The intron RNA from the predator appeared inside the dead archaeal cells
In the quiet metabolism of a two-decade-old microbial culture, scientists have witnessed something evolution rarely makes visible: a predatory bacterium leaving fragments of its own genetic identity inside the cells of its archaeal prey. The discovery, centered on a self-splicing RNA element called a group I intron, offers the first direct observation of mobile genetic material crossing the boundary between biological domains not through viruses or plasmids, but through the intimate violence of predation itself. It is a reminder that the flow of genetic information through life is stranger and more promiscuous than our tidy categories suggest.
- For decades, horizontal gene transfer between bacteria and archaea was inferred from evolutionary incongruence — now, for the first time, it has been watched happening in real time inside individual cells.
- The predatory bacterium Ca. Velamenicoccus archaeovorus clings to its archaeal host and, during the act of killing, appears to flood the dying cell with its own intron RNA — a molecular fingerprint left at the scene.
- The intron signal is vanishingly rare — roughly one molecule per 20,000 mature ribosomal RNAs — yet its specificity is unambiguous: it appears only in dead prey cells, never in living neighbors protected by structural plugs within the filament.
- Circular RNA, which resists enzymatic degradation, and a reverse transcriptase carried by the predator together suggest a plausible pathway by which this RNA could be converted back into DNA and permanently written into the archaeal genome.
- The finding reframes extracellular RNA not merely as a signaling or competitive tool, but as a vehicle for genetic colonization — one that may quietly reshape microbial genomes across evolutionary time in slow, closed ecosystems.
In a culture flask fed only once a year on the hydrocarbon limonene, maintained for more than twenty years, something was unfolding at a scale invisible to the naked eye. A predatory ultramicrobacterium, Candidatus Velamenicoccus archaeovorus, was clinging to the surface of filamentous methane-producing archaea and, in the act of killing them, leaving behind a piece of itself.
Researchers noticed that certain cells within the archaeal filaments appeared hollowed — their ribosomal RNA gone, their membranes intact, their DNA preserved — the signature of recent death. What drew attention was not the death itself, but what the dead cells contained: RNA from the predator's own genome. Specifically, a group I intron embedded in the bacterium's ribosomal RNA gene had accumulated inside the prey cells. Using fluorescent probes and a technique called CARD-FISH, the team could visualize this intron RNA directly, watching it appear in dead archaeal cells while living neighbors — shielded by structural plugs within the filament — showed no trace of it.
The signal was faint. Transcriptome analysis placed the intron at roughly one molecule per 20,000 mature ribosomal RNAs, yet the sequencing reads confirmed it was real and spliced — a discrete molecular entity persisting after separation from its host transcript. The likely explanation involves the intron's capacity to form stable circular RNA after splicing, a form resistant to enzymatic degradation. Electron micrographs showed direct cytoplasmic contact between predator and prey, with no membrane barrier — an open channel through which circular RNA, and possibly the predator's reverse transcriptase enzyme, could pass.
If that enzyme enters the prey cell alongside the intron RNA, it could theoretically convert the RNA back into DNA and integrate it into the archaeal genome — a complete horizontal gene transfer accomplished not by a virus or a plasmid, but by the mechanics of eating. The discovery adds a new category to the known repertoire of extracellular RNA and suggests that group I introns may have evolved an RNA-based transposition pathway rooted in predation. In the slow, recycling world of an anaerobic enrichment culture, such transfers — rare in any given moment — may accumulate into something that reshapes genomes across geological time.
In a slowly growing culture maintained for over two decades, fed only once a year on the natural hydrocarbon limonene, something unexpected was happening at the microscopic scale. A predatory bacterium no larger than a grain of sand was leaving its genetic fingerprints inside the dead cells of its archaeal prey. The discovery, made through careful visualization of RNA molecules inside individual cells, reveals a previously unknown pathway for genes to move between organisms—one that operates not through viruses or plasmids, but through the raw mechanics of predation itself.
The predator is Candidatus Velamenicoccus archaeovorus, an ultramicrobacterium that lives as an epibiont, clinging to the surface of larger cells. Its prey is Methanothrix soehngenii, a filamentous archaeon that produces methane as part of the culture's slow metabolic cycle. The researchers noticed something odd: individual cells within the archaeal filaments showed signs of death—no detectable ribosomal RNA, a hollowed appearance under staining, less cellular material visible in images—yet their DNA and lipids remained intact. This suggested that something was being transferred from predator to prey during the act of predation itself.
