快猫短视频

All great crested newts have a genetic flaw that dooms half of their offspring to die while still in the egg. How they evolved such a disadvantageous trait has long been a mystery, but now we have an unexpected answer.

鈥淚t鈥檚 fair to say that everything we expected or predicted turned out to be wrong,鈥� says at Leiden University in the Netherlands.

The fact that many crested newts die in the egg was first noticed in 1821. At first it was assumed this was due to the conditions the eggs were kept in, but by the 1980s it was traced to a chromosomal abnormality. 鈥淔ifty per cent of your resources just lost,鈥� says Wielstra. 鈥淭hat鈥檚 just crazy.鈥�

Most animals have two copies of each chromosome, one from each parent. Usually, the only pair of chromosomes that aren鈥檛 identical in size are the sex chromosomes. But in crested newts, chromosome 1 comes in a large form and a small form.

Offspring that inherit two copies of the small form die because they are missing key genes found in the large form. Those that inherit two large forms also die because they have too many copies of those genes. Only offspring with one large and one small form have the right number of genes and survive.

This is known as a balanced lethal system, and it is found in all species in the genus Triturus, which consists of around nine species of crested and marbled newts. 鈥淭hey all evolved from a single ancestor around 24 million years ago, and that ancestor must have had the balanced lethal system,鈥� says team member also at Leiden University.

The leading hypothesis for how this came about, which France and Wielstra expected to confirm, is that the large and small forms of chromosome 1 were once sex chromosomes. Instead, the team鈥檚 genetic studies revealed that these forms were generated by a single massive mutation.

During the formation of eggs or sperm, matching chromosomes swap sections. As this happened, a massive chunk of one chromosome 1 got transferred to the other copy, generating the two forms.

This possibility was previously dismissed because there appeared to be no reason why the mutant chromosomes would survive and spread. 鈥淭hat was something that we were really stumped by for a long time,鈥� says France.

Then they realised offspring that inherited a normal chromosome 1 and a large or small form might be able to survive but wouldn鈥檛 be entirely healthy. If these animals interbred, however, some of their offspring would inherit the large and small form and be perfectly healthy because they have the right number of genes.

So, a population with the balanced lethal system could persist alongside the original species because any interbreeding would result in unhealthy offspring. In this way, a new species could arise within two generations of the mutation. 鈥淏y the standard for speciation that鈥檚 pretty instantaneous,鈥� says France.

at the Swiss Federal Research Institute (WSL), says these findings point to a single massive mutation, but she is less convinced by the explanation for how it persisted.聽 鈥淓ven their own simulations suggest that the probability that this would happen is very low,鈥� she says.

France counters that if it were more likely, there would be more animals with this kind of balanced lethal system. But we know only of a handful so far, none which originated in this way. 鈥淚t makes sense that there鈥檚 a low chance of it because it鈥檚 a very strange and unlikely system,鈥� he says.

Many questions remain unanswered. For instance, nowadays Triturus species only have to compete with smaller newt species that they often eat. But when the balanced lethal system evolved, they would have been competing with a nearly identical species that had twice as many offspring.

鈥淪o it鈥檚 not as much of a handicap as it seems today, but when it evolved it was a massive handicap, which is really the hardest bit to explain,鈥� says France.

It is possible their survival is down to fortuitous events, he says, such as some environmental disaster wiping out the ancestral species.