LOTS OF LEAVESS. cheesemaniae (left) sports rather simple leaves, which branch only once. A single DNA letter difference in its genetic code appears to cause the leaves on S. galapagense (right) to fork and branch into a series of leaflets.Courtesy of Systematic Botany Monographs/D. Spooner, I. Peralta

A single gene may act like a genetic dimmer switch, fine-tuning leaf variation between tomato plants. The gene reveals another way evolution can increase natural variation, and hints at the genetic basis of species distinction.

Plant biologist Neelima Sinha and her colleagues looked at two tomato plants from the Galapagos Islands. The leaves of the tomato plant Solanum galapagense look like snowflakes — branching and forking into a series of smaller leaflets. A close relative Solanum cheesemaniae boasts more demure leaves that branch only once.

The team found that the intricate leaf pattern of S. galapagense is caused by a single deletion from its genetic code, says Sinha of the University of California, Davis. The snipped gene spurs S. galapagense to produce more of a protein needed for new leaf formation, the team reports in the May Current Biology.

The study links taxonomy, which scientists use to categorize different species, to genetic differences, says David Spooner, a plant biologist at the University of Wisconsin–Madison who was not involved in the research. He notes that the work characterized the genetic basis for the main taxonomic species distinction — leaf variation — between S. galapagense and S. cheesemaniae.

“This is a wonderful study,” Spooner says. “It gets to the question, how much genetic change does it take to make a species?”

Sinha and her colleagues found that the plant gene acts like a molecular dimmer switch, fine-tuning the amount leaves branch. While that in itself is not unexpected, what’s exciting is the way it does so. The variation in S. galapagense’s blueprint is in a promoter region — a kind of genetic foreman in the DNA that encourages more protein production downstream. Instead of changing the chemical makeup of the end product, the genetic tweak simply pumps up its output.

Scientists used to think that evolution only proceeded gradually and that any visible change in an organism’s physical appearance would involve complex genetic changes. But the new work is one of a growing number of recent studies that reveals how tiny changes to the DNA code can cause dramatic shifts in physical appearance.

It also reveals another way evolution increases natural variation. Since plants that grow in shade tend to have more leaf surface area to collect the dimmer sunlight, this single change in leaf pattern might have enabled S. galapagense to thrive in a shadier inland habitat than its strictly coastal cousin. “In the Galapagos these two tomatoes have very distinct environments,” Sinha says. “They’re adapted to different shade and water habitats.”

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