Breathing to avoid oxygen?

Insects' discontinuous respiratory cycles may reduce oxidative damage, investigators suggest

By | February 7, 2005

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."

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