A flower’s ability to mimic the sexually attractive traits of pollinators to attract them to its nectar has long fascinated scientists.
Flowers can’t buy glasses and a wig to create a disguise, so how they came to pull off such convincing dupes, using only an existing toolkit of DNA, is baffling.
Orchids are the most infamous flower for insect mimicryand may look and smell irresistible to an involuntary Casanova insect, whose brief rendezvous between the petals leaves the insect laden with orchid pollen, which it inevitably spreads to other flowers.
Orchids developed this trait so long ago, and with such dominant success, that all individuals without these traits are long gone.
This is why scientists turned to the South African daisy Diffuse Gorteriawhose sexual mimicry appeared relatively recently in the history of evolution, and is not consistent for all flowers of the species.
The petals of this daisy range from pale yellow to bright red-orange, some with spots that form a circle around the center of the flower.
The appearance of these flowers varies widely, and in some the spots have turned into green-black bumps that, to a fly, look exactly like its female companion, waiting seductively on the petals.
The evolution of the variety of features in this species was recently mapped to determine the order in which traits evolved to result in such convincing deception: first it revealed color, then random positioning, then texture.
New searchled by evolutionary biologist Roman Kellenberger of the University of Cambridge, is investigating how three sets of genes, which once had nothing to do with attraction to horn flies, became part of its salacious strategy.
“This daisy hasn’t developed a new ‘make a fly’ gene”, plant biologist and lead author Beverley Glover from the University of Cambridge said.
“He brought together existing genes, which are already doing other things in different parts of the plant, to create a complicated spot on the petals that tricks male flies.”
One gene moves iron around the plant, another causes root hairs to grow, and a third controls flower production.
Genes that move iron create dots that guide pollinators to the center of the flower for a nectar reward, a time-tested pollination strategy that flowers relied on long before sexual mimicry arose.
The resulting pigment combination (carotenoids, which create the yellow-orange color, and the dark blue-violet of anthocyanins) is a blue-green black hue – the exact color we associate with a fly’s shell.
A set of genes typically implicated in limiting flower production was then repurposed, like an “off switch” for nectar guide points, causing the “false flies” to appear in seemingly random positions on the petals. .
If the strategy for attracting pollinators is to advertise the flower’s nectar, then a clear ring of spots is a bit like a billboard for a parking lot. But when the strategy is sexual mimicry, it seems that for flies, less is more.
The sequence of genes typically involved in the production of a plant’s root hairs are responsible for the real-life dupe, causing the three-dimensional shape and texture of the daisy’s fly-like petal spots.
The protein produced by this gene, EXPA, relaxes usually rigid plant cell walls, causing “irreversible cell expansion”. In the petal, this protein causes existing hairs to swell, giving its surface texture.
This gene was almost entirely out of action in developing leaves and unspotted florets; moderately expressed in developing roots; and strongly in development of spotted flowers.
But researchers know that this gene is involved in the “puffing up” of what would otherwise look like a cardboard cutout of a female fly, because it’s only expressed in the petals of the most convincing flowers, and even among those here the expression is almost entirely absent in the petals without spots or bumps.
You can imagine how much extra action the flowers with this 3D upgrade must get.
“Male flies don’t stay long on flowers with single spots, but they are so convinced by these false flies that they spend more time trying to mate and rub more pollen on the flower – helping to pollinate it”, Kellenberger said.
It seems that the coming together of three sets of existing genes has created an evolutionary shortcut for these masters of disguise.
This study was published in Current biology.