Sex in (floral) advertising


A honeybee acting as the wing...lady for a flower of the goat willow (aka pussy willow) Salix caprea. Image by Böhringer Friedrich via Wikimedia commons.

A honeybee acting as the wing…lady for a flower of the goat willow (aka pussy willow) Salix caprea. Image by Böhringer Friedrich via Wikimedia commons.

Having a wingman can be helpful, but for many plants it’s absolutely crucial. Flowering plants don’t have smoky bars, speed dating or eHarmony. They have to rely entirely on their tiny wing—well, I guess “men” isn’t really appropriate. But unlike your witty friend who backs you up in the bar, pollinators don’t help plants with their dating life out of friendship alone. They need something in return, and flowers flaunt their assets to advertise the sweet rewards awaiting a helpful bee. In a paper published this March in the journal PLoS One, scientists studying a particular type of flower have discovered that bees are much more attracted to boys.

The majority of flowering plants have both male and female flowers in the same plant, or have both boy and girl parts in the same flower. But about 6% of flowering plant species are dioecious, meaning that they have separate male and female individuals. One such plant is the pussy willow (aka goat sallow, aka goat willow) Salix caprea. This plant uses its flowers to attract honeybees, and rewards them with nectar, so that the bees can pick up pollen from male plants and spread it around to female plants. But just how is this three-way coordinated?

Stefan Döetterl and colleagues set out to test the popular hypothesis that, because male plants have more gametes (ie: pollen) to go around than females, they should invest more resources in advertising their sexy flowers to bees. This also insures that bees are more likely to visit male flowers first, so that they have some pollen to drop on the female flowers’ stigmas when they visit. This hypothesis is based on the fact that male fertility in these plants is only limited by how many mates their pollen can come into contact with. Female fertility, on the other hand, is limited by the number of stigmas (1 per flower). However, this is not universally true, particularly in species where females don’t get enough pollen to fertilize all their eggs. But is it true in pussy willows?

The researchers first wanted to find out which part of the flower (the look or the smell) was more important to bees.  Next, they asked which sex (if both aren’t equal) puts the most work into attracting bees.

Analyzing plant color and fragrance is a lot more work than just stopping to smell the flowers. On sunny days, way back in 2006 and 2007, Dr. Döetterl and his colleagues went out to collect flowers. Sounds nice enough. But then they brought them back to the lab to throw various amounts of science at them.

Figure 1 from Dötterl et al. 2014 showing the test cylinders used in the study.

Figure 1 from Dötterl et al. 2014 showing the test cylinders used in the study.

Color was measured using computer software after picking apart all the flower parts that could be seen by bees and sticking them onto black electrical tape. But the collection of fragrance is my favorite part. After testing the flowers to see which had a more attractive scent to bees, the researchers basically trapped the scents of flowers by putting them in an oven bag for 10 minutes and then collecting the air in special tubes. As a control, they also collected the nearby air to figure out which components were from the flowers and which weren’t.

The researchers tested visual and olfactory cues to figure out if bees were more excited by pretty flowers or smelly flowers. They set up 2 test cylinders (see picture) with the test samples inside 3 meters away from a beehive that contained bees who had never before met a willow of any kind. In one cylinder, flowers were hidden from sight, but their fragrance filled the air. In the other, the bees could only see, but not smell, the flowers. Then they let the bees loose and watched them for 40 minutes, switching the location of the cylinders half way through. They did their best to count the bees that showed a preference to one or the other cylinder (bees are notoriously difficult to pick out of a crowd). In separate experiments, the researchers compared both scent and color to a negative control (empty cylinder) and all the combinations in between.

As it turns out, bees aren’t that superficial. They’re attracted more to a flower’s fragrance (you could call it personality; no one’s going to stop you) than just the look of the flower. Obviously, they preferred if they could both see and smell the flowers, but given the choice between the two, smell won out.

The next step was to figure out whether the yellow male flowers or green female flowers were more attractive to bees. If the bees only had access to the smell of the flowers, they didn’t seem able to tell the difference between males or females. But when they could see the flowers, they preferred males. Their preference for boys was even more obvious when the bees could both see and smell the flowers.

The scientists speculate that the bees are responding to the higher contrast between the yellow pollen and the leaves compared to the similar coloring in female plants between leaves and flowers. Their analysis also showed that male flowers emit more smell, though made up of the same chemicals as females. However, the bees weren’t able to distinguish the sex of flowers based on smell alone (at least not using the methods described in the paper).

This is his 'come hither' look. Image by Kurt Stuber via Wikipedia.

This is his ‘come hither’ look. Image by Kurt Stuber via Wikipedia.

It’s also important to point out that the bees used in these experiments had never encountered these plants before. It’s possible that more experienced bees would be able to tell males and females apart on scent alone once they learned that only male flowers have delicious pollen as well as sweet, sweet nectar.

The results of the study support the original hypothesis, that males are working harder to attract bees than females. In pussy willows, both bees and wind are used to spread pollen around, so females likely don’t need to increase the amount of pollen they’re getting. The guys, on the other hand, are always looking for ways to get their pollen out to as many ladies as possible. This could potentially explain the higher fragrance production in males. The next step is to figure out how they make smellier flowers and whether bees actually do learn to love the smell of these boys over time.

 

Reference:

Dötterl S, Glück U, Jürgens A, Woodring J, & Aas G (2014). Floral Reward, Advertisement and Attractiveness to Honey Bees in Dioecious Salix caprea. PloS one, 9 (3) PMID: 24676333

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