Light-induced hormone conversion of T4 to T3 regulates photoperiodic response of gonads in birds

Seasonal food intake and energy balance: Neuronal and non-neuronal control mechanisms

Animals inhabiting temperate and high latitudes undergo drastic seasonal changes in energy storage, facilitated by changes in food intake and body mass. Those seasonal changes in the animal's biology are not mere consequences of environmental energy availability but are anticipatory responses to the energetic requirements of the upcoming season and are actively timed by tracking the annual progression in photoperiod. In this review, we discuss how photoperiod is used to control energy balance seasonally and how this is distinct from energy homeostasis. Most notably, we suggest that photoperiodic control of food intake and body mass does not originate from the arcuate nucleus, as for homeostatic appetite control, but is rather to be found in hypothalamic tanycytes. Tanycytes are specialized ependymal cells lining the third ventricle, which can sense metabolites from the cerebrospinal fluid (e.g. glucose) and can control access of circulating signals to the brain. They are also essential in conveying time-of-year information by integrating photoperiod and altering hypothalamic thyroid metabolism, a feature that is conserved in seasonal vertebrates and connects to seasonal breeding and metabolism. We also discuss how homeostatic feedback signals are handled during times of rapid energetic transitions. Studies on leptin in seasonal mammals suggest a seasonal shift in central sensitivity and blood-brain transport, which might be facilitated by tanycytes. This article is part of the Special Issue on "Food intake and feeding states".

Mouse Models in Circadian Rhythm and Melatonin Research

This contribution reviews the role of inbred and transgenic mouse strains for deciphering the mammalian melatoninergic and circadian system. It focusses on the pineal organ as melatonin factory and two major targets of the melatoninergic system, the suprachiasmatic nuclei (SCN) and the hypophysial pars tuberalis (PT). Mammalian pinealocytes sharing molecular characteristics with true pineal and retinal photoreceptors synthesize and secrete melatonin into the blood and cerebrospinal fluid night by night. Notably, neuron-like connections exist between the deep pinealocytes and the habenular/pretectal region suggesting direct pineal-brain communication. Control of melatonin biosynthesis in rodents involves transcriptional regulation including phosphorylation of CREB and upregulation of mPer1. In the SCN, melatonin acts upon MT1 and MT2 receptors. Melatonin is not necessary to maintain the rhythm of the SCN molecular clockwork, but it has distinct effects on the synchronization of the circadian rhythm by light, facilitates re-entrainment of the circadian system to phase advances in the level of the SCN molecular clockwork by acting upon MT2 receptors and plays a stabilizing role in the circadian system as evidenced from locomotor activity recordings. While the effects in the SCN are subtle, melatonin is essential for PT functions. Via the MT1 receptor it drives the PT-intrinsic molecular clockwork and the retrograde and anterograde output pathways controlling seasonal rhythmicity. Although inbred and transgenic mice do not show seasonal reproduction, the pathways from the PT are fully intact if the animals are melatonin proficient. Thus, only melatonin-proficient strains are suited to investigate the circadian and melatoninergic systems.

Differential Regulation of Opsin Gene Expression in Response to Internal and External Stimuli

Determining how internal and external stimuli interact to determine developmental trajectories of traits is a challenge that requires the integration of different subfields of biology. Internal stimuli, such as hormones, control developmental patterns of phenotypic changes, which might be modified by external environmental cues (e.g. plasticity). Thyroid hormone (TH) modulates the timing of opsin gene expression in developing Midas cichlid fish (Amphilophus citrinellus). Moreover, fish reared in red light accelerate this developmental timing compared to fish reared in white light. Hence, we hypothesized that plasticity caused by variation in light conditions has coopted the TH signaling pathway to induce changes in opsin gene expression. We treated Midas cichlids with TH and crossed this treatment with two light conditions, white and red. We observed that not only opsin expression responded similarly to TH and red light but also that, at high TH levels, there is limited capacity for light-induced plasticity. Transcriptomic analysis of the eye showed that genes in the TH pathway were affected by TH, but not by light treatments. Coexpression network analyses further suggested that response to light was independent of the response to TH manipulations. Taken together, our results suggest independent mechanisms mediating development and plasticity during development of opsin gene expression, and that responses to environmental stimuli may vary depending on internal stimuli. This conditional developmental response to external factors depending on internal ones (e.g. hormones) might play a fundamental role in the patterns of phenotypic divergence observed in Midas cichlids and potentially other organisms.

Avian migration clocks in a changing world

Avian long-distance migration requires refined programming to orchestrate the birds' movements on annual temporal and continental spatial scales. Programming is particularly important as long-distance movements typically anticipate future environmental conditions. Hence, migration has long been of particular interest in chronobiology. Captivity studies using a proxy, the shift to nocturnality during migration seasons (i.e., migratory restlessness), have revealed circannual and circadian regulation, as well as an innate sense of direction. Thanks to rapid development of tracking technology, detailed information from free-flying birds, including annual-cycle data and actograms, now allows relating this mechanistic background to behaviour in the wild. Likewise, genomic approaches begin to unravel the many physiological pathways that contribute to migration. Despite these advances, it is still unclear how migration programmes are integrated with specific environmental conditions experienced during the journey. Such knowledge is imminently important as temporal environments undergo rapid anthropogenic modification. Migratory birds as a group are not dealing well with the changes, yet some species show remarkable adjustments at behavioural and genetic levels. Integrated research programmes and interdisciplinary collaborations are needed to understand the range of responses of migratory birds to environmental change, and more broadly, the functioning of timing programmes under natural conditions.

Photoperiod, but not progesterone, has a strong impact upon the transcriptome of the medio-basal hypothalamus in female goats and ewes

Photoperiod is the main environmental driver of seasonal responses in organisms living at temperate and polar latitudes. Other external cues such as food and temperature, and internal cues including hormones, intervene to fine-tune phasing of physiological functions to the solar year. In mammals, the medio-basal hypothalamus (MBH) is the key integrator of these cues, which orchestrates a wide array of seasonal functions, including breeding. Here, using RNAseq and RT-qPCR, we demonstrate that molecular components of the photoperiodic response previously identified in ewes are broadly conserved in does (female goats, Capra hircus), with a common core of ∼50 genes. This core group can be defined as the "MBH seasonal trancriptome", which includes key players of the pars tuberalis-tanycytes neuroendocrine retrograde pathway that governs intra-MBH photoperiodic switches of triiodothyronine (T3) production (Tshb, Eya3, Dio2 and SlcO1c1), the two histone methyltransferases Suv39H2 and Ezh2 and the secreted protein Vmo1. Prior data in ewes revealed that T3 and estradiol (E2), both key hormones for the proper timing of seasonal breeding, differentially impact the MBH seasonal transcriptome, and identified cellular and molecular targets through which these hormones might act. In contrast, information regarding the potential impact of progesterone (P4) upon the MBH transcriptome was nonexistent. Here, we demonstrate that P4 has no discernible transcriptional impact in either does or ewes. Taken together, our data show that does and ewes possess a common core set of photoperiod-responsive genes in the MBH and conclusively demonstrate that P4 is not a key regulator of the MBH transcriptome.

