The first big study of daily rhythms in fruit flies outdoors doesn’t match some of the basic results from decades of lab tests.
Fruit flies flittering in lab containers have revealed much about how light can set the master molecular clock that ticks out a daily beat in living organisms. Yet watching daily rhythms in fruit flies caged outdoors reveals regular surges in activity not seen in the lab, says geneticist Rodolfo Costa of the University of Padova in Italy. And certain patterns of activity seen in the lab don’t show up in the real world, he and his colleagues report online April 4 in Nature.
A major difference, he says, is that the typical increase in fruit fly motion as day dawns doesn’t seem to need a built-in clock in the real world. Flies with genetic mutations that disable their biological clocks don’t join in the usual laboratory bustle of activity before lights-on. Yet outdoors they perk up and get moving just like clock-normal flies. “This was something really unexpected,” Costa says.
“We are not saying that everything that has been done until now is useless,” he adds. But some of the assumptions based on laboratory experiments, he says, should be expanded to account for behavior in nature.
“The new study very nicely illustrates the risks of extrapolating from laboratory studies to natural conditions,” says neuroscientist and chronobiologist F. Rob Jackson of Tufts University School of Medicine in Boston.
Decades of laboratory work suggest that many organisms — molds, cyanobacteria, mice, plants, people and so on — share genetic mechanisms that create a master 24-hour rhythm influencing cell processes. Understanding what drives this rhythm and what derails it holds promise for treating sleep disorders, managing the stress of shift work and optimizing medical treatments — as well as understanding how the rhythms of life evolved on a periodic planet.
In indoor lab studies, when lights flip on in the morning and flip off at night, fruit flies are active around dawn and dusk. Genetically normal flies start fidgeting and moving about several hours before lights-on and then settle down during the day for a siesta. They get moving again as lights-off nears. Adding dim simulated moonlight shifts some of their activity to the night.
To translate laboratory work to outdoor experiments, Costa and his colleagues set both natural and mutant flies in clear containers in Padova, with automatic equipment to monitor their activity. For comparison, collaborator Charalambos Kyriacou of the University of Leicester in England set up a parallel fly outfit in his children’s old outdoor playhouse.
Outdoor environments expose flies to a much more complicated mix of cues. Light appears and dims gradually. Intensity and color change. Temperature rises and falls.
Unlike in the lab experiments, the scientists saw the outdoor flies go through an afternoon burst of activity. Instead of being a dawn/dusk animal, “fruit flies are diurnal,” Costa says. Also he saw no moonlight effect. And in some cases, temperature apparently trumped light as a cue.
A difference between indoor and outdoor flies is not that surprising, says geneticist Paul Hardin, who directs the Center for Biological Clocks Research at Texas A&M University in College Station. What those differences turned out to be though, did strike him as unexpected. “This will be a notable paper moving forward,” he says.