AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

KAPPA PARTICLES IN PARAMECIUM

• T. M. Sonneborn described the inheritance of some cytoplasmic particles known as kappa and their relation to nuclear gene in the common cillate protozoan, Paramecium aurelia.
• There are two strains of Paramecium. They are killer and sensitive.
• Killer strain produces a toxic substance called paramecin that is lethal to other individuals called "sensitives" .
• The production of paramecin in killer type is controlled by certain cytoplasmic particles known as kappa particles. The sensitive strains lack these particles.
• The kappa particles are transmitted through the cytoplasm.
• The existence, production and maintenance of kappa particles are controlled by a dominant gene ‘K’ present in the nucleus. However, ‘K’ cannot initiate the production of kappa in the total absence of kappa in the cytoplasm.
• When a Paramecium of killer strain is having the genotype “KK” or (K+) conjugates with the Paramecium of non-killer strain having the genotype “kk”, the exconjugants are all heterozygous for “Kk” genes.
• The development of a particular type depends upon the duration of cytoplasmic exchange
  • If conjugation is normal, i.e., lasts only for a short time, and no exchange of cytoplasm takes place between the two, both killers and non-killers (sensitive) are produced.
  • However in rare or prolonged conjugation (i.e., lasting for long time) the cytoplasmic bridge between the two conjugants is larger. In such cases, in addition to the nuclear material, the cytoplasmic materials are also exchanged.
  • During this cytoplasmic exchange, the kappa particles present in the cytoplasm of the killer type enter the non-killer type and convert it into a killer type. So all the offspring produced by the exconjugants are killer type.

• This shows that a Paramecium becomes a killer when it receives kappa particles and it becomes a sensitive when it does not receive kappa particles.