Circadian synchronization determines critical day length for seasonal responses.

Abstract 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 (T22 11L  = 6L:4D:1L:11D; T24 11L  = 6L:4D:1L:13D,T24 12L  = 6L:5D:1L:12D, T24 13L  = 6L:6D:1L:11D; T26 12L  = 6L:5D:1L:14D). Photoinduction at genetic and phenotypic levels in T24 12L and T24 13L , not T24 11L , 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 T22 11L , not T22 6L , group. Similarly, CD appeared to be delayed under T26, with no photoinduction in the T26 12L 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 (T22 6L ), 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.