Flexibility exists, but loyalty wins.
For sixty-two million years, tortoise beetles and the bacterium Stammera capleta have woven themselves into one of nature's most complete interdependencies — a bond so deep that neither organism can persist without the other. Researchers at the Max Planck Institute have now probed the edges of this ancient arrangement, asking whether evolution's long work could be undone by deliberate exchange. What they found speaks to a truth older than the experiment itself: life builds loyalty into its deepest partnerships, and flexibility, when it exists, is not the same as freedom.
- A 62-million-year symbiosis is put to the test when scientists transplant bacteria between beetle species that have never shared a microbial partner.
- Foreign bacteria survive inside new hosts but trigger immune alarms and metabolic disruption — the body recognizes the stranger, even when it tolerates it.
- Closely related donor species cause less upheaval, revealing that evolutionary distance is not abstract but written in the chemistry of recognition.
- The sharpest boundary emerges at reproduction: no transplanted bacterium, however compatible, manages to pass itself to the next generation — the maternal transmission system holds firm.
- The findings ripple outward, suggesting that the specificity seen in human gut microbiomes may itself be the residue of millions of years of coevolution, not mere chance.
For sixty-two million years, tortoise beetles and a bacterium called Stammera capleta have sustained one of biology's most complete partnerships. The beetles cannot digest food without these microbes; the bacteria cannot reproduce outside their hosts. The bond is passed maternally through sphere-shaped structures attached to the eggs themselves — an inheritance written into the architecture of reproduction.
Dr. Hassan Salem and his team at the Max Planck Institute asked a deceptively simple question: how fixed is this ancient arrangement? To find out, they transplanted bacteria from one tortoise beetle species into the eggs of another. The results were layered. Foreign bacteria could survive inside a new host's gut and support some degree of development — but the outcome depended heavily on how closely related the donor and recipient species were. Distantly related pairings provoked stronger immune responses and only partial growth. The evolutionary gap between species, it turned out, has a biological texture.
Yet the clearest limit appeared at the moment of inheritance. None of the transplanted bacteria — not even those from closely related species — succeeded in passing to the next generation. The maternal transmission system that had functioned without interruption for millions of years rejected every foreign candidate. First author Dr. Inès Pons-Guillouard, whose experimental work anchored the study published in Nature Communications, noted that the very fact exchange was possible at all suggested these partnerships are not completely rigid — but the failure of transmission revealed that evolution has layered multiple recognition and filtering mechanisms to reinforce the original bond.
The implications reach beyond beetles. The team sees in this system a model for understanding why gut microbiomes across many species, including humans, show the specificity they do — not as accident, but as the accumulated result of long coevolution. With three thousand tortoise beetle species yet to be studied, and some among them significant crop pests, the research carries both theoretical depth and practical possibility.
For sixty-two million years, tortoise beetles and a bacterium called Stammera capleta have been locked in an intimate partnership so complete that neither can survive without the other. The beetles cannot digest their food without these microbes living inside them. The bacteria cannot reproduce outside their beetle hosts. It is a relationship written into the very biology of both organisms, passed from mother beetle to offspring through peculiar sphere-shaped structures attached to the eggs themselves. But the question that drew Dr. Hassan Salem and his team at the Max Planck Institute to study these beetles was deceptively simple: just how fixed is this ancient bond? Could the same bacterial species thrive inside a different beetle host? Or has evolution locked these partners so tightly together that swapping them would be impossible?
To find out, the researchers did something audacious. They took bacteria from one tortoise beetle species and transplanted them into the eggs of another, using those same sphere-shaped structures as vehicles for the exchange. What they discovered was a world of nuance. The foreign bacteria could survive inside the new host's digestive system. They could function, after a fashion. But success came in degrees. Bacteria from closely related beetle species behaved almost like the original strain, allowing the young beetle to develop normally. Bacteria from distantly related species triggered stronger immune responses and metabolic upheaval in the developing insect, only partially supporting its growth. The gulf between species mattered.
Yet there was a harder limit. None of the transplanted bacteria—not even those from closely related species—managed to pass themselves on to the next generation. The beetles could harbor foreign microbes during their own lifetime, but when it came time to seed the next generation, the partnership failed. The maternal transmission system that had worked flawlessly for millions of years rejected the newcomers. This is where the real story emerges: flexibility exists, but loyalty wins.
Dr. Inès Pons-Guillouard, the first author of the study published in Nature Communications, led much of this delicate experimental work. She expressed surprise at what the research revealed. The fact that bacteria associated with their hosts for sixty-two million years could be experimentally exchanged at all suggested that these partnerships, while ancient, were not completely rigid. Yet the failure of transmission to offspring showed something equally important: evolution has built multiple layers of specificity into these relationships. Partner recognition systems, transmission bottlenecks that filter which microbes pass down, and competition-based selection all work together to reinforce the original pairing.
The implications extend far beyond beetles. These findings illuminate how gut microbes operate across many species, including humans. They suggest that the specificity researchers observe in human microbiomes—the way certain bacteria thrive in certain people—may not be accidental but rather the result of millions of years of coevolution and selection. The tortoise beetle system, with its clear maternal transmission and extreme interdependence, offers a window into how these long-term partnerships are built and maintained.
Salem noted that the work would not have been possible without the technical expertise of his team, particularly laboratory manager Christiane Emmerich, whose skills helped overcome the challenges of such delicate experimental work. He also signaled where the research is heading: while this study focused on a single clade of insects, expanding to other insect families would test whether these patterns hold broadly across the invertebrate world. There are three thousand species of tortoise beetles alone, and some of them—both adults and larvae—are destructive pests on crops like sweet potatoes. Understanding how their microbial partnerships work could have practical consequences as well as theoretical ones.
Notable Quotes
What surprised me most is that it is possible to experimentally exchange symbionts associated with their hosts over the last 62 million years.— Dr. Inès Pons-Guillouard, first author of the study
Exploring ways we could expand our research to include additional insect families would provide an important opportunity to test how broadly these patterns apply across the invertebrate world.— Dr. Hassan Salem, senior author and JIC group leader
The Hearth Conversation Another angle on the story
So these beetles have bacteria they literally cannot live without. How did that happen in the first place?
Over millions of years, the bacteria became so useful for digestion that beetles that lost them died out. The ones that kept them survived. Eventually, the partnership became so tight that the bacteria's genome actually shrank—they don't need to do much on their own anymore because the beetle does so much for them.
And the mother passes them to babies through these sphere structures on the eggs. That's not how most bacteria spread.
Right. It's inheritance, like DNA. The beetle controls which bacteria get passed down, which is the opposite of how bacteria usually work—they just try to infect everything. This system is so controlled that it becomes a bottleneck. Only certain bacteria make it through.
So when they tried swapping bacteria between species, why did the foreign ones fail to pass on?
The transmission system is tuned to recognize the original partner. It's like a lock and key. A different key might fit in the lock, might even turn it a little, but it doesn't open the door the same way. The beetle's body can tolerate the foreign bacteria for a while, but when it comes time to package them for the next generation, the system rejects them.
Does this tell us anything about human microbiomes?
It suggests that the specificity we see in human gut bacteria isn't random. If beetles and bacteria can be so tightly matched after sixty million years, then the bacteria living in your gut right now might be there because of millions of years of selection too. Your body recognizes them. They recognize you.
What happens next with this research?
They want to test whether this pattern holds across other insect families. If it does, it means this isn't just a quirk of tortoise beetles—it's a fundamental principle of how long-term symbiosis works.