title: 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 1h 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 12h in buntings under T24. Compared to T24, exposure to T22 advanced CD by 1h, 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 1h during the dark period in each cycle, not with the 6h main light periods falling 2h earlier each successive 24hday 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.
authors.affiliation: DST-IRHPA Center for Excellence in Biological Rhythms Research, IndoUS Center for Biological Timing, Department of Zoology,University of Delhi, Delhi 110 007, India.
authors.forename: Gaurav
authors.initials: G
authors.lastname: Majumdar
authors.affiliation: DST-IRHPA Center for Excellence in Biological Rhythms Research, IndoUS Center for Biological Timing, Department of Zoology,University of Delhi, Delhi 110 007, India.
authors.forename: Amit Kumar
authors.initials: AK
authors.lastname: Trivedi
authors.affiliation: Department of Zoology, MMH College Ghaziabad 201009, India.
authors.forename: Neelu Jain
authors.initials: NJ
authors.lastname: Gupta
authors.affiliation: DST-IRHPA Center for Excellence in Biological Rhythms Research, IndoUS Center for Biological Timing, Department of Zoology,University of Delhi, Delhi 110 007, India. Electronic address: vkumar@zoology.du.ac.in.
authors.forename: Vinod
authors.initials: V
authors.lastname: Kumar
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D002097:C-Reactive Protein / Q000235:genetics / Q000378:metabolism*; D002940:Circadian Rhythm / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation / Q000502:physiology*; D007987:Gonadotropin-Releasing Hormone / Q000235:genetics / Q000378:metabolism; D007031:Hypothalamus / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D008297:Male; D017440:Photoperiod; D012333:RNA, Messenger / Q000378:metabolism; D012621:Seasons; D020308:Songbirds; D018709:Statistics, Nonparametric; D037322:Thyrotropin, beta Subunit / Q000235:genetics / Q000378:metabolism
chemical_list: D004268:DNA-Binding Proteins; D012333:RNA, Messenger; D037322:Thyrotropin, beta Subunit; D007987:Gonadotropin-Releasing Hormone; D002097:C-Reactive Protein
keywords: Bunting; Critical day length; Photoinducible phase; Photoperiodic response; T-cycle