One Health: Circadian Medicine Benefits Both Non-human Animals and Humans Alike

Circadian biology's impact on human physical health and its role in disease development and progression is widely recognized. The forefront of circadian rhythm research now focuses on translational applications to clinical medicine, aiming to enhance disease diagnosis, prognosis, and treatment responses. However, the field of circadian medicine has predominantly concentrated on human healthcare, neglecting its potential for transformative applications in veterinary medicine, thereby overlooking opportunities to improve non-human animal health and welfare. This review consists of three main sections. The first section focuses on the translational potential of circadian medicine into current industry practices of agricultural animals, with a particular emphasis on horses, broiler chickens, and laying hens. The second section delves into the potential applications of circadian medicine in small animal veterinary care, primarily focusing on our companion animals, namely dogs and cats. The final section explores emerging frontiers in circadian medicine, encompassing aquaculture, veterinary hospital care, and non-human animal welfare and concludes with the integration of One Health principles. In summary, circadian medicine represents a highly promising field of medicine that holds the potential to significantly enhance the clinical care and overall health of all animals, extending its impact beyond human healthcare.

Dissociating Mechanisms That Underlie Seasonal and Developmental Programs for the Neuroendocrine Control of Physiology in Birds

Long-term programmed rheostatic changes in physiology are essential for animal fitness. Hypothalamic nuclei and the pituitary gland govern key developmental and seasonal transitions in reproduction. The aim of this study was to identify the molecular substrates that are common and unique to developmental and seasonal timing. Adult and juvenile quail were collected from reproductively mature and immature states, and key molecular targets were examined in the mediobasal hypothalamus (MBH) and pituitary gland. qRT-PCR assays established deiodinase type 2 (DIO2) and type 3 (DIO3) expression in adults changed with photoperiod manipulations. However, DIO2 and DIO3 remain constitutively expressed in juveniles. Pituitary gland transcriptome analyses established that 340 transcripts were differentially expressed across seasonal photoperiod programs and 1,189 transcripts displayed age-dependent variation in expression. Prolactin (PRL) and follicle-stimulating hormone subunit beta (FSHβ) are molecular markers of seasonal programs and are significantly upregulated in long photoperiod conditions. Growth hormone expression was significantly upregulated in juvenile quail, regardless of photoperiodic condition. These findings indicate that a level of cell autonomy in the pituitary gland governs seasonal and developmental programs in physiology. Overall, this paper yields novel insights into the molecular mechanisms that govern developmental programs and adult brain plasticity.

Effect of immunization against OPN5 on the reproductive performance in Shan Partridge ducks under different photoperiods

Photoperiod is an important environmental factor that influences seasonal reproduction behavior in birds. Birds translate photoperiodic information into neuroendocrine signals through deep brain photoreceptors (DBPs). OPN5 has been considered candidate DBPs involved in regulating seasonal reproduction in birds. We found that OPN5 could mediate light to regulate the follicle development in ducks. In this study, we further verified the effect of OPN5 on follicular development in Shan Partridge ducks by immunizing against the extracellular domain (ECD) of OPN5. We investigated the specific regulatory mechanism of photoperiod mediated by OPN5 on the reproductive activity of ducks. The trial randomly divided 120 Shan Partridge ducks into 3 groups with different treatments: the immunization of OPN5 group was done at d0, d15, d30, and d40 with 1 mL of vaccine containing OPN5 protein (thus containing 1, 1, 0.5, and 0.5 mg of OPN5-KLH protein), and the control group (CS and CL groups) was injected at the same time with the same dose of OPN5-uncontained blank vaccine. The group of CS (900 lux), OPN5 (600 lux), and CL (600 lux) lasted for 40 d in 12 L:12 D photoperiods, respectively. Then, the groups of CS, OPN5, and CL subsequently received 12 L:12 D, 12 L:12 D, and 17 L:7 D light treatments for 33 d, respectively. The ducks were caged in 3 constant rooms with the same feeding conditions for each group, free water, and limited feeding (150 g per duck each day). Duck serum and tissue samples were collected at d 40, d 62, and d 73 (n = 12). It was found that before prolonged light, the group of immunization (group OPN5) and the group of strong light intensity (group CS) were higher than the group of CL in egg production. Subsequent to prolonged light, the group CL in egg production rose about the same as the group immunization, while the strong light group (group CS) was lower. Group OPN5 increased the ovarian index of ducks, and both the immunization of group OPN5 and group CL (extended light) increased the thickness of the granular layer and promoted the secretion of E2, P4, LH, and PRL hormones. Compared with group CS, group CL and OPN5 increased the mRNA level and protein expression of OPN5 in the hypothalamus on d 62 and d 73 (P < 0.05). The gene or protein expression patterns of GnRH, TRH, TSHβ, DIO2, THRβ, VIP, and PRL were positively correlated with OPN5, whereas the gene expression patterns of GnIH and DIO3 were negatively correlated with OPN5. The results showed that immunization against OPN5 could activate the corresponding transmembrane receptors to promote the expression of OPN5, up-regulate the expression of TSHβ and DIO2, and then regulate the HPG axis-related genes to facilitate the follicular development of Shan Partridge ducks. In addition, in this experiment, prolonging the photoperiod or enhancing the light intensity could also enhance follicle development, but the effect was not as significant as immunizing against OPN5. Our results will offer beneficial data and more supportive shreds of evidence in favor of elucidating the role of OPN5 in relation to photoperiods and reproduction.

The dynamic landscape of chromatin accessibility and active regulatory elements in the mediobasal hypothalamus influences the seasonal activation of the reproductive axis in the male quail under long light exposure

BACKGROUND

In cold and temperate zones, seasonal reproduction plays a crucial role in the survival and reproductive success of species. The photoperiod influences reproductive processes in seasonal breeders through the hypothalamic-pituitary-gonadal (HPG) axis, in which the mediobasal hypothalamus (MBH) serves as the central region responsible for transmitting light information to the endocrine system. However, the cis-regulatory elements and the transcriptional activation mechanisms related to seasonal activation of the reproductive axis in MBH remain largely unclear. In this study, an artificial photoperiod program was used to induce the HPG axis activation in male quails, and we compared changes in chromatin accessibility changes during the seasonal activation of the HPG axis.

RESULTS

Alterations in chromatin accessibility occurred in the mediobasal hypothalamus (MBH) and stabilized at LD7 during the activation of the HPG axis. Most open chromatin regions (OCRs) are enriched mainly in introns and distal intergenic regions. The differentially accessible regions (DARs) showed enrichment of binding motifs of the RFX, NKX, and MEF family of transcription factors that gained-loss accessibility under long-day conditions, while the binding motifs of the nuclear receptor (NR) superfamily and BZIP family gained-open accessibility. Retinoic acid signaling and GTPase-mediated signal transduction are involved in adaptation to long days and maintenance of the HPG axis activation. According to our footprint analysis, three clock-output genes (TEF, DBP, and HLF) and the THRA were the first responders to long days in LD3. THRB, NR3C2, AR, and NR3C1 are the key players associated with the initiation and maintenance of the activation of the HPG axis, which appeared at LD7 and tended to be stable under long-day conditions. By integrating chromatin and the transcriptome, three genes (DIO2, SLC16A2, and PDE6H) involved in thyroid hormone signaling showed differential chromatin accessibility and expression levels during the seasonal activation of the HPG axis. TRPA1, a target of THRB identified by DAP-seq, was sensitive to photoactivation and exhibited differential expression levels between short- and long-day conditions.

CONCLUSION

Our data suggest that trans effects were the main factors affecting gene expression during the seasonal activation of the HPG axis. This study could lead to further research on the seasonal reproductive behavior of birds, particularly the role of MBH in controlling seasonal reproductive behavior.

