Circadian synchronization determines critical day length for seasonal responses

A photoperiodic species initiates fat deposition (in migrants) and gonadal recrudescence in response to a specific duration of natural daylight, called critical day length (CD), when light extends in the inductive phase of the endogenous circadian rhythm of photoinducibility (CRP). The molecular basis of species-specificCD, determined by the entrainment of the CRP, has been poorly understood. To investigate this, we measured expression levels of genes implicated in the photoperiod-induced changes in reproduction (EYA3, TSH beta, DIO2, DIO3, GNRH and GNIH) and metabolism (SIRT1, HMGCR, FASN and PPAR alpha) in photosensitive redheaded buntings subjected to light-dark cycles of varying period lengths (T-photocycles). Buntings were exposed to six T22, T24 or T26 photocycles, with 1 h additional light at night falling at different phases of the entrained CRP (T2211L = 6L:4D:1L:11D; T2411L = 6L:4D:1L:13D,T2412L = 6L:5D:1L:12D, T2413L = 6L:6D:1L:11D; T2612L = 6L:5D:1L:14D). Photoinduction at genetic and phenotypic levels in T2412L and T2413L, not T2411L, groups confirmed CD being close to 12 h in buntings under T24. Compared to T24, exposure to T22 advanced CD by 1 h, as evidenced by photoinduction in the T2211L, not T226L, group. Similarly, CD appeared to be delayed under T26, with no photoinduction in the T2612L group. Further, to show that induction of response under a T-photocycle was because of the interaction of inductive phase of the CRP with 1 h during the dark period in each cycle, not with the 6 h main light periods falling 2 h earlier each successive 24 h day in a T22 paradigm, a group of buntings was exposed to 6L:16D (T226L), to which they did not respond. The mRNA expression of genes, particularly TSH beta, DIO2, DIO3 and PPAR alpha, was significantly correlated with changes in reproductive and metabolic phenotypes. These results suggest CRP-entrainment based genetic regulation of the CD, and extend the idea that synchronization with environment is a critical measure in a seasonal species for its temporal adaptation in the wild.