What is Cytoplasmic inheritance?
It is a type of genetic transmission where traits are inherited through genes located in the cytoplasm rather than in the cell nucleus.
The inheritance of Kappa particles in Paramecium
aurelia is one of the classical examples of cytoplasmic inheritance.
It was discovered by T.M. Sonneborn in 1943
Watch our video here: Kappa Particles in Paramecium
There are two strains of Paramecium. Killer Strain (called Killers)
produce a toxic substance called paramecin that kills other strains
(called Sensitives).
"Killer" Strain requirement
For a Paramecium to be a "Killer," 2 things are required
• The Nuclear Gene (K): A dominant nuclear gene K must be present (KK or Kk) homozygous and heterozygous condition.
• The Cytoplasmic Particles (Kappa): These are actual physical particles (now known to be endosymbiotic bacteria called Caedibacter taeniospiralis) located in the cytoplasm.
The Rules:
1. K gene without Kappa: The Paramecium is Sensitive.
2. Kappa without K gene: The Kappa particles later disappear, and the Paramecium becomes Sensitive.
3. K gene + Kappa: The Paramecium is a Killer. (both conditions needed)
Inheritance During Conjugation
The duration of conjugation determines the pattern of inheritance.
Case 1: Brief Conjugation (Normal)
• During a normal, quick conjugation, the two Paramecia exchange nuclear material only.
• The Result: A Killer (KK + Kappa) mates with a Sensitive (kk). They swap nuclei and both become Kk. (Nuclear transfer only)
• No transfer of cytoplasm, the Killer stays a Killer, and the Sensitive remains Sensitive.
• The F2 Generation: The offspring of the Killer remain Killers, and the offspring of the Sensitive remain Sensitives.
Case 2: Prolonged Conjugation
If the two Paramecia undergoes prolonged conjugation, a cytoplasmic bridge forms, allowing cytoplasm (and Kappa particles) to flow from the Killer to the Sensitive.
• The Result: Both individuals now have the Kk genotype and Kappa particles.
• The Outcome: Both Paramecia become Killers.
The role of K gene
Now we know that Kappa particles in the cytoplasm are actually an endosymbiotic
bacteria called Caedibacter taeniospiralis living within the cytoplasm of the Paramecium.
For the multiplication and survival of kappa particles, Host must have Nuclear Gene (K)either in homozygous KK or heterozygous condition Kk.
Why is this Cytoplasmic Inheritance?
The physical location of the genetic factor that determines the "Killer" trait (the kappa particle) is inside the cell's cytoplasm, not inside its nucleus.
Non-Mendelian: The trait does not
segregate in a 3:1 ratio in the offspring
because it depends on the physical presence of cytoplasm, not just the genes.
Reference
Sonneborn TM. Gene and Cytoplasm: I. The Determination and Inheritance of the Killer Character in Variety 4 of Paramecium Aurelia. Proc Natl Acad Sci U S A. 1943 Dec;29(11):329-38.
