The following points highlight the two main cases of cytoplasmic inheritance. The cases are: 1. Inheritance of Plastids 2. Kappa Particles of Paramoecium.
Cytoplasmic Inheritance: Case # 1.
Inheritance of Plastids:
The cytoplasm of plants bears minute pigments called plastids. Plastids arise from smaller DNA particles of the ovum and multiply by division. During the formation of gamete a plastid enters the egg and the pollen receives none. Thus plastid is entirely a maternal contribution.
Cytoplasmic inheritance of plastids has been studied in Four-o’clock plant, Mirabilis. Albomaculatus race of this plant have variegated leaves with white and green patches Sometimes a branch is formed which bears leaves which are either green or white or variegated.
A seed from a uniformly green branch produces a plant which is totally green and a seed from a white branch grows into a white seedling which dies before maturity. Similarly the variegated branch gives rise to variegated off-springs.
The pollen is devoid of plastid and cannot influence the inheritance of plastid. Thus the colour depends entirely upon the egg and what the egg produces is dependent upon the plastid of the cytoplasm. Thus it is an evidence of cytoplasmic inheritance.
Cytoplasmic Inheritance: Case # 2.
Kappa Particles of Paramoecium:
Presence of certain particles in the cytoplasm and their role in inheritance speaks in favour of cytoplasmic inheritance. These particles are called “Plasmagenes”. Some races of Paramoecium aurelia possess a substance in their cytoplasm. The substance is called “Paramecin or Kappa particles”.
Races of Paramoecium with Kappa particles in the cytoplasm are designated as killer strain. Other members of the race which do not have Kappa particles are sensitive to the Kappa particles. Recent studies have shown that presence of Kappa particles is dependent upon the presence of a dominant gene K in the killer strains. Further, the Kappa particles are made up of nucleic acid and protein.
Fission is the usual mode of reproduction in Paramoecia. Killer individuals in such cases produce killer offsprings and sensitive individuals produce sensitive off-springs. If killer strain is mated with the sensitive strain normally the products that follow such a mating do not show any blending that is killer parent produce killers and sensitive parent produce sensitives.
This shows that the exchange of nuclear genes during mating has nothing to do with the inheritance of Kappa particles.
In case the conjugation between the killer and sensitive strains become prolonged and there occurs considerable cytoplasmic exchange along with nuclear exchange, all the ex-conjugants become killers. This shows that Kappa particles if enter into the cytoplasm of sensitive strains make them killers.
When killers KK conjugate with non-killer KK for a prolonged period and exchange cytoplasmic Kappa particles, the ex-conjugants become all Kk heterozygous and become killers as K is a dominant gene (Fig. 2.30).