Dutch impressionist Vincent van Gogh obsessed himself with sunflowers until his death in 1890, later earning avid collectors tens of millions of dollars for single paintings.

These days, researchers have developed their own sunflower obsession: Solving the genetic origin of mutant "teddy bear" sunflowers depicted in van Gogh's ochre-splashed canvases.

"If you look at sunflowers as a whole, a family called Asteraceae, it's the largest and most successful flowering plant family on Earth," said plant biologist John Burke at the University of Georgia.

"There's a lot of floral variation going on, largely due to changes in symmetry. We think the genes controlling this symmetry could play an extremely important role in their success," said Burke, co-author of a study of sunflowers published online Mar. 29 in PLoS Genetics.

Sunflowers are crucial sources of vegetable oil and other food, but their large yellow heads aren't single flowers. They are instead aggregates of hundreds or thousands of smaller flowers called florets, which typically grow in two types.

Ray florets are petal-like flowers that line the edge of a sunflower's head and can't develop seeds. Disc florets are tube-like flowers which can grow seeds, and they fill in the centers of sunflowers.

'These traits are kind of transient. They may come and go, depending on what kind of florets pollinators are attracted to.'But other floret arrangements abound. Some sunflowers grow all ray-like florets, a variety called double-flowered, while others sprout only disc-like florets, a variety called tubular. In-between types like the "teddy bear" grow a mixture of the two florets and are prized by ornamental growers (and van Gogh, of course).

In 2008 Burke and his team described a family of sunflower genes they thought might lead to the abnormal floret varieties.

But in the new study, they dug deeper. They cross-bred multiple sunflower varieties – typical, double-flowered and tubular – to trace any genes tied to their appearance.

They discovered two different mutations to a single gene, called HaCYC2c, could in part guide floret symmetry growth. "When we started, we had some good gene candidates but didn't know which genes were involved," Burke said. "We knew it must be a gene that affects symmetry, and we suspected genes in this family, but we didn't know which one or ones in particular."

"Just like humans, sunflowers have two copies of every gene," Burke said. Two copies of one kind of mutation to HaCYC2c, called "tubular," causes all of the florets to resemble disc florets, even those lining the edge of a flower.

"When you have two 'double' mutant copies, you get double-flowered heads with all ray-like florets," Burke said.

Burke couldn't say which exact combination of mutated gene copies led to the green-centered, double-flowered "teddy bear" sunflowers in van Gogh's famous series, but he said the HaCYC2c gene plays a big role.

What's more, some other aggregate flowers display the floret behavior seen in sunflowers, but don't appear to rely on the same mutation. The results suggest that entirely different genes in the same aggregate-flower family can influence floret type in the same way.

"That tells us these traits are kind of transient," Burke said. "They may come and go, depending on what kind of florets pollinators are attracted to."

Images: 1) "Sunflowers," 1889, by Vincent van Gogh. (Kennisland/Flickr 2) Three sunflower arrangements (top) and their floret types (bottom). Wild sunflowers have typical outer ray and disc florets (left), double-flowered sunflowers have mostly ray-like mutant florets (middle), and tubular sunflowers grow only disc-like florets (right).

Citation: "Genetic Analysis of Floral Symmetry in Van Gogh's Sunflowers Reveals Independent Recruitment of CYCLOIDEA Genes in theAsteraceae." By Mark A. Chapman, Shunxue Tang, Dörthe Draeger, Savithri Nambeesan, Hunter Shaffer, Jessica G. Barb, Steven J. Knapp and John M. Burke. PLoS Genetics, published online Mar. 29, 2012. DOI: 10.1371/journal.pgen.1002628