Why You Are Not Only Your Genes:
Sonia Sultan and Plant Adaptability
byJane Lewis
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Click to visit Professor Sultan's department at Wesleyan University. Includes a link to her personal webpage from which all photos were taken. |
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Prof. Sultan majored in the history and philosophy of science at Princeton. Click the photo to check out another Synthesis article by Peter Hagan on Prof. Joseph Rouse, who teaches those subjects |
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"I always hated science, as a child and in high school," confesses plant biologist and Wesleyan professor Sonia Sultan. "It seemed like most of what I was taught as science seemed to be about memorizing a lot of arbitrary facts, [and not] asking questions or challenging things, which is what science is really all about."
Sultan first became interested in plants when she took a forestry course her senior year in high school. The course was designed for people who planned to work in national parks, and it inspired her to get an internship at the Arnold Arboretum after she graduated. The Arnold Arboretum is a Harvard-affiliated collection of trees and woody plants from around the world, and Sultan worked there for several months, working on the grounds and mapping trees.
The next year, Sultan went to Princeton, partially because she had been offered a job in the greenhouse there. Even though her interest in plants continued, she didn't major in Biology, since the emphasis in elementary courses still seemed to be on memorization rather than inquiry. Instead, Sultan majored in the history and philosophy of science, which, she believes, has been invaluable to her career as a scientist. Thinking about science from a historical perspective taught her that "there are different ways of interpreting and explaining the same phenomena [which are] culturally specific and arbitrary to some extent. When you study science with that insight, you don't fool yourself that you've got some sort of ultimate truth in your hands."
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| "Genes are not a blueprint. Genes are not a program." |
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All organisms have a genotype, which is their genetic makeup, and a phenotype, which is their outward appearance and behavior. Most people tend to think of one as a translation of the other, like a computer language and the program it describes. Yet Sultan and others strongly object to this viewpoint. "Genes are not a blueprint," says Sultan, "genes are not a program," and what appears to us as phenotype is far, far broader than just DNA sequences. Sultan's research has shown that plants with the same DNA may vary widely depending on their environments, and they will change themselves specifically in response to environmental changes. The name for this adaptability of plants is phenotypic plasticity.
Who Needs Darwin?
As a graduate student in Cambridge, Massachusetts, Sultan read zealously about plants, searching for a general "way to think about plants and how they operated in their environments." One of her common walking routes in Cambridge took her by a vacant parking lot where weeds grew out of the cracked pavement against the brick wall of a building, some in shade and some in full sunlight. One day, Sultan noticed that the weeds growing in shade looked drastically different from those in the sun: they looked like typical shade plants, even though they were the same species as their sun-drenched companions. This variation may seem obvious, but Sultan turned her observation into an interesting question: "if the weeds can do this just by perceiving their environment, then why do they need to go through natural selection and be genetically adapted to their environment?"
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| Two Polygonum seedlings from Sultan's lab. Like the parking lot plants, they were grown in different light levels. The one on the left was grown in shade and grew larger leaves. The one on the right was grown in strong sunlight and has smaller leaves. |
The prevailing view of evolution, a combination of Darwinian adaptation and Mendelian genetics, tends to edit out this phenotypic flexibility. Neo-Darwinism, or the New Synthesis, begins with Darwin's insight that organisms' interactions with their environments define which traits are passed on to the next generation. This idea is combined with Mendel's discovery that traits correspond to specific alleles on specific genes, and that those alleles combine according to mathematical principles. Neo-Darwinism says that evolutionary change can be expressed by change in alleles over time. Instead of looking at traits, such as a bird's wing size or color, you can simply follow the presence or absence of alleles for those traits.
The problem with this view, according to Sultan, is that it often leads to gross oversimplification. The metaphor of genes as a blueprint is just that, says Sultan. "A metaphor. . . .Genes do not determine anything. Genes influence." Geneticists know that genes do not have a one-to-one relationship with traits, although that shorthand is prevalent in common understanding of genetics. Rather, many genes combine to influence one trait. Eye color, for example, is influenced by several individual genes.
Genes are inflexible in that they determine the limits of an organism. As Sultan puts it, "if you have the genome of a chicken, you will not grow ears and a tail like a dog." Within those limits, however, there is a broad range of possibilities. Appearance is affected by a combination of an organism's genes, its present environment, and all the past environments that the organism has experienced. So in reality, eye color is "a reflection of genetic information . . . and what your eyes have done up until yesterday and what your eyes are doing today and what your eyes will be doing tomorrow." This kind of complexity is merely "a nuisance to the Neo-Darwinian view," Sultan observes.
Sultan was treated as a nuisance when she first began asking her question about the parking lot weeds, but she soon discovered that she was not alone in her idea. In 1965, an English botanist named Anthony Bradshaw had written a paper about phenotypic plasticity, this ability of plants to adapt to their environments without going through the process of natural selection. Although Sultan was slightly disappointed to learn that someone else had gotten to the idea long ago, this paper meant that her observation was not nonsense. What she shared with Bradshaw was a pragmatic and complex approach to plant biology that went beyond one gene, one trait.
