Circadian clock and seasonal adaptation of two high latitude \(Drosophila\) species of the \(virilis\) group

Circadian rhythms are physiological and behavioral rhythms that occur within a period of about a day and are controlled by an endogenous mechanism called circadian clock, which among insects is best studied in the fruit fly Drosophila melanogaster. There are other timed biological rhythms, including annual rhythms that occur each year in response to seasonal changes and anticipation of unfavorable conditions. In chronobiology (the study of biological rhythms and timing), a long-standing question has been whether the circadian clock is involved in measuring the hours of light or darkness that trigger seasonal responses. This thesis investigates two northern European fly species of the virilis group, Drosophila ezoana and Drosophila littoralis, which are closely related to the model organism D. melanogaster but in contrast, show a strong light-dependent seasonal response (diapause). Here, I show that both species have a functioning circadian clock although strongly damped compared to that of D. melanogaster. While low latitude D. littoralis strains show rhythmic activity under constant conditions, D. ezoana is rather arrhythmic but appears to use its circadian clock as time reference for measuring night length. The differences in the clock strength might reside in their circadian neuronal network, as the small ventrolateral neurons (s-LNv), considered the main pacemaker neurons of the circadian network, showed important differences between the virilis group and D. melanogaster. The reduced expression of the neuropeptide Pigment Dispersing Factor (PDF) in these neurons could explain their weakly rhythmic locomotor activity. In both species, Pdf is also expressed in some neurosecretory neurons (CRZ neurons) in the dorsal brain, which are not part of the clock network and also appear to play a role in diapause. D. littoralis is known for showing differences in diapause according to its latitude of origin. Indeed, southern strains whose diapause is triggered by longer nights, show a stronger clock, while the northern strains whose diapause is triggered under shorter nights, have a weaker clock. Further differences are also found in the PDF level which is higher in the s-LNv of the southern strains but, higher in the CRZ neurons of the northern D. littoralis.