Anticipation of favorable and adverse environmental conditions is a substantial evolutionary advantage for life on Earth. Consequently, endogenous clocks have developed in almost all higher organisms in order to track seasonal or diel rhythms of abiotic or biotic factors. The evolution of circadian clocks resulted from the periodically recurring light-dark cycle of a terrestrial day (24 hours). Depending on the latitude, the annual variation of the photoperiod (length of the daily light phase) alternates to different degrees. While a 12-hour light phase is persistent throughout the year at the equator, organisms in arctic latitudes face very long photoperiods in summer and very short ones in winter. Species that are endemic to such high-latitude regions hence require plastic circadian oscillators that are capable of coping with these extreme changes. A well-studied circadian clock in animals is that of the fruit fly Drosophila melanogaster. The versatile toolset of circadian research in Drosophila comprises molecular, immunocytochemical, and behavioral techniques and can easily be transferred to other fruit fly species. The rich phylogeny and variable geographical distribution of the species predestinate the genus Drosophila to investigate the effects of latitude on circadian plasticity. In the past decade, several high-latitude drosophilids from the virilis group have already been studied and exhibited clear delays in evening activity during long photoperiods. At the same time, they showed strong arrhythmic behavior in constant darkness, indicating a weak circadian oscillator that enables this high plasticity. In the first part of the study, I sought to broaden the view on the capacity of Drosophilidae to adapt to high-latitude light conditions by including various species and strains from different subgenera, species groups, and geographical regions. Overall, I investigated 52 species and found that the ability to extend the phase angle between morning and evening peak of activity (short ΔΨM-E) during long photoperiods strongly correlates with their average latitudinal distribution, i.e., circadian plasticity increased with latitude. Not all of the high-latitude species with a long ΔΨM-E displayed the weak rhythmic behavior found in the virilis group in constant darkness, nor do they show the virilis group-specific expression pattern of the clock factors PIGMENT DISPERSING FACTOR (PDF) and CRYPTOCHROME (CRY). In these species, other mechanisms must be at work generating a high degree of circadian plasticity. One of these mechanisms could be the reduction of circadian light sensitivity and was the subject of the second part of this thesis: In D. melanogaster, two alleles of the clock gene timeless (tim) exist, s-tim and ls-tim. The ls-tim allele gives rise to the L-TIM isoform, which is less sensitive towards light-induced degradation mediated by CRY. The ls-tim allele has previously been shown to provide fitness advantages at high latitudes. We used about 40 wild-type strains of D. melanogaster with different s-tim/ls-tim compositions and challenged them with long photoperiods. Importantly, the strains monomorphic for the ls-tim allele performed best in delaying the evening activity peak to dusk in long photoperiods and also exhibited an increased amount of rhythmicity in constant light conditions. This indicates that attenuation of TIM degradation in response to light can be another technique of drosophilids to adapt the circadian clock to the photic conditions of high-latitude summers. In the third part of this study, I focused on D. funebris, a high-latitude species that is remarkably rhythmic in constant light. This behavior resembled that of cry mutants or some ls-tim strains from D. melanogaster. Therefore, I checked if the canonical light input pathway of D. funebris is similarly impaired. Surprisingly, the evidence I found rather refuses the involvement of the respective genes cry, jetlag (jet), and tim. Hence, this species must have adopted a distinct mechanism that enables its circadian plasticity and, in particular, rhythmic behavior in constant light. Overall, I could prove that Drosophila species from high latitudes have developed different techniques to adapt their circadian clock to the challenges of life in these regions. Future research could decipher these mechanisms more precisely.
Peter Deppisch (Fri,) studied this question.