With each influenza infection, an immense number of new viral particles are produced, and it is easy to imagine them as exact replicas of the original. In reality, many of these viral particles lack crucial segments of the genetic blueprint.
Researchers tracked the spread of influenza over dozens of generations and discovered that these defective viruses are far from useless. The findings of the study were published in the journal Nature Communications.
One variant even developed a mutation that hinders the successful development of the virus itself, revealing an unexpected force that shapes the course of infection. Scientists refer to these defective versions as faulty genomes—portions of viral code with large sections removed from their middle.
When replicating its eight gene segments, the influenza virus often discards long stretches from one of them. What remains is too short to independently construct a complete virus.
On its own, such a genome is a dead end. However, if it ends up inside a cell already infected with a full-length virus, it hijacks the virus’s replication machinery to produce even more copies of itself. This suppressive effect, known as interference, has been studied for several decades.
Defective genomes can reduce the amount of functional virus released by a cell and are associated with milder infections. Until now, it remained unclear how they behave over many generations.
To track this phenomenon, the team cultivated influenza A virus in cell cultures and transferred it from one batch of cells to another 72 times.
At each stage, they sequenced the entire genetic makeup of the viral population, recording millions of genomes simultaneously.
The work was conducted by Christopher B. Brooke from the University of Illinois at Urbana-Champaign (UIUC) along with a team from the Singapore Agency for Science, Technology and Research (A*STAR).
Propagating the virus through numerous cycles in this way accelerates its evolution within a Petri dish, condensing many rounds of competition into a single experiment. In each generation, damaged and full-length genomes competed for the same limited resources, and the analysis results revealed which ones prevailed.
Initially, the situation was chaotic. Early results identified hundreds of different defective genomes, each missing a slightly different fragment. There was no clear leader, but then the number of contenders dwindled.
In subsequent generations, this vast diversity vanished until only one or two defective genomes dominated the population. Out of hundreds of candidates, only a handful remained. Because this reduction repeated itself in later experiments, the researchers concluded it was not random.
An earlier study had observed defective particles rising and falling in a similar manner, but without explaining the underlying cause. The next mystery was what gave these “survivor” particles their advantage.
The outcome was determined by a single mutation—a change of one letter in the virus’s genetic code that consistently reappeared. Across different experiments, the defective genomes that eventually became dominant carried the same tiny alteration. Independent populations ultimately converged on the same result.
When the researchers tested specimens with this change, the results were striking. The mutated versions replicated themselves more aggressively and suppressed the virus’s activity more strongly than copies without it. A small modification made them more efficient at both multiplication and sabotage.
Scientists have long known that the influenza virus produces defective genomes and that some of them interfere with the functionality of the intact virus. However, prior to this study, no one had precisely identified the specific recurring mutation that enhances this interference. The repeated emergence of this phenomenon indicated a predictable evolutionary path rather than a random one.
A defective genome cannot survive on its own, so its success depends on exploiting the very virus that created it. It appears that the beneficial change simply amplified this advantage, allowing one variant to overtake all competing nonfunctional copies.
It is exactly this self-destructive behavior that makes defective genomes attractive to drug developers. Because they suppress the activity of the intact virus, researchers have spent years investigating whether they could be used as antiviral agents—packaged as therapeutic particles and administered to patients, as described in one review.
This study identifies the single change that makes a defective copy most harmful to the intact virus, providing developers with a precise feature to incorporate into a treatment.
Instead of relying on random defective copies, this concept has already shown encouraging results in early studies.
In animal experiments, doses of these interfering particles protected mice from influenza that would otherwise have been lethal, and a paper on genetically engineered versions reported strong protection with low toxicity.
Two points are now clear. The scattered copies of the influenza virus are not random debris, and they follow a predictable path that culminates in one or two winners.
A single recurring modification makes this preparation particularly effective at combating the intact virus. For drug developers, this transforms a vague strategy into a well-defined target.
Rather than using whatever defective genomes the virus happens to produce, researchers can deliberately craft the most destructive version, incorporating the efficiency-boosting mutation from the start.
The same vulnerability may extend to other viruses that generate defective copies, including more dangerous respiratory infections. For a virus that reinvents itself every season, a weakness embedded in its own genome is a rare and dependable target.
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