[ RadSafe ] Why insects are "radioresistant?" Less oxygen.

[John Jacobus]

>From THE SCIENTIST February 7, 2005

Breathing to avoid oxygen?
Insects' discontinuous respiratory cycles may reduce
oxidative damage, investigators suggest 
| By Graciela Flores
 
Insects use discontinuous patterns of gas exchange as
a means of avoiding the toxic effects of oxygen,
researchers propose in Nature this week. 

The report puts forward a novel interpretation for a
mechanism that has puzzled scientists for decades. The
peculiar type of breathing exhibited by some insects
is a cyclical pattern of opening and closing the
spiracles—apertures that connect the respiratory
tracheal system with the exterior. 

"Two main hypotheses have been proposed to explain
discontinuous breathing," senior author Timothy
Bradley of the University of California at Irvine told
The Scientist. One suggests that closing the spiracles
reduces respiratory water loss, Bradley explained. The
other suggests that the discontinuous breathing
pattern may have evolved initially in underground
insects as a means of better excreting carbon dioxide.


To investigate the phenomenon, Bradley and co-author
Stefan Hetz measured oxygen levels within the trachea
of the moth Attacus atlas at a range of ambient oxygen
concentrations, and found that they remained
constantly low. "Our hypothesis is that, in fact,
discontinuous breathing is a means of reducing oxygen
uptake, and maintaining a low oxygen concentration in
the trachea to reduce oxygen damage," he said.

"It's a really neat work, very well done," said Allen
Gibbs of the University of Arizona, Tucson, who was
not involved in the study," but I'm not convinced that
they really have shown that discontinuous gas exchange
cycles are an adaptation to combat oxidative damage." 

Nonetheless, the work is a good demonstration that
insects are actually sensing and regulating internal
oxygen levels, Gibbs added. "What the authors really
need to show is that when insects leave the spiracles
open, or when they don't use this pattern of
breathing, there is more oxidative damage than when
they do use this pattern."

Thorsten Burmester of the University of Mainz in
Germany, who wrote a related News and Views article,
welcomed the study. "This paper brings to mind that
oxygen is not always good for us. The tracheal system
is a very efficient system, and this means that
normally the insect gets a lot of oxygen. If the
insect is not active, it needs to avoid too much
oxygen, and that's the reason it closes the spiracles.
That is the important point of this paper." 

John Lighton of the University of Nevada, Reno, and
proponent of the carbon dioxide–centered "underground"
hypothesis, told The Scientist the Nature work was
interesting. "When I first heard of it I thought it
was off the wall, but I learned that when an idea is
really unusual, that's often a good sign that there is
something to it." 

"The water loss hypothesis is not dead, but I would
say it's probably quite sick," said Lighton, who did
not participate in the research. "For the underground
hypothesis, there is a lot of indirect evidence, but
no direct experimental confirmation." Lighton stressed
the heuristic value of the new hypothesis. "It doesn't
really matter if it is right or not, it will generate
debate, and it will make people think."

Bradley, however, did not dismiss alternative
explanations. "Discontinuous ventilation under some
circumstances does save water, as the water loss model
proposes; and closing and then opening the spiracles
may facilitate the rapid loss of carbon dioxide, as
the underground model suggests," he said. "But the
question that we were trying to address is, what is
the real fundamental reason that insects
discontinuously ventilate? And what are the
evolutionary forces that led to that pattern? For
that, the answer seems to be to keep oxygen out." 

Links for this article
S. K. Hetz et al., "Insects breathe discontinuously to
avoid oxygen toxicity," Nature, 433: 516-9, February
3, 2005.
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal
/v433/n7025/abs/nature03106_fs.html 

Timothy J. Bradley
http://ecoevo.bio.uci.edu/Faculty/Bradley/Bradley.html


Allen G. Gibbs
http://eebweb.arizona.edu/faculty/gibbs/ 

Thorsten Burmester
http://www.uni-mainz.de/FB/Biologie/Zoologie/abt2/thorsten/thor
sten_english.htm 

T. Burmester, "A welcome shortage of breath," Nature,
433: 471-2, February 3, 2005.
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal
/v433/n7025/full/433471a_fs.html 

John Lighton
http://www.sablesys.com/john-lighton-lab/index.html 
 
©2004 The Scientist, unless otherwise stated 


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"Baltimore is actually a very safe city if you are not involved in the drug trade."
DR. PETER BEILENSON, the city's health commissioner.

-- John
John Jacobus, MS
Certified Health Physicist
e-mail:  crispy_bird at yahoo.com


		
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