Pollenless dandelions may affect the future of insects

Dandelions in Sweden reproduce without pollination, and their seeds are genetic copies of the plants they grow on. Yet most dandelions still produce pollen. Recently, however, more and more pollenless dandelions have been discovered, a development that could reduce the availability of food for many insects.

Pollenless dandelions. Photo: Lina Enell/Stockholm University

“From an evolutionary perspective, it would make much more sense for dandelions to stop producing pollen,” says Yannick Woudstra, postdoctoral researcher at the Department of Ecology, Environment and Plant Sciences at Stockholm University.

In a field at Stockholm University, he has found an unusually large number of pollenless dandelions. He is now investigating how quickly they spread and what consequences this may have for springtime insects.

Can spread more quickly

From a distance, the flowers look the same, but with a hand lens it is possible to see that some dandelions have yellow pollen grains on their small pistils, while others are completely bare.

Stopping pollen production may give dandelions an advantage. In greenhouse experiments, pollenless dandelions have been compared with asexual dandelions that still produce pollen. When grown under the same conditions, the pollenless plants began producing more flowers, and could therefore also form more seeds.

“If a pollenless dandelion produces more seeds than its neighbours, it can in principle spread quickly. Over time, an entire field could become dominated by pollenless dandelions,” says Yannick Woudstra.

Yannick Woudstra is a postdoctoral researcher at the Department of Ecology, Environment and Plant Sciences.

Why dandelions produce pollen

If it takes both energy and resources for dandelions to produce pollen that they do not actually need, why do most of them continue to do so?

“It has to do with their origin. Sexual dandelions still exist in southern Europe, where they can grow close to asexual dandelions. When pollen from an asexual dandelion fertilises a sexual dandelion, new asexual microspecies can arise,” says Yannick Woudstra.

The new microspecies can then spread northwards. Because they originate from sexual dandelions, they often produce pollen, even though they do not need it themselves to form seeds. This may slow the development towards entirely pollenless populations.

Pollenless dandelions appear to be more common near the coast and further north in Sweden. Researchers do not yet know why this is the case.

An important resource for springtime insects

For the plant it may be an advantage to stop producing pollen. For insects, the development may have other consequences.

Dandelions flower early, when many bees, bumblebees and other insects are becoming active and there are still few other flowering plants. In early spring, dandelions can provide up to 80 per cent of the pollen available to insects in meadows, and are therefor an extremely important resource.

“If pollenless dandelions become more common, it could reduce the availability of pollen for insects that are already under pressure,” says Yannick Woudstra.

A common plant with unexpected complexity

Yannick Woudstra looking at dandelions with a hand lens.

Dandelions are often seen as simple and familiar. But according to Yannick Woudstra, they are unusually complex. In Sweden, there are hundreds of described forms, sometimes called microspecies. In practice, they are groups of genetically identical dandelions that reproduce by cloning themselves.

“The more I learn about dandelions, the more fascinated I become. What first seemed like an ordinary plant turned out to contain a wealth of evolutionary questions,” he says.

His advice to anyone with a garden is to let dandelions grow, especially in spring.

“Dandelions have an undeservedly bad reputation. They are little suns in the grass,” says Yannick Woudstra.

How to recognise a pollenless dandelion

Look closely at the small parts in the centre of the flower. A dandelion with pollen has yellow pollen grains on its pistils. A pollenless dandelion lacks these yellow grains and may have darker or slightly malformed pistils.

An ordinary hand lens with about ten times magnification is often enough to see the difference.