The tick-tock of an animal's daily clock can be set, in part, by beneficial bacteria.
At night, the bacterium Vibrio fischeri glows in a compartment in the underbelly of its symbiotic host, the Hawaiian bobtail squid (Euprymna scolopes; pictured), hiding the squid's shadow in the moonlight. Margaret McFall-Ngai at the University of Wisconsin—Madison and her colleagues have found that the light-producing bacteria also cause the squid to boost expression of its light-responsive gene escry1 in the colonized compartment. This boost, which helps to set daily rhythms, did not occur in bacteria-free squid, even if they were exposed to blue light to mimic bacterial glow. But when researchers provided squid with light as well as bits of the bacterium's cell envelope, escry1 cycling returned. The researchers suggest that microbiota could also set daily rhythms in mammals.
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Symbionts set squid's clock. Nature 496, 273 (2013). https://doi.org/10.1038/496273c