Kappa Particles in Paramecium

Sonneborn and his co-workers discovered that some strains of the ciliate Paramecium aurelia carry self-replicating cytoplasmic particles called kappa particles, and that these strains — called 'killer' Paramecia — secrete a toxin, paramecin, that is lethal to other, non-carrier strains, called 'sensitives'. Kappa particles are now understood to behave as symbionts rather than parasites, since they do not harm the host cell that carries them; in fact they are themselves a bacterium, Caedobacter taeniospiralis, with its own DNA that replicates autonomously inside the host's cytoplasm, and a killer Paramecium can harbour hundreds of them at once.

Critically, maintaining kappa particles inside the cytoplasm also requires a dominant nuclear gene, K — a Paramecium with nuclear genotype kk cannot sustain kappa particles even if some were somehow present, and without the dominant K allele, kappa particles simply disappear from the cytoplasm over successive generations. This dual requirement (a nuclear gene plus the cytoplasmic particles themselves) is what let Sonneborn design a decisive experiment: when a killer Paramecium (KK) conjugates with a sensitive one (kk), both resulting exconjugants become nuclear heterozygotes (Kk) — genetically identical, and by nuclear genotype alone, both 'should' be capable of being killers. But whether both actually become killers turns out to depend entirely on how long conjugation lasts. A brief conjugation exchanges only nuclear material, not cytoplasm, so only the exconjugant that already had kappa particles in its own cytoplasm remains a killer, while its partner (now Kk in genotype but with no kappa particles of its own) stays sensitive. A prolonged conjugation, by contrast, allows the two Paramecia's cytoplasm to mix fully, so kappa particles pass into both exconjugants, and both lineages become killers. This result confirms that the killer trait itself is determined cytoplasmically (by the physical presence of kappa particles), even though a dominant nuclear gene is a separate, necessary condition for those particles to persist at all — the trait is neither purely nuclear nor purely cytoplasmic, but depends on both together. …