Is the circalunar clock derived from photoperiodic diapause - or vice versa?

Life on earth evolved in the presence of recurring environmental cycles, e.g. the tides, the day-night cycle, the seasons and the lunar cycle. In adaptation to those, organisms evolved endogenous time keeping systems - so called biological clocks. Non-circadian clocks, including the circalunar clock which anticipates the lunar cycle, remain understudied. In this thesis we conducted a comprehensive literature review of circatidal and circalunar clocks in insects and of circalunar clocks across the tree of life. We demonstrate that circalunar clocks in different organisms rely on different functional principles, potentially indicating multiple evolutionary origins of circalunar clocks. We highlight that the experimental evidence indispensable for studying circalunar clocks is largely lacking. Consequently, we experimentally investigated the functional principle of the circalunar clock in the marine midge Clunio marinus. We found, that the circalunar clock in C. marinus is a circalunar counter based on counting circadian clock cycles. This result strengthens the idea that the circalunar clock in marine midges is mechanistically different from that of marine annelids. We hypothesized that the circalunar counter in marine midges may be derived from a photoperiodic counter. Therefore, in C. marinus we compared the transcriptomes of two phenotypes which are downstream of the circalunar clock and the photoperiodic counter, i.e. the circalunar developmental arrest and diapause. We found that both developmental arrests occur in the same larval substage and potentially differ in their energy metabolism. Finally, we present the first annotated genome on chromosome scale for the midge Clunio tsushimensis. This resource constitutes the basis for comparative genomic analysis in order to study adaptations of the circalunar clock in different species and to understand why Clunio genomes are highly compact.