Life under one tree's rule?
Over the next five years, researchers in Utah and Arizona will work to determine how the genetic traits of cottonwood trees affect other life forms in the area.
| FLAGSTAFF, ARIZ.
Tom Whitham and trees go way back. As a youngster, he walked along rows of trees, picking out ones that had an oddly shaped branch or some other peculiar trait. His family, which has been in the nursery business since 1863, would try to propagate these trees to see if the differences he spotted were genetic - a key step to developing and patenting new varieties.
"Very early on, I came to the realization that, hey, there's a lot of genetic variation out here," says Dr. Whitham, director of Northern Arizona University's Merriam-Powell Center for Environmental Research.
Now, Whitham and colleagues in the US, Canada, and Australia are on the trail of a more ambitious proposition - that a gene or a set of genes from a single species can lay the foundation for an entire ecosystem.
That idea borders on heresy for many biologists, who argue that any one gene plays only a small role in the complex interactions of ecosystems. But if the researchers are right, then the genetic diversity of one dominant species can have a huge impact on the biological riches of its surroundings. And conservationists may have to take a new look at everything from preserving endangered species to evaluating the effects of genetically modified crops.
"When we talk about conserving genetic diversity, it's all focused around rare and endangered species," Whitham says.
"Nobody has really argued the importance of preserving genetic diversity in dirt- common species. If you want to conserve a community, you really need to be worried about conserving the genetic variation in dominant species."
The test subjects for the theory: cottonwoods in the American West. Previous experiments by Whitham and his collaborators suggested that a single cottonwood gene could influence thousands of organisms in the larger ecosystem.
Now, in cooperation with the US Bureau of Reclamation, the state of Utah, and local conservation groups, Whitham and his colleagues are providing thousands of cottonwood saplings for planting along segments of the Colorado and Weber Rivers that these groups are trying to restore to ecological health. These will become test sites for the experiment as well. In addition, the team is using thousands of other, more mature cottonwood in stands at sites in Utah and Arizona. Armed with the fully sequenced cottonwood genome, the group over the next five years will closely measure how the expression of genetic traits in different varieties of cottonwoods affects everything from formation and decomposition of leaves to the variety of microbes, insects, and other organisms in the area.
The notion that organisms in an ecosystem interact and rely on each other has a long pedigree, acknowledges Richard Lindroth, an entomologist at the University of Wisconsin and one of the project's principal investigators. Since "genetics underlies everything in biology, we would think that at some level all of these interactions and associations have a genetic component."
But trying to establish links genetically - especially tying them to a specific gene or segment of a genome in a dominant species - has been viewed as a fool's errand, Dr. Lindroth suggests.
"Ecologists have long thought that as you work your way up from genes to a whole organism to whole populations of organisms to a whole communities of different species of organisms, the effects of a particular gene would be completely swamped out" by other factors, he says. "You wouldn't find the trail of a particular gene and track it all the way back to a particular plant."
But that isn't necessarily so, he continues. "In ecologically dominant species such as cottonwood, you can have specific genes that have a profound impact on the entire community. You don't lose the trail of cause and effect as you bring in additional species or the effects of the abiotic environment."
The evidence comes in a series of experiments with cottonwood trees in the lab and in the field. Cottonwoods are of particular interest because they represent the dominant species in ecosystems along the banks of streams and rivers in the western US. Only about 3 percent of the these habitats are left, Whitham says. These threatened habitats are "hot spots of biodiversity."
In February, researchers led by Jennifer Schweitzer of Northern Arizona University (NAU) published results of a study that looked at the effect of tannin concentrations on leaf-decomposition in soils. Tannins represent a genetically based trait that offer trees a defense against disease, foraging, and environmental stress.
Working with two types of cottonwoods and their hybrids along Utah's Weber River, the group found that differing tannin concentrations in fallen leaves from each type of cottonwood could explain up to 65 percent of the variation they saw in the amount of nitrogen returned to the soil. The results suggested that tannin levels had an effect on the microorganisms breaking down the leaves.
A month later, NAU researcher Joseph Bailey and colleagues published results of work along the Weber showing that beavers had a marked preference for cottonwood varieties based on their tannin concentrations - the lower the tannin levels, the tastier the wood. Because beavers can reengineer an ecosystem by building dams, a single genetic trait in a dominant species could in effect indirectly sculpt a landscape.
Last month, a group led by University of Maryland entomologist Gina Marie Wimp published the results of work that looked at the range of genetic diversity in cottonwood stands and how tightly that diversity correlated with genetic diversity in the ecological community the stands inhabited. The results suggested that the greater the genetic diversity in individual cottonwoods, the greater the diversity in the insect community linked to a given tree. The team found that variations in genetic diversity in the trees accounted for 60 percent of the variation in genetic diversity in the attendant bug population.
Based on these and other experiments, Whitham and his colleagues are taking the next step - to see if they can map entire associations within cottonwood- dominated communities and trace them back to a particular gene or part of the tree's genome. "That's where it gets contentious," Lindroth notes.
Beyond the feasibility of such a mapping effort, the notion that a community can evolve genetically raises eyebrows.
"Historically, in community biology, a lot of the previous research has assumed that communities are just assemblages of organisms and that natural selection can only work at the individual level. At higher levels of organization things just fall together as a result of individual selection," says Stephen Shuster, a zoologist and colleague of Whitham's at NAU.
Here, the team hopes to tease out evidence that genetic interactions among species can evolve distinct characteristics within a natural community.
Given the sheer scale of the experiment, the results "may not turn out the way we hypothesize," Lindroth says. Even so, the work done so far leads to several take-home messages, he says. The first: "Individual genetic factors can have a more pronounced effect on a community than we ever dreamed they could."
This, he says, leads to another, cautionary observation. With genetically modified organisms, "if you insert a gene, the fact that the gene is one of many millions in a complex ecosystem doesn't necessarily mean it's not going to have a profound effect. What we are showing is that certain genes can have very significant effects."