When threatened by a virus, bread mould attacks its own DNA

Bread mould mutates its own DNA as a way of fighting off infections, new research suggests.

While most organisms try to stop their DNA from mutating, scientists from the UK and China have discovered a common fungus found on bread behaves differently.

Although mutations can enable species to adapt, most are harmful, and so evolutionary biologists have suggested natural selection will always act to reduce the mutation rate.

Recent work by Professor Laurence Hurst, of the Milner Centre for Evolution at the University of Bath, and Sihai Yang, Long Wang and colleagues at Nanjing University, China, have found that a type of bread mould called Neurospora crassa is an exception to the rule.

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Prof Hurst said: “Many organisms have a problem with transposable elements, otherwise called jumping genes.

“These are virus-like bits of DNA that insert themselves into their host’s DNA, copy themselves and keep on inserting – hence the name jumping genes.

“Organisms have found different ways of combating this nuisance, many of which try to prevent the transposable elements from expressing their own genes.

“Neurospora has evolved a different solution – it hits them exceptionally hard with mutations to rapidly degrade them.”

It would be like opening up the back of a watch, stabbing at all the cog wheels that look a bit similar and expecting the watch to still function

Professor Laurence Hurst, University of Bath

The study published in the Journal Genome Biology found that the mould distinguishes jumping genes from its own DNA by detecting two or more copies of the same bit of DNA.

The fungus then attacks the jumping genes by mutating them in a process called repeat-induced point mutation (RIP).

Researchers found that each base pair in the Neurospora genome has about a one in a million chance of mutating every generation.

Prof Hurst said: “This was a real surprise to us – any organism that hits its own genes with that many mutations is likely one that will not persist for very long.

“It would be like opening up the back of a watch, stabbing at all the cog wheels that look a bit similar and expecting the watch to still function."

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“Our findings show that Neurospora has not only a high mutation rate but is also a massive outlier," said Prof Hurst.

“It appears to use RIP to destroy transposable elements from viruses, but at a cost, with considerable collateral damage.

“This organism thus goes against the standard theory for mutation rate evolution which proposes that selection should always act to reduce the mutational burden.

“It is the exception that proves the rule.”