Variations In Length Of Day Research Articles

The effect of gravitational and pressure torques on Titan's length-of-day variations

Cassini radar observations show that Titan's spin is slightly faster than synchronous spin. Angular momentum exchange between Titan's surface and the atmosphere over seasonal time scales corresponding to Saturn's orbital period of 29.5 year is the most likely cause of the observed non-synchronous rotation. We study the effect of Saturn's gravitational torque and torques between internal layers on the length-of-day (LOD) variations driven by the atmosphere. Because static tides deform Titan into an ellipsoid with the long axis approximately in the direction to Saturn, non-zero gravitational and pressure torques exist that can change the rotation rate of Titan. For the torque calculation, we estimate the flattening of Titan and its interior layers under the assumption of hydrostatic equilibrium. The gravitational forcing by Saturn, due to misalignment of the long axis of Titan with the line joining the mass centers of Titan and Saturn, reduces the LOD variations with respect to those for a spherical Titan by an order of magnitude. Internal gravitational and pressure coupling between the ice shell and the interior beneath a putative ocean tends to reduce any differential rotation between shell and interior and reduces further the LOD variations by a few times. For the current estimate of the atmospheric torque, we obtain LOD variations of a hydrostatic Titan that are more than 100 times smaller than the observations indicate when Titan has no ocean as well as when a subsurface ocean exists. Moreover, Saturn's torque causes the rotation to be slower than synchronous in contrast to the Cassini observations. The calculated LOD variations could be increased if the atmospheric torque is larger than predicted and or if fast viscous relaxation of the ice shell could reduce the gravitational coupling, but it remains to be studied if a two order of magnitude increase is possible and if these effects can explain the phase difference of the predicted rotation variations. Alternatively, the large differences with the observations may suggest that non-hydrostatic effects in Titan are important. In particular, we show that the amplitude and phase of the calculated rotation variations are similar to the observed values if non-hydrostatic effects could strongly reduce the equatorial flattening of the ice shell above an internal ocean.

Effect of mantle and ocean tides on the Earth's rotation rate

Aims. We aim to compute the rate of increase of the length of day (LOD) due to the axial component of the torque produced by the tide generating potential acting on the tidal redistribution of matter in the oceans and the solid Earth. Methods. We use an extension of the formalism applied to precession-nutation in a previous work to the problem of length of day variations of an inelastic Earth with a fluid core and oceans. Expressions for the second order axial torque produced by the tesseral and sectorial tide-generating potentials on the tidal increments to the Earth’s inertia tensor are derived and used in the axial component of the Euler-Liouville equations to arrive at the rate of increase of the LOD. Results. The increase in the LOD, produced by the same dissipative mechanisms as in the theoretical work on which the IAU 2000 nutation model is based and in our recent computation of second order effects, is found to be at a rate of 2.35 ms/cy due to the ocean tides, and 0.15 ms/cy due to solid Earth tides, in reasonable agreement with estimates made by other methods.

Moult strategies of Cory’s Shearwaters Calonectris diomedea borealis: the influence of colony location, sex and individual breeding status

The replacement of old feathers is essential for birds, but it is also an energy-demanding task. As moult usually does not coincide with other stressful events in its annual cycle, such as reproduction and migration, the bird can optimise its use of time and energy allocated to different activities. There are very few studies comparing the moult strategies of populations with similar breeding calendars but occurring in areas of different habitat quality. Cory’s Shearwaters Calonectris diomedea have a partial moult–breeding overlap, an unusual phenomenon among pelagic seabirds. We have compared the moult schedules in Cory’s Shearwater colonies located in distinct environments (pelagic vs. coastal) and show that moult–breeding overlap is less extensive on Selvagem Grande, situated in deep oceanic waters, than on Berlenga, situated on the continental shelf. Colony attendance of failed breeders, most of which were moulting, was lower at Selvagem Grande than at Berlenga, which suggests that the feeding areas of birds from the former site are more distant from the colony. Failed breeders started to moult earlier than individuals still raising a chick, and breeding status had a stronger influence on determining the onset of wing-feather moult than colony location. Despite published evidence that internal circannual rhythms and external cues, such as variation in daylength, are important factors regulating moult schedules, it is clear that birds retain a considerable flexibility that allows them to respond to external factors in order to strategically manage time and energy in a way that is thought to maximise their fitness.

