trait due to presence of a cytoplasmic factor called Kappa. The killer strain can destroy the sensitivite strains growing in culture which do not have Kappa by liberating a toxic substance paramecin. Killer strains are not killed by their own paramecin.

Paramecin has two kinds of nuclei, a small micronucleus and a very large macronucleus which is highly polyploid and irregular in shape and behaviour during cell division. Only the micronucleus behave according to mendelian principles. Paramecium has three modes of reproduction. The first is a simple mitotic division called binary fission. The second method is conjugation here two protozoans divide meiotically to form four micronuclei in each cell; out of these, three nuclei degenerates and only one remains which divides by mitosis to produce two genetically identical haploid nuclei in each paramecium. During conjugation only one of the two  haploid nuclei is exchanged through a cytoplasmic bridge formed between the two ciliates. The cells than separates as two Exconjugants.

The third method of reproduction is called autogamy. Here a single paramecium divides meiotically and by the same process that occurs in conjugation, two identical haploid nuclei are formed which fuse to form a diploid organism. As there was no genetic exchange, the diploid paramecium is Homozygous.

One noteworthy feature of the sensitive strains is that they are not killed by paramecin while they are in the process of conjugation. This has an advantage because it allows the investigator to perform cross between the killer and sensitive strains. The two strains can be distinguished morphologically as killers have granular cytoplasm and sensitive are clear. When a cross is made between killer and a sensitive paramecium (each made homozygous by autogamy), there is exchange of genetic material through conjugation. This is followed by separation of the two genetically identical Exconjugants. It is found that killer Exconjugants produce only killer paramecia and the sensitivite traits are not controlled by mendelian genes.

If the heterozygous (KK) killer Exconjugants is inbred to another heterozygous killer, it produce three-Quarter killer (1KK and 2kk) and one quarter sensitivites (kk). But if the sensitivite Exconjugant (Kk) is crossed to another heterozygous sensitivite, it results in all sensitivite progeny even through their genotypes are in the ratio of 1KK:2Kk:1kk. The results suggest non-chromosomal inheritance of killer trait.

The final proof regarding inheritance of killer trait was obtained by modifying the experiment in the following way. The cross between killer and sensitivite was prolonged, allowing enough time for exchange of cytoplasm to take place. In this way, some of the Kappa particles could move  from killer into the sensitivite strain. Att the resulting progeny of such a cross consisted of killers thus confirming the cytoplasmic inheritance of Kappa particles.

The below experiment can be performed in such a way that paramecia divide very rapidly by controlling the nutrient conditions. Under such conditions, a Homozygous killer strains (kk) containing Kappa particles can produce a few individuals that are sensitivite and without Kappa particles. The explanation is that Kappa particles cannot multiply as rapidly as the cells and become fewer in number in comparison with the number of paramecium cells. Due to their reduced number Kappa particles are not passed on to some members of the progeny at all.

It was found that although Kappa particles are transmitted cytoplasmically, yet they require a dominant K gene for maintenance. The K gene cannot initiate the presence of Kappa particles about 0.2 micron in diameter and have ability to reproduce independent of the nucleus. They have their own DNA, can multiply and produce the substance paramecin.

Fig : Conjugation in Paramecium demonstrating extranuclear inheritance of Killer trait

Fig : exchange of nuclear genes

Fig : exchange of both nuclear genes and cytoplasm

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