There is no rose without thorns, the old saying goes. But to botanists, there is no rose with thorns — the spiky outgrowths of a rose stem are called “prickles” and are biologically distinct from the stiff, woody thorns of other plants.

Prickles are a remarkable example of evolution repeating itself. In the past 400 million years, plants evolved them 28 different times. Roses grow prickles on their stems, whereas others grow them on their leaves or their fruits. Grasses grow tiny prickles on their flowering tufts. Solanum atropurpureum, a wild relative of potatoes that grows in Brazil, has prickles so nasty that they’ve earned it two fearsome nicknames: Purple devil and Malevolence.

A new study published in Science sheds some light. Over and over again, each species took advantage of a single gene. The discovery opens up the possibility of altering the DNA of plants to remove their prickles, making some wild plants easier to grow as crops.

Zachary Lippman, a plant geneticist at Cold Spring Harbor Laboratory, US, got curious about prickles as he studied a group of crops that includes potatoes, tomatoes and eggplants. While the crops are all prickle-free, some of their wild relatives are covered in the little daggers. Early farmers must have bred the prickles out of the wild plants as they started using them for crops.

“Who wants to cultivate eggplants that you can eat if there are these prickles poking you all the time?” he asked.

The loss of prickles led Lippman and his colleagues to wonder how the spikes had evolved in the first place. It’s likely that prickles arose in many plants as a defence against animals that would devour them. But prickles can serve other purposes, too. Some plants use them to hook onto surfaces while climbing. Some wild grasses use them to hook their seeds onto the fur of passing animals.

Charles Darwin recognised that the same trait can evolve many times over in the history of life — a process known as convergence. Bats and birds, for example, both evolved wings for flying. But they took different evolutionary paths to the same destination: bat wings evolved as membranes that grew between their fingers, whereas birds evolved wings made of feathers.

To understand how plants converged on prickles, Lippman and his colleagues crossed one variety of domesticated eggplant with its wild, prickly relative. Some of the hybrids had lots of prickles, while others had none.

Comparing the DNA of the plants, the scientists found that a specific variant of a gene called LOG was only carried by plants with prickles. Botanists had long known that LOG genes help produce a hormone that signals plant cells to grow. But in the wild eggplant, a special version of LOG allowed prickles to grow.

Lippman ran the same experiment on two other kinds of eggplants, crossing them with their wild relatives. Each time, the researchers discovered a LOG variant was responsible for prickles in the wild plants.

“We found that it was the same gene that had independently accumulated mutations in all three lineages to lose those prickles,” Lippmann said.

It might seem puzzling that mutating a LOG gene could take away prickles without harming the eggplant as a whole. After all, LOG genes are known to be essential for plants growing at all.

It turns out that plants have evolved many copies of LOG genes over millions of years. Lippman and his colleagues speculated that one copy of the LOG gene evolved in the wild ancestors of eggplants to make prickles, while the other versions went on with their original job of helping the plants grow.

He and his colleagues wondered if LOG genes were important to prickles in other plants as well. In dozens of species, they discovered evidence of the same link. Blocking a LOG gene in roses, for example, interfered with the development of prickles. “We can make these giant prickles turn into tiny little buds,” Lippman said.

The new findings could help scientists transform wild plants into domesticated crops, Lippman said. He and his colleagues tested this possibility on a wild Australian plant sometimes called the desert raisin. The sweet fruit is normally prickly. But after the researchers snipped the LOG gene from its DNA, it became free of prickles.

Vivian Irish, an evolutionary biologist at Yale University, US, who was not involved in the new study, said it showed that convergence can happen in a way that Darwin didn’t envision. Each time prickles arose, plants did not evolve an entirely new way to make them. They just borrowed the same gene again and again.

“Innovation in many cases might simply reflect reusing old genes in new ways,” Irish said.

NYTNS