Two clocks in the brain: an update of the morning and evening oscillator model in Drosophila

Circadian clocks play an essential role in adapting the activity rhythms of animals to the day-night cycles on earth throughout the four seasons. In many animals, including the fruit fly Drosophila melanogaster, two separate but mutually coupled clocks in the brain -morning (M) and evening (E) oscillators- control the activity in the morning and evening. M and E oscillators are thought to track dawn and dusk, respectively. This alters the phase-angle between the two oscillators under different day lengths, optimally adapting the animal's activity pattern to colder short and warmer long days. Using excellent genetic tools, Drosophila researchers have addressed the neural basis of the two oscillators and could partially track these to distinct clock cells in the brain. Nevertheless, not all data are consistent with each other and many questions remained open. So far, most studies about M and E oscillators focused on the influence of light (photoperiod). Here, we will review the effects of light and temperature on the two oscillators, will update the present knowledge, discuss the limitations of the model, and raise questions that have to be addressed in the future.