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Flashy Courtship Takes Gas
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NO on a Summer's Eve
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One knew, perhaps, that the gas nitric oxide (NO) is a constituent
of automobile exhaust and a big player in the formation of smog, and is no
laughing matter, such as nitrous oxide, N2O. But who knew that
NO is crucial chemistry in the flash of a firefly as it searches for sex
(or dinner) in the dark of a summer night?  | | Female firefly gives a flash response to suitor. | | S. Lewis | Sara
M. Lewis '75, BI '90, associate professor of biology at Tufts University,
and spouse Thomas Michel '77, M.D., associate professor of medicine at Harvard
Medical School and chief of cardiology at the Harvard-affiliated West Roxbury
VA Hospital, have collaborated with scientists of other disciplines and with
their sons, specimen collectors Benjamin and Zachary, to show that a firefly
uses nitric oxide to control the flash of its lantern, a spectacular natural
phenomenon. This is the same molecule, Michel notes, "that in humans controls
blood pressure, penile erection, and the formation of memories, among diverse
other roles." Nitric oxide, a small, diffusible, highly reactive molecule,
is often deployed by the body as a biological signal and regulator. Michel
and colleagues at Harvard-affiliated Brigham and Women's Hospital were able
to describe how the gas helps control blood pressure by dilating blood vessels.
Naturally, he talked about his work at the dinner table with Lewis, who is
an evolutionary ecologist and insect neurophysiologist interested in fireflies.
The couple and Barry Trimmer, a biologist colleague at Tufts, wondered whether
nitric oxide might have something to do with how fireflies turn their lanterns
on and off. The three of them asked June Aprille, a cell biologist at Tufts,
for her thoughts on the matter. The four roped in David Dudzinski, a student
at Harvard Medical School, to build a tiny gas chamber for firefly research.
These four, with three others, in time announced flash findings in Science
magazine. "No single one of us could have made the discovery," says Michel,
"and ours is a nice example of the power of interdisciplinary collaboration." Thousands
of specialized light-producing cells called photocytes in the lantern of
the firefly—a beetle, actually, not a fly—have in their inner regions structures
that contain chemicals that generate light when turned on by oxygen. But
the edges of the photocytes are also densely packed with organelles called
mitochondria, "which are quite famous as the oxygen-consuming power plants
of almost all cells," says Aprille. Ordinarily, the mitochondria use up the
oxygen piped in through tracheal air tubes to make energy for all cellular
activities. But when the firefly wants to flash, it generates a nerve pulse
that releases nitric oxide—the team found the enzyme that produces NO lying
right next door to the firefly's light-generating apparatus—and when NO is
present, the use of oxygen by the millions of respiring mitochondria comes,
say the scientists, to a screeching halt. Oxygen is then able to pass through
the mitochondria to the interior of the cell where it reacts with the light-producing
chemicals to produce luminescence. Flash. As the NO signal decays, the mitochondria
power up again and begin to gobble up oxygen, which turns the lantern off.
All of this happens in a fraction of a second.  | | Thomas Michel and Sara Lewis, with their sons, Zachary (left) and Benjamin, poised for research en famille outside their home in Lincoln, Massachusetts. | | Jon Chase / Harvard News Office | "What
is life? It is the flash of a firefly in the night," said the dying Blackfoot
warrior Crowfoot. Yes, but what is the meaning of the flash? "Fireflies are
very romantic beasts," says Lewis. "Their whole adult life is spent courting."
The courtship system is based on flash communication. A firefly can flash
once a second, and each flash lasts about half a second. All of the hundreds
of known species of firefly time their flashes differently for identification
purposes, and male and female flash patterns within species also differ.
Thus, one species will flash at a regular, sedate rate, while another will
emit a periodic machine-gun burst of multiple flashes. Flashes, some fireflies
learn, aren't always an invitation to the dance. Female Photuris fireflies
know how to imitate the flashes of females of other species, luring males
of those species, whom they eat. All that glitters is not gold. Who
knew what a gas is nitric oxide? "Only within the past dozen years have we
begun to learn the major role that nitric oxide plays in the internal regulatory
system of human beings," says Michel. "It is at work in the heart, blood
vessels, brain, penis, liver, lungs, eyes, and likely in every other human
organ." In many of its good works, it functions as a relaxer. Thus, release
of NO in the kidneys relaxes the smooth muscle in the walls of blood vessels,
increasing blood flow and, thereby, the rate of filtration. Nitroglycerine,
often prescribed to reduce the pain of angina, works by generating nitric
oxide, which relaxes the walls of the coronary arteries. During sexual excitation,
nerve endings near the blood vessels of the penis release NO, allowing the
vessels to relax and blood to pool, producing an erection. (Viagraź works
by amplifying the effects of nitric oxide.) According to the self-styled home page of NO (www.academicpress.com/no/),
"From diabetes to hypertension, cancer to drug addiction, stroke to intestinal
motility, memory and learning disorders to septic shock, sunburn to anorexia,
male impotence to tuberculosis, there is probably no pathological condition
where nitric oxide does not play an important role." Website address for this firefly research:
http://ase.tufts.edu/biology/firefly
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