A freshwater ciliate navigates along Earth's magnetic field lines, orienting itself in chemically stratified water to reach the oxygen concentration it prefers. This magnetotaxis is not a product of the ciliate's own genome. The magnetic sense comes from endosymbiotic bacteria — specifically, sulphate-reducing Desulfovibrionales that biomineralize bundles of bullet-shaped magnetite crystals inside the ciliate's body.
The finding, reported in Nature Communications (2026), reveals that the ciliate hosts four populations of endosymbiotic bacteria, all showing genome reduction. The genomes are shrinking — the bacteria are losing the genes for independent life while retaining the genes for magnetite synthesis. They cannot survive outside the host. They are no longer visitors. But they are not yet organelles.
This is the same trajectory that produced mitochondria and chloroplasts. An alpha-proteobacterium entered an archaeal cell roughly two billion years ago and never left; its genome contracted from thousands of genes to a few dozen; it became the mitochondrion. A cyanobacterium did the same in the ancestor of all plants. In both cases, what was once a complete organism became a component — a sense organ for energy production or photosynthesis. The transition is understood retrospectively, from finished examples.
The ciliate's endosymbionts are the transition caught in progress. The bacteria are still distinct organisms — they have their own, if reduced, genomes. But they exist only inside the host, they provide a capability the host cannot produce alone, and their genomes are actively losing the machinery of independence. The compass is still a separate creature. But it is becoming a part.
What makes this structurally distinct from the classical cases is the capability being transferred. Mitochondria transferred metabolism. Chloroplasts transferred photosynthesis. Here, the transfer is perception — the ability to sense a feature of the physical environment. The ciliate can orient in a magnetic field because something inside it, something that is still technically another organism, builds the crystals that make orientation possible. The boundary between the organism and its sense organ is the same boundary between two species.
The endosymbiotic origin of organelles is usually presented as a completed event — something that happened two billion years ago. The ciliate's magnetosymbionts show it happening now: genome reduction in real time, dependence deepening in real time, a borrowed capability becoming an intrinsic one. The compass is being absorbed. Eventually it will just be part of the ciliate's body, the way mitochondria are part of ours, and no one will remember it was ever a separate organism with its own genome and its own evolutionary history.