Manarch Migratory Mechanism
Stanley A. Gorodenski
stan_gorodenski at asualumni.org
Fri May 30 18:18:13 EDT 2003
Hot off the press from Science:
Illuminating the Circadian Clock in Monarch
Oren Froy,* Anthony L. Gotter,* Amy L. Casselman, Steven M. Reppert
Migratory monarch butterflies use a time-compensated Sun compass to
navigate to their
overwintering grounds in Mexico. Here, we report that constant light,
circadian clock function at both the behavioral and molecular levels in
disrupts the time-compensated component of flight navigation. We
further show that
ultraviolet light is important for flight navigation but is not
required for photic entrainment of
circadian rhythms. Tracing these distinct light-input pathways into the
brain should aid our
understanding of the clock-compass mechanisms necessary for successful
Another hot off the press from Science:
Monarchs Check Clock to Chart Migration Route
People who need to consult maps, radio traffic reports, or the Global
Positioning System to navigate from one city to another should stand in
awe of monarch butterflies. They migrate thousands of kilometers to a
small winter retreat in Mexico. These intrepid travelers use the sun as
a guide, but exactly how has been a mystery. Now researchers demonstrate
that monarchs depend on an internal clock to determine their course.
Studies of birds 50 years ago indicated that long-distance migrants
"need an accurate
internal timepiece" to use the sun as a compass, says Steven Reppert, a
neurobiologist at the University of Massachusetts Medical School in
Worcester. Without a clock, the sun would prove an unreliable landmark
as it moves across the horizon; a clock allows animals to compensate for
this apparent motion and maintain a direct course.
By combining genetic and behavioral studies, as reported on page 1303,
Reppert and his colleagues established that monarchs use their internal
timepiece, apparently set by genes that are important to circadian
rhythms, to fly in the right direction. The work "shows provocative
links between the circadian machinery and the compass mechanism required
for seasonal migration," says Gene Robinson, a neurobiologist at the
University of Illinois, Urbana-Champaign.
To demonstrate that monarchs have internal clocks and that these clocks
are set by
daylight, Reppert's collaborator Oren Froy tested monarchs' emergence
chrysalises. They normally emerge in the morning, but exposure to
constant light instead of daily light-dark regimens caused the
butterflies to emerge throughout the day, indicating that their clocks
had been disrupted.
Next, Anthony Gotter, also part of Reppert's team, looked at flight
patterns during lab simulations of a normal day-night cycle, one in
which light was advanced 6 hours, and one in which night never came.
Once the butterflies were accustomed to these light regimens, Gotter
tested their behavior outside using a flight simulator, a 150-liter
barrel in which the butterfly flies while tethered to a tungsten wire
connected to a computer. The monarchs could track the sun's path as it
moved over the top of the simulator, which was open to the sky.
Butterflies whose lab light cycle corresponded to local daytime headed
southwest. They had been collected in Massachusetts at the beginning of
the study, and "that seems to be the genetically programmed direction
that they need to go to get to Mexico," Reppert says.Next the
researchers looked at monarchs whose exposure to light was advanced by 6
hours, so the "day" in the lab lasted from
1 a.m. to 1 p.m. Outside, these would-be migrants shifted their
orientation by 115º and headed southeast. Like birds whose circadian
rhythms have been fooled, the time-shifted insects failed to interpret
the sun's position correctly. During the morning,
they acted as though the sun were in the west rather than the east.
Butterflies that had been in constant light indoors simply headed
straight for the sun no matter where it was in the sky.
To see whether gene expression corresponded to light-induced changes in
behavior, Reppert and his colleagues measured RNA from a gene called
period (per). Fruit fly researchers had identified per as a key gene for
setting the internal clock, and
Reppert's group isolated the gene in monarchs. The per gene's activity
was high during dark periods and low during light times. Butterflies
exposed to constant light for several days showed no swings in per
Taken together, the experiments demonstrate that the internal clock,
perhaps timed by per activity, tells monarchs how to calibrate their
movements against the sun. "It's the first molecular entrée" into the
clock-assisted migration, says Charalambos
Kyriacou, a behavioral geneticist at the University of Leicester, U.K.
But it's no slam dunk. Orley Taylor, an entomologist at the University
of Kansas, Lawrence, is not sure the work demonstrates that the clock
guides navigation. "The data would be more conservatively interpreted as
phototactic [light-based] orientation,"
In any case, many mysteries of monarch migration remain. For instance,
monarchs from different parts of the eastern United States and Canada
all end up in the same Mexican wintering grounds. They follow different
bearings and start their journeys at different times, a feat that is
hard to reconcile with a simple, species-wide clock, says Taylor. A
further complication is that each year at least three generations of
butterflies emerge before the last one flutters to Mexico in the fall.
No one knows what triggers
migration in that generation, says Reppert, but the circadian clock
might help them recognize shortened day lengths and therefore the
changing of the seasons.
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