Clues to Health in “Smartweeds”
Sonia Sultan, a slim and dark-haired biologist, kicks off her sandals and begins to water one of the bushy green Polygonum plants growing in Wesleyan’s research greenhouse. She intends to compare it to its relatives growing in the wild on a bright Cape Cod beach.
By studying growth under different conditions, Sultan has shown that a plant’s development depends on environmental factors and not solely on its genetic makeup. The significance of this work extends far beyond the plants she studies, many of which are common weeds. Sultan’s insights are helping medical scientists understand how, even in the womb, environmental influences may play an unsuspected and detrimental role in human health, leading to serious diseases such as hypertension and diabetes.
Ever since Watson and Crick described the structure of DNA in 1953, scientists have focused primarily on the role of genes in determining traits of living things. The role of environment in development has taken a back seat. Sultan and her colleagues argue that environmental factors deserve a closer look. Surprisingly, environment can change development in ways that extend across generations.
“The general public is probably unaware how profoundly environmental influences shape development and physiology in both animals (including humans) and plants,” says Sultan. “Genes alone do not determine everything about us.”
She and a group of international colleagues recently published their ideas in the journal Nature. The article raises significant?and as yet unanswered?questions about public health.
The Nature paper had its origins in fields as diverse as evolutionary ecology, physical anthropology, and medical epidemiology. The key finding, according to the authors, is that many organisms show great adaptability in responding to environmental stresses, a phenomenon known as “plasticity.”
For example, the freshwater shrimp species Daphnia shows plasticity when a mother is exposed to chemical traces of a predator. Her offspring are born with a defensive “helmet” that protects them from that predator. If the predator has disappeared, however, this extra structure becomes a liability in competing to capture food. What is adaptive in one environment can actually reduce success if conditions change.
Sultan’s interest in plasticity dates to her graduate work at Harvard in the mid-’80s. As she strolled by a vacant parking lot one day, she made an important connection when she stooped down to look at several weeds growing out of the cracked pavement. Some were in the shade and some were bathed in full sunlight. The weeds in the shade had developed broader leaves, which would catch more light, while the weeds in the sun had formed narrower leaves, which would conserve water. These adaptive differences were due to plasticity, since both plants belonged to the same species.
“I realized that if genetically similar plants could do well in the sun and shade simply by virtue of flexible responses, then flexibility is an important part of how species cope with different environments,” she says. “Organisms don’t have to have specific genes to survive in certain environments.”
Today, Sultan continues to study the adaptability of plants and how this affects their ecology and evolution.
“I prefer working with plants because you can clone them,” she says. “This produces much more convincing results since any differences between genetically identical plants must arise from environmental differences, and not from genetic ones.”
Much of Sultan’s experimental work takes place inside Wesleyan’s research greenhouse. A rooftop weather station feeds information to an elaborate computer system that adjusts vents, shade curtains, and heating and cooling machinery to maintain optimal climatic conditions. One area Sultan and her students are studying is cross-generational plasticity?the way mother plants alter their offspring in response to environmental conditions.
In the weedy species called Polygonum persicaria (or “smartweed”), Sultan has found that mother plants grown in the shade make smaller fruits by building them with thinner outer walls, but the size of the embryonic plant’s nutrient reserves remain the same.
“Due to growing in low light, where they photosynthesize less, the mother plants economize on the outer covering of the seeds, but they maintain the more essential high level of nutrient supply to the new plant,” she explains. “The seedlings also make shorter roots, favoring shoot growth, which would maximize photosynthetic surface area under limited light. In this experiment, Sultan’s seedlings were particularly well suited to the shaded environment their mothers encountered.
“It used to be thought that the effects of environmental stress on a mother animal or plant would just result in smaller or weaker offspring,” she says. “In the past eight years or so, it’s been recognized that certain plant and animal species show specific, adaptive responses to these kinds of stresses in the way they make their offspring.”
Do humans respond in a similar way?
According to Sultan and her colleagues, humans evolved so that the mother would signal to her fetus what environment the child could expect, and that child would develop a body appropriate to that environment. If a mother has poor nutrition, for example, the fetus would develop the kind of metabolism and digestive system designed to conserve calories.
That’s the thrust of the Nature paper. David Barker, a leading epidemiologist who heads the Developmental Origins of Health and Disease faculty at England’s University of Southampton, says the study of developmental plasticity in people is just beginning.
Humans?just like the Daphnia shrimp?may be adversely affected if the mother prepares the fetus for an environment that the child does not encounter. A pregnant woman in poor nutritional condition may unwittingly signal to her unborn baby that it is about to enter a harsh world. If so, this “weather forecast” from the mother’s body may result in her baby being born with characteristics such as a small body and a modified metabolism that help it to cope with a shortage of food. If the child later has lots to eat, this may trigger health problems such as obesity. Barker’s research shows that people with low birth weights who grow up in affluent environments are at increased risk of developing coronary heart disease, type 2 diabetes, and hypertension. Those born as heavier babies and brought up in affluent environments enjoy a much-reduced risk.
The authors of the Nature paper say that a critical public health issue is whether measures exist that would help individuals whose characteristics were set early in life for an environment that subsequently changed. If so, physicians may be able to help some individuals avert chronic diseases. The group met in Italy last year to discuss ways that a mother’s nutritional health can affect the development of her child and his or her health as an adult.
Coauthor Patrick Bateson, former head of the Department of Zoology at the University of Cambridge and a fellow of Britain’s Royal Society, says that people in developing countries where conditions are changing, such as India and China, are particularly susceptible to the problems caused by intergenerational plasticity. An essential public health measure, he suggests, is careful control of diet after birth, along with enhanced nutrition for young women.
He credits plant scientists, including Sultan, with leading the way toward a better understanding of plasticity and its implications for healthy or unhealthy development.
As with all of her research, Sultan’s ultimate goals remain the same. “I want to change the way people think about organisms,” she says. “I want them to question and challenge their ideas.”
