More About Biological Clocks: Fly Circadian Activity
The fruit fly Drosophila melanogaster has been an essential experimental
model for both early genetic studies of circadian rhythms and more recent
examinations of molecular mechanisms. Researchers have devised methods
to record daily activity fluctuations in individual flies. This animation
series shows four experiments that compare the activity patterns of a
wild-type fly keeping a normal schedule with those of a mutant fly apparently
following a 19-hour internal clock. Furthermore, the role of light and
dark in "re-setting" the biological clock of these two types of flies
is also demonstrated.
This animation is based on the research of HHMI investigator Michael
Experiment 1: Wild-type flies are most active at dusk and dawn.
A single fruit fly is placed into a small glass tube. This tube is put
into a device that has an infrared beam (invisible to the fly) aimed across
the tube. Any motion of the fly that interrupts the beam registers as
This activity is then plotted on an "actogram," which shows the amount
of activity over the course of a 24-hour cycle. In an actogram, the 12
hours of light are represented by the light-colored bar at the top of
the graph, and the 12 hours of dark by the dark-colored segment of the
bar. Light and dark are controlled artificially and are represented in
these animations by the turning on and off of the light bulb.
The first animation illustrates how a wild-type fly responds to a regular
light-dark (LD) cycle. Its first bout of activity is at "dawn," just before
and just after the light turns on. Its second bout of activity is at "dusk,"
again at the transition from light to dark. This second bout of activity
lasts for a longer time, and its amplitude is greater than that of the
morning activity bout.
For this animation, 24 hours of activity have been compressed into 5
seconds, with a total of four days being measured. In addition, although
an actual fly does move somewhat during its quiet periods, the animation
represents the fly as being motionless to make it easier to discern the
difference between quiet and active periods.
Experiment 2: Flies in constant dark conditions have a slightly lengthened
In this experiment, the fly is kept in constant dark conditions (DD).
This is also referred to as a "free-running" condition; the fly has no
environmental cues to tell it where it is in its 24-hour cycle.
Under these conditions, a wild-type fly maintains both activity bouts,
although the amplitude of the morning bout is greatly reduced. The other
notable difference in a free-running wild-type fly is that its activity
cycle is close to, but not exactly, 24 hours. The period is approximately
24.6 hours, so its activity bouts shift gradually later and later over
successive days. This can be seen on the actogram by the activity bouts
gradually shifting to the right.
Experiment 3: perS mutants have a shorter period of activity
in constant dark conditions.
The perS mutant, under free-running DD conditions, exhibits a
shortened period in its daily activity, which can be shown by looking
at an actogram of a perS mutant over a series of days. Its free-running
behavior has a shortened period between its two activity bouts. Since
its cycle is only 19 hours rather than 24, the activity bouts shift to
the left with each successive day.
Experiment 4: perS mutants are entrainable in light-dark conditions
but maintain a shortened activity cycle.
Placing the perS mutant back into light-dark conditions returns
the fly's morning activity bout to full amplitude and brings back a regular
24-hour cycle to its activities. However, the mutant fly's evening bout
is advanced in time, occurring before the light-dark switchover. This
shows that the mutant fly's circadian rhythms, although different from
wild-type flies, are still present and can be "entrained" to respond directly
to light stimulus from the environment.
Fly Circadian Activity Background
Living organisms have evolved internal timekeeping mechanisms to synchronize
behavior and physiology with the cycles of day and night. These biological
clocks have been found in organisms as diverse as fungi, fruit flies,
hamsters, and humans.
This animation shows a real-time schematic demonstration of how circadian
rhythms researchers record the daily activity of fruit flies. The activity
charts, or "actograms," of a normal fly are compared with those of a fly
that has a mutant gene important in maintaining a 24-hour period. Thus,
the behavior, or phenotype, of a fly can be correlated with mutations
at the level of the gene.
This animation was designed in conjunction with HHMI's 2000 Holiday Lectures
on Science, Clockwork Genes: Discoveries in Biological Time (www.holidaylectures.org).
Fly Circadian ActivityTeaching Tips
The animations in this section have a wide variety of classroom applications.
Use the tips below to get started but look for more specific teaching
tips in the near future. Please tell us how you are using the animations
in your classroom by sending e-mail to firstname.lastname@example.org.
Use the animations to make abstract scientific ideas visible and
Explain important scientific principles through the animations. For
example, the biological clocks animations can be used to demonstrate
the fundamentals of transcription and translation.
Make sure that students learn the material by repeating sections
of the animations as often as you think necessary to reinforce underlying
scientific principles. You can start, restart, and play back sections
of the animations.
Urge students to use the animations in accordance with their own
learning styles. Students who are more visually oriented can watch
the animations first and read the text later, while others might prefer
to read the explanations first and then view the graphics.
Incorporate the animations into Web-based learning modules that you
create to supplement your classroom curricula.
Encourage students to incorporate the animations into their own Web-based
Fly Circadian Activity Credits
Director: Dennis Liu, Ph.D.
Scientific Direction: Michael Rosbash, Ph.D.
Scientific Content: Donna Messersmith, Ph.D.
Animator: Chris Vargas