"Lean, Green, Eating Machine " Subject of Research

Mary Chamberlin, associate professor of biological sciences specializing in animal physiology, seems to be happiest when she is surrounded by little green crawling caterpillars, of which she has dozens. Actually, the tobacco hornworm (Manduca sexta) does more eating than crawling.

"This is a lean, green, eating machine," Dr. Chamberlin said. "That's all it does as the larval stage of the moth." In 1994, Dr. Chamberlin received a three- year grant for nearly $250,000 from the National Science Foundation to conduct research on the caterpillars. Her research project, "Changes in Insect Epithelial Metabolism and Active Ion Transport During Larval Growth and Development," involves the bright green, soft-bodied pest which tobacco growers have battled for years and which many gardeners will recognize as a prodigious consumer of their tomato plants.

"The tobacco hornworm is frequently used for research in development, endocrinology, feeding behavior, and so forth," Dr. Chamberlin said. "I'm interested in the midgut. The job of all caterpillars, is to eat, to store up nutrients and fat for the subsequent developmental stages, the pupa (commonly called a chrysalis) and the moth. The pupal stage is the quiescent, developmental stage, while the moth is the sex stage. A moth or butterfly drinks but it doesn't really eat much and the pupa doesn't eat at all."

Caterpillars grow quite rapidly. Manduca (the genus name, which means "chewer") increases in mass 10,000-fold in sixteen days. At maximum caterpi1lar size it weighs about ten grams and is about three inches long, at which time it undergoes metamorphosis into a pupa.

"Basically, the caterpi1lar is a bag of blood with a gut running through it, that goes through four larval molts," Dr. Chamberlin explained. "My research has two aims. The first is studying the developmental changes in the midgut, because we don't know what goes on inside the animal during larval-larval molting and metamorphosis to the pupa. I look at metabolism and active ion transport in the midgut, because the midgut not only digests and absorbs organic solutes, but it also has to transport a lot of inorganic ions.

The other part of the developmental work is to look at what happens from the larval stage to the pupal stage, because at that stage the larval midgut is destroyed and replaced by a pupal midgut. As you would expect, the pupal midgut is a lot smaller. Big changes go on in the midgut as cells that will become the pupal midgut differentiate. I'm looking at the changes that occur before pupation when the animal is "committed" to pupate by a series of molecular events that reprogram cells. A manifestation of the "commitment peak" is development of a dark-blue stripe down its back. This is called the "wandering stage," and that is the point when you realize the caterpillars have disappeared from your garden. It's gone to the wandering stage, stopped feeding, crawled off the plant, buried itself in the ground, and pupated. The other aspect of my research on the caterpillar midgut involves looking at the coupling between active ion transport and metabolism. I think I have a niche.

Dr. Chamberlin did postdoctoral work at Duke University on the link between ion transport and metabolism in kidney tubules. She said she is probably the only person working on the metabolism of insect epithelia (the epithelia are the cell layers that separate the inside from the outside of the body in both invertebrates and vertebrates). "I'm interested in midgut epithelium from a bioenergetics point of view-that is, how is energy made and how is it partitioned between various uses. I've found you can turn off ion transport and the metabolic rate of the tissue doesn't seem to change, so it's not clear what is going on."

Animal experimentation involves housing, feeding, and acquiring the animal, which can be a challenge apart from the research. Dr. Chamberlin said she and Frank Horodyski, assistant professor of molecular biology, who also works with Manduca faced a crisis when their supplier of caterpillar eggs went out of business. "We realized we had to start producing our own, so we established a breeding colony. We have a tobacco plant where the female moths lay their eggs. Then we collect the eggs and raise the caterpillars from them. The insects have been obliging and it has been quite successful! It's pretty easy to feed Manduca because this caterpillar will eat commercially prepared food, an advantage because many insects will refuse anything but their natural food."

Dr. Chamberlin said her research could lead to a better understanding of how toxins work in an insect's midgut, which in turn could provide information on insect control. "Caterpillars have alkaline rather than acid stomachs, and eventually I'd like to learn more about how that alkaline secretion is made. In a general way this type of research can also shed more light on cellular and subcellular development in humans. Insects are the dominant animal life form on our planet-a conservative estimate is about 10 million species, of which we can identify less than 1 million. Much of our basic understanding of cellular function has come from studies of insects."