How snapdragon genes shape colours to guide bees

In the wild, flowers need to catch attention. A snapdragon uses yellow and magenta patches on its petals to show a passing bee precisely where to land.

A team of biologists has now revealed how much goes into producing just one of those colours. The yellow is not simply splashed on: its shade is carefully blended.

The research was carried out by Professor Enrico Coen’s group at the John Innes Centre (JIC).

Partners at the Institute of Science and Technology Austria (ISTA) contributed to the fieldwork and numerical analysis.

Two kinds of snapdragon

In the Pyrenees, two kinds of the same snapdragon grow close together. Their colours are almost mirror images of one another.

One has yellow flowers with a small magenta dot at the bee’s entrance. The other reverses the pattern, bearing magenta flowers with a small yellow dot.

For a bee, the patterns are more than attractive. Yellow and magenta activate different parts of its eye, making each dot a conspicuous signpost.

The two kinds mostly remain separate in the wild. Each occupies its own section of hillside, and they meet only in a narrow strip.

A patch that fades

Yellow is not distributed uniformly across the flower. It is brightest at the landing spot and becomes fainter towards the petal’s edge.

In magenta flowers, the colour fades quickly, leaving a neat yellow dot. In yellow flowers, it fades gradually, spreading colour across the entire petal.

The rate of fading matters because a clear boundary prevents yellow from bleeding into magenta and obscuring the message the bee sees.

That separation is crucial to the bee. A small, distinct dot and a broad wash of colour offer two very different signals for landing.

Blurred colours confuse bees

Getting the pattern wrong comes at a cost. When yellow spreads into magenta, both colours become muddied and the bee is left uncertain where to go.

A bee that cannot find its way may move on. For a plant whose survival depends on pollination, a clear signal has considerable value.

Distinct, sharply separated colours make the whole flower easier to read. That clarity draws a busy bee down from the air.

Many genes shape snapdragon colours

The shape of the fading pattern is determined not by a single gene, but by a group working together.

Seven genes control the two colours: three govern magenta and four govern yellow.

One of the yellow genes had escaped detection until now. The researchers finally identified it and named it FLAVIA.

There was a reason it had been so easy to miss. It lies in a section of the plant’s genes that undergoes very little reshuffling, which kept it hidden for years.

Effects that stack up

The biggest surprise was the way the genes interact. Their small effects do not merely add together independently.

Instead, each builds on what came before, acting only on the colour left by the others. In combination, a few gentle changes produce a steep drop in yellow.

The process is somewhat like applying thin coats of paint. Each successive coat covers less bare wall than the one before, yet the light layers still create a deep, even shade.

Some of these genes make changes so slight that a human eye would scarcely notice them. Together, however, they do most of the shaping.

A natural test zone

The two kinds meet in one area, where their genes mix. Flowers there emerge in unusual blends, including orange and white.

A gardener might find them appealing, but bees are less enthusiastic. Their mixed-up colours send weak signals, so the blends do not spread far.

That resistance keeps the mixing zone narrow, at only about 0.97 kilometres across. Its limited width allowed the team to assess how strongly nature favours each gene.

Genes that produced the greatest colour changes were held most firmly in place. Those with subtler effects could drift over a somewhat wider area.

Snapdragon colours evolved slowly

The process did not happen quickly. The two kinds have been meeting for at least a hundred years, and probably much longer.

Throughout that period, natural selection has continuously sifted the genes. The resulting colour is so finely tuned that the adjustment is almost invisible to us.

When survival is at stake, small advantages accumulate. Given enough time, even a gene that offers a modest benefit can prevail.

Why the bee decides

Ultimately, the bee makes the judgement. A shade that appears faint to us can be a distinct signal to an insect searching for flowers.

“We have looked at how fascinating colour patterns in nature are formed and shaped, the genes that make such patterns and how their working together has been selected over evolutionary time to favour the dance and visit of bees,” said Dr Desmond Bradley, the study’s lead author.

“Together these four genes act together to precisely hone the yellow gradient. Some genes have very subtle effects, but each contributes to the gradient pattern that is monitored by the bee.”

“Even those patterns which we could hardly discern can be distinguished by the bee in the hybrid zone.”

Beyond the flower bed

Fading patterns of this kind appear throughout the living world. They shape butterfly wing markings and even help establish the earliest form of a fly egg.

For a long time, nobody understood how nature could tune these patterns so precisely. The flower provides a clue that may apply in many other cases, with numerous genes each contributing a small effect.

It suggests a strategy nature may use repeatedly: large, complete patterns can arise from many tiny, separate steps.