Bold claim: half of Triturus newt offspring die due to a stubborn, ancient genetic trap that should have vanished long ago—and yet it persists for at least 25 million years. That paradox sits at the heart of a fascinating discovery by Leiden researchers, who uncovered a mysterious DNA error in crested and marbled newts that defies simple evolutionary logic. PhD candidate James France uncovered new clues that illuminate how this balancing lethal system remains alive in nature.
In Triturus newts, offspring outcomes are split: half inherit one version of chromosome 1, and the other half inherit the alternate version. Both versions are necessary for survival, so when offspring inherit the same version from both parents by chance, those individuals typically perish. In effect, a substantial fraction of the next generation is doomed right from the start. This would seem sufficient to erase the system over time, yet the mechanism endures.
“We would expect such a mistake to disappear through evolution,” notes Ben Wielstra, who supervised France’s doctoral work on crested and marbled newts. “But this system has persisted for tens of millions of years.” The researchers wondered why a seemingly harmful arrangement persists, and they focused on what makes both chromosome versions essential: each carries missing large, unique DNA segments. A viable genome requires one complete set from each version, so animals with only one version lack crucial genes.
A leading hypothesis from the 1980s proposed that the error arose from a rare, faulty exchange between chromosomes. The new study provides the first direct evidence supporting that view: a single mutation created a huge deletion on chromosome 1 in both versions, paired with a corresponding duplication on the opposing version. In other words, the system emerged when a vast block of DNA was lost on both versions, while the same region was duplicated elsewhere on the other version.
But how does such a harmful arrangement propagate? France used computer models to simulate small, isolated populations—like newts living in separate ponds—where natural selection acts less efficiently. In those settings, harmful variants can slip through and become established. “As soon as the system is in place, it maintains itself. It’s a kind of evolutionary trap,” Wielstra explains. In small populations, unusual events can occur: if a population that harbors the faulty system later colonizes new habitats, the disadvantage can spread.
Despite the mechanism’s persistence, such systems remain rare, with only a handful of species showing similar dynamics. Triturus newts are the most thoroughly studied case, though analogs exist in certain plants and insects.
The trap is notoriously hard to remove once rooted: when a healthy newt mates with one carrying the system, many offspring end up with an imbalanced gene copy number, often resulting in lethality. “Once the system is in place, it maintains itself,” Wielstra reiterates, highlighting how an evolutionary trap can become self-sustaining.
The research demanded extensive lab work, notably given the sheer size of newt genomes—about ten times larger than the human genome. Sequencing everything directly would be impractical, so teams in Poland and Serbia bred multiple newt species and tracked how thousands of genes were inherited. By identifying which regions were consistently missing or duplicated, researchers could map the system’s structure. It was a colossal, multi-year effort, and the pandemic added further challenges. Still, the work reveals how strikingly natural laws can operate in surprising ways.
What lies ahead is to pinpoint the exact genes that are essential, and to understand which developmental switches are missing in embryos that fail to survive. Modern genetics offers potential tools to explore this deeper. “As an evolutionary biologist, the goal is to understand what’s happening. Can the system be reversed to make an animal healthy again? Or could a healthy animal be made to carry the same error to learn why it dies?” Wielstra muses.
This research is driven by pure curiosity about evolution’s quirks. “Evolution is like gravity: it happens, sometimes for the good, sometimes for the disastrous,” Wielstra says. These newts demonstrate just how unexpectedly natural laws can unfold.
The project culminated with James France defending his thesis, Comparative genomics of the balanced lethal system in Triturus newts, on April 3, 2025, in Leiden’s Academy Building, under the supervision of Ben Wielstra and Michael Richardson. This work was supported by the scholarly community and reflects a snapshot of evolving scientific understanding, with the full details available through the cited sources.
If you’re curious about how rare genetic traps shape biodiversity—and whether similar dynamics operate in other species—share your thoughts in the comments. Do you see this as a remarkable quirk of evolution, or a cautionary tale about how fragile balance can be in biology? Also, what potential implications might this have for conservation or genetic research more broadly?