Constraints on core‐mantle electromagnetic coupling from torsional oscillation normal modes

Decadal axial angular momentum variations in the Earth's core are believed to be carried by the normal modes of torsional oscillations. Coupling with the mantle transfers angular momentum to the latter, leading to changes in length of day (LOD). Electromagnetic stresses at the core‐mantle boundary (CMB) may be an important coupling mechanism as well as a source of dissipation for torsional oscillations. In this work, we investigate whether the observed spectra of fluid core velocities and LOD variations can be both explained in terms of the normal modes of torsional oscillations when the only coupling with the mantle is through electromagnetic stresses. We show that this explanation may be true when the magnetic field at the CMB is based on a downward continuation of surface observations, provided the conductance at the bottom of the mantle does not greatly exceed 108 S and small wavelength field features do not contribute more than approximately 25% of the total radial field at the CMB. A larger conductance or a higher amplitude radial magnetic field results in a damping of the normal modes of torsional oscillation that is sufficiently large that they should not be detectable. In particular, we show that this is the case for the conductance and radial magnetic field that are inferred from the Earth's forced nutations. If these constraints are correct, the decadal periodicities in the fluid velocity and LOD must then represent the preferred frequencies of the excitation mechanism of torsional oscillations rather than the signature of the free modes.

Relationship between MTNR1A melatonin receptor gene polymorphism and seasonal reproduction in different goat breeds

The reproductive activity of goats bred in temperate latitude follows a seasonal pattern, influenced by annual variation in day length. Daily variation in pineal melatonin secretion is the neuroendocrine signal recognized by animals through the link between this hormone and melatonin receptor 1a (MTNR1A). A total of 345 goats of different breeds (225 Sarda, 30 Saanen, 30 Chamois Coloured, 30 Maltese and 30 Nubian) with a kidding period in October–December or January–March were analysed to verify if a link exists between the structure of the receptor gene and reproductive activity. The main part of exon II of MTNR1A gene was amplified by PCR and then digested with MnlI and RsaI to prove the presence of restriction sites. Sequencing of 20 cloned samples and 20 purified samples permitted comparison with previously published sequences. No polymorphism was found using MnlI enzyme, as all 345 samples showed the cleavage site in position 605 and all the goats were MM genotype. However, using RsaI enzyme, some Sarda goats, showed a polymorphic site in position 53. Nine Sarda goats were R/r genotype, lacking this cleavage site only in one allele, while the other animals, both Sarda and the other breeds, presented the cleavage site in both the alleles and were thus R/R genotype. No r/r genotype was found in any of the breeds. In Sarda goats the allelic frequency was 0.98 for R allele and 0.02 for r allele; genotypic frequency was 96.00% for R/R genotype and 4.00% for R/r genotype. A strong link emerged from statistical analysis ( P < 0.001) between R/r genotype and reproductive activity, which was strongly influenced by photoperiod. Sequencing indicated six nucleotide changes that did not induce any amino acid change. Data showed that polymorphism was present and that it influences reproductive activity only in the Sarda breed.

Gonadal hormone-dependent and -independent regulation of immune function by photoperiod in Siberian hamsters

Siberian hamsters (Phodopus sungorus) exhibit changes in reproductive and immune function in response to seasonal variations in day length. Exposure to short days induces gonadal regression and inhibits testosterone secretion. In parallel, short days enhance immune function: increasing leukocyte numbers and attenuating cytokine and behavioral responses to infection. We examined whether photoperiodic changes in leukocyte phenotypes and sickness behaviors are dependent on concurrent photoperiodic changes in gonadal function. Male hamsters were gonadectomized or sham-gonadectomized and either exposed to short days (9 h light/day; SD) or kept in their natal long-day (15 h light/day; LD) photoperiod for 10-13 wk. Blood samples were obtained for leukocyte enumeration, and hamsters were challenged with bacterial LPS, which induced behavioral (anorexia, reductions in nest building) and somatic (weight loss) sickness responses. Among gonad-intact hamsters, exposure to SD increased total and CD62L+ lymphocytes and CD3+ T lymphocytes in blood and significantly attenuated LPS-induced sickness responses. Independent of photoperiod, castration alone increased total and CD62L+ lymphocyte and CD3+ T lymphocyte numbers and attenuated somatic and anorexic sickness responses. Among castrated hamsters, SD exposure increased lymphocyte numbers and suppressed sickness behaviors. In castrated hamsters, the magnitude of most immunological effects of SD were diminished relative to those evident in gonad-intact hamsters. The SD phenotype in several measures of immunity can be instated via elimination of gonadal hormones alone; however, photoperiodic effects on immune function persist even in castrated hamsters. Thus, photoperiod affects the immune system and neural-immune interactions underlying sickness behaviors via gonadal hormone-dependent and -independent mechanisms.

