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Buzz Pollination

Buzz pollination is a highly specialized process involving certain plant and bee species. Buzz pollination takes place when flowers with a certain anther morphology, known as poridical anthers, are grasped by a pollinator. When the bee attaches to the flower, it shakes it's body at a certain frequency, which creates a buzzing noise, to cause the anthers to release pollen through a small opening. Several of these plant species produce fruits we commonly eat. These plant species depend on specific bee species to efficiently extract pollen from the anthers. Buzz pollinated plants are commonly pollinated by bees that are found in two genera, Bombus and Xylocopa. The relationship between these flower types and their pollinators is partially explained by coevolution strategies of the plant to maximize reproductive output. The earliest evidence of this phenomenon is seen in the Cretaceous period..

Flower Morphology

Plants that rely on buzz pollination have a unique anther shape compared to other flora. The anthers of a flower are where sperm is produced and develops into pollen. This pollen leaves the plant to pollinate a stigma, or female reproductive structure, of another flower, either by wind or animal pollinators. Eventually a pollen tube grows down the stigma to fertilize the ovule. In buzz pollinated plants, this process can only happen if pollinators visit the flowers to extract pollen. However, only a select few insect species are able to pollinate these plants.The flower morphology of buzz pollinated plants is different from other flora that do not use this type of pollination. The anthers are completely sealed expect for a small pore at the top or have very small slits that open along the sides. The pores and slits are small enough that insects cannot easily enter the anther, but large enough pollen can exit. Because of this shape, they are often referred to as poridical anthers. These poridical anthers are only able to release pollen when vibrated at a specific frequency. The stigmas of these flowers are often located below the anthers. This is could be an evolutionary strategy to prevent self-fertilization, also known as selfing, by creating distance between the anthers and the stigma.

Buzz Pollination Mechanism

While many flowers can be pollinated by several different types of insects, buzz pollinated plants are extremely limited. Buzz pollinated plants can only be pollinated by bees, more specifically bees from the genera Bombus and Xylocopa. These bees are known as the bumble bees and carpenter bees, although some other bees are able to buzz pollinate (see examples below).These bees have evolved a certain behavior that allows them to successfully extract pollen from this particular group of plants. The bees land on a flower and grasp the anthers with their legs. While holding onto the anthers, they contract their flight muscles which vibrates the anthers. The frequency of this vibration triggers the flower to release pollen; the pollen shoots out of the anther through the pore and sticks to the hair on the bumblebee’s abdomen and legs. The bee will then fly to the next flower, and when it lands it will brush against the stigma transferring pollen from one plant to another resulting in pollination and eventually fertilization. The process is repeated as the bee continues to visit flowers collecting pollen.

Examples of Buzz Pollinating Bee Species

• Bombus terresris
• Xyolocopa frontalis
• Euglossa sp.
• Melipona sp.
• Protandrena sp.
• Nomia sp.
• Oxea sp.

Buzz Pollinated Crops

Buzz pollination is a highly specialized process between plants and their pollinators, and only certain plants possess the physical traits that make this mode of pollination possible. Approximately 6-8% of flowering plants, roughly 15,000 to 20,000 species, are buzz pollinated worldwide, several of which we rely on for food. Buzz pollinated crops grown on a large scale for human consumption, and are the fruits are commonly see in grocery stores.

Examples of Crops that Utilize Buzz Pollination

• Potato (Solanum tuberosum)
• Tomato (Solanum lycopersicum)
• Eggplant (Solanum melongena)
• Pumpkin (Cucurbita pepo)
• Zucchini (Cucurbita pepo var. cylindrica)
• Blueberry (Vaccinium corymbosum)
• Cranberry (Vaccinium oxycoccos)

Evolutionary Origins of Buzz Pollination

The earliest evidence of ancestors of plants that use this mode of pollination in the fossil record has been dated to the Cretaceous period. Also, some extant flora such as members of the family Myrtaceae show a spectrum of anther shapes including poridical anthers and are thought to resemble some morphological aspects of ancestral buzz pollinated flowers. As these plants have evolved complex floral structures, pollinators have coevolved with these plants.

Although pollination results from the bees visiting these flowers, this is not the primary reason they visit plants with poridical anthers. Pollen contains a substantial amount of protein compared to nectar, the sugary liquid the majority of plants produce as a reward for their animal pollinators. Bees eat pollen as well as make a paste with it to feed their larvae. The pollen paste is then sealed into the nest to create a reserve for the young bees. Bees rely on this resource for food; therefore they are also dependent on flowers that produce substantial amounts of accessible pollen, including flowers with poridical anthers. Bees from Bombus and Xylocopa are thought to pollinate these flowers because their adaptive behavior allows them easily extract pollen that is less available to other insects. Since bees have a source of plentiful pollen that they do not have compete with other insects for, they are more likely to visit these flowers. This then allows the flowers to more successful reproductively because the plants maximize their pollen dispersal with each bee visit, and less pollen is lost. The relationship between buzz pollinated plants and bees benefits both groups and could be why poridical anthers have been successful evolutionarily. Pollinator and flower relationships have been observed in Orpheum frutescens, a small shrub that has poridical anthers. Bees visited these plants outside of the University of Cape Town and continued to visit the plants even when all of the pollen had been extracted. Although the bees did not know the pollen was depleted, they continued to associate those particular flowers with a source of food. O. frutescens would benefit from these multiple visits as the plants continue to produce pollen during the flowering season.