Seasonal clock

Whereas the rotation of the Earth around its axis causes the alternation of day and night with a period of 24 hrs, as a consequence of its axial tilt almost all regions on Earth are affected by cycling conditions throughout the year. The Tropics experience wet/dry seasons, temperate regions are affected by relatively mild changes in photoperiod and temperature, whereas Polar regions go through extreme changes in photoperiod (figure below). Organisms therefore have to cope with dramatic environmental changes every time the Earth completes a revolution around the Sun: cycling food and water availability (or accessibility), presence of predator species, accessibility to shelters/nests, capability to camouflage with the changing environment.

Figure from Wilczek et al., 2010. Genetic and physiological bases for phenological responses to current and predicted climates. Philosophical Transactions of the Royal Society B.

Failure to adapt to these environmental challenges is catastrophic, therefore timing is vital.
The overwintering strategy of Drosophila melanogaster is diapause, a neuroendocrinally controlled interruption of metabolism and reproduction which allows flies to escape winter in time (as opposed to in space eg migration). It involves profound changes in adult metabolism which precede the beginning of unfavourable conditions.
Drosophila displays a quite shallow diapause phenotype, but given the wide arrange of genetics tools available it is still a valuable model organism for the study of seasonal characters. We study the propensity of flies to enter diapause in different latitudes and the underlying genetic basis, which incidentally includes circadian clock genes, as well as how photoperiod and low temperature interact to induce diapause.