Winter day lengths enhance T lymphocyte phenotypes, inhibit cytokine responses, and attenuate behavioral symptoms of infection in laboratory rats

Annual variations in day length (photoperiod) trigger changes in the immune and reproductive system of seasonally-breeding animals. The purpose of this study was to determine whether photoperiodic changes in immunity depend on concurrent photoperiodic responses in the reproductive system, or whether immunological responses to photoperiod occur independent of reproductive responses. Here we report photoperiodic changes in enumerative, functional, and behavioral aspects of the immune system, and in immunomodulatory glucocorticoid secretion, in reproductively non-photoperiodic Wistar rats. T-cell numbers (CD3+, CD8+, CD8+CD25+, CD4+CD25+) were higher in the blood of rats housed in short as opposed to long-day lengths for 10 weeks. Following a simulated bacterial infection ( Escherichia coli LPS; 125 μg/kg) the severity of several acute-phase sickness behaviors (anorexia, cachexia, neophobia, and social withdrawal) were attenuated in short days. LPS-stimulated IL-1β and IL-6 production were comparable between photoperiods, but plasma TNFα was higher in long-day relative to short-day rats. In addition, corticosterone concentrations were higher in short-day relative to long-day rats. The data are consistent with the hypothesis that photoperiodic regulation of the immune system can occur entirely independently of photoperiodic regulation of the reproductive system. In the absence of concurrent reproductive responses, short days increase the numbers of leukocytes capable of immunosurveillance and inhibition of inflammatory responses, increase proinflammatory cytokine production, increase immunomodulatory glucocorticoid secretion, and ultimately attenuate behavioral responses to infection. Seasonal changes in the host immune system, endocrine system, and behavior may contribute to the seasonal variability in disease outcomes, even in reproductively non-photoperiodic mammals.

Synchronous flowering linked to changes in solar radiation intensity

Synchronous flowering linked to changes in solar radiation intensity

Long Term Variations in the Length of Day and Climatic Change

Summary The long-period (greater than about 10 yr) variations in the length-of-day (LOD) observed since 1820 show a marked similarity with variations observed in various climatic indices; periods of acceleration of the Earth corresponding to years of increasing intensity of the zonal circulation and to global-surface warming: periods of deceleration corresponding to years of decreasing zonal-circulation intensity and to a global decrease in surface temperatures. The long-period atmospheric excitation functions for near-surface geostrophic winds, for changes in the atmospheric mass distribution and for eustatic variations in sea level have been evaluated and correlate well with the observed changes in the LOD; although their total effect represents only about 10 per cent of the required excitation. The computed excitation lags the LOD variations by about 10-15 yr. Upper atmospheric winds or other meteorological related factors appear to be quite inadequate to provide the additional driving force that is required if the LOD changes are of meteorological origin. Instead, it appears that the LOD fluctuations and the climatic variations on a time scale of 20-30 yr may have a common origin as has been suggested by Anderson. That is, the results suggest that indirect solid Earth effects on climate may be important.

The Drosophila Circadian Network Is a Seasonal Timer

Previous work in Drosophila has defined two populations of circadian brain neurons, morning cells (M-cells) and evening cells (E-cells), both of which keep circadian time and regulate morning and evening activity, respectively. It has long been speculated that a multiple oscillator circadian network in animals underlies the behavioral and physiological pattern variability caused by seasonal fluctuations of photoperiod. We have manipulated separately the circadian photoentrainment pathway within E- and M-cells and show that E-cells process light information and function as master clocks in the presence of light. M-cells in contrast need darkness to cycle autonomously and dominate the network. The results indicate that the network switches control between these two centers as a function of photoperiod. Together with the different entraining properties of the two clock centers, the results suggest that the functional organization of the network underlies the behavioral adjustment to variations in daylength and season.

