Researchers in the
X-ray hutch. Left to
right: Wah-Keat Lee
(Argonne National
Laboratory), Brent
Sinclair, Steve Roberts (now of Central
Michigan University),
Arun Rajamohan
(now of North Dakota
State University),
Allen Gibbs (
University of Nevada at
Las Vegas) and Jake
Socha (Virginia Tech).
Filming Freezing Flies
Why can some insects survive freezing, while others can’t?
The answer seems to be related to how
freezing occurs, according to research
conducted at the University of Western
Ontario (Canada) using images obtained
with the Advanced Photon Source (APS)
from Argonne National Laboratory
(PLoS ONE 4( 12): e8259; doi: 10.1371/
journal.pone.0008259).
To investigate the ways that various flies freeze, the researchers filmed
the formation and spread of ice in real
time as the flies (as larvae) froze using
high-energy X-rays from the APS. They
compared Chymomyza amoena, an insect
native to Ontario that survives freezing,
with the fruit fly often used for research,
Drosophila melanogaster.
Brent Sinclair, an assistant professor
in the university’s biology department,
From a practical standpoint, figuring
out the mechanism could have immedi-
ate applications, since fruit flies are used
in thousands of labs. Maintaining some-
thing like 250,000 different fly popula-
tions is a hassle. Also, the reproductive
speed that makes flies an attractive
research animal becomes a problem as
spontaneous mutations can creep into
the line of flies. Sinclair says, “They are
not as stable a resource as we would like.”
It would be more convenient if the flies
could be stored in a freezer when they
weren’t needed.
The current model for how insects
survive freezing suggests that ice starts
to form between cells. All the other
things in solution—ions, sugars, proteins,
etc.—are pushed out from the ice lattice,
The Western
Ontario group
wanted to watch
Fly larva in the process of freezing. (Left) Phase-contrast X-ray
image. (Right) Composite of subtraction images of the frames
where the gut appeared to freeze (orange) and the initial freezing
frame (green).
Gut
Trachea
Jake Socha, Virginia Tech
ice form inside the insects in real time
and see if there is anything fundamentally different about how ice forms in
larvae that can survive freezing and those
that can’t. The flies that survive consistently freeze at higher temperatures than
those that don’t—which suggests that
the insects have some control over when
and where ice starts crystallizing.
Other techniques (such as MRI and
confocal imaging) exist for looking
inside frozen insects, but those don’t give
the temporal resolution that the researchers needed for watching changes every
few seconds. The APS provided this as
well as parallel 15-keV X-rays, chosen to
optimize image clarity. The larvae were
attached to a thermocouple, and the
temperature was reduced by about half
a degree per minute. The X-rays didn’t
appear to perturb the process. (See video
of the process attached to the PLoS
ONE paper or at www.youtube.com/
watch?v=m07CKU1XGdk.)
Sinclair would like to find ways to
use contrast agents to tell different sorts
of soft tissues apart. Since the images
depend on density differences, the imagery shows only the difference between
“air in the tracheal systems, wet stuff
and ice,” he explains. For now, Sinclair is
continuing to investigate the biochemistry of freezing—and thawing.
Yvonne Carts-Powell ( yvonne@nasw.org) is
a freelance science writer who specializes in
optics and photonics.
