Flowers wear colours to target their customers – the pollinating insects

Flowering Plants feature a wondrous collection of colours, the primary purpose of colour, which is to attract insect pollinators, but this may not have been the original function of flower colour. In a study published in 2020, Dr. Paula J. Rudall, head of the Department of Comparative Plant and Fungal Biology at the Royal Botanic Gardens, suggests that one of the primary compounds which allow flowering Plants to have their diverse colours—anthocyanin pigments, was originally acquired in Plants to help protect growing tissues.

According to Rudall, anthocyanin pigments were first built up in seed-producing Plants (gymnosperms), which predate flowering Plants (angiosperms) and include Gingko, conifers, and cycads. These pigments gave some gymnosperms’ seed cone bracts – specialized, a red colour, scaly leaves and also provided photoprotection, thermoregulation, or some sort of defence against herbivores. Around 200-240 million years ago, when flowering Plants branched off from gymnosperms on the evolutionary tree, flower colour was co-opted as a way to draw insect pollinators.

In 2012, a research team from Australia had demonstrated that the evolution of flowers was driven by the preferences of bees. There are thousands of species of bees around the world, and they all share a common visual pattern—their eyes are sensitive to blue, ultraviolet, and green wavelengths of the light spectrum. Based on this ancient visual pattern, most of the flowering Plants gradually designed their flowers to attract bees—the most efficient pollinators.

Bees are not the only pollinators in the market; besides bees, so many insects also actively take part in this most important and sophisticated process. According to some experts, flies are the second common species of pollinators, and then the birds.

Insects’ colour preferences influence their choice

Many flowering Plants depend on drawing pollinators to reproduce, so based on the preferences of the pollinators, the appearance of their flowers has evolved to cater to the pollinators.

Based on several studies, now it is a fact that bees prefer the bluish spectrum of light, and most of the flowering Plants that need efficient pollinators like bees as a mediator produce blue flowers or use blue halo to create an optical trick to attract bees. Scientists also observed that hoverflies (Eristalis Tenax) are much comfortable at distinguishing between different shades of yellow than between different blues.

In Macquarie Island of Australia, there are no bees, so the Plant communities on this island evolved with different pollinators. Flies are the only animal pollinator on this island. Flowers have predominately evolved into yellow shades to suit animal pollination.

Generalization or specialization

According to one school of thought, flower evolution is driven by competition between flowering Plants. In this concept, different species might have very different colours from one another to increase their chances of being reliably identified and then efficiently pollinated. According to experts, this is a bit like how exclusive brands seek customers by having readily identifiable branding.

An alternative proposal to competition is facilitation. Plants may share preferred colour signals to attract a higher number of specific insects. This explanation is like how some competing businesses can do considerably better by being physically close together to attract many consumers.

In 2021, a study conducted by an international research team demonstrated how flower colour signalling has dynamically evolved depending on the availability of insect pollinators. In Victoria, flowers have concentrated on evolving colour signals preferred by their pollinators. They observed that orchids pollinated by flies are typically yellowish-green, while closely related orchids pollinated by bees have more bluish and purple colours. The flowers appeared to share the preferred colours of their main pollinator, compatible with a facilitation hypothesis.

On Macquarie Island, where flies are the only pollinators, flower colours diverge from each other – but still stay within the range of the flies’ preferred colours. This is consistent with a competition strategy, where differences between Plant species allow flies to more easily identify the colour of recently visited flowers.

When both bee and fly pollinators are present, flowers pollinated by flies appear to “filter out” bees to reduce the number of ineffective and opportunistic visitors. For example, in the Himalayas, specialized Plants require flies with long tongues to access floral rewards. This is similar to when a shop wants to exclusively attract customers interested in their specific product range.

Conclusion

Now scientists also know from research that flowers can evolve and change colours to suit the local pollinators. Flowers can thus change colours if, instead of bees pollinating flowers, flies with different colour preferences and perceptions dominate the community.

According to researchers, these findings on the evolution of flower colour, along with novel precision agriculture techniques, can help in using flies and other insects as alternative pollinators of crops.

References

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6. Paudel, R. B., et al. (2015). Out of Africa: evidence of the obligate mutualism between long corolla tubed Plant and long-tongued fly in the Himalayas. Ecology and Evolution, [online] Volume, 5(22), p. 5240-5251. Available at: https://doi.org/10.1002/ece3.1784 [Accessed 10th October 2021].
7. Rudall P. J. (2020). Colorful cones: how did flower color first evolve?. Journal of experimental botany, [online] Volume, 71(3), p. 759–767. Available at: https://doi.org/10.1093/jxb/erz479 [accessed 10th October 2021].
8. Smith, B. (2016). Macquarie Island gives scientists a view of the ecosystem without bees. The Sydney Morning Herald, [online] Available at: https://www.smh.com.au/technology/macquarie-island-gives-scientists-a-view-of-ecosystem-without-bees-and-birds-20160407-go0fvu.html [Accessed 10th October 2021].