Biologists discover a gene essential for the development of emblematic ancestors’ spurs

Every now and then over the course of life’s history a new trait evolves that leads to an explosion of diversity in a group of organisms. Take wings, for example. Each group of animals that made them evolve transformed into a multitude of different species: birds, bats, insects and pterosaurs. Scientists call these “key innovations”.

Understanding the development of key innovations is essential to understanding the evolution of the astonishing array of organisms on Earth. Most of them occurred in the distant past, which makes them difficult to study from a genetic perspective. Fortunately, a group of plants have acquired such a characteristic over the past millions of years.

The columbines, with their elegant nectar spurs, promise scientists an opportunity to study the genetic changes that underlie a key innovation. After much research, Scott Hodges, professor at UC Santa Barbara, Evangeline Ballerini, research associate, and their coauthors at Harvard University identified a gene essential for the development of these structures. And to their knowledge, this is one of the first key innovations for which a critical developmental gene has been identified. Their findings appear in the journal PNAS.

The researchers named the gene after Gregg Popovich, head coach of the San Antonio Spurs basketball team. “This gene is a transcription factor, which means that it controls the development of stimulation in columbines by regulating the activity of other genes,” Ballerini explained. “So I chose the name POPOVICH because as a coach Popovich controls the development of the San Antonio Spurs, in a sense, by regulating the activity of his players.”

The evolution of spurs in ancestors from ancestors seems to have led to a rapid expansion of the genre. About 70 species have evolved over the past 5-7 million years, compared to its sister genus without spurs, which has only four species among its members.

And columbines aren’t the only spur flowers. The trait has evolved independently in many different plants, including nasturtiums, larkspurs, and impatiens. “And in each of those groups, those who have spurs have a lot more species than their closest relatives who don’t have spurs,” Hodges said.

“We believe that the diversity is related to the evolution of this spur because the spur produces nectar, which attracts animal pollinators,” Ballerini said. Changing the length or shape of the spur changes the animals that can pollinate the flower. “Bees only move pollen between bee flowers, hummingbirds only move pollen between hummingbird flowers, so you don’t switch genes between these two different populations.” Eventually, the two can split into different species.

The question the researchers were trying to answer was how innovations like these develop in the first place. “If we can find genes that are important in the development of a key innovation, it will help us understand this type of process,” said Hodges.

“In most of these cases, like in the example of wings with birds, bats and insects, these evolved so long ago that it is difficult to find a particular gene that was essential. to change this trait, ”he added. “Here we have a fairly recent origin of a key innovation, only 5-7 million years ago, and it’s a pretty straightforward trait, so it’s a little simpler.”

Find POPOVICH

Since columbines have evolved so recently, most of them can form fertile hybrids with each other. In the 1950s and 1960s, a Polish geneticist crossed a spurless species – aptly named the spurless columbine – with its spurless cousins. She found that in the first generation of offspring all had spurs, but these self-pollinated resulted in a second generation where the lack of a spur reappeared in a quarter of the plants.

This ratio was crucial for the work of Hodges and Ballerini half a century later. This simple fraction suggested that a single gene controlled the development of spurs. But columbines have about 30,000 genes, and only one was the gene they were looking for.

Following in the footsteps of his predecessor, Hodges also crossed the spurless columbine with a spurred species and then self-pollinated the offspring. But unlike the previous experience, Ballerini and Hodges now had the tools to research the genetic code of plants.

Ballerini sequenced the genome of each of the nearly 300 second-generation plants and looked for cases in which plants without spurs had inherited two copies from their spurless grandparent. This limited the search to around 1,100 genes on one of the plant chromosomes.

Still, 1,100 genes are a lot to sort through. “There was no guarantee that these methods would lead us to the gene we were looking for,” Ballerini said. “There was certainly quite a bit of work in all the experiments and analyzes, but in the end there was also a bit of luck.”

Ballerini examined gene expression during five stages of early petal development in spurless columbine and three other spur species. She sequenced all the genes turned on at each stage and looked for consistent differences between spurless and spur plants. Eventually, with the contribution of one of her collaborators at Harvard, Ballerini suspected that she had identified the correct gene. It was always turned off in spurless species, turned on in spurless species, and was one of 1,100 genes previously identified as associated with spurless flowers in the genetic cross. Now was the time to test his hypothesis.

She used a genetically engineered virus to reverse the expression of the gene in question as well as a critical gene for the production of red pigment. This way, they could tell which petals were affected just by looking at the color.

Wherever POPOVICH was sidelined, the flowers developed tiny spurs. But the length of the spur depends on both the number and the size of the cells. The researchers therefore worked with collaborators to count the number and measure the length of each cell making up these tiny spurs.

“The longer spurs had more cells and the shorter spurs had fewer cells,” Hodges noted. “The gene must therefore have acted by affecting the number of cells produced.”

Ballerini remembers sitting in her office after finishing her last scans. She started pitching potential gene names to graduate student Zac Cabin, another sports enthusiast. “At the same time, Zac and I turned to each other and we both said ‘POPOVICH! »», She remembers. The name seemed perfectly fitting. “And that leaves open the possibility that if we identify other genes at play in stimulus development, we may name them after some of the Spurs players.”

A path to new discoveries

While the identification of POPOVICH is certainly successful, the real value of the discovery lies in what it reveals about the evolution of key innovations. Prior to this work, none of the plant groups with well-known genomes also made spurs. “We didn’t know where to start,” Hodges said. “This discovery gives us a foothold.”

“Once we identify a gene – like this gene, which seems to be the key in the spur-forming process – then we can start to understand all of the components,” he added. The team can now begin to search for genes regulated by POPOVICH and those regulating POPOVICH. “It’s a place to start to understand the whole system.”

Although researchers are not sure how POPOVICH works in other groups of plants, it appears to influence the number of leaflets that grow on ball clovers. Columbines also express the gene in their leaves; perhaps it was recruited from the leaves to the development of the petals, suggested Ballerini.

The new adaptations don’t appear out of nowhere, she explained. “When you evolve a new structure, you usually aren’t evolving a whole new gene.” In general, organisms reuse or add a purpose to an existing gene.

The authors are also interested in identifying the genes involved in the second phase of spur formation: the elongation of cells in the spur cup.

“These are things that we will want to do now that we have identified this gene,” Hodges said. “And since it’s a transcription factor, it has to have particular genes that it affects. The next logical step would be to identify the targets for this gene, and that would tell us a lot more about how it works. “

The researchers expressed their gratitude to Harvey Karp, who generously funded the Karp Discovery Award that made their research possible. “We really couldn’t have done this project without him,” Ballerini said.