A fruit fly in its natural habitat
In circadian rhythm research, the single best-studied organism is probably the fruit fly. It was through grinding up the heads of countless flies that scientists discovered molecular clock genes, which were then found to play similar roles in humans. But when it come to how actual fly behavior changes in a 24-hour period, a recent study questions whether conventional lab wisdom is wrong. A new paper in Nature put those same lab-bred fruit flies in a natural habitat and observed bursts of activity at unexpected times.
Two set of experiments, one in Italy and another right in the backyard of the lead researcher in England, found that flies are diurnal. That means they’re most active during the day, specifically the afternoon with small upticks in activity during dawn and dusk. While this may not sound exciting by itself, it upends decades of lab research that said fruit flies take a “siesta” during the day and have dramatic bursts of morning and evening activity. This behavior is so well-accepted that there are neuron clusters of the fly brain called morning and evening oscillators whose activity corresponds to the bursts.
Outdoors, however, temperature fluctuations and the gradual rising or setting of the sun offer much richer information than a temperature-controlled incubator where lights flick on at ZT 0 and off at ZT 12. (ZT stands for Zeitgeber Time, which means “time giver” in German.) Since the morning burst of activity comes just before sunrise, scientists had thought it was governed by an internal molecular clock. Data from this new study suggest this morning activity is actually set by warming temperatures leading up to sunrise.
While its results are surprising, this paper doesn’t necessarily invalidate previous research on daily cycles. Circadian rhythms are regulated two different ways: internally with a molecular clock or with external cues such as light and, according to these studies, temperature. One particularly neat part of this study elucidates looks at how these two mechanisms interact by comparing flies with different forms of the period gene. “Short” mutants (with the perS version of the gene) have a shortened circadian cycle, and “long” mutants (perL) the opposite. When these flies were let loose outdoors, perS mutants reached their afternoon activity burst earlier and perL mutants later than normal flies. From these data, it seems that temperature determines when flies wake up, but the molecular clock counts down the time for buzzy afternoon activity.
The authors go on to show that the flies’ outdoor activity pattern can be replicated by mimicking natural light and temperature patterns in lab. This has a real potential to impact fly behavior research. If scientists want to make arguments about evolutionarily relevant behaviors in flies, they don’t want to do it using flies that rest and wake at odd times. It would be like studying human circadian rhythms in sleep-deprived college students—actually, that happens too, but that’s another story.
[via The Scientist]
Image via Flickr / Silversyrpher