Expression of mRNAs Encoding Hypothalamic Small Proteins, Neurosecretory Protein GL and Neurosecretory Protein GM, in the Japanese Quail, Coturnix japonica

Neurosecretory protein GL (NPGL) and neurosecretory protein GM (NPGM) are novel neuropeptides that have been discovered in the hypothalamic infundibulum of chickens. NPGL and NPGM play important roles in lipid metabolism in juvenile chickens. The physiological functions of NPGL and NPGM in sexually mature birds remain unknown. The Japanese quail (Coturnix japonica) seems to be an appropriate model for analyzing NPGL and NPGM during sexual maturity. However, studies on NPGL or NPGM have yet to be reported in the Japanese quail. In the present study, we identified cDNAs encoding precursor proteins of NPGL and NPGM in the quail hypothalamus. In situ hybridization revealed that NPGL mRNA-expressing cells in the hypothalamus were localized in the infundibular nucleus and median eminence, and NPGM mRNA-expressing cells were only found in the mammillary nucleus. Immunohistochemistry revealed that NPGM-like immunoreactive cells were distributed in the mammillary nucleus, whereas NPGL-like immunoreactive cells were not detected in the hypothalamus. Real-time PCR analysis indicated that the expression of NPGL mRNA was higher in the hypothalamus of females than in that of males, and NPGM mRNA expression showed no sex differences. NPGL and NPGM mRNA expression in males was upregulated after 24 h of food deprivation. In females, only NPGM mRNA expression was increased by fasting. These results suggest that the physiological functions of NPGL and NPGM are different in quail, and these factors are involved in sex differences in energy metabolism.

Seasonality of Reproduction in a Subtropical Free-Living Finch Amandava amandava: Plasticity of Adenohypophyseal Gonadotropes, Lactotropes, and Thyrotropes

INTRODUCTION

This study sought to decipher the mechanism of transitions between life-history stages in a seasonally reproducing subtropical finch, Amandava amandava delineating the plasticity of the gonadotropes (LH cells), lactotropes (PRL cells), and thyrotropes (TSH cells) in the pituitary gland including the pars tuberalis, with regard to the in situ expression, morphological characteristics, and alteration in the plasma levels of hormones.

METHODS

Immunohistochemistry of LH, PRL, TSH cells, morphometry and densitometry of expressed hormones (Image J software analysis), and ELISA for plasma hormonal levels were performed.

RESULTS

LH, PRL, and TSH cells showed remarkable plasticity during the annual seasonal reproductive cycle. In the PT, all the 3 cell types were detected during the breeding phase, with additional detection of the TSH immunoreactivity during the pre-breeding and the PRL immunoreactivity during post-breeding phases. Pars distalis (PD) expressions and the plasma levels of the LH and TSH were at the peak during the breeding phase, but the PRL peak was during the post-breeding phase. In addition to PRL in the neurohypophysis and in the median eminence, hypothalamic PRL, and TSH were also elucidated.

CONCLUSIONS

This study suggests activation of the gonadal axis by the PT TSH which might transduce seasonal cues, but not specifically photoperiod, in the birds of the tropics/subtropics. Post-breeding phase sustained high plasma TSH and peak plasma PRL might coordinate the transition to the non-breeding phase including the trigger of parental care as the later hormone assigned with. Hypothalamic TSH and PRL might influence events of seasonality through central modulation.

Photoperiodic physiology of summer breeding birds and a search for the role of eye

Photoperiod regulation of gonadal cycles is well studied and documented in both birds and mammals. Change in photoperiod is considered as the most effective and important cue to time the initiation of the annual physiological cycles in birds. Approaching of long days (as observed in summer months), signal long-day breeding birds to initiation reproduction and other related functions. Birds and other non-mammalian vertebrates use the extraocular photoreceptors which may be present in the mediobasal hypothalamus (MBH) or associated regions to measure the photoperiodic time and so are different from mammals where only the eyes are lone photoreceptive organs. The downstream signaling involves thyroid responsive genes playing a crucial role in mediating photoperiodic signals in both birds and mammals. Role of eyes in the avian seasonal cycle has been a questionable issue with evidences both favoring and negating any role. We propose that morphological as well as physiological data argue that retinal photoreceptors can participate in gonadal cycle, at least in the quail and duck. The present review details the studies of photoneuroendocrine control of gonadal axis in birds and review evidences to decipher the role eyes in photoperiodic mediated physiologies in birds.

The ageing thyroid: implications for longevity and patient care

Thyroid hormones have vital roles in development, growth and energy metabolism. Within the past two decades, disturbances in thyroid hormone action have been implicated in ageing and the development of age-related diseases. This Review will consider results from biomedical studies that have identified the importance of precise temporospatial regulation of thyroid hormone action for local tissue maintenance and repair. Age-related disturbances in the maintenance of tissue homeostasis are thought to be important drivers of age-related disease. In most iodine-proficient human populations without thyroid disease, the mean, median and 97.5 centile for circulating concentrations of thyroid-stimulating hormone are progressively higher in adults over 80 years of age compared with middle-aged (50-59 years) and younger (20-29 years) adults. This trend has been shown to extend into advanced ages (over 100 years). Here, potential causes and consequences of the altered thyroid status observed in old age and its association with longevity will be discussed. In about 5-20% of adults at least 65 years of age, thyroid-stimulating hormone concentrations are elevated but circulating concentrations of thyroid hormone are within the population reference range, a condition referred to as subclinical hypothyroidism. Results from randomized clinical trials that have tested the clinical benefit of thyroid hormone replacement therapy in older adults with mild subclinical hypothyroidism will be discussed, as well as the implications of these findings for screening and treatment of subclinical hypothyroidism in older adults.

FSHβ links photoperiodic signaling to seasonal reproduction in Japanese quail

Annual cycles in daylength provide an initial predictive environmental cue that plants and animals use to time seasonal biology. Seasonal changes in photoperiodic information acts to entrain endogenous programs in physiology to optimize an animal's fitness. Attempts to identify the neural and molecular substrates of photoperiodic time measurement in birds have, to date, focused on blunt changes in light exposure during a restricted period of photoinducibility. The objectives of these studies were first to characterize a molecular seasonal clock in Japanese quail and second, to identify the key transcripts involved in endogenously generated interval timing that underlies photosensitivity in birds. We hypothesized that the mediobasal hypothalamus (MBH) provides the neuroendocrine control of photoperiod-induced changes in reproductive physiology, and that the pars distalis of the pituitary gland contains an endogenous internal timer for the short photoperiod-dependent development of reproductive photosensitivity. Here, we report distinct seasonal waveforms of transcript expression in the MBH, and pituitary gland and discovered the patterns were not synchronized across tissues. Follicle-stimulating hormone-β (FSHβ) expression increased during the simulated spring equinox, prior to photoinduced increases in prolactin, thyrotropin-stimulating hormone-β, and testicular growth. Diurnal analyses of transcript expression showed sustained elevated levels of FSHβ under conditions of the spring equinox, compared to autumnal equinox, short (<12L) and long (>12L) photoperiods. FSHβ expression increased in quail held in non-stimulatory short photoperiod, indicative of the initiation of an endogenously programmed interval timer. These data identify that FSHβ establishes a state of photosensitivity for the external coincidence timing of seasonal physiology. The independent regulation of FSHβ expression provides an alternative pathway through which other supplementary environmental cues, such as temperature, can fine tune seasonal reproductive maturation and involution.

