Population-specific association of Clock gene polymorphism with annual cycle timing in stonechats

Timing is essential for survival and reproduction of organisms across the tree of life. The core circadian clock gene Clk is involved in the regulation of annual timing events and shows highly conserved sequence homology across vertebrates except for one variable region of polyglutamine repeats. Clk genotype varies in some species with latitude, seasonal timing and migration. However, findings are inconsistent, difficult to disentangle from environmental responses, and biased towards high latitudes. Here we combine field data with a commongarden experiment to study associations of Clk polymorphism with latitude, migration and annualcycle timing within the stonechat species complex across its transequatorial distribution range. Our dataset includes 950 records from 717 individuals from nine populations with diverse migratory strategies. Gene diversity was lowest in resident African and Canary Island populations and increased with latitude, independently of migration distance. Repeat length and annualcycle timing was linked in a populationspecific way. Specifically, equatorial African stonechats showed delayed timing with longer repeat length for all annualcycle stages. Our data suggest that at low latitudes with nearly constant photoperiod, Clk genotype might orchestrate a range of consistent, individual chronotypes. In contrast, the influence of Clk on annualcycle timing at higher latitudes might be mediated by its interactions with genes involved in (circadian) photoperiodic pathways.