For more than a year scientists investigated the development of six groups of mice in an outdoor enclosure. Credit: © MPI f. Ornithology
Mice with deviant internal rhythms due to a genetic mutation have fewer offspring and shorter life spans than normal conspecifics whose rhythms follow the 24hr cycle of a day more accurately. This discovery was made by a team of scientists led by researchers from the Max Planck Institute for Ornithology and Princeton University. Almost all living things possess internal clocks that govern periods of sleep and waking, and ensure these processes are in synchrony with night and day. This circadian clock evolved to allow the anticipation of regular daily events. Sunlight aligns the internal clock with the 24-hr-rhythm of the rotation of the earth.
Mutations in certain genes can disrupt the internal clock so that it runs out of sync with the day-night cycle. In mice, a mutation called tau is known to alter daily rhythms: mice carrying this mutation run through their day about 2 hours faster than normal mice. Scientists studied the biological fitness of such mice with deviant circadian rhythms in a large outdoor enclosure for over a year, where they were exposed to natural predators. At the beginning of the study the researchers divided 238 mice into 6 groups. For each group they housed an identical mix of mice without the mutation together with mice carrying either 1 or 2 copies of the mutation in their genes. Each mouse was equipped with a transponder, so that the scientists could monitor their activity rhythms at feeders. Mice with 1 or 2 copies of the mutation showed aberrant daily rhythms.
Mice without the mutation were observed to live longer and to produce more offspring than mice with the mutation that showed abnormal rhythms. As a consequence, after <1 year the prevalence of the mutation in the population dropped from an 50% in the starting population to only about 20% in the last cohort that was studied. This finding led the researchers to conclude that strong selection pressures must exist against the tau mutation in a natural environment. “Our findings highlight the fundamental importance of circadian clocks for the biological fitness of living beings. This has never been shown that clearly”, summarizes Michaela Hau.
http://www.mpg.de/9814892/internal-clock-fitness
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