One Lab, Infinite Environments
Sultan's pragmatic approach led to yet more complexity when she set out to test it. Her goal as a graduate student was to test genetically identical plants in different environments and see whether they behaved differently from one another and if so, how. But what plants should she study?
After much searching, she chose plants of the genus Polygonum as the ideal subjects. Polygonum is a large genus of weeds that grows worldwide and survives in many different environments, which indicated to Sultan either local speciation or phenotypic plasticity. Polygonum plants are annuals, which means that their life cycle is easy to observe, and that cloning them is a simple matter of cutting off a piece from one plant and putting it in soil.
Then came the question of what variables to study. Sultan's taste for realism led her to the most fundamental factors of plant life: light, water, and nutrients. She finds these factors interesting both because they are fundamental to plant life, and because "even within a location, any one plant will receive variation [in these factors]. Those are aspects of environmental variation that plants deal with all the time." Unlike many scientists, Sultan does not study plants in extreme environments, but varies the factors to approximate real environments.
Sultan's findings with varied light levels are some of the best examples of her work. In one 1993 study, ten genotypes of Polygonum Persicaria were cloned and grown at three different light levels. In each different genotype, the plants had the same reaction to lower light levels: they concentrated on making their leaves wider, so they could catch more precious sunlight. Conversely, the plants at high light levels had smaller leaves and larger root systems in order to conserve water. In effect, Sultan proved in the lab what she had suspected in the Cambridge parking lot.
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| A Polygonum subject grown in a study of how root systems react to varying water levels. This one was grown in flooded conditions. |
Other studies of a similar nature have shown that plants will adapt many other features depending on their environments. Root systems change, for example, in varying levels of water, and metabolisms change in varied nutrient levels. Sultan is currently looking at how plant environments can affect plant offspring as well. One study examines how seeds which have lain dormant for a while and then germinate can affect evolution.
In future studies, Sultan hopes to examine how competition affects plasticity, since it is a "very ubiquitous and fundamental aspect of plant life." Yet even considering such a study highlights the subtlety involved in Sultan's research. "You have to make a lot of decisions about what you're calling your control," she muses. "What species, and what particular genotypes of that species [am I going] to use, and what environments am I going to do the test in? The whole idea of a control is a very problematic one."
Sultan's work defies more than one seemingly simple and widely accepted notion. First, she does away with the standard "genes equal traits" model, and then she questions the whole concept of a "control." In science, a control represents a neutral environment, a background against which other things are measured. The concept of a control is a defining feature of lab biology.
"People agree on certain standardized protocols so they don't have to encounter the consequences of environmental variation," Sultan counters, "but that doesn't mean there's such a thing as a neutral environment." In the same way that messy reality interferes with a pure genotype-phenotype correspondence, it prevents everything from being truly, purely standard. Human error and the variability of the natural world will always cause slight deviations, and Sultan's work shows that organisms are extremely sensitive to any type of change. Not only are plants highly variable depending upon their environments; those environments are infinitely variable.
It's a Plastic World
The phenotypic plasticity of plants is a fundamental element in how we understand biological behavior, and recognizing its importance means more than just changing textbooks. Sultan is quick to point out that "at this point in the earth's history, we're engendering the loss of many, many species. Therefore, it's very interesting to try to ask is what properties are there in species that are likely to be able to survive what we're doing, and what are the properties of species we're likely to lose."
Weeds, particularly the Polygonum species that Sultan studies, seem hardly to notice the horrors of human disruption. Some weeds even thrive off of human destruction, occupying areas where other species have been cleared out. And the property that allows these weeds to stick around? Plasticity.
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Figure One illustrates change in leaf sizes of several species of Polygonum at varying levels of light. The three steep lines are very plastic species of Polygonum, which changed their leaves drastically with light changes. Naturally, these species are some of the most common varieties of Polygonum, and they are found in numerous locations. The shallow line represents a less plastic species, one which is more rare than the others.
Compared to other plants, not to mention animals, even the less plastic weeds are highly adaptable. "The species we're likely to lose," asserts Sultan, "and that we have been losing, animals, are species that are very ecologically narrow. They have to eat a certain thing, they have to live in a certain type of environment. You screw that up for them, they're gone."
Much of Sultan's funding comes from projects that are specifically targeted at conservation. She recently received a large grant from the Mellon foundation, which has a program called Conservation and the Environment for researchers looking at natural ecosystems and at plants in particular. Other funding comes from a USDA program that studies the population biology of weeds. Sultan expressed dismay over America's "agribusiness" and the country's history of pesticide use, and she views this program as a promising alternative:
"Finding sustainable strategies . . . and getting away from the lines of technology. That's their interest." And hers. And, most importantly, everyone else's.
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