The petals of a range of flowers harbour repeated patterns of nanostructures that show similar levels of disorder across species. This degree of disorder produces a blue halo of scattered light that helps bees to find flowers. See Article p.469
The ability to effectively pass genetic material to a sexual partner is a powerful driver of evolution. Humans and other mobile organisms have evolved so that sexual partners are attracted to one another. But immobile organisms such as plants must rely on an intermediary carrier — bees carrying genetic material in pollen, for instance. These carriers are crucial to the survival of immobile species, and have co-evolved closely with them1,2. Moyroud et al.3 report on page 469 that diverse flowering plants have evolved to produce a 'blue halo' of colour that attracts bumblebees (Bombus terrestris).
Pollinators use a combination of olfactory and visual cues to find flowers4,5. For bees, the colours and shapes of flowers are probably the dominant discriminatory factors. However, the idea that bees can see colours has been challenged by studies6,7 of photoreception and spectral sensitivity, which showed that bee eyes are relatively insensitive to most colours, except blue.
Colour commonly originates from pigment molecules that absorb some of the spectra of white light, letting others pass through, which are then visible. Blue seems to be a difficult colour to make from pigments alone, although the reasons for this are still being investigated. As such, blue is relatively uncommon in flowering plants (collectively known as angiosperms), compared to other colours.
The question of what drives bees to the many flowers that are not blue has become an intriguing area of interest. Non-blue flowers can attract bees as efficiently as blue ones, and this ability seems to be independent of size, fragrance or apparent coloration. Thus, there are clearly other cues involved in successful and efficient flower-finding by bees.
The colours of angiosperms can also be influenced by repeated patterns of nanostructures on the surface of petals. The mixed colour spectra in sunlight interact with these nanostructure motifs, which physically separate the spectra such that only particular wavelengths are reflected. The distance between and the size of the motifs determines output colour. Blue can easily be made through this structural coloration process, because the typical size of nanostructures is conducive to scattering blue spectra. Nanostructures can also be the basis of iridescence, providing optical hues — tints of colour added to a petal's base colour to produce discrete signals that are sometimes better observed under certain conditions or angles of illumination.
To investigate bee attraction to angiosperms, Moyroud et al.3 carefully analysed the nanostructures of a dozen flower species that vary in colour and are evolutionarily distant from one another. As expected, the surfaces of the flowers showed different nanostructural repeat motifs, resulting in different colours or hues. The authors confirmed previous reports8,9,10 that the organization of each of these repeat motifs shows some disorder. Surprisingly, they found that the level of disorder was similar across all 12 species, and so was independent of the fundamental nanostructural pattern that gave rise to each flower's primary colour or hue. Therefore, the degree of disorder seems to be evolutionarily conserved and a phenomenon common to otherwise divergent species. The disorder might have originated from a single common ancestor and been propagated through the various evolutionary branches, but it cannot be excluded that the phenomenon has arisen independently several times.
What would be the benefit of evolutionarily conserving the degree of disorder in nanostructural arrangements? Moyroud et al. hypothesized that, rather than being an imperfection, disorder is associated with a clear visual output and function. Indeed, they went on to demonstrate that the disordered nanostructures generate a blue 'halo' of light when rays of sunlight hit the flower at certain angles. The halo (which can best be seen by the human eye against dark-coloured petals) appears as though mixed with the base colour of the flower (Fig. 1).
Moyroud et al. found blue haloes consistently produced across flowers of different colours. They showed that the blue halo has a well-defined spectral range to which bees are sensitive. The authors created synthetic flowers that had small surface gratings designed to mimic the disorder that generates a blue halo. They then performed behavioural experiments with bumblebees, measuring their foraging speeds and visiting counts to the synthetic flowers. The bees were more attracted to the synthetic flowers with blue haloes than to ones without them.
Finally, the researchers tested the ability of the blue halo to attract bees to synthetic flowers of three different base colours, to assess the effect of contrasting backgrounds. A blue background was most attractive to bees. The blue halo did not improve foraging to blue flowers (as expected, because bees can see the blue background). By contrast, it did improve foraging on yellow and black backgrounds. Together, the researchers' data provide comprehensive evidence that the blue halo is the key visual signal that attracts bees.
Thus, a mystery has been resolved. Flowers that might have different base colours for various ecological reasons, from attracting or repelling organisms to scattering deleterious spectra of light, can also have a conspicuous blue halo to attract bees. The blue halo originates from an orderly disordered motif that might have been preserved throughout the evolution of the angiosperm lineage, securing reproductive success over millions of years.
Moyroud and colleagues' paper brings together multiple disciplines — bee behavioural ecology, plant evolutionary biology, optical modelling and materials science. Such unusual merging of expertise makes the work appealing, because it enables the researchers to approach investigation of this fundamental biological process in the most robust manner possible.
The study clearly has ecological implications. In the future, perhaps bee populations and their pollination success could be better managed using bioinspired synthetic surfaces, or selected flower species that have strong blue haloes. However, it will first be necessary to determine the relevance of the blue halo under natural conditions. Moreover, it will be interesting to determine the halo's importance — if any — for attracting other insects. Do plants that are not pollinated by bees use blue? Do plants that feed on insects use blue to attract their prey? There is still much to learn about the ecological value of this optical phenomenon.
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