Fenologia de espécies arbóreas zoocóricas em uma floresta psamófila no sul do Brasil

Os fenômenos biológicos estão sujeitos a fatores abióticos que variam de acordo com a posição geográfica, especialmente relacionados à sazonalidade do ambiente. O presente estudo avaliou por dois anos as fenofases vegetativas e reprodutivas de nove espécies em uma floresta psamófila no sul do Brasil. Foram avaliados os eventos de queda foliar, brotamento, floração e frutificação em indivíduos localizados em diferentes pontos da área de estudo. A queda de folhas foi maior no mês de julho, sendo correlacionada com a menor temperatura média mensal e o menor comprimento do dia. As espécies apresentaram um aumento nas taxas de brotação após a queda foliar, atingindo maior atividade de setembro a novembro. Floração e frutificação apresentaram ritmos variados, sendo que houve maior produção de flores durante outubro e produção de frutos maximizada em novembro. A ausência de estação seca definida sugere que as espécies não experimentam restrições hídricas em intervalos regulares. Por outro lado, os eventos fenológicos vegetativos e reprodutivos estão estreitamente relacionados às variações anuais de temperatura média e comprimento do dia.

Different Neural Melatonin‐Target Tissues are Critical for Encoding and Retrieving Day Length Information in Siberian Hamsters

Siberian hamsters exhibit several seasonal rhythms in physiology and behaviour, including reproduction, energy balance, body mass, and pelage colouration. Unambiguous long- and short day lengths stimulate and inhibit reproduction, respectively. Whether gonadal growth or regression occurs in an intermediate day length (e.g. 14 h L : 10 h D; 14L), depends on whether the antecedent day lengths were shorter (10L) or longer (16L). Variations in day length are encoded by the duration of nocturnal pineal melatonin secretion, which is decoded at several neural melatonin target tissues to control testicular structure and function. We assessed participation of three such structures in the acquisition and retrieval of day length information. Elimination of melatonin signalling to the nucleus reuniens (NRe), but not to the suprachiasmatic nucleus (SCN) or paraventricular thalamus (PVt), interfered with the acquisition of a long day reproductive response, whereas the obscuring of melatonin signals to the SCN and the NRe, but not to the PVt, interfered with the photoperiod history response. The SCN and NRe contribute in different ways to the melatonin-based system that mediates seasonal rhythms in male reproduction.

Seasonal resource tracking by Amazonian hummingbirds

Many ecological theories predict a close match between food resources and the number and diversity of consumers but comparatively few empirical studies have simultaneously quantified seasonal variation in an entire assemblage and its resource base. Hummingbirds provide an excellent model system to examine this relationship because their primary food resource is conspicuous and easily quantified. Over a 2‐year period I used mist‐netting and direct observation of birds feeding at flowering plants to record the abundance of 16 species of hummingbird in lowland Amazonian rainforest. Although there was very little annual variation in temperature and daylength, both rainfall and river level showed significant seasonal variation. At the study site this resulted in an annual flood lasting for over 5 months, inundating much of the area to a depth of 2 m. I recorded marked seasonal patterns in flowering phenology and arthropod abundance, which correlated with river level and rainfall, respectively. In turn, these seasonal fluctuations in resource abundance correlated very closely with variation in hummingbird species richness, hummingbird abundance and the total biomass of hummingbirds in the assemblage. This study is the first to demonstrate tracking of seasonal resource variation in equatorial rainforest and provides evidence that hummingbirds are sensitive to resource availability at spatial and temporal scales much greater than previously documented.

Control of annual reproductive cycle in the subtropical house sparrow (Passer domesticus): evidence for conservation of photoperiodic control mechanisms in birds

BackgroundIn many birds, day length (=photoperiod) regulates reproductive cycle. The photoperiodic environment varies between different seasons and latitudes. As a consequence, species at different latitudes may have evolved separate photoperiodic strategies or modified them as per their adaptive need. We studied this using house sparrow as a model since it is found worldwide and is widely investigated. In particular, we examined whether photoperiodism in house sparrows (Passer domesticus) at 27°N, 81°E shared features with those exhibited by its conspecifics at high latitudes.ResultsInitial experiment described in the wild and captive conditions the gonad development and molt (only in captives) cycles over a 12-month period. Both male and female sparrows had similar seasonal cycles, linked with annual variations in day length; this suggested that seasonal reproduction in house sparrows was under the photoperiodic control. However, a slower testis and attenuated follicular growth among captives indicated that other (supplementary) factors are also involved in controlling the reproductive cycle. Next experiment examined if sparrows underwent seasonal variations in their response to stimulatory effects of long day lengths. When birds were transferred every month over a period of 1 year to 16 hours light:8 hours darkness (16L:8D) for 17–26 weeks, there was indeed a time-of-year effect on the growth-regression cycle of gonads. The final experiment investigated response of house sparrows to a variety of light-dark (LD) cycles. In the first set, sparrows were exposed for 31 weeks to photoperiods that were close to what they receive in between the period from sunrise to sunset at this latitude: 9L:15D (close to shortest day length in December), 12L:12D (equinox, in March and September) 15L:9D (close to longest day length in June). They underwent testicular growth and regression and molt in 12L and 15L photoperiods, but not in 9L photoperiod. In the second set, sparrows were exposed for 17 weeks to photoperiods with light periods extending to different duration of the daily photosensitivity rhythm (e.g. 2L:22D, 6L:18D, 10L:14D, 14L:10D, 18L:6D and 22L:2D). Interestingly, a slow and small testicular response occurred under 2L and 10L photoperiods; 6L:18D was non-inductive. On the other hand, 14L, 18L and 22L photoperiods produced testicular growth and subsequent regression response as is typical of a long day photostimulation.ConclusionSubtropical house sparrows exhibit photoperiodic responses similar to that is reported for its population living at high latitudes. This may suggest the conservation of the photoperiodic control mechanisms in birds evolved over a long period of time, as a physiological strategy in a temporally changing environment ensuring reproduction at the best suited time of the year.

