Bacterial Endosymbionts of Killer Paramecium

Paramecium is a protozoan that cruises around in freshwater and feeds by swallowing bacteria whole and digesting them. This lifestyle has led to the development of killer strains of Paramecium that use bacteria as a biological weapon to kill other, sensitive strains of Paramecium. Such killer Paramecium produce a toxin that kills all sensitive Paramecium that get too close. Killers are immune to their own toxin, but different brands of killer Paramecium exist that kill each other. Killer Paramecium contain kappa particles in their cytoplasm (Fig. 19.06). These are actually endosymbi-otic bacteria (Caedibacter) that grow and divide inside the larger, eukaryotic cell. The kappa particles are inherited by cytoplasmic or maternal inheritance, like mitochondria. Unlike mitochondria, kappa particles are still obviously bacteria. Some strains of Caedibacter can still grow and divide outside the Paramecium, although most types are obligate symbionts.

Besides harboring Caedibacter, the ability to kill also depends on a gene found in the nucleus of the Paramecium. The gene has two alternate forms: K and k. Parame-cium is diploid, so any individual has two copies of each gene and may be KK, Kk or kappa particle Endosymbiotic bacteria (Caedibacter) that grow and divide inside killer Paramecium killer Paramecium Paramecium that contains kappa particles in the cytoplasm, which it uses to kill other strains of Paramecium mutualism Form of symbiosis where both partners benefit

Paramecium A type of free-living protozoan that feeds on bacteria parasitism Form of symbiosis where one organism lives at the expense of the other pathogen Parasite that seriously incapacitates or kills its host

FIGURE 19.06 Killer Paramecium contains Caedibacter

(A) The Caedibacter or kappa particles are found in the cytoplasm of the Paramecium. (B) Kappa particles are symbiotic in many strains of Paramecium, yet they have their own DNA and divide like typical bacteria.

FIGURE 19.06 Killer Paramecium contains Caedibacter

(A) The Caedibacter or kappa particles are found in the cytoplasm of the Paramecium. (B) Kappa particles are symbiotic in many strains of Paramecium, yet they have their own DNA and divide like typical bacteria.

Cell wall of kappa broken down toy digestion

Cell wall of kappa broken down toy digestion

Kappa (swallowed by sensitive Paramecium)

FIGURE 19.07 Sensitive Paramecium is Killed by R-Bodies

Kappa particles contain a protein toxin coiled into a crystal known as an R-body. When the membrane of the kappa particle is digested, the toxin is free to uncoil and releases the toxin protein. Once released inside its victim, the toxin kills the Paramecium.

Kappa (swallowed by sensitive Paramecium)

FIGURE 19.07 Sensitive Paramecium is Killed by R-Bodies

Kappa particles contain a protein toxin coiled into a crystal known as an R-body. When the membrane of the kappa particle is digested, the toxin is free to uncoil and releases the toxin protein. Once released inside its victim, the toxin kills the Paramecium.

kk. Since the K allele is dominant, possession of one or two K alleles (i.e., KK or Kk) allows the kappa particles to grow and divide inside the Paramecium. However, kk individuals lose the kappa particles. Thus a killer must have both the cytoplasmic kappa particle and at least one copy of the nuclear K allele.

All Paramecium cells without kappa particles are sensitive, whether they are KK, Kk or kk. Killing occurs when a few kappa particles are liberated from killer strains into the culture medium. The sensitive Paramecium strains swallow the kappa particles, believing them to be harmless digestible bacteria. Inside the kappa particle are toxic proteins coiled into crystals known as R-bodies. When the kappa particle is digested by a sensitive Paramecium, the R-body is liberated, uncoils, releases the toxin and kills the Paramecium (Fig. 19.07).

R-bodies Toxic proteins that form crystals inside the kappa particles of killer Paramecium

The R-body protein is actually encoded by a gene belonging to a bacterial plasmid or a defective bacterial virus, not by the Caedibacter chromosome. So a toxin made by a virus infecting the symbiotic Caedibacter has been domesticated and diverted to the purpose of killing sensitive Paramecium.

Symbiotic bacteria living inside insects provide their hosts with essential amino acids.

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