Variation in responses to photoperiods and temperatures in Japanese medaka from different latitudes

Seasonal changes are more robust and dynamic at higher latitudes than at lower latitudes, and animals sense seasonal changes in the environment and alter their physiology and behavior to better adapt to harsh winter conditions. However, the genetic basis for sensing seasonal changes, including the photoperiod and temperature, remains unclear. Medaka (Oryzias latipes species complex), widely distributed from subtropical to cool-temperate regions throughout the Japanese archipelago, provides an excellent model to tackle this subject. In this study, we examined the critical photoperiods and critical temperatures required for seasonal gonadal development in female medaka from local populations at various latitudes. Intraspecific differences in critical photoperiods and temperatures were detected, demonstrating that these differences were genetically controlled. Most medaka populations could perceive the difference between photoperiods for at least 1 h. Populations in the Northern Japanese group required 14 h of light in a 24 h photoperiod to develop their ovaries, whereas ovaries from the Southern Japanese group developed under 13 h of light. Additionally, Miyazaki and Ginoza populations from lower latitudes were able to spawn under short-day conditions of 11 and 10 h of light, respectively. Investigation of the critical temperature demonstrated that the Higashidori population, the population from the northernmost region of medaka habitats, had a critical temperature of over 18 °C, which was the highest critical temperature among the populations examined. The Miyazaki and the Ginoza populations, in contrast, were found to have critical temperatures under 14 °C. When we conducted a transplant experiment in a high-latitudinal environment using medaka populations with different seasonal responses, the population from higher latitudes, which had a longer critical photoperiod and a higher critical temperature, showed a slower reproductive onset but quickly reached a peak of ovarian size. The current findings show that low latitudinal populations are less responsive to photoperiodic and temperature changes, implying that variations in this responsiveness can alter seasonal timing of reproduction and change fitness to natural environments with varying harshnesses of seasonal changes. Local medaka populations will contribute to elucidating the genetic basis of seasonal time perception and adaptation to environmental changes.

The DNA methylation status of the serotonin metabolic pathway associated with reproductive inactivation induced by long-light exposure in Magang geese

BACKGROUND

Domestic geese are seasonal breeders and have the lowest reproductive capacity among all poultry species. Magang geese is a topical short-day breeder, short photoperiod exposure stimulates its reproductive activity while long photoperiod inhibits. To explore epigenetic change that could influence reproductive activity, we performed whole genome bisulfite sequencing and transcriptome sequencing in the hypothalamus at three reproductive stages during long-light exposure in male Magang geese.

RESULTS

A total number of 10,602 differentially methylated regions (DMRs) were identified among three comparison groups. We observed that the vast majority of DMRs were enriched in intron regions. By integrating the BS-sequencing and RNA-seq data, the correlation between methylation changes of CG DMRs and expression changes of their associated genes was significant only for genes containing CG DMRs in their intron. A total of 278 DMR-associated DEGs were obtained among the three stages. KEGG analysis revealed that the DMR-associated DEGs were mainly involved in 11 pathways. Among them, the neuroactive ligand-receptor interaction pathway was significantly enriched in both two comparisons (RA vs.RD and RD vs.RI); the Wnt signaling pathway, apelin signaling pathway, melanogenesis, calcium signaling pathway, focal adhesion, and adherens junction were significantly enriched in the RA vs. RI comparison. In addition, the expression level of two serotonin-metabolic genes was significantly altered during reproductive axis inactivation by the methylation status of their promoter region (TPH2) and intron region (SLC18A2), respectively. These results were confirmed by Bisulfite sequencing PCR (BSP), pyrosequencing, and real-time qPCR, indicating that serotonin metabolic signaling may play a key role in decreasing the reproductive activity of Magang geese induced by long-light exposure. Furthermore, we performed a metabolomics approach to investigate the concentration of neurotransmitters among the three stages, and found that 5-HIAA, the last product of the serotonin metabolic pathway, was significantly decreased in the hypothalamus during RI.

CONCLUSIONS

Our study reveals that the methylation status of the serotonin metabolic pathway in the hypothalamus is associated with reproductive inactivation, and provided new insight into the effect of DNA methylation on the reproductive regulation of the hypothalamus in Magang geese.

Genetic Control of Avian Migration: Insights from Studies in Latitudinal Passerine Migrants

Twice-a-year, large-scale movement of billions of birds across latitudinal gradients is one of the most fascinating behavioral phenomena seen among animals. These seasonal voyages in autumn southwards and in spring northwards occur within a discrete time window and, as part of an overall annual itinerary, involve close interaction of the endogenous rhythm at several levels with prevailing photoperiod and temperature. The overall success of seasonal migrations thus depends on their close coupling with the other annual sub-cycles, namely those of the breeding, post-breeding recovery, molt and non-migratory periods. There are striking alterations in the daily behavior and physiology with the onset and end of the migratory period, as shown by the phase inversions in behavioral (a diurnal passerine bird becomes nocturnal and flies at night) and neural activities. Interestingly, there are also differences in the behavior, physiology and regulatory strategies between autumn and spring (vernal) migrations. Concurrent molecular changes occur in regulatory (brain) and metabolic (liver, flight muscle) tissues, as shown in the expression of genes particularly associated with 24 h timekeeping, fat accumulation and the overall metabolism. Here, we present insights into the genetic basis of migratory behavior based on studies using both candidate and global gene expression approaches in passerine migrants, with special reference to Palearctic-Indian migratory blackheaded and redheaded buntings.

The study of selection signature and its applications on identification of candidate genes using whole genome sequencing data in chicken - a review

Chicken is a major source of protein for the increasing human population and is useful for research purposes. There are almost 1,600 distinct regional breeds of chicken across the globe, among which a large body of genetic and phenotypic variations has been accumulated due to extensive natural and artificial selection. Moreover, natural selection is a crucial force for animal domestication. Several approaches have been adopted to detect selection signatures in different breeds of chicken using whole genome sequencing (WGS) data including integrated haplotype score (iHS), cross-populated extend haplotype homozygosity test (XP-EHH), fixation index (F_ST), cross-population composite likelihood ratio (XP-CLR), nucleotide diversity (Pi), and others. In addition, gene enrichment analyses are utilized to determine KEGG pathways and gene ontology (GO) terms related to traits of interest in chicken. Herein, we review different studies that have adopted diverse approaches to detect selection signatures in different breeds of chicken. This review systematically summarizes different findings on selection signatures and related candidate genes in chickens. Future studies could combine different selection signatures approaches to strengthen the quality of the results thereby providing more affirmative inference. This would further aid in deciphering the importance of selection in chicken conservation for the increasing human population.

Seasonal Adaptation: Geographic Photoperiod-Temperature Patterns Explain Genetic Variation in the Common Vole Tsh Receptor

The vertebrate photoperiodic neuroendocrine system uses the photoperiod as a proxy to time the annual rhythms in reproduction. The thyrotropin receptor (TSHR) is a key protein in the mammalian seasonal reproduction pathway. Its abundance and function can tune sensitivity to the photoperiod. To investigate seasonal adaptation in mammals, the hinge region and the first part of the transmembrane domain of the Tshr gene were sequenced for 278 common vole (Microtus arvalis) specimens from 15 localities in Western Europe and 28 localities in Eastern Europe. Forty-nine single nucleotide polymorphisms (SNPs; twenty-two intronic and twenty-seven exonic) were found, with a weak or lack of correlation with pairwise geographical distance, latitude, longitude, and altitude. By applying a temperature threshold to the local photoperiod-temperature ellipsoid, we obtained a predicted critical photoperiod (pCPP) as a proxy for the spring onset of local primary food production (grass). The obtained pCPP explains the distribution of the genetic variation in Tshr in Western Europe through highly significant correlations with five intronic and seven exonic SNPs. The relationship between pCPP and SNPs was lacking in Eastern Europe. Thus, Tshr, which plays a pivotal role in the sensitivity of the mammalian photoperiodic neuroendocrine system, was targeted by natural selection in Western European vole populations, resulting in the optimized timing of seasonal reproduction.

