Arctic reindeer, which live most of the year in 24-hour darkness or daylight, may lack an internal clock common to most organisms, according to research published online today (March 11) in Current Biology.

Reindeer Image: Per Harald Olsen/Wikimedia

The study found no evidence of cyclic changes in reindeer gene expression, consistent with behavioral evidence that the arctic animals do not rely on such daily rhythms. But the fact that the researchers only investigated two clock genes in one cell type is not conclusive evidence that reindeer completely lack a circadian clock, said Michael Menaker from the University of Virginia, who was not involved in the study. "If it were shown that there was no circadian organization anywhere -- that would be very surprising," he said.

Most organisms are thought to have a strong circadian clock, observable in behavioral patterns and the cyclic expression of clock genes, which scientists believe confer an evolutionary advantage. Plants, for example, will have their "photosynthetic machinery up and running just before light is appearing," rather than starting after first light, Urs Albrecht from the University of Fribourg in Switzerland, who wasn't involved in the study, explained in an email.

But in areas where the sun is constantly present or absent for most of the year, a gene expression cycle that follows a 24 hour period may not offer any advantages to an organism, and thus some arctic species may have evolved mechanisms to suppress it. Reindeer, for example, feed and then ruminate -- digesting while resting or sleeping -- over cycles of 2-3 hours. "For reindeer, food source appears not to be time-of-day dependent," Albrecht said.

To investigate the molecular basis of this phenomenon, Andrew Loudon from the University of Manchester in the UK and colleagues, took skin cells from reindeer and inserted a fluorescent tag next to two prominent clock genes, Bmal and Per2, which maintain their cyclic expression for up to 7 days in culture. The researchers tracked the expression of the clock genes by their fluorescent flicker, and saw no regular cyclical pattern, suggesting that arctic animals may have developed a way to suppress such rhythms. In contrast, the same marker inserted in mouse skin cells revealed signs of a 24-hour oscillation in gene expression.

"Up to now we believed that every organism has a strong circadian clock that is manifest in the transcription [of] genes involved in circadian timing," said Loudon. The results of this study, however, suggest that such a clock may not operate in some arctic species.

But "fibroblasts are only one type of cell," said Menaker, and other cell types may still exhibit rhythmic gene expression cycles. "Animals in the arctic have suppressed circadian rhythm" in terms of their behavior, Menaker said, but not necessarily their physiology.

Whether physiological rhythms change in other animals that live in other extreme environments, like those at the bottom of the ocean, is still unclear, Menaker said, but studying such questions can begin to inform how changes in circadian clocks might affect people. "We're all exposed to environments that we weren't really designed for," such as extended periods of light during the workday, or the clock shifts of jet lag, which may have implications for human health. Night-shift workers, for example, can experience sever clock deregulation, which has been strongly correlated with an increased risk of cancer.


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