Wildflower Colors Tell Butterflies How To Do Their Jobs

The recipe for making one species into two requires time and some kind of separation, like being on different islands or something else that discourages gene flow between the two budding species.

In the case of common Texas wildflowers that share meadows and roadside ditches, color-coding apparently does the trick.

Duke University graduate student Robin Hopkins has found the first evidence of a specific genetic change that helps two closely related wildflowers avoid creating costly hybrids. It results 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.

"There are big questions about evolution that are addressed by flower color," said Hopkins, who successfully defended her doctoral dissertation just weeks before seeing the same work appear in the prestigious journal Nature.

What Hopkins found, with her thesis adviser, Duke biology professor Mark Rausher, is the first clear genetic evidence for something called reinforcement in plants. Reinforcement keeps two similar proto-species moving apart by discouraging hybrid matings. Flower color had been expected to aid reinforcement, but the genes had not been found.

In animals or insects, reinforcement might be accomplished by a small difference in scent, plumage or mating rituals. But plants don't dance or choose their mates. So they apparently exert some choice by using color to discourage the butterflies from mingling their pollen, Hopkins said.

Where Phlox drummondii 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. "It was 2 in the morning when I figured this out," she said. "I almost woke up my adviser."

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

Hopkins will be continuing her research as a visiting scientist at the University of Texas, and the clear message from all of her advisers is "follow the butterflies. Everyone wants to know more about the butterflies!"

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

By Karl Leif Bates

CITATION:
"Identification of two genes causing reinforcement in the Texas wildflower Phlox drummondii," Robin Hopkins and Mark D. Rausher. Nature, Advance Online Publication, Jan. 9, 2011 DOI:10.1038/nature09641

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Shifts In Soil Bacteria Linked to Wetland Restoration Success

A new study led by Duke University researchers finds that restoring degraded wetlands -- especially those that had been converted into farm fields -- actually decreases their soil bacterial diversity.

But that’s a good thing, say the study’s authors, because it marks a return to the wetland soils’ natural conditions.

“It sounds counter-intuitive, but our study shows that in restored wetlands, decreased soil bacterial diversity represents a return to biological health,” said Wyatt H. Hartman, a Ph.D. candidate in wetlands and environmental microbiology at Duke’s Nicholas School of the Environment.

“Our findings are novel because they are the opposite of the response seen in terrestrial ecosystems, where restoration improves conditions from a more barren, degraded state,” said Curtis J. Richardson, director of the Duke University Wetland Center and professor of resource ecology at the Nicholas School. Richardson is Hartman’s faculty adviser.

Their report on the study will be published online this week by Friday in The Proceedings of the National Academy of Sciences.

Soils in undisturbed wetlands present harsh conditions, with elevated acidity and low oxygen and nutrient availability in which fewer bacterial groups can survive and grow, they explained. In comparison, former wetlands that have been drained, limed and fertilized for farming host greater soil bacterial diversity because they present conditions more suitable for bacterial growth.

“The bacterial communities in these fields almost resemble those found in wastewater treatment plants,” Hartman noted.

Soil bacteria are essential to wetland functions that are critical to environmental quality, such as filtering nutrients and storing carbon. “The mixture of bacterial groups in wetland soils can reflect the status of wetland functioning, and the composition of these populations is as telling as their diversity,” Richardson said.

Measuring whether the right mix of bacteria is returning to a restored wetland can be a valuable biological indicator scientists can use to evaluate restoration success, he added.

“We found that one of the simplest and most promising indicators of restoration success was the ratio of Proteobacteria, which have the highest affinity for nutrient-rich environments, to Acidobacteria, which have the highest tolerance for poor conditions,” Hartman said.

The researchers determined soil bacterial composition and diversity within restored wetlands, agricultural fields and undisturbed wetlands across North Carolina's coastal plain. They sampled these paired land-use categories across three distinct types of wetlands: pocosin bogs, floodplain swamps and backwater swamps that were not connected to streams.

Samples were also taken from sections of the Everglades, the largest wetland in the United States, where a $10.9 billion effort is now underway to remediate the effects of agricultural runoff.

“We identified bacterial groups by their evolutionary relationships, which were determined by sequencing DNA extracted from soils,” Hartman said. “This approach allowed us to capture a much greater diversity of bacteria than would be possible using conventional laboratory culturing, which works for only a small fraction of the 10,000 to 1 million species of bacteria that can be found in a single cubic centimeter of soil.”

Previously, researchers have used genetic techniques to target known organisms or bacterial groups in wetland soils, he said. “But this study is unique in that we used these methods to capture the full range of bacterial groups present, and determine how their composition shifts with land-use changes and restoration.”

“These types of findings can only be obtained in studies done on sites that have been restored and studied over a number of years and assessed with these modern techniques,” Richardson said.

Wetlands filter and reduce nutrients and pollutants from agricultural and urban runoff as well as improve water quality and store around 25 percent of the world’s soil carbon, while covering only 4 to 6 percent of its land mass.

More than half of original wetland acreage in the U.S. has been destroyed or degraded, but some has been restored in recent decades under the federal government’s “no net loss” policy.

“Re-establishment of microbial communities indicates a restoration of the biological functions of soils. This study across a wide range of wetlands is the first to establish that shifts in soil bacteria populations may be a key marker of restoration success,” Richardson said.

Rytas Vilgalys, professor of biology at Duke, and Gregory L. Bruland, assistant professor of soil and water conservation at the University of Hawaii at Manoa, were co-authors on the report. Bruland received his Ph.D. from the Nicholas School in 2004; Richardson was his faculty adviser.

The study was funded by a Duke University Wetland Center Case Studies Endowment and a National Science Foundation Graduate Research Fellowship.

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