The Circadian Clock, Nutritional Signals and Reproduction: A Close Relationship

The circadian rhythm, which is necessary for reproduction, is controlled by clock genes. In the mouse uterus, the oscillation of the circadian clock gene has been observed. The transcription of the core clock gene period (Per) and cryptochrome (Cry) is activated by the heterodimer of the transcription factor circadian locomotor output cycles kaput (Clock) and brain and muscle Arnt-like protein-1 (Bmal1). By binding to E-box sequences in the promoters of Per1/2 and Cry1/2 genes, the CLOCK-BMAL1 heterodimer promotes the transcription of these genes. Per1/2 and Cry1/2 form a complex with the Clock/Bmal1 heterodimer and inactivate its transcriptional activities. Endometrial BMAL1 expression levels are lower in human recurrent-miscarriage sufferers. Additionally, it was shown that the presence of BMAL1-depleted decidual cells prevents trophoblast invasion, highlighting the importance of the endometrial clock throughout pregnancy. It is widely known that hormone synthesis is disturbed and sterility develops in Bmal1-deficient mice. Recently, we discovered that animals with uterus-specific Bmal1 loss also had poor placental development, and these mice also had intrauterine fetal death. Furthermore, it was shown that time-restricted feeding controlled the uterine clock's circadian rhythm. The uterine clock system may be a possibility for pregnancy complications, according to these results. We summarize the most recent research on the close connection between the circadian clock and reproduction in this review.

Long-day stimulation increases thyroid-stimulating hormone expression and affects gonadal development in chub mackerel

For seasonal breeders, photoperiodic changes are important signals that mark the start of the breeding season. Thyroid-stimulating hormone (TSH) is a glycoprotein hormone that not only promotes the secretion of thyroid hormone but also plays a key role in regulating seasonal reproduction in birds and mammals. However, whether TSH activation has been implicated as a seasonal indicator in fish breeding has not been fully investigated. In this study, we isolated tshb as a starting point to elucidate the effect of photoperiodic changes on the activation of the reproductive axis of chub mackerel. The isolated tshb was classified as tshba, which is widely conserved in vertebrates. The quantitative PCR results showed that tshb was strongly expressed in the pituitary. When female and male chub mackerel with immature gonads were reared for six weeks under different photoperiodic conditions, the gonads developed substantially in the long-day (LD) reared fish compared to those in the short-day reared fish. Real-time PCR results showed that the expression level of tshb in the pituitary gland was significantly elevated in the LD group. Although there was no difference in the gonadotropin-releasing hormone 1 gene expression level in the preoptic area of the brain, follicle-stimulating hormone and luteinizing hormone gene expression levels in the pituitary were also significantly elevated in the LD group. In conclusion, TSH is a potential mediator of seasonal information in the reproductive endocrine axis and may induce gonadal development during the breeding season of chub mackerel.

Single-cell RNA sequencing revealed the liver heterogeneity between egg-laying duck and ceased-laying duck

BACKGROUND

In the late phase of production, ducks untimely cease laying, leading to a lower feed conversion. Liver plays a vital role in the synthesis and transport of yolk materials during egg formation in birds. However, the molecular mechanism of liver in ceased-laying duck is far from clear, higher resolution and deeper analysis is needed. Sing-cell RNA-sequencing of 10 × Genomics platform can help to map the liver single cell gene expression atlas of Shaoxing duck and provide new insights into the liver between egg-laying and ceased-laying ducks.

RESULTS

About 20,000 single cells were profiled and 22 clusters were identified. All the clusters were identified as 6 cell types. The dominant cell type is hepatocyte, accounted for about 60% of all the cells. Of note, the heterogeneity of cells between egg-laying duck and ceased-laying duck mainly occurred in hepatocytes. Cells of cluster 3 and 12 were the unique hepatocyte states of egg-laying ducks, while cells of cluster 0 and 15 were the unique hepatocyte states of ceased-laying ducks. The expression mode of yolk precursor transporters, lipid metabolizing enzymes and fibrinogens were different in hepatocytes between egg-laying duck and ceased-laying duck. APOV1, VTG2, VTG1, APOB, RBP, VTDB and SCD might be activated in egg-laying ducks, while APOA1, APOA4, APOC3, FGB and FGG might be activated in ceased-laying ducks.

CONCLUSIONS

Our study further proofs that APOV1 and APOB play key roles in egg production, rather than APOA1 and APOA4. It is also the first to detect a correlation between the higher expression of APOC3, FGB, FGG and ceased-laying in duck.

A brain for all seasons: An in vivo MRI perspective on songbirds

Seasonality in songbirds includes not only reproduction but also seasonal changes in singing behavior and its neural substrate, the song control system (SCS). Prior research mainly focused on the role of sex steroids on this seasonal SCS neuroplasticity in males. In this review, we summarize the advances made in the field of seasonal neuroplasticity by applying in vivo magnetic resonance imaging (MRI) in male and female starlings, analyzing the entire brain, monitoring birds longitudinally and determining the neuronal correlates of seasonal variations in plasma hormone levels and song behavior. The first MRI studies in songbirds used manganese enhanced MRI to visualize the SCS in a living bird and validated previously described brain volume changes related to different seasons and testosterone. MRI studies with testosterone implantation established how the consequential boost in singing was correlated to structural changes in the SCS, indicating activity-induced neuroplasticity as song proficiency increased. Next, diffusion tensor MRI explored seasonal neuroplasticity in the entire brain, focusing on networks beyond the SCS, revealing that other sensory systems and even the cerebellum, which is important for the integration of sensory perception and song behavior, experience neuroplasticity starting in the photosensitive period. Functional MRI showed that olfactory, and auditory processing was modulated by the seasons. The convergence of seasonal variations in so many sensory and sensorimotor systems resembles multisensory neuroplasticity during the critical period early in life. This sheds new light on seasonal songbirds as a model for unlocking the brain by recreating seasonally the permissive circumstances for heightened neuroplasticity.

Photoperiodic regulation of avian physiology: From external coincidence to seasonal reproduction

Seasonal cycles of environmental cues generate variation in the timing of life-history transition events across taxa. It is through the entrainment of internal, endogenous rhythms of organisms to these external, exogenous rhythms in environment, such as cycling temperature and daylight, by which organisms can regulate and time life history transitions. Here, we review the current understanding of how photoperiod both stimulates and terminates seasonal reproduction in birds. The review describes the role of external coincidence timing, the process by which photoperiod is proposed to stimulate reproductive development. Then, the molecular basis of light detection and the photoperiodic regulation of neuroendocrine timing of seasonal reproduction in birds is presented. Current data indicates that vertebrate ancient opsin is the predominant photoreceptor for light detection by the hypothalamus, compared to neuropsin and rhodopsin. The review then connects light detection to well-characterized hypothalamic and pituitary gland molecules involved in the photoperiodic regulation of reproduction. In birds, Gonadotropin-releasing hormone synthesis and release are controlled by photoperiodic cues via thyrotropin-stimulating hormone-β (TSHβ) independent and dependent pathways, respectively. The review then highlights the role of D-box and E-box binding motifs in the promoter regions of photoperiodic genes, in particular Eyes-absent 3, as the key link between circadian clock function and photoperiodic time measurement. Based on the available evidence, the review proposes that at least two molecular programs form the basis for external coincidence timing in birds: photoperiodic responsiveness by TSHβ pathways and endogenous internal timing by gonadotropin synthesis.

