September 24, 2013
Understanding How Our Seasonal Clocks Work Using Siberian Hamsters
April Flowers for redOrbit.com - Your Universe Online
Humans, and many other animals, have internal clocks and calendars that help them regulate behavior, physiological functions and biological processes. Scientists have extensively studied the mechanisms that inform our daily functions, known as circadian rhythms. However, they know very little about the internal timekeeping mechanisms that inform seasonal functions.
A new international study, led by the University of Chicago, reveals that this measurement of seasonal time has an epigenetic component -- which refers to an alteration in gene expression that occurs without a change in the sequence of DNA molecules.
The research, published online in the early edition of Proceedings of the National Academy of Sciences (PNAS), used Siberian hamsters as test subjects. Siberian hamsters only breed in the late spring and early summer, when days are the longest. The study revealed the molecular mechanism behind how these hamsters avoid breeding in the fall and winter. This mechanism allows the hamsters to prevent births during the cold, resource-scarce winter months.
Exposure to short periods of daylight decreases DNA methylation in the hypothalamus of the hamsters. DNA methylation occurs when the expression of a gene is altered when a methyl group -- one carbon atom and three hydrogen atoms -- attaches to the gene. In the hamsters, DNA methylation alters the gene that shuts down the hamster's reproductive competency.
"Our findings reveal, for the first time, that DNA methylation, acting as a dynamic mechanism in the mammalian brain, plays a central role in the animal's psychological and physiological orientation to the length of the day," said Brian Prendergast, professor in psychology at the University of Chicago. "But this study is only the tip of the iceberg because it helps to illuminate the poorly understood area of animal seasonality, the internal time-keeping mechanism that regulates animal life."
Prendergast added that human beings are remarkably similar, like other mammals and vertebrates. He cited a long, astonishing list of universal human experiences that have a seasonal component, such as birth, death, suicide, viral infections, mortality from bacterial infections, sleep patterns and sudden infant death syndrome (SIDS), even though industrial societies buffer humans from contributory factors.
"Many of these experiences are related to the length of day," he said, "so it's critical to learn how photoperiodic time measurement works in animals."
For the purpose of studying seasonal timekeeping, Siberian hamsters are very good subjects because of the seasonal changes in their body. For example, as days shorten in the fall, males lose approximately 30 percent of their body mass, their fur molts and their testes decrease significantly in size.
"The changes are so profound that the hamster almost looks like another animal," Prendergast said.
The study findings reveal how such seasonal changes are linked to the hamster's reactions to the constantly changing length of day, as well as revealing that these changes are linked to winter-like melatonin levels and that the process is reversible.
"DNA methylation is a key mechanism by which day length and melatonin levels exert seasonal control over the expression of the deiodinase enzyme known as dio3, and dio3 expression likely acts as a key step for the maintenance of reproductive competency during the breeding season," said Tyler Stevenson, senior lecturer at the Institute of Biological and Environmental Sciences at the University of Aberdeen in Scotland.
Only human beings have clocks, calendars, computers and smartphones. According to Prendergast, however, the internal, physiological mechanisms of other animals are just about as accurate as those devices.
"Natural selection has led to organisms that have formed an internal representation of time," he said, "and science is making progress on better understanding this biological timekeeping."