RNA Brakes May Stabilize a Cellular Symbiosis

In its cytoplasm, the aquatic microbe Paramecium bursaria carries hundreds of green Chlorella algae that provide photosynthetic food. New research hints at what helps to stabilize this relationship.

Seen through a microscope, the hairy, slipper-shaped aquatic microbe Paramecium bursaria often looks as if it is bursting at the seams with tiny green marbles. Yet the verdant spheres are a different organism altogether: Chlorella, an alga that occasionally takes refuge within the confines of the paramecium’s cushy cell membrane. Each species can survive on its own, but the two frequently and repeatedly engage in an endosymbiotic partnership. In exchange for providing defense and nutrients, the paramecium demands that its algal tenants share the food they photosynthesize.

The conventional wisdom is that a paramecium refrains from digesting the algae only because it would lose out on the sugars that they create. But new work posted to the biorxiv.org preprint server suggests that the algae may also be protected by a stabilizing fail-safe system: If a ravenous paramecium makes a meal of its resident, digested bits of the algae’s RNA may interfere with the host’s ability to grow and reproduce. Understanding the dynamics between paramecia and Chlorella could yield insights into innumerable other endosymbioses found in nature and may even offer hints about what helped sustain the original endosymbiosis that is thought to have produced eukaryotes (complex cells with organelles).

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