Flowers use adjustable ‘paint by numbers’ petal designs to attract pollinators

The study, by researchers at the University of Cambridge’s Sainsbury Laboratory also found that bees prefer larger bullseyes over smaller ones and fly 25% faster between artificial flower discs with larger bullseyes – potentially boosting efficiency for both bees and blossoms. 

Patterns on the flowers of plants guide insects, like bees, to the centre of the flower, where nectar and pollen await, enhancing the plant's chances of successful pollination. Despite their importance, surprisingly little is known about how these petal patterns form and how they have evolved into the vast diversity we see today, including spots, stripes, veins, and bullseyes. 

Using a small hibiscus plant as a model, researchers compared closely related plants with the same flower size but three differently sized bullseye patterns featuring a dark purple centre surrounded by white – H. richardsonii (small bullseye covering 4% of the flower disc), H. trionum (medium bullseye covering 16%) and a transgenic line (mutation) of H. trionum (large bullseye covering 36%). 

They found that a pre-pattern is set up on the petal surface very early in the flower’s formation long before the petal shows any visible colour. The petal acts like a 'paint-by-numbers' canvas, where different regions are predetermined to develop specific colours and textures long before they start looking different from one another. 

The research also shows plants can precisely control and modify the shape and size of these patterns using multiple mechanisms, with possible implications for plant evolution. By fine-tuning these designs, plants may gain a competitive advantage in the contest to attract pollinators or maybe start attracting different species of insects. 

These findings are published in Science Advances. 

Dr Edwige Moyroud, who leads a research team studying the mechanisms underlying pattern formation in petals, explained: “If a trait can be produced by different methods, it gives evolution more options to modify it and create diversity, similar to an artist with a large palette or a builder with an extensive set of tools. By studying how bullseye patterns change, what we are really trying to understand is how nature generates biodiversity.” 

Lead author Dr Lucie Riglet investigated the mechanism behind hibiscus petal patterning by analysing petal development in the three hibiscus flowers that had the same total size but different bullseye patterns. 

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She found that the pre-pattern begins as a small, crescent-shaped region long before the bullseye is visible on tiny petals less than 0.2mm in size. 

Dr Riglet said: “At the earliest stage we could dissect, the petals have around 700 cells and are still greenish in colour, with no visible purple pigment and no difference in cell shape or size. When the petal further develops to 4000 cells, it still does not have any visible pigment, but we identified a specific region where the cells were larger than their surrounding neighbours. This is the pre-pattern.” 

These cells are important because they mark the position of the bullseye boundary, the line on the petal where the colour changes from purple to white – without a boundary there is no bullseye! 

A computational model developed by Dr Argyris Zardilis provided further insights, and combining both computational models and experimental results, the researchers showed that hibiscus can vary bullseye dimensions very early during the pre-patterning phase or modulate growth in either region of the bullseye, by adjusting cell expansion or division, later in development. 

Dr Riglet then compared the relative success of the bullseye patterns in attracting pollinators using artificial flower discs that mimicked the three different bullseye dimensions. Dr Riglet explained: “The bees not only preferred the medium and larger bullseyes over the small bullseye, they were also 25% quicker visiting these larger flower discs. Foraging requires a lot of energy and so if a bee can visit 4 flowers rather than 3 flowers in the same time, then this is probably beneficial for the bee, and also the plants.” 

The researchers think that these pre-patterning strategies could have deep evolutionary roots, potentially influencing the diversity of flower patterns across different species. The next step for the research team is to identify the signals responsible for generating these early patterns and to explore whether similar pre-patterning mechanisms are used in other plant organs, such as leaves. 

This research not only advances our understanding of plant biology but also highlights the intricate connections between plants and their environments, showing how precise natural designs can play a pivotal role in the survival and evolution of species. 

For example, H. richardsonii, which has the smallest bullseye of the three hibiscus plants studied in this research, is a critically endangered plant native to New Zealand. H. trionum is also found in New Zealand, but not considered to be native, and is widely distributed across Australia and Europe and has become a weedy naturalised plant in North America. Additional research is needed to determine whether the larger bullseye size helps H. trionum attract more pollinators and enhance its reproductive success. 

Reference 
Lucie Riglet, Argyris Zardilis, Alice L M Fairnie, May T Yeo, Henrik Jönsson and Edwige Moyroud (2024) Hibiscus bullseyes reveal mechanisms controlling petal pattern proportions that influence plant-pollinator interactions. Science Advances. DOI: 10.1126/sciadv.adp5574