Mirror Image: Flowers with opposite styles have a fling

Flowers’ female sexual organ, known as the style, sometimes leans to the left or right. Researchers have just found evidence of a gene underlying this style predilection, and it’s the first gene known to influence right-left orientation of any plant trait.

OPPOSITES ATTRACT. The left-leaning and right-leaning female organs of the species Solanum rostratum are indicated by the arrows. Pollen from a left-styled flower tends to fertilize a right-styled one. William G. Wilson/Nature

Working with left- and right-handed plants of the species Heteranthera multiflora, Linley K. Jesson and Spencer C.H. Barrett at the University of Toronto have carried out breeding experiments and report evidence that a single gene controls floral orientation. Other experiments hint at an evolutionary role for this trait.

Scientists have known for over a century that some plants sport flowers with mirror-image orientations. Such flowers are found in more than a dozen plant families, and researchers have long debated the evolutionary significance of style handedness, and even whether it’s genetically based.

Many botanists have theorized that left- and right-leaning styles reduce self-fertilization of plants, though data for this have been scarce. When an individual plant’s pollen fertilizes its own flowers, genetic variation in a population can decline, decreasing a species’ resilience to both disease and environmental change.

In flowers with deflected female styles, part of the pollen-carrying male sexual organ is bent in the opposite direction, says Jesson. Botanists have conjectured that this reduces the likelihood that insects collecting pollen from a flower will end up depositing it on the style of same-handed flowers. However, when these insects visit flowers with the opposite orientation, they would deposit that pollen in just the right spot.

“Most animals have a very standard behavioral pattern of how they interact with plants,” so they usually approach flowers from the same direction, explains W. John Kress, head of botany at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C. It’s quite possible for small differences in floral orientation to control where pollen is picked up and deposited, he says.

After discovering that style orientation is genetically determined, Jesson, who’s now at Victoria University of Wellington in New Zealand, and Barrett set out to test the conjecture. The pair chose a tomato relative called Solanum rostratum, which normally has flowers with both right- and left-leaning styles on every plant.

The researchers created three groups of plants. The first group was composed of plants with flowers of both types. A second group had either all its right-styled or all its left-styled flowers removed to create plants of a single orientation. In the third group, the flowers’ styles were artificially straightened. The scientists then permitted all the plants to be visited by free-ranging bumble bees.

The scientists later collected seeds and genetically characterized them to determine whether they were the result of self-pollination. The tests also revealed the handedness of the pollen-donating parents.

In the June 13 Nature, the scientists conclude that the handedness in flower orientation does indeed reduce self-fertilization. “Pollen from a left-handed flower was much more likely to be deposited onto a right-handed flower [and vice versa], as researchers had thought,” says Jesson. Plants with flowers all in the same orientation were least likely to self-fertilize.

The team also found that a single plant carrying flowers of both orientations was less likely to self-fertilize than a plant with straightened styles was. In the straight-styled plants, all of the flowers can donate and receive pollen from one another, say the researchers. In contrast, plants with both left- and right-leaning styles have respectively fewer donor-receiver pairs.

The existence of left- and right-handed flowers has fascinated botanists for years, but until now, there had been little data to back up the theory that the style orientation worked to limit self-fertilization, says Kress.

John Pickrell is a freelance writer based in Sydney and the author of Flames of Extinction: The Race to Save Australia’s Threatened Wildlife.

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