Cockroaches had absorbed more than a hundred times that amount.
For millions of years, cockroaches and their inner bacterial companions have been engaged in a quiet, intimate exchange — one that has left tens of thousands of foreign genetic fragments embedded in the cockroach genome. Researchers at the University of Sydney have now measured the full scale of this borrowing, finding it dwarfs anything previously recorded in complex animal life. The discovery invites us to reconsider where one organism ends and another begins, and whether survival is less about inheritance than it is about unlikely partnership.
- Scientists expected to find traces of bacterial DNA in cockroaches — they did not expect to find 40,485 fragments, more than a hundred times the previous record for any complex animal.
- The sheer volume of transferred genetic material went undetected for so long because researchers had been scanning for protein-coding genes, missing the vast silent archive of smaller, non-coding fragments hiding in plain sight.
- Some of these borrowed fragments have persisted for nearly 29 million years, raising the urgent question of whether they are quietly powering cockroach resilience or simply along for the ride.
- The research team is now working to determine which fragments are functional, which are neutral, and whether the same bacterial gene-lending is occurring across the broader animal kingdom — a task that could rewrite evolutionary biology.
Cockroaches have long been celebrated for their near-indestructible nature, but a team at the University of Sydney has found something unexpected beneath that reputation: tens of thousands of DNA fragments that originated not in the cockroach lineage at all, but in bacteria.
The source is Blattabacterium cuenoti, a bacterium that has lived inside cockroaches for millions of years, helping them metabolize nitrogen. Scientists already knew about this symbiotic arrangement. What they did not know was that the bacteria had been steadily transferring pieces of their own genetic code directly into cockroach cells through a process called horizontal gene transfer. Analyzing the complete genomes of 18 cockroach and termite species, the researchers counted 40,485 distinct bacterial DNA fragments — shattering the previous record for complex animals, which stood at fewer than 300.
The discovery was made possible by looking where others hadn't: at small, non-coding fragments that earlier studies had overlooked. These pieces ranged widely in size, and some have been lodged in the cockroach genome for at least 28.7 million years — a longevity that hints they may be serving some purpose, though the researchers are careful not to assume so. The fragments could be genuinely useful, quietly neutral, or only marginally harmful — evolution's tolerance for ambiguity on full display.
The broader implication is perhaps the most striking. Horizontal gene transfer was once considered a largely bacterial phenomenon, exotic and rare in complex animals. But improving genomic tools are revealing it as far more widespread. If cockroaches have absorbed this much foreign DNA from a single bacterial partner, similar exchanges may be occurring across the animal kingdom at a scale yet unmeasured. The next phase of research will ask which of these fragments actually function — and whether other species harbor their own hidden libraries of borrowed genetic material, quietly reshaping what it means to be a distinct organism at all.
Cockroaches are famous for their resilience—creatures that seem to survive anything, from poison to crushing blows. Much of that legendary toughness comes from their DNA, which carries instructions for neutralizing toxins and regrowing lost limbs. But a team of researchers at the University of Sydney has uncovered something far stranger hiding in the cockroach genome: tens of thousands of DNA fragments that don't belong to cockroaches at all.
These borrowed genes come from bacteria called Blattabacterium cuenoti, organisms that have lived inside cockroaches for millions of years, helping them process nitrogen. Scientists knew about this partnership. What they didn't know was that the bacteria had been quietly transferring vast amounts of their own genetic material directly into their host's cells—a process called horizontal gene transfer, or HGT. When the researchers analyzed the complete genomes of 18 cockroach and termite species, they found 40,485 distinct fragments of bacterial DNA scattered throughout the cockroach genome. The previous record for horizontal gene transfer in any complex animal was fewer than 300 fragments. Cockroaches had absorbed more than a hundred times that amount.
The scale of the discovery became clear only because the researchers looked for something previous studies had missed: small, non-coding DNA fragments that don't build proteins. These fragments had been invisible to earlier scans focused on larger, more obvious genetic material. The bacterial DNA pieces ranged from 93 to 4,900 units in length, and some of them had been sitting in the cockroach genome for at least 28.7 million years—a span of time suggesting they might actually be doing something useful.
How does this transfer happen? Unlike the normal inheritance of genes from parent to offspring, horizontal gene transfer occurs when cells from different species come into extremely close contact. In this case, cockroach cells absorbed loose pieces of bacterial DNA floating around inside them. For the insect absorbing these fragments, the result could be a more versatile and robust genome—gaining molecular capabilities it wouldn't otherwise possess. But the researchers are careful to note a crucial caveat: they don't yet know what most of these transferred fragments actually do. Some may provide real benefits. Others might be neutral, simply persisting because they're not harmful enough for evolution to eliminate them. A few could even be slightly damaging, but not damaging enough to matter.
The discovery points to something larger. Horizontal gene transfer was long thought to be primarily a bacterial phenomenon, rare in complex animals and plants. But as scientific techniques improve, researchers are finding it everywhere. Many animals maintain symbiotic relationships with bacteria—living arrangements where both partners benefit. If cockroaches have absorbed this much bacterial DNA, the same process could be happening across the animal kingdom on a scale no one has yet measured.
The next phase of research will focus on two questions. First, the team wants to determine whether any of those 40,485 fragments are actually functional—whether they're contributing to cockroach survival, adaptation, or any other trait. Second, they plan to extend their investigation to other species that harbor similar bacterial partners, looking for evidence of the same kind of genetic borrowing. What emerges from this work could reshape how scientists understand evolution itself, revealing that the boundaries between species are far more permeable than previously believed, and that survival sometimes depends not on what you inherit from your ancestors, but on what you steal from your neighbors.
Notable Quotes
The persistence of numerous inserts over millions of years indicates that they may have assumed functional roles in both genes and intergenic regions, are effectively neutral, or are only slightly deleterious.— University of Sydney researchers, published paper
The Hearth Conversation Another angle on the story
So cockroaches have been absorbing bacterial DNA for millions of years. Does that mean they're part bacteria now?
Not quite. The DNA is there, integrated into their genome, but we don't know if most of it's actually being used. It's more like they've collected a vast library of genetic instructions they may or may not be reading.
But some of it must be doing something, right? Otherwise why would it stick around for 28 million years?
That's the working hypothesis. DNA that's actively harmful usually gets weeded out by evolution. If these fragments have persisted that long, they're either useful, neutral, or only slightly damaging. The researchers just don't know which yet.
How did anyone miss this before? Forty thousand fragments is enormous.
Previous studies were looking for large, protein-coding genes. These fragments are small and don't build proteins. It's like searching for furniture in a house when you're only looking at the walls—you miss everything in between.
Could this be happening in other animals too?
Almost certainly. Any animal with a long-term symbiotic relationship with bacteria could be doing this. We're probably just now developing the tools to see it.