Temporal neuronal control of diapause in the high-latitude fly, Drosophila littoralis.

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Timely preparation for the coming winter is essential for survival, especially for small animals such as insects. Food reserves must be built up in good time before the onset of hostile environmental conditions, reproduction must be stopped and resistance to cold and drought stress must be increased. Insects from high-latitudes spend the winter itself in a state of diapause, during which food intake largely ceases, metabolism is drastically lowered, and lifespan is increased. The signal to go into diapause is the decreasing day length (photoperiod) in summer-autumn, and it is assumed that the circadian clock provides the internal reference value for measuring day length. However, this has been clearly demonstrated only for few insect species. Furthermore, it is largely unknown how the circadian clock network in the brain communicates with the hormonal centers controlling diapause. The goal of this project is to (1) elucidate the function of the circadian clock in measuring daylength in the northern fly species Drosophila littoralis, which exhibits a strong and well-studied photoperiodic response, and (2) comparatively decipher the communication pathways between the circadian clock and hormonal centers in the pars intercerebralis/ lateralis in D. littoralis and D. melanogaster. We showed that the molecular clock and clock network are well conserved in both fly species, but that there are striking differences in the use of neuropeptides. These differences may explain the strong photoperiodic response of D. littoralis flies from the north. Flies from Finland (Kilpisjärvi, 69°N) enter diapause at a critical daylength of ~18h, while flies from more southern regions (Caucasus, 42°N) behave similarly to D. melanogaster and sometimes remain reproductive at a daylength of 12h. Interestingly, southern D. littoralis flies show similarities with D. melanogaster in both neuropeptide expression in circadian clock neurons and rhythmic behavior. This allows a correlative analysis between neuropeptide expression, rhythmic behavior, and critical daylength for diapause initiation in D. littoralis flies from different latitudes. Furthermore, we plan to simulate the neuropeptide expression pattern of northern and southern D. littoralis flies in D. melanogaster and investigate diapause and rhythmic behavior in the transgenic flies. The synaptic and neuropeptidergic connections between clock neurons and hormonal centers in the pars intercerebralis/lateralis will be elucidated in D. melanogaster by trans-Tango and TANGO-Map MKII. Ultimately, we will use CRISPR/Cas9 in D. littoralis to generate mutants in selected clock and neuropeptide genes to elucidate the role of the circadian clock in daylength measurement. We expect that our studies will contribute significantly to our understanding of daylength measurement and the seasonal control of winter dormancy by circadian clocks.