Metabolic control of transcriptional-translational control loops (TTCL) by circadian oscillations in the redox- and phosphorylation state of cells

Evolution from prokaryotic to eukaryotic organisms was paralleled by a corresponding evolution in energy metabolism. From primeval fermentation, energy conservation progressed to anaerobic photosynthesis and then to carbon dioxide fixation with acceptance of electrons by water and the evolution of oxygen. In a progressively oxygenic biosphere, respiration developed with oxygen as a terminal electron acceptor. Evolving life was paralleled by a corresponding evolution of tropospheric O2/CO2 composition and the feedback of oxygen on life processes via reactive oxygen and reactive nitrogen species, which as signalling molecules became crucial for the control of development of pro- and eukaryotic living systems. Adaptation to the seasonal variation in daylength resulted in photoperiodic control of development with a circadian rhythm in energy conservation and transformation to optimise energy harvesting by photosynthesis. Photosynthesis on the other hand acts as a light-dependent metabolic regulator via redox signals in addition to specific photoreceptors like phytochromes and cryptochromes. Finally, redox control integrates rhythmic gene expression in chloroplasts, mitochondria and the nucleus. The circadian rhythmic cell (cyanobacterial and eukaryotic) is a hydro-electro-chemical oscillator synchronised by the daily light – dark cycle with temporal compartmentation of metabolism and a network of metabolic sequences to compensate for oxidative stress in adapting to the light environment e.g. by separating N-fixation from oxygen production. In Chenopodium rubrum L. a circadian rhythm in overall energy transduction has been observed. This rhythm results from an oscillatory network between glycolysis and oxidative phosphorylation coupled to photophosphorylation. This network produces a circadian rhythm in adenylate energy charge and redox state (NADP/NADPH2). The nucleotide ratios themselves could, as rate effectors in compartmental feedback, fulfil the requirements for precise temperature-compensated time keeping. The integration of metabolic activity of Chenopodium plants on a hydraulic-electrochemical level is represented by a diurnal rhythm in compound surface membrane resting potential. Using molecular genetic techniques, research of the last 30 years has come to the conclusion that the core oscillator of circadian systems should reside in transcriptional and translational control loops (TTCL) involved in feedback regulation of clock genes. Considering the evolution of metabolic networks in response to environmental constraints, we proposed (Wagner & Cumming 1970; Wagner et al. 1998) that circadian rhythms in redox state and phosphorylation potential, as an output from the network of energy transduction (Singh 1998), should be gating the TTCL for the circadian rhythmic production of proteins needed in the metabolic networks. A similar concept has been advanced for metabolic control of human circadian rhythms, assuming that the redox state of cells should be the driving effector (Rutter et al. 2002) of the physiological clock.

Core surface flow modelling from high-resolution secular variation

S U M M A R Y Data from the Orsted and CHAMP satellites have allowed modelling of the geomagnetic secular variation (SV ) with unprecedented accuracy. The spectrum of the SV is consistent with generation by advectively dominated processes. Based on the SV model, it is not possible to reject the frozen-flux hypothesis, but the spectrum of the SV implies that a conclusive test of frozen-flux is not possible. We parametrize the effects of diffusion as an expected misfit in the flow prediction due to departure from the frozen-flux hypothesis; at low spherical harmonic degrees, this contribution dominates the expected departure of the SV predictions from flow to the observed SV , while at high degrees the SV model uncertainty is dominant. We construct fine-scale core surface flows to model the SV . Flow non-uniqueness is a serious problem because the flows are sufficiently small scale to allow flow around non-uniqueness contours. Nevertheless, we find evidence to support previously suggested polar vortices. For this model of field and SV , predicted variations in length of day from modelled core angular momentum vary over a large range, although there is evidence that this effect is reduced with longer time-series of magnetic data and better parametrization of the external magnetic field.

