It's not easy being a fly. You don't live long, you eat lots of feces and sometimes, when you're just trying to get laid, you get tricked by a flower into being its artificial sperm donor.
Meet the South African beetle daisy (Gorteria diffusa), an unassuming orange flower that populates Namaqualand, a blooming desert in southwest Africa. Upon closer inspection, it reveals a remarkable case of plant sexual deception, a form of pseudocopulation. This is when a plant grows something that looks almost exactly like a bug or other pollinator, as a way of attracting an unwitting host to spread pollen and generate new plants.
This is a fairly common behavior in the plant kingdom, but almost every example is found in orchids, a type of flower that numbers over 28,000 species. Orchids are a robust example of an evolutionary success story, filling nearly endless niches and evolving specialized relationships with the animals around it. Take for example Ophrys sphegodes, the early spider orchid, which has flowers resembling a spider's abdomen, but also reproduces the sex pheromones of certain mining bees. Two mimics in one!
In the case of beetle daisies, one of the rare non-orchid sexual deceivers, their blooms sometimes feature dark, raised petal spots at the base of their yellow-orange florets. If you squint — or have the vision of a bug — they look exactly like dark flies resting on the flower. There's even hairy bumps and green-black pigment surrounding some reflective white dots that imitate an insect exoskeleton. Maybe it wouldn't trick you, but to a male bombyliid fly (Megapalpus capensis), it looks like the real deal.
And when it spots a G. diffusa flower, the male will swoop down and try to be, uh, more than friends with the sham fly. All that frustrated buzzing around stirs up a lot of pollen, which helps it spread to the next blossom. It's not entirely surprising that this fly is the dominant pollinator for these flowers.
It's a pretty neat trick for a plant, but also pretty baffling to botanists, who have been intrigued by beetle daises for centuries. A new genetic analysis in the journal Current Biology lays bare how the flower pulls this off, which helps explain broader complex trends in evolution.
Not every variety of G. diffusa has the same fly-like pattern. Some sport only a few counterfeits on select petals, others display an entire ring of them. By sequencing the genome of different strains, researchers from the University of Cambridge illuminated how the flower alters its pigment and the shape of its petals.
"Plant sexual deception has fascinated biologists for centuries, but the evolution of sexually deceptive flowers remains somewhat enigmatic due to their extreme specialization and the absence of intermediately deceptive forms," the authors wrote, meaning that we don't really see any examples of plants that only go halfway on pseudocopulation. It's all or nothing when it comes to tricking arthropods into mating with you.
"This daisy didn't evolve a new 'make a fly' gene," the study's lead author, professor Beverley Glover of the University of Cambridge's department of plant sciences, said in a statement. "Instead it did something even cleverer — it brought together existing genes, which already do other things in different parts of the plant, to make a complicated spot on the petals that deceives male flies."
This joining of genes is called co-option, an old concept in evolutionary biology that has been used to explain why some features in nature seem like miracles. There are a lot of gaps in the fossil record, due to how difficult it is for them to form in the first place. But while the record is still pretty extensive, it can sometimes seem like one species jumped into another with no transition.
Co-option explains this as a sort of genetic recycling, in which a trait isn't discarded but altered to do something else. A classic example is feathers in birds, which originally evolved in some dinosaurs for display and protection from the elements, but later became useful for flight.
This new analysis presents three genes the beetle daisies have co-opted: one for its color, one for its bulging shape and one for its placement. The more convincing the fake fly appears, the more strongly these genes were expressed. And it seems to tie these genetic expressions together at the same time.
This trait seems to have evolved relatively recently, only about 1.5 to 2 million years ago. That's not very long in geological terms — in fact, Homo sapiens probably first emerged around the same time. And the fact that this flower still provides nectar — a sort of consolation prize for the poor, lied to fly — points to the fact that G. diffusa hasn't used this trick long enough to rely on deception alone.
"We'd expect that something as complex as a fake fly would take a long time to evolve, involving lots of genes and lots of mutations," one of the other study authors, Roman Kellenberger, a postdoctoral researcher in the University of Cambridge's Department of Plant Sciences, said in the same statement. "But actually by bringing together three existing sets of genes it has happened much more quickly."
It's far too early to tell, but in a future scenario where flies outnumber other pollinators like bees and butterflies — a real risk exacerbated by climate change — this strategy may prove extremely beneficial. There are still some unanswered questions about this flower and its evolution. Using genetic engineering, the authors report, it may be possible to test the theory how these genes allow G. diffusa to build its own decoys. This research can help illuminate some of the complexities of co-option, including how it starts, refines and affects other traits. Evolution is a dynamic process that can sometimes defy logic, but even trickster flowers from South Africa have something to teach us about the intricacies of life on Earth.
