January 27, 2009

Biology professor Elizabeth Marin examines a colony of fruit flies in her lab

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By Julia Ferrante

The next time you get the urge to swat those pesky little insects hovering over the fruit bowl, you may want to reconsider.

Humans and fruit flies have a lot in common – not the least of which is similar genetics.

"Flies are a supreme developmental model organism," said Elizabeth Marin, an assistant professor of biology at Bucknell University. "Almost every gene identified in humans is in the fly. We're definitely more complex than flies, but there's a lot we can learn from something so simple."

Marin studies the central nervous system development in Drosophila melanogaster, also known as the fruit fly. Her research focuses on a structure in the insect’s brain called the "mushroom body," a bundle of thousands of neurons formed by just four stem cells.

Although fruit flies are not as complex as some other insects with mushroom bodies, such as bees and wasps, they are capable of learning and memory, Marin said. She and groups of researchers at other universities are examining how neurons develop in fruit flies over time. The knowledge eventually could shed light on similar questions about humans.

"We are also interested in evolutionary questions," Marin said. "I'd like to see how the mushroom body evolved in insects, primarily by looking at insects that have less-complex mushroom bodies."

Short life, long history
Marin has been working with fruit flies since she was an undergraduate at the University of California in San Diego. She began studying the olfactory system of the fruit fly, which lives only a few months under the best conditions, while pursuing her doctorate at Stanford University. Marin continued her work through a Damon Runyon postdoctoral fellowship at the University of Washington in Seattle. Her current research at Bucknell builds on that work. 

Fruit flies are born with a certain set of neurons, but the structure and function of those neurons changes during their short life span, Marin said.

"There is a massive reorganization during metamorphosis when the larva transitions to an adult," she said. "The larvae have a nervous system that is much smaller and simpler. The neurons have to change and reorganize to be used by an adult."

Growth through the stages
Only about 5 percent of a fly's neurons are present in the newly hatched larvae; the other 95 percent are born during larval and pupal stages to function exclusively in the adult. Most previous studies have focused on the small number of neurons born in the embryo. Marin studies the neurons used in adult fruit flies, looking at what controls their identity and connectivity and why different types of neurons are produced at specific times.

Marin and her students are conducting experiments to determine the role of hormones, diet, and temperature in growth of the mushroom body in fruit flies, which forms from four stem cells and appears to be a four-fold symmetric structure, Marin explained. The cells divide and produce three types of neurons in a fixed sequence. The researchers have learned that the sequential generation of neuron types is linked with the development of the whole organism, but they don't yet know how.

Poor nutrition could spur development
Another research group has found that larvae, when given only a sugar-water solution, are deprived of protein and stop growing outwardly. The mushroom body stem cells, however, continue to divide under these conditions and make new neurons.

Conversely, researchers can stop development of the mushroom body but allow larvae to continue growing by manipulating a mutant gene whose protein product is nonfunctional at high temperatures, Marin said. The researchers then dissect the nervous system and use antibody staining to see how many neurons are made and whether they are of the early or later born types.

"The question is: Is the mushroom body cell type controlled by the stage of the animal or by some kind of internal timing mechanism?" Marin said.  "We have found that it is definitely linked to the stage of the animal and are currently trying to identify the hormone that is responsible."

Researchers are trying to find out why the nervous system continues to grow when the fruit fly is deprived of good nutrition. One theory is that fruit flies take longer to reach adulthood under these conditions and therefore develop larger and more complex nervous systems, possibly as a necessity to recognize scarce food sources. If given a better diet, they grow more quickly but don't have as much time to develop complex nervous systems.

 "When food is abundant, they're fat and happy, and they don’t need to be as smart," Marin said.

Contact: Division of Communications


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