Fickle flowers use color to choose

DUKE (US) — Plants use color to discourage butterflies from mingling their pollen, according to new research.

Animals and insects accomplish reinforcement—a process that keeps two similar proto-species moving apart by discouraging hybrid matings—by a small difference in scent, plumage, or mating rituals.

But plants don’t dance or choose their mates—instead they exert choice by using color.

texas_map_2

A map of Texas shows the approximate range of Phlox drummondii, which appears light blue to the west, and Phlox cuspidata, another light blue flower, to the east. Where the two ranges overlap in the center, drummondii takes on a reddish hue. (Credit: Robin Hopkins)

“There are big questions about evolution that are addressed by flower color,” says Duke University graduate student Robin Hopkins, who successfully defended her doctoral dissertation just weeks before seeing the same work appear in the journal Nature.

With her thesis adviser, biology professor Mark Rausher, Hopkins found the first evidence of a specific genetic change that helps two closely related wildflowers avoid creating costly hybrids, resulting in one of the normally light blue flowers being tagged with a reddish color to appear less appetizing to the pollinating butterflies which prefer blue.

Flower color had been expected to aid reinforcement, but Hopkins’ findings are the first clear genetic evidence.

Where Phlox drummondii, a common Texas wildflower, lives by itself, it has a periwinkle blue blossom. But where its range overlaps with Phlox cuspidata, which is also light blue, drummondii flowers appear  darker and more red.

Some individual butterflies prefer light blue blossoms and will go from blue to blue, avoiding the dark reds. Other individual butterflies prefer the reds and will stick with those. This “constancy” prevents hybrid crosses.

Hybrid offspring between drummondii and cuspidata turn out to be nearly sterile, making the next generation a genetic dead-end. The persistent force of natural selection tends to push the plants toward avoiding those less fruitful crosses, and encourages breeding true to type. In this case, selection apparently worked upon floral color.

Hopkins was able to find the genes involved in the color change by crossing a light blue drummondii with the red in greenhouse experiments. She found the offspring occurred in four different colors in the exact 9-to-3-to-3-to-1 ratios of classical Mendelian inheritance.

From there, she did standard genetics to find the exact genes. The change to red is caused by a recessive gene that knocks out the production of the plant’s one blue pigment while allowing for the continued production of two red pigments.

Even where the red flowers are present, about 11 percent of each generation will be the nearly-sterile hybrids. But without color-coding, that figure would be more like 28 percent, Hopkins says.

Why and how the butterflies make the distinction has yet to be discovered.

The research was supported by grants from the National Science Foundation.

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