Ranunculus flowers produce numerous carpels via evolutionarily rewiring of the canonical AGAMOUS-WUSCHEL developmental module

Most flowers are built with a precise, repeating rhythm. They start with a small, dome-shaped cluster of cells at the tip of a stem, known as a floral meristem. This cluster acts as a construction site, churning out the flower's parts in a specific order: first the outer protective leaves, then the colorful petals, followed by the male stamens, and finally the female carpels at the very center. In the vast majority of flowering plants, this construction site shuts down the moment the female parts begin to form. The result is a flower with many male parts but only a few female ones, a pattern that has held true for millions of years. Scientists have long understood that a specific genetic switch, involving a protein that maintains the construction site and another that tells it to stop, controls this timing. When the stop signal arrives, the site flattens out, and no new parts can be added.

But nature occasionally breaks the rules. In some plants, the construction site refuses to close. Instead of stopping after a few female parts, it keeps growing, churning out dozens or even hundreds of carpels. This creates a flower that looks like a dense, fuzzy cone rather than a typical bloom. For a long time, researchers did not know how these plants managed to keep their construction sites open while others shut them down. Was it a simple mistake in the genetic code, or was there a more complex rewiring of the instructions? A new study on the buttercup family, specifically looking at a species called Ranunculus sceleratus, has finally revealed the answer. The researchers found that this plant did not just break the stop signal; it completely rewired the genetic machinery to turn the stop signal into a go signal, allowing the flower to produce an extraordinary number of female parts.

The researchers began by comparing the cellular behavior of Ranunculus sceleratus, which produces over 150 carpels, with a close relative, Nigella damascena, which produces only about five. Using a technique that allows them to watch living cells divide and expand in real time, they observed a striking difference. In the plant with few carpels, the cells at the tip of the flower stop dividing and expanding as soon as the female parts appear, causing the construction site to flatten and close. In the plant with many carpels, the cells continue to divide and expand vigorously even after the first female parts have formed. This sustained activity keeps the construction site dome-shaped and spacious, providing room for hundreds of new carpels to develop. The study showed that the cells in the high-carpel flower were not just dividing more often; they were also growing larger, creating a much bigger surface area to hold the extra organs.

To understand why the cells kept growing, the team looked at the genes responsible for managing the construction site. In most plants, a gene called WUS acts as the engine that keeps the site active, while a gene called AG acts as the brake, turning off WUS when the female parts are ready. In the plant with few carpels, the brake works perfectly: once the female parts start, the AG gene silences the WUS gene, and construction stops. However, in the plant with many carpels, the researchers found that the plant had two copies of the WUS gene instead of one, a result of an ancient duplication event. More importantly, the way these genes behaved had changed. One copy of the gene continued to work in the center of the flower, keeping the site open. The other copy, however, had moved its activity to the male parts of the flower, the stamens.

The most surprising discovery was how the brake gene, AG, interacted with these new copies of the WUS gene. In normal plants, AG suppresses WUS. But in this buttercup, the AG gene actually activated one of the WUS copies. Instead of shutting down the construction site, the presence of the male parts now signaled the site to keep going. The researchers traced this change back to the DNA sequence just before the gene started. In the ancestor of these plants, this region did not have a specific binding site for the AG protein. But in the lineage that led to the high-carpel buttercup, two new binding sites appeared in this region. These new sites allowed the AG protein to latch onto the gene and turn it on, rather than turn it off. This single change in the genetic instruction manual flipped the relationship between the two genes from a stop signal to a go signal.

The study confirms that this rewiring is the key to the flower's unique shape. When the researchers silenced the AG genes in the high-carpel plant, the flower stopped producing so many carpels and became smaller, resembling its low-carpel relatives. When they silenced the WUS genes, the construction site collapsed entirely, and the flower failed to develop properly. The evidence suggests that the evolution of this plant did not require a complete overhaul of its genetic system. Instead, a simple duplication of a gene, followed by the addition of two small binding sites in its DNA, was enough to rewire a fundamental developmental loop. This allowed the flower to ignore the usual timing cues and continue building female parts long after the rest of the plant would have stopped.

This finding offers a clear example of how complex new traits can arise through small, specific changes in the genetic code. It shows that the diversity of flower shapes we see in nature is not just a matter of random mutation, but often the result of precise evolutionary tinkering with the switches that control growth. By turning a stop signal into a go signal, this buttercup has managed to create a floral structure that is both rare and highly specialized. The research highlights that the rules governing how plants build their bodies are flexible, and that a few extra letters in a DNA sequence can lead to a dramatic transformation in the final form of a living thing.