Control and adaptability of seasonal changes in behavior and physiology of latitudinal avian migrants: Insights from laboratory studies in Palearctic-Indian migratory buntings

Twice-a-year migrations, one in autumn and the other in spring, occur within a discrete time window with striking alterations in the behavior and physiology, as regulated by the interaction of endogenous rhythms with prevailing photoperiod. These seasonal voyages are not isolated events; rather, they are part of an overall annual itinerary and remain closely coupled to the other annual subcycles, called seasonal life history states (LHSs). The success of migration depends on appropriate timing of the initiation and termination of each LHS, for example, reproduction, molt, summer nonmigratory, preautumn migratory (fattening and weight gain), autumn migratory, winter nonmigratory (wnM), prevernal (spring) migratory (fattening and weight gain), and spring migratory LHSs. Migration-linked photoperiod-induced changes include the body fattening and weight gain, nocturnal Zugunruhe (migratory restlessness), elevated triglycerides and free fatty acids, triiodothyronine and corticosterone levels. Hypothalamic expression of the thyroid hormone-responsive dio2 and dio3, light-responsive per2, cry1, and adcyap1 and th (tyrosine hydroxylase, involved in dopamine biosynthesis) genes also show significant changes with transition from wnM to the vernal migratory LHS. Concurrent changes in the expression of genes associated with lipid metabolism and its transport also occur in the liver and flight muscles, respectively. Interestingly, there are clear differences in the behavioral and physiological phenotypes, and associated molecular changes, between the autumn and vernal migrations. In this review, we discuss seasonal changes in the behavior and physiology, and present molecular insights into the development of migratory phenotypes in latitudinal avian migrants, with special reference to Palearctic-Indian migratory buntings.

Seasonality in tropical birds

The survival of offspring depends on environmental conditions. Many organisms have evolved with seasonality, characterized as initiation-termination-reinitiation of several physiological processes (i.e., body fattening, molt, plumage coloration, reproduction, etc). It is an adaptation for the survival of many species. Predominantly seasonal breeders use photoperiod as the most reliable environmental cue to adapt to seasonal changes but supplementary factors like temperature and food are synergistically involved in seasonal processes. Studies from diverse vertebrate systems have contributed to understanding the mechanism involved in seasonal reproduction at the molecular and endocrine levels. Long-day induced thyrotropin (thyroid-stimulating hormone) released from the pars tuberalis of the pituitary gland triggers local thyroid hormone activation within the mediobasal hypothalamus. This locally produced thyroid hormone, T3, regulates seasonal gonadotropin-releasing hormone secretion. Most of the bird species studied are seasonal in reproduction and linked behavior and, therefore, need to adjust reproductive decisions to environmental fluctuations. Reproductive strategies of the temperate zone breeders are well-documented, but less is known about tropical birds' reproduction and factors stimulating the annual breeding strategies. Here, we address seasonality in tropical birds with reference to seasonal reproduction and the various environmental factors influencing seasonal breeding.

Oviduct Transcriptomic Reveals the Regulation of mRNAs and lncRNAs Related to Goat Prolificacy in the Luteal Phase

Simple Summary

The kidding number is an important reproductive trait in domestic goats. The oviduct, as one of the most major organs, is directly involved in the reproductive process, providing nutrition and a location for early embryonic development. The current study provides genome-wide expression profiles of mRNA and long noncoding RNAs (lncRNAs) expression in Yunshang black goat, a new breed of meat goat bred in China with a high kidding number. During the luteal phases, oviduct mRNAs and lncRNAs associated with high- and low-fecundity Yunshang black goats were identified, and their potential biological functions were predicted using GO, KEGG, and GSEA enrichment analysis. These findings shed light on the oviduct-based prolificacy mechanism in goats.

Abstract

The oviduct is associated with embryo development and transportation and regulates the pregnancy success of mammals. Previous studies have indicated a molecular mechanism of lncRNAs in gene regulation and reproduction. However, little is known about the function of lncRNAs in the oviduct in modulating goat kidding numbers. Therefore, we combined RNA sequencing (RNA-seq) to map the expression profiles of the oviduct at the luteal phase from high- and low-fecundity goats. The results showed that 2023 differentially expressed mRNAs (DEGs) and 377 differentially expressed lncRNAs (DELs) transcripts were screened, and 2109 regulated lncRNA-mRNA pairs were identified. Subsequently, the genes related to reproduction (IGF1, FGFRL1, and CREB1) and those associated with embryonic development and maturation (DHX34, LHX6) were identified. KEGG analysis of the DEGs revealed that the GnRH- and prolactin-signaling pathways, progesterone-mediated oocyte maturation, and oocyte meiosis were related to reproduction. GSEA and KEGG analyses of the target genes of DELs demonstrated that several biological processes and pathways might interact with oviduct functions and the prolificacy of goats. Furthermore, the co-expression network analysis showed that XLOC_029185, XLOC_040647, and XLOC_090025 were the cis-regulatory elements of the DEGs MUC1, PPP1R9A, and ALDOB, respectively; these factors might be associated with the success of pregnancy and glucolipid metabolism. In addition, the GATA4, LAMA2, SLC39A5, and S100G were trans-regulated by lncRNAs, predominantly mediating oviductal transport to the embryo and energy metabolism. Our findings could pave the way for a better understanding of the roles of mRNAs and lncRNAs in fecundity-related oviduct function in goats.

Impact of food restriction on the medio-basal hypothalamus of intact ewes as revealed by a large-scale transcriptomics study

In mammals, the medio-basal hypothalamus (MBH) integrates photoperiodic and food-related cues to ensure timely phasing of physiological functions, including seasonal reproduction. The current human epidemics of obesity and associated reproductive disorders exemplifies the tight link between metabolism and reproduction. Yet, how food-related cues impact breeding at the level of the MBH remains unclear. In this respect, the sheep, which is a large diurnal mammal with a marked dual photoperiodic/metabolic control of seasonal breeding, is a relevant model. Here, we present a large-scale study in ewes (n = 120), which investigated the impact of food restriction (FRes) on the MBH transcriptome using unbiased RNAseq, followed by RT-qPCR. Few genes (~100) were impacted by FRes and the transcriptional impact was very modest (<2-fold increase or < 50% decrease for most genes). As anticipated, FRes increased expression of Npy/AgRP/LepR and decreased expression of Pomc/Cartpt, while Kiss1 expression was not impacted. Of particular interest, Eya3, Nmu and Dio2, genes involved in photoperiodic decoding within the MBH, were also affected by FRes. Finally, we also identified a handful of genes not known to be regulated by food-related cues (e.g., RNase6, HspA6, Arrdc2). In conclusion, our transcriptomics study provides insights into the impact of metabolism on the MBH in sheep, which may be relevant to human, and identifies possible molecular links between metabolism and (seasonal) reproduction.

Transcriptome analyses of nine endocrine tissues identifies organism-wide transcript distribution and structure in the Siberian hamster

Temperate zone animals exhibit seasonal variation in multiple endocrine systems. In most cases, peripheral organs display robust switches in tissue involution and recrudescence in mass. Our understanding of the molecular control of tissue-specific changes in seasonal function remains limited. Central to this problem is the lack of information on the nucleic acid structure, and distribution of transcripts across tissues in seasonal model organisms. Here we report the transcriptome profile of nine endocrine tissues from Siberian hamsters. Luteinizing hormone receptor expression was localized to gonadal tissues and confirmed previous distribution analyses. Assessment of the prolactin receptor reveal relatively high abundance across tissues involved in reproduction, energy, and water homeostasis. Neither melatonin receptor-1a, nor -1b, were found to be expressed in most tissues. Instead, the closely related G-protein coupled receptor Gpr50 was widely expressed in peripheral tissues. Epigenetic enzymes such as DNA methyltransferase 3a, was widely expressed and the predominant DNA methylation enzyme. Quantitative PCR analyses revealed some sex- and tissue-specific differences for prolactin receptor and DNA methyltransferase 3a expression. These data provide significant information on the distribution of transcripts, relative expression levels and nucleic acid sequences that will facilitate molecular studies into the seasonal programs in mammalian physiology.

