The role of pollinator preference to maintain pollen colour variation

It’s not just flower colour that can vary in a plant. Pollen changes in colour too. For example, pollen colour in Campanula americana varies from deep purple in western populations to white and light-purple pollen in eastern populations. Why the difference?

As Hempel de Ibarra and Somanathan mention in their commentary, it’s not our eyes that are the issue. It’s bees’ eyes, and they work in quite a different way. High-contrast pollen can help guide a pollinator to the target. But it’s not all about contrast. Thermal tolerances mean that pollen that’s viable in one location may be at a disadvantage in another.

Ison and colleagues set out to see if pollinators are helping shape the colour of pollen. They did this a flight cage, some C. americana flowers and a lot of patience. The botanists trained the foragers. They did this by removing nectar from plants with white pollen, replacing the nectar with water. In plants with purple pollen, they loaded the nectaries up with a sucrose-water mix. The only cue for the insects to follow was the different colour of pollen. When the flowers were prepared, they released the bees, Bombus impatiens. The team found that they could train the bees to favour the purple-pollen plants, showing the bees could distinguish by pollen colour. They then went out and looked at wild populations of C. americana.

They found that C. americana‘s specialist pollinator, Megachile campanulae preferred purple-pollen plants. In particular, they looked for pollen-bearing male-phase flowers. Other bees had no preference without training. This preference matters as prior research has shown that Megachile removes twice as much pollen per visit as other bees. They can get through a lot of pollen. Ison and colleagues say that a preference for purple pollen could remove it from reproduction, favouring whiter pollen. So how is purple pollen surviving? Ison et al. conclude in their paper that pollen has another factor acting on it: “[S]election against purple pollen is opposed by abiotic selection favouring purple pollen since it is more heat resistant. These opposing selective forces may help to maintain pollen colour variation throughout C. americana‘s range, with a prevalence of white and light-colour pollen in the eastern part of the range where abiotic selection is probably relaxed.”