Sep. 23, 2013 — Many animals, including humans, have internal clocks and calendars to help them regulate behavior, physiological functions and biological processes. Although scientists have extensively studied the timekeeping mechanisms that inform daily functions (circadian rhythms), they know very little about the timekeeping mechanisms that inform seasonal functions.
New research to be published this week in the online early edition of the Proceedings of the National Academy of Sciences shows, for the first time, that this measurement of seasonal time has an epigenetic component. Epigenetics refers to an alteration in gene expression that occurs without a change in the sequence of DNA molecules.
The research used Siberian hamsters, which only breed in the late spring and early summer, when days are the longest. It revealed the molecular mechanism behind how these hamsters avoid breeding in the fall and winter, thereby preventing births during the cold, resource-scarce winter months.
Here's how the mechanism works: Exposure to short periods of daylight decreases DNA methylation in the hamsters' hypothalamus. (With DNA methylation, a methyl group -- one carbon atom and three hydrogen atoms -- attaches to a gene, thereby altering its expression.) In turn, the decreased DNA methylation stimulates the expression of a gene that shuts down the hamster's reproductive competency.
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