A cross-taxonomic perspective on the integration of temperature cues in vertebrate seasonal neuroendocrine pathways

The regulation of seasonality has been an area of interest for decades, yet global climate change has created extra urgency in the quest to understand how sensory circuits and neuroendocrine control systems interact to generate flexibility in biological timekeeping. The capacity of temperature to alter endogenous or photoperiod-regulated neuroendocrine mechanisms driving seasonality, either as a direct cue or through temperature-dependent effects on energy and metabolism, is at the heart of this phenological flexibility. However, until relatively recently, little research had been done on the integration of temperature information in canonical seasonal neuroendocrine pathways, particularly in vertebrates. We review recent advances from research in vertebrates that deepens our understanding of how temperature cues are perceived and integrated into seasonal hypothalamic thyroid hormone (TH) signaling, which is a critical regulator of downstream seasonal phenotypic changes such as those regulated by the BPG (brain-pituitary-gonadal) axis. Temperature perception occurs through cutaneous transient receptor potential (TRP) neurons, though sensitivity of these neurons varies markedly across taxa. Although photoperiod is the dominant cue used to trigger seasonal physiology or entrain circannual clocks, across birds, mammals, fish, reptiles and amphibians, seasonality appears to be temperature sensitive and in at least some cases this appears to be related to phylogenetically conserved TH signaling in the hypothalamus. Nevertheless, the exact mechanisms through which temperature modulates seasonal neuroendocrine pathways remains poorly understood.

Effects of spring warming on seasonal neuroendocrinology and activation of the reproductive axis in hibernating arctic ground squirrels

Many animals adjust the timing of seasonal events, such as reproduction, molt, migration, and hibernation, in response to interannual variation and directional climate-driven changes in temperature. However, the mechanisms by which temperature influences seasonal timing are relatively under-explored. Seasonal timing involves retrograde signaling in which thyrotropin (TSH) in the pars tuberalis (PT) alters expression of thyroid hormone (TH) deiodinases (Dio2/Dio3) in tanycyte cells lining the third ventricle of the hypothalamus. This, in turn, affects the availability of triiodothyronine (T3) within the mediobasal hypothalamus - increased hypothalamic T3 restores a summer phenotype and activates the reproductive axis in long-day breeders. Recently, we showed that retrograde TH signaling is activated during late hibernation in arctic ground squirrels (Urocitellus parryii) held in constant darkness and constant ambient temperature. Sensitivity of seasonal pathways to non-photic cues, such as temperature, is likely particularly important to hibernating species that are sequestered in hibernacula during spring. To address this issue, we exposed captive arctic ground squirrels of both sexes to an ecologically relevant increase in ambient temperature (from -6°C to -1°C) late in hibernation and examined the effects of warming on the seasonal retrograde TSH/Dio/T3 signaling pathway, as well as downstream elements of the reproductive axis. We found that warmed males tended to have higher PT TSHβ expression and significantly heavier testis mass whereas the TSH/Dio/T3 signaling pathway was unaffected by warming in females, although warmed females exhibited a slight decrease in ovarian mass. Our findings suggest that temperature could have different effects on gonadal growth in male and female arctic ground squirrels, which could lead to mismatched timing in response to rapid climate change.

An updated look at the mating system, parental care and androgen seasonal variations in ratites

Androgens modulate multiple key aspects of male reproduction, from morphology to mating behavior. Across animals the seasonal patterns of androgens are tightly linked to many of the species' life-history traits and their evolution. One popular framework to address this issue has been the Challenge Hypothesis, which proposed a testosterone-mediated trade-off between mating and parental care in males. Given the lack of empirical support, especially in birds, this hypothesis has been recently revisited (Challenge Hypothesis 2.0), integrating aspects such as male-female interactions and the diversity of reproductive systems in birds. Ratites constitute the most basal avian group (Palaeognathae: ratites together with Tinamiformes) and have certain characteristics that make them unique. They are flightless and generally have promiscuous mating systems with communal nests and male-only parental care (nest building, incubation and chick rearing). Furthermore, male testosterone concentrations remain high during the entire parental care period. Here we review the reproductive biology of ratites, integrating information on seasonal variations in parental care, social interactions and androgen levels across the group, in light of the Challenge Hypotheses and the Challenge Hypothesis 2.0 (there are no seasonal hormonal data for Tinamiformes, therefore they are not included in this review). We also discuss the constraints that could explain the lack of experimental approaches in behavioral endocrinology across ratites. I hope this review will motivate further research on this basal group of birds and further our understanding of the evolution of the mechanisms in the endocrine system that underly reproductive behavior across birds.

Environmental disruption of reproductive rhythms

Reproduction is a key biological function requiring a precise synchronization with annual and daily cues to cope with environmental fluctuations. Therefore, humans and animals have developed well-conserved photoneuroendocrine pathways to integrate and process daily and seasonal light signals within the hypothalamic-pituitary-gonadal axis. However, in the past century, industrialization and the modern 24/7 human lifestyle have imposed detrimental changes in natural habitats and rhythms of life. Indeed, exposure to an excessive amount of artificial light at inappropriate timing because of shift work and nocturnal urban lighting, as well as the ubiquitous environmental contamination by endocrine-disrupting chemicals, threaten the integrity of the daily and seasonal timing of biological functions. Here, we review recent epidemiological, field and experimental studies to discuss how light and chemical pollution of the environment can disrupt reproductive rhythms by interfering with the photoneuroendocrine timing system.

Unraveling the Role of Thyroid Hormones in Seasonal Neuroplasticity in European Starlings (Sturnus vulgaris)

Thyroid hormones clearly play a role in the seasonal regulation of reproduction, but any role they might play in song behavior and the associated seasonal neuroplasticity in songbirds remains to be elucidated. To pursue this question, we first established seasonal patterns in the expression of thyroid hormone regulating genes in male European starlings employing in situ hybridization methods. Thyroid hormone transporter LAT1 expression in the song nucleus HVC was elevated during the photosensitive phase, pointing toward an active role of thyroid hormones during this window of possible neuroplasticity. In contrast, DIO3 expression was high in HVC during the photostimulated phase, limiting the possible effect of thyroid hormones to maintain song stability during the breeding season. Next, we studied the effect of hypothyroidism on song behavior and neuroplasticity using in vivo MRI. Both under natural conditions as with methimazole treatment, circulating thyroid hormone levels decreased during the photosensitive period, which coincided with the onset of neuroplasticity. This inverse relationship between thyroid hormones and neuroplasticity was further demonstrated by the negative correlation between plasma T3 and the microstructural changes in several song control nuclei and cerebellum. Furthermore, maintaining hypothyroidism during the photostimulated period inhibited the increase in testosterone, confirming the role of thyroid hormones in activating the hypothalamic-pituitary-gonadal (HPG) axis. The lack of high testosterone levels influenced the song behavior of hypothyroid starlings, while the lack of high plasma T4 during photostimulation affected the myelination of several tracts. Potentially, a global reduction of circulating thyroid hormones during the photosensitive period is necessary to lift the brake on neuroplasticity imposed by the photorefractory period, whereas local fine-tuning of thyroid hormone concentrations through LAT1 could activate underlying neuroplasticity mechanisms. Whereas, an increase in circulating T4 during the photostimulated period potentially influences the myelination of several white matter tracts, which stabilizes the neuroplastic changes. Given the complexity of thyroid hormone effects, this study is a steppingstone to disentangle the influence of thyroid hormones on seasonal neuroplasticity.

