A mammal's annual reproductive cycle is controlled by reversible changes to DNA.

Siberian hamsters (Phodopus sungorus) breed during the longest days of the year; shorter days trigger a series of hormonal changes that cause their gonads to shrink. The change is tied to the nocturnally produced hormone melatonin, but how the hormone alters gene activity has been unclear.

Tyler Stevenson, now at the University of Aberdeen, UK, and Brian Prendergast of the University of Chicago in Illinois found that both shorter days and corresponding changes in melatonin reduce methylation of a key gene in the hypothalamus to launch the gonad-shrinking cascade.

The study is the first to show that cycles of DNA modification underlie seasonal changes, the authors say.

Proc. Natl Acad. Sci. USA http://doi.org/nz8 (2013)