Grasses steal genes from their neighbours

British researchers discover that grasses can use a process called lateral gene transfer to shortcut their evolution

Scientists at the University of Sheffield in the United Kingdom have discovered that grasses can shortcut their evolution by taking genes from their neighbours in a system called lateral gene transfer.

The researchers were studying the photosynthesis of grasses, the chemical process by which plants make their food from carbon dioxide and water in the presence of sunlight and chlorophyll. They focused on the tropical grass Alloteropsis semialata, also known as black seed grass or cockatoo grass. The majority of temperate grasses use the ancestral (C3) photosynthesis pathway but cockatoo grass has populations that use C3 photosynthesis and C4 photosynthesis, a modified pathway that improves photosynthesis efficiency in warm, dry conditions and used by many tropical grasses.

While reconstructing the evolutionary history of photosynthesis pathways, researchers discovered two genes acquired by the study plant from distantly related grass species. It led to decoding the grass’s genome to see what other genes had been acquired from its neighbours.

“This is the first time that a large number of genes from the nuclear genome have been shown to be transferred among plants that are not involved in host-parasite interactions,” said Luke Dunning, postdoctoral research associate with the university’s animal and plant sciences department.

He said certain traits of grasses may make LGT more likely, such as wind-pollinated characteristics and the high density stands they often grow in. However, he said researchers are detecting them more frequently in grasses, which suggests LGT may also be a product of the available genetic resources for this plant group.

Researchers found grasses absorbed useful genetic information from neighbouring grasses to out-compete others and survive in a variety of habitats. The scientists looked at some of the most economically and ecologically important plants including wheat, corn, rice, barley, sorghum and sugar cane.

“During evolution, a majority of mutations and genetic variations are lost,” said Dunning. “If they have no dramatic advantage, then they may never reach a high frequency in the population and can drift away. Every once in a while, a gene of adaptive significance, will give an individual an advantage and they will go on to produce more seeds than surrounding individuals, increasing the proportion of individuals with the laterally acquired gene in the next generation. If really advantageous, this survival of the fittest process may ultimately result in all individuals of a species having the stolen gene.”

Dunning said a species that has been living in an extremely dry habitat for millions of years may have genes that have been optimized for drought resistance. If a wetland species colonized that habitat and acquired the drought-optimized genes, it could be an advantage and they could effectively skip millions of years of evolution.

The mechanism behind these gene transfers remains unknown, but researchers have two theories they plan to test.

One of those theories is illegitimate pollination.

“The grass species involved are too distantly related to hybridize,” said Dunning. “However, pollen from one species can still land on the flowers of the other, and the pollen tube may grow, which can transfer small fragments of DNA into a fertilized embryo. The resulting seed will then have the genome of the parents (both of the same species), with a little extra genetic information acquired from the distantly related species.”

The other theory is root-to-root contact. Many of the grasses studied can reproduce through rhizomes, which contain stem cells that form the rest of the plant. If the grasses grow in high density, the roots of one species penetrate the rhizome of another and small amounts of DNA could be incorporated into the plant stem cells and plant tissue, including the flowers, to make the next generation of seeds. Any seed arising from that process will possess the foreign genetic material.

A major question now is whether LGT exists with genetically modified plants.

“We have not detected transgenes from GM crops in wild species,” said Dunning. “What we have shown is that wild species are trading genes, and the genes they receive can have adaptive consequences for disease resistance, environmental adaptation etc.”

Dunning pointed out that the research shows that plants are exchanging advantageous genes naturally, so genetically modified grasses are occurring naturally.

“One could almost say that GM crops are not as unnatural as generally perceived.”

He said previous studies on the chances of traits from artificially developed GM crops moving into non-GM plants have focused only on hybridization with closely related wild relatives, and the risk was deemed to be very low in most cases.

This new work, however, shows that gene flow in grasses is not restricted to close relatives and it can occur across the grass family through LGT.

Future research should help determine whether the process occurs in every grass species or is restricted to grasses with specific traits. Researchers are also expected to look at the conditions that facilitate their spread.

The report was published in the Proceedings of the National Academy of Sciences.

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