Midnight/midday-synchronized expression of cryptochrome genes in the eyes of three teleost species, zebrafish, goldfish, and medaka

Photoperiodic responses are observed in many organisms living in the temperate zones. The circadian clock is involved in photoperiodic time measurement; however, the underlying molecular mechanism for detection of the day length remains unknown. We previously compared the expression profiles of the Cryptochrome(Cry) genes in the zebrafish eye and reported that Cry1ab has a double peak with variable expression duration depending on the photoperiod. In this study, to understand commonalities and differences in the photoperiodic responses of ocular Cry genes, we identified Cryptochrome genes in two other teleost species, goldfish and medaka, living in temperate zones, and measured ocular Cry mRNA levels in all of the three species, under different photoperiods (long-day [14 h light: 10 h dark] and short-day [10 h light: 14 h dark] and in constant darkness. Cry1ab mRNA levels did not show dual peaks in goldfish or medaka under the examined conditions; however, the mRNA expression profiles of many Crys were altered in all three species, depending on the day length and light condition. Based on their expression profiles, Cry mRNA peaks were classified into three groups that better synchronize to sunrise (light-on), midnight/midday (middle points of the dark/light periods), or sunset (light-off). These results suggest the presence of multiple oscillators that oscillate independently or a complex oscillator in which Cry expression cycles change in a photoperiod-dependent manner in the eye.

OPN5 Regulating Mechanism of Follicle Development Through the TSH-DIO2/DIO3 Pathway in Mountain Ducks Under Different Photoperiods

Abstract: Photoperiod is an important environmental factor that influence seasonal reproduction behavior in bird. Birds translates photoperiodic information into neuroendocrine signals through deep brain photoreceptors (DBPs). OPN5 has been considered as candidate DBPs involving in regulation of seasonal reproduction in birds. However, little is known about the effect of OPN5 in non-seasonal breeding birds. Thus, we pondered on whether OPN5 regulating follicular development through TSH-DIO2/DIO3 system responds to different photoperiods in non-seasonal laying ducks. As an ideal non-seasonal breeding bird, a total of 120 mountain ducks were randomly divided into three groups and treated respectively to a different photoperiod: group S (8 L:16D), group C (17 L:7D), and group L (24 L:0D). The ducks were caged in a fully enclosed shelter with the same feeding conditions for each group, free water and limited feeding (150 g per duck each day). Samples were collected from each group at d 0, d 5, d 8, d 20, and d 35 (n = 8). The ducks in 24 h photoperiod had the highest laying rate and the lowest feed-to-egg ratio, while the ducks in 8 h photoperiod had the lowest laying rate and the highest feed-to-egg ratio. Long-day photoperiod for 24 h significantly increased the ovarian index and GnRH, LH, E2, and P4 levels in serum; short-day photoperiod for 8 h increased testosterone levels in serum. Compared with 8 h photoperiod, long-day photoperiod significantly or highly significantly increased the mRNA level and protein expression of OPN5 in the hypothalamus of long-day photoperiod on d 35 (p &lt; 0.05). The gene or protein expression patterns of GnRH, TRH, TSHβ, DIO2, THRβ, VIP, and PRL were positively correlated with OPN5, whereas the gene expression patterns of GnIH and DI O 3 were negatively correlated with OPN5. The results revealed that OPN5 mediated the effect of light on follicular development through the TSH-DIO2/DIO3 pathway, the expression of OPN5 increased with light duration and improved the efficiency of the HPG axis to promote follicular development in mountain ducks.

Revisiting TSHβ's Role in Avian Seasonal Reproduction, Insights and Challenges from Mammalian Models

The core neuroendocrine pathways regulating seasonal reproduction in vertebrates were characterized over a decade ago. This has led to the development of a "consensus" model of seasonal reproduction that appears to be largely conserved across mammals, birds, amphibians, reptile and fish. This model centers around the photoinduced increase in TSHβ expression in the pars tuberalis (PT) of the anterior pituitary gland as the key transducer of photic information from sensory cells to the critical switch in hypothalamic deiodinase enzyme expression that drives changes in localized thyroid hormone signaling. These changes in localized thyroid hormone signaling in the medial basal hypothalamus ultimately activate the reproductive axis. This model has in turn been consistently supported by studies in a variety of taxa. As such it has become the definitional standard against which subsequent work is compared, particularly in the non-mammalian literature. However, as new studies move away from the handful of canonical model systems and begin to explore the effects of naturalistic rather than artificial photoperiod manipulations a more nuanced picture has begun to emerge. Yet, progress in elucidating the detailed events of reproductive photostimulation has been uneven across the research community. In this perspective I draw on emerging data from studies in free living animals that challenges some of the established assumptions of the avian consensus model of reproduction. Specifically, the role of TSHβ and its dissociation from deiodinase signaling. I then discuss how these apparently surprising findings can be contextualized within the context of the mammalian seasonal literature. In turn this ability to contextualize from the mammalian literature highlights the breadth of the current gap I our understanding of the molecular neuroendocrine mechanisms of seasonality in mammals versus birds and other non-mammalian seasonal breeders.

Hypothalamic remodeling of thyroid hormone signaling during hibernation in the arctic ground squirrel

Hibernation involves prolonged intervals of profound metabolic suppression periodically interrupted by brief arousals to euthermy, the function of which is unknown. Annual cycles in mammals are timed by a photoperiodically-regulated thyroid-hormone-dependent mechanism in hypothalamic tanycytes, driven by thyrotropin (TSH) in the pars tuberalis (PT), which regulates local TH-converting deiodinases and triggers remodeling of neuroendocrine pathways. We demonstrate that over the course of hibernation in continuous darkness, arctic ground squirrels (Urocitellus parryii) up-regulate the retrograde TSH/Deiodinase/TH pathway, remodel hypothalamic tanycytes, and activate the reproductive axis. Forcing the premature termination of hibernation by warming animals induced hypothalamic deiodinase expression and the accumulation of secretory granules in PT thyrotrophs and pituitary gonadotrophs, but did not further activate the reproductive axis. We suggest that periodic arousals may allow for the transient activation of hypothalamic thyroid hormone signaling, cellular remodeling, and re-programming of brain circuits in preparation for the short Arctic summer.

Arctic ground squirrels hibernating in darkness activate the pars tuberalis - hypothalamus thyroid hormone signaling pathway, remodel hypothalamic tanycytes, and activate the reproductive axis.

Sex-differences in Phenology: A Tinbergian Perspective

Shifts in the timing of cyclic seasonal life-history events are among the most commonly reported responses to climate change, with differences in response rates among interacting species leading to phenological mismatches. Within a species, however, males and females can also exhibit differential sensitivity to environmental cues and may therefore differ in their responsiveness to climate change, potentially leading to phenological mismatches between the sexes. This occurs because males differ from females in when and how energy is allocated to reproduction, resulting in marked sex-differences in life-history timing across the annual cycle. In this review, we take a Tinbergian perspective and examine sex differences in timing of vertebrates from adaptive, ontogenetic, mechanistic, and phylogenetic viewpoints with the goal of informing and motivating more integrative research on sexually dimorphic phenologies. We argue that sexual and natural selection lead to sex-differences in life-history-timing and that understanding the ecological and evolutionary drivers of these differences is critical for connecting climate-driven phenological shifts to population resilience. Ontogeny may influence how and when sex differences in life-history timing arise because the early-life environment can profoundly affect developmental trajectory, rates of reproductive maturation, and seasonal timing. The molecular mechanisms underlying these organismal traits are relevant to identifying the diversity and genetic basis of population- and species-level responses to climate change, and promisingly, the molecular basis of phenology is becoming increasingly well-understood. However, because most studies focus on a single sex, the causes of sex-differences in phenology critical to population resilience often remain unclear. New sequencing tools and analyses informed by phylogeny may help generate hypotheses about mechanism as well as insight into the general "evolvability" of sex differences across phylogenetic scales, especially as trait and genome resources grow. We recommend that greater attention