Wednesday, 22 March 2017

Pollination Mystery Unlocked by Stirling Bee Researchers


Bees latch on to similarly-sized nectarless flowers to unpick pollen - like keys fitting into locks, according to a new study by researchers at the University of Stirling.

The research shows the right size of bee is needed to properly pollinate a flower. The bee fits tightly with the flower's anthers, to vibrate and release the pollen sealed within.

"We found that a pollinator's size, compared to the flower, significantly influences how much pollen is deposited." said Dr Mario Vallejo-Marin, from Stirling's Faculty of Natural Sciences.

Experts found more pollen grains are deposited when the pollinator's body is the same size or wider than the space between the flower's reproductive organs.

"Some plants, particularly those that are buzz-pollinated - a technique where bees hold onto the flower and vibrate to shake out the pollen - require a close physical interaction between their floral sexual organs and their visitors. The closer the bee fits to the flower, allowing it to touch both the male and female sexual organs, the more efficiently the insect can transfer pollen between plants." said Dr Vallejo-Marin.

Bees that are too small, relative to the size of the flower, transfer fewer pollen grains to other flowers and act 'pollen thieves', extracting the pollen they need without pollinating the flower.

Flowers need pollinators to collect and transport pollen to fertilise other flowers and trigger fruit and seed production.

Former Stirling PhD researcher Dr Lislie Solís-Montero, who works in El Colegio de la Frontera Sur (ECOSUR) research centre in Mexico, added: "Bees that are too small in relation to the distance between a flower's sexual organs behave as pollen thieves - removing pollen, but depositing very little.

"Our findings will help understand natural populations of nightshade and whether a visitor acts as a pollinator, or a pollen thief. This is not only relevant in its native range in Mexico, but also in the invasive species which is found right around the world. Surprisingly, visits by smaller bees were associated with more seeds being produced, indicating that more pollen does not necessarily create more seeds. Seed production may also depend on the quality of the pollen and different kinds of pollen grains competing to germinate. However, by identifying whether visiting bees and complex flowers match physically, we can predict whether these bees are likely to be effective pollen carriers or not." said Dr Vallejo-Marin.

The ecologists used a species of nightshade plant (Solanum rostratum), which was buzz-pollinated with captive bumblebees of varying sizes. They recorded the number of visits received, pollen deposition, and fruit and seed production. Varieties of nightshade include potatoes, tomatoes and peppers.

The abstract of the study reads:

Some pollination systems, such as buzz-pollination, are associated with floral morphologies that require a close physical interaction between floral sexual organs and insect visitors. In these systems, a pollinator's size relative to the flower may be an important feature determining whether the visitor touches both male and female sexual organs and thus transfers pollen between plants efficiently. To date, few studies have addressed whether in fact the “fit” between flower and pollinator influences pollen transfer, particularly among buzz-pollinated species. Here we use Solanum rostratum, a buzz-pollinated plant with dimorphic anthers and mirror-image flowers, to investigate whether the morphological fit between the pollinator's body and floral morphology influences pollen deposition. We hypothesized that when the size of the pollinator matches the separation between the sexual organs in a flower, more pollen should be transferred to the stigma than when the visitor is either too small or too big relative to the flower. To test this hypothesis, we exposed flowers of S. rostratum with varying levels of separation between sexual organs, to bumblebees (Bombus terrestris) of different sizes. We recorded the number of visits received, pollen deposition, and fruit and seed production. We found higher pollen deposition when bees were the same size or bigger than the separation between anther and stigma within a flower. We found a similar, but not statistically significant pattern for fruit set. In contrast, seed set was more likely to occur when the size of the flower exceeded the size of the bee, suggesting that other postpollination processes may be important in translating pollen receipt to seed set. Our results suggest that the fit between flower and pollinator significantly influences pollen deposition in this buzz-pollinated species. We speculate that in buzz-pollinated species where floral morphology and pollinators interact closely, variation in the visitor's size may determine whether it acts mainly as a pollinator or as a pollen thief (i.e., removing pollen rewards but contributing little to pollen deposition and fertilization).

References
- Solís-Montero, L., & Vallejo-Marín, M. (2017). Does the morphological fit between flowers and pollinators affect pollen deposition? An experimental test in a buzz-pollinated species with anther dimorphism Ecology and Evolution DOI: 10.1002/ece3.2897

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