It’s freecycling, but for DNA. Grasses routinely pass genes from one plant to another, even if they belong to distantly related species.
“We’ve shown that lateral gene transfer is a widespread process in grasses,” says Luke Dunning at the University of Sheffield in the UK. The finding adds to the evidence that DNA can be transferred from one complex organism to another, rather than only being inherited, and that this can benefit the recipient.
Biologists have known for decades that single-celled organisms like bacteria can pass genes in this way, a process called lateral gene transfer or horizontal gene transfer. But as recently as 20 years ago, it was thought that this didn’t happen in organisms with more complex cells, known as eukaryotes – the group that includes all animals, plants and fungi.
“People thought it was completely restricted to bacteria and didn’t happen in eukaryotes,” says Dunning. “It’s probably only been 10 to 15 years that that’s really shifted.” Nowadays many eukaryotic examples are known, such as a plant gene that has crossed into insects.
Most studies of this phenomenon have focused on isolated examples: for example, in 2019 Dunning’s team showed that a grass called Alloteropsis semialata had 59 laterally transferred genes.
To find out how widespread such gene transfer really is, Dunning’s team studied the genomes of 17 grass species, some of which have been evolving independently of one another for 50 million years. These included food crops like Asian rice, common wheat and foxtail millet.
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The team found that 13 of the 17 species carried laterally transferred genes – indicating widespread transfer. In total 170 genes had been transferred.
“As more and more genomes of eukaryotes are sequenced, we’re seeing so many examples of horizontal gene transfer,” says Julia Van Etten at Rutgers University in New Jersey. She co-authored a 2020 study estimating that about 1 per cent of the genes in the single-celled eukaryotes called protists are the result of lateral gene transfer.
For every 10,000 genes in the grass genomes, the team estimates 3.72 are detectably laterally transferred. “But that is a massive underestimate,” says Dunning, because only some transferred genes will be favoured by natural selection and become common in a population. “It’s probably an ongoing process happening all the time, and then you’re only going to fix one or two.”
The team found that lateral transfer was more common among closely related species, perhaps because their genes are more compatible. But it still happened in the least related ones.
Transfers were also more common in grasses that had rhizomes – underground stems that can send out roots and shoots beneath the surface. “They are tissues that allow plants to asexually reproduce,” says Dunning. “If you get any foreign DNA into that rhizome, when the plant regenerates it’s in every cell of that clone, including the flowers, and that’s how it gets into the germline.”
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“The million-dollar question is to find out how it’s happening,” says Dunning. The grasses aren’t hybridising with each other, as the DNA would look very different if they were. He suggests that in many cases pollination by wind might be a factor. “Potentially you could have illegitimate pollination where you only get a small bit of DNA transferred from an outside species”, instead of a true hybrid, he says.
Although laterally transferred genes only make up a small percentage of eukaryote genomes, they may still be having major impacts on evolution, says Van Etten. For example, she works with red algae that live in hot, toxic environments. “They’re thriving because they’ve horizontally acquired genes for arsenic detoxification and mercury detoxification, and they’re able to take up sugars so they don’t have to photosynthesise all the time. They’ve changed their entire lifestyle.”
It may be that lateral gene transfers underpin some of the traits found in domestic strains of crop grasses like wheat, says Dunning. That is speculation, but if it is confirmed, it will mean lateral gene transfer has helped us create the crops that now feed us.
Journal reference: New Phytologist, DOI: 10.1111/nph.17328