The effect of season on serum testosterone concentrations in dogs

Photoperiod and environmental temperature are two important factors that may influence the reproductive cycle of various species. The objective of this study was to investigate seasonal influences on serum testosterone concentrations in dogs in a tropical zone, where the variation in day length between winter and summer solstice was approximately 2.5 h. Blood samples were collected every 15 days from seven adult dogs over a 14-month interval and serum testosterone concentrations were determined by radioimmunoassay. The year was divided into four seasons and mean testosterone concentrations for each season were related to the mean environmental temperature and rainfall during that season. Mean testosterone concentrations were 1.81 ng/mL (winter 2002), 1.93 ng/mL (spring 2002), 1.31 ng/mL (summer 2003), 2.02 ng/mL (autumn 2003) and 1.93 ng/mL (winter 2003). The temperature ranged from 10.2 to 32.8 °C and the rainfall from 33 to 476 mm. Serum testosterone concentrations were lower in summer 2003 than in both spring 2002 ( P = 0.05) and autumn 2003 ( P = 0.016). In a tropical zone, a combination of high temperature and substantial rainfall may have reduced serum testosterone concentrations in dogs.

CO/FT Regulatory Module Controls Timing of Flowering and Seasonal Growth Cessation in Trees

Forest trees display a perennial growth behavior characterized by a multiple-year delay in flowering and, in temperate regions, an annual cycling between growth and dormancy. We show here that the CO/FT regulatory module, which controls flowering time in response to variations in daylength in annual plants, controls flowering in aspen trees. Unexpectedly, however, it also controls the short-day-induced growth cessation and bud set occurring in the fall. This regulatory mechanism can explain the ecogenetic variation in a highly adaptive trait: the critical daylength for growth cessation displayed by aspen trees sampled across a latitudinal gradient spanning northern Europe.

Evolution of a Length Polymorphism in the Human PER3 Gene, a Component of the Circadian System

Period homologue 3 (PER3) is a component of the mammalian circa-dian system, although its precise role is unknown. A biallelic variable number tandem repeat (VNTR) polymorphism exists in human PER3, consisting of 4 or 5 repeats of a 54-bp sequence in a region encoding a putative phosphorylation domain. This polymorphism has previously been reported to associate with diurnal preference ("morningness" and "eveningness") and delayed sleep-phase syndrome. We have investigated the global allele frequencies of this variant in ethnically distinct indigenous populations. All populations were polymorphic, with the shorter (4-repeat) allele ranging in frequency from 0.19 (Papua New Guinea) to 0.89 (Mongolia). To investigate if allele frequency has been influenced by natural selection, the authors 1) tested for a correlation with latitude and mean annual insolation (incident sunlight energy), using classical markers to correct for historical population differentiation; and they 2) compared allele-frequency difference between European American, African American, and East Asian populations, as measured using F(ST), to an empirical null distribution of F(ST)values based on a genome-wide dataset of single nucleotide polymorphisms (SNPs) of presumed neutral loci that were previously typed by The SNP Consortium. The variation in allele frequencies between indigenous populations did not show a pattern that would indicate selective pressure on PER3resulting from day-length variation or mean annual insolation, and the allele-frequency difference between European Americans, African Americans, and East Asians was not an outlier when compared to the distribution for presumed neutral SNPs. We therefore find no evidence for differential or balancing selection in the contemporary pattern of global PER3allele frequencies.

Photoperiod differentially regulates gene expression rhythms in the rostral and caudal SCN

At temperate latitudes, organisms use annual changes in daylength (photoperiod) to synchronise seasonal physiology and behaviour. In mammals, photoperiodic responses depend on a master neural circadian pacemaker in the hypothalamic suprachiasmatic nuclei (SCN) [1]. Because circadian rhythm generation depends upon the rhythmic RNA expression of a small group of canonical clock genes [2], it is possible to study the effects of light on oscillations within the SCN by in situ hybridisation histochemistry.