Most admirers of van Gogh’s iconic “Sunflower” paintings gaze upon the golden inflorescences without any awareness of the scientific conundrum they pose. But researchers from the University of Georgia have finally cracked the case with a paper published in PLoS Genetics.
The puzzle begins with the fact that all flowers are either radially or bilaterally symmetrical. A buttercup is an example of radial symmetry; it looks the same no matter how you rotate it. An orchid, on the other hand, has bilateral symmetry, like a human face–the left and right sides look the same, but you can tell whether it’s right side up or upside down.
Here’s the sneaky thing: a lot of seemingly radially symmetrical flowers are actually clusters of tiny bilaterally symmetrical flowers, or florets. In fact, this is true of one of the biggest flower families, Asteraceae, which includes such familiar friends as dandelions, daisies and, yes, sunflowers.
Then the sunflower makes things extra complicated by building its cluster out of two kinds of florets: bilaterally symmetrical ray florets, and radially symmetrical disk florets. This may sound confusing, but it’s obvious as soon as you look for it: the classic sunflower is a ring of petals (ray florets) surrounding a big disk that will become filled with seeds (fertilized disk florets). The ray florets are infertile–they’re just there to help attract pollinators.
Now at last we can consider van Gogh, and his double-flowered sunflowers. They’re mutants.
A double-flowered mutant has no true disk florets, only concentric rings of ray florets–a profusion of petals. Consequently, the plant loses a lot of its fertility. You might wonder, can the opposite occur? Indeed, in tubular-rayed mutants ray florets are replaced with radialized, fertile disk florets.
Mark Chapman and his colleagues have just discovered that one particular gene, called HaCYC2c, causes both mutations. If HaCYC2c is over-expressed, it creates double-flowered van Goghs. If the gene’s function is lost, however, you get tubular-rayed flowers.
I particularly love this study because at least the first part of their methods is totally accessible to anyone who’s studied Mendelian crosses in high school biology. See:
Okay, maybe it’s a bit tricky. If you want to puzzle it out but you’re rusty on Mendel, here’s a primer.
Of course, you don’t need to understand Mendelian crosses–or the super-sophisticated genetic mapping that Chapman et al. use later–to appreciate van Gogh’s art. Nor do you need to be an Impressionist fan to appreciate sunflower genetics.
But I think we can all appreciate that it’s not often a famous painting is included in Figure 1 of a scientific paper.