The best bouquets are often a riot of colours and shapes. But, how can a single wild flower appear in 14 different versions of itself ?
The South African daisy species GORTERIA DIFFUSA comes in varied hues replete with petals of “notched yellow, rounded red or jagged peach” with centres of yellow, orange, green or black. Even more incredible is the fact that the same flower comes in 14 distinct forms, making it, perhaps, “the most varied flower on earth”.
The truth is we don’t yet know, not exactly. But research suggests this single mysterious daisy is at the centre of an evolutionary struggle, involving some tiny bee flies, sex and the art of deception. And this struggle has created the astonishing variety of forms we see today in this single species.
Flowering plants create blooms to attract pollinators, mostly small insects that carry pollen away, allowing the plants to reproduce. Plant breeders can, of course, artificially create varieties of one type of flower.
But, in the wild, its the arrival of new pollinators that can drive the evolution of “floral designs”. Flowers that appear different, may attract new pollinators and produce more offspring. That creates a “selection pressure” for flowers to evolve new colours and shapes and can lead to one flower species eventually splitting into two new species.
But this “multi-coloured daisy” doesn’t fit this pattern at all. That’s because all 14 forms of the flower are pollinated by the same insect —— the small bee fly (MEGAPALPUS CAPENSIS). Something else is going on, and there is a clue in the “flower’s structure”.
Although it can present itself in “14 distinct forms”, there are actually just 3 “basic” or “functional” types of this South African Daisy. The 1st type is called the “feeding form”. These daisies provide nectar and pollen and their petals are “quite plain”, of “a single colour”. On the “feeding form” it is just a flat pigmented surface. There is no “three-dimensionality”, explains Dr. Marinus de Jager, a post-doctoral researcher at Stellenbosch University in South Africa. Male and female bee flies simply feed from these flowers.
A 2nd type is called the “inspection form”. These flowers also provide nectar and pollen for male and female bee flies. But, they do something else too. The flower also produces “spots” on its petals, that “mimic female bee flies”. These attract the attention of the male bee flies. “They’ll just quickly land on one of these “small black spots” ………. and then take off again,” says Jager, “but the flies aren’t that fooled by the spots that they try to mate them. It’s not full-blown mating behaviour”.
The 3rd type is the “most deceptive”. It also has spots, but they do not appear on every petal. That better replicates the natural random pattern that would be formed by the temporary landing of one or more female bee flies. The spots are also more complex, being “3-dimensional” and “raised off the surface”, including ridges ———- adaptations that make them appear more like a real bee fly. The presence of these spots is itself remarkable. Until recently, “Orchids were the only plant group thought to often use sexual mimicry to deceive and attract pollinators”.
These complex spots have long intrigued botanists who, first suggested they mimicked beetles, leading to the daisy’s common name, still used today ———- the BEETLE DAISY. —— In 1997, botanists examined these spots using a scanning electron microscope. They found that the “spots reflect ultraviolet (UV) light” in the same way as a bee fly’s body does. They also found that cells, around the centre of each spot, have light-reflecting tips, making the spot appear to sparkle in the sunlight.
But, were the spots really attracting the bee flies ? Experiments were done to find out. They removed “spotted” versus “unspotted” petals from flowers and then gave the bee flies a choice. The result —– fewer spots meant fewer visits. That suggests the males are sexually attracted to these spots, and by trying to mate with them, increasing the chances they will pollinate the flower.
More recently, Jager and collaborator Dr. Allan Ellis investigated whether the sexual preferences of the flies might be driving the evolution of so many different varieties of the BEETLE DAISY. Females, they learnt, prefer simple spots and avoid UV highlights. Males, in contrast, prefer textured, glossy, visually complex, UV highlighted spots.
Yet, visual signs are not the only messages the flowers are sending out. “Floral odour” also seems to be another lure. But in contrast to what is seen in Orchids, it is the female bee fly that finds this odour attractive, suggesting we do not yet fully understand what has driven the REMARKABLE VARIATION IN THIS SINGLE FLOWER SPECIES.
Leaving us with a “complex, enigmatic and beautiful” flower, and some remaining questions about HOW IT CAME TO BE.
—————–Leslie Evans Ogden (BBC)