While mutations can arise in the genome and have little consequence, sometimes they are beneficial and selected to remain in an organism. In most cases, genetic errors have negative consequences for an organism, so biology prefers not to mutate the genome. Sometimes, cells can even detect and repair undesirable mutations. But researchers have now found that a common fungus called Neurospora crassa, which grows on bread, and is shown in the video below, actively causes mutations so that it can fight off viral transposable elements, which can disrupt genomic sequences. In the process of mutating these elements, it mutates its own DNA too. This work has been reported in Genome Biology.
"Many organisms have a problem with transposable elements, otherwise called jumping genes," said Professor Laurence Hurst, Director of the Milner Centre for Evolution at the University of Bath. "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 combatting 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."
In this study, the researchers found that the mold can differentiate between the viral jumping genes and its own DNA by searching for bits of DNA that are repetitive and then attacking the jumping genes by mutating them. The process is called Repeat-Induced Point mutation (RIP).
RIP impacts the genome of the fungus; the scientists sequenced the genomes of fungal parents and offspring over many generations. They looked for mutations in these genomes, and determined that every base pair of the Neurospora genome has around a one-in-a-million chance of mutation every generation.
"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 cogwheels that look a bit similar and expecting the watch to still function," noted Hurst.
"Our findings show that Neurospora has not only a high mutation rate but is also a massive outlier. It appears to use RIP to destroy transposable elements but at a cost, with considerable collateral damage," Hurst continued. "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."