The researchers focused on a specific molecular target: a group I intron embedded in the predator's 23S ribosomal RNA gene. Introns are self-splicing genetic elements found in many prokaryotes, and they are known to be mobile—capable of moving between organisms. But how they move between different species has remained largely theoretical, inferred from the incongruence of genetic sequences across evolutionary trees rather than observed directly. The team designed fluorescent probes to visualize this intron RNA inside cells, using a technique called catalyzed reporter deposition-fluorescence in situ hybridization, or CARD-FISH. They could watch, quite literally, where the intron RNA was located.
What they found was striking: the intron RNA from the predatory bacterium appeared inside the dead archaeal cells. The signal was faint—the intron was detected in only a tiny fraction of the prey cells—but it was there, and it was specific. When they used a reverse-complement probe as a negative control, no signal appeared. The intron RNA was only found in cells that had lost their own ribosomal RNA, the hallmark of death. Viable archaeal cells sitting adjacent to dead ones, separated by structural plugs within the filament that act as a kind of passive defense system, showed no intron signal. The dead cells, by contrast, still contained DNA but now also harbored genetic material from their killer.
To confirm this finding with independent evidence, the researchers analyzed the culture's transcriptome—the complete set of RNA molecules present. They found intron RNA at a mean coverage of 14 reads, compared to 263,770 reads for the mature 23S rRNA. This meant that for every 20,000 mature ribosomal RNA molecules, only about one unspliced transcript or intron molecule was present. The intron-exon boundaries themselves appeared in individual sequencing reads, with 23 reads bridging the start of the intron and 20 reads bridging the end, confirming that the intron was being spliced out and persisting as a separate molecule.
The mechanism behind this transfer likely involves the intron's ability to form stable circular RNA molecules after splicing. Unlike linear RNA, which is rapidly degraded, circular RNA resists breakdown by cellular enzymes. When the predator's cytoplasm comes into direct contact with the prey cell—transmission electron micrographs showed open contact between the two organisms, with no membrane barrier—the circular intron RNA can slip across. The predator also carries a reverse transcriptase enzyme, a protein that can convert RNA back into DNA. If this enzyme also enters the prey cell during predation, the intron RNA could theoretically be converted back into DNA and integrated into the archaeal genome, completing a horizontal gene transfer.
This discovery expands the known repertoire of extracellular RNA molecules. Until now, researchers understood extracellular RNA primarily as signaling molecules and metabolic inhibitors—small RNAs that bacteria release to communicate with their environment or suppress the activity of competitors. The intron RNA adds a new category: mobile genetic elements that can physically move between cells during predation, potentially establishing themselves in new genomes. The finding suggests that group I introns may have evolved an RNA-based transposition pathway, one that operates through the predator-prey relationship itself. In the slow, closed world of an anaerobic enrichment culture, where predators accumulate and necromass is recycled endlessly, such transfers may occur with enough frequency to shape the genetic landscape over time.
Notable Quotes
Our observations provide in vivo evidence of the mobility of intron RNA between cells of different organisms. This is a prerequisite for a transposition into a foreign species, for a horizontal gene transfer.— Study authors
The Hearth Conversation Another angle on the story
Why would a predator bother transferring its own intron into a dead cell? What's the advantage?
That's the question, isn't it. The intron itself doesn't benefit the predator directly. But the intron is a mobile element—it wants to spread. If it can hitchhike into a new host during predation, it increases its own chances of persistence. The predator is just the vehicle.
So the intron is using the predator to get into the prey?
Exactly. The predator kills the archaeon, opens up its cytoplasm, and suddenly the intron RNA—which is stable in circular form—can slip across. If the prey cell's reverse transcriptase machinery is still functional, or if the predator's reverse transcriptase enters too, the intron can be converted back to DNA and integrated.
But the prey is dead. How does it benefit from receiving a new gene?
It doesn't. But its genome might. If the intron integrates before the cell fully degrades, it becomes part of the genetic record. And in a culture like this, where cells are constantly dying and being recycled, that genetic material could be scavenged by other organisms.
So this is horizontal gene transfer, but through predation rather than through viruses?
Yes. We've always known introns move between species—the evidence is in the trees, in the incongruence of genetic sequences. But we've never actually seen it happen. This is the first direct observation of an intron RNA leaving one organism and entering another.
What happens to the intron once it's inside the dead cell?
It circularizes, which makes it stable. It sits there, resistant to degradation. Whether it gets converted back to DNA and integrated depends on what enzymes are present in that dying cell. But the fact that we can detect it at all means it's persisting long enough to be found.
Does this change how we think about gene transfer in microbes?
It opens a door. We've been focused on phages and plasmids as vectors. But predation is everywhere in microbial communities. If introns can move this way, what else can? It suggests that the boundaries between organisms are more permeable than we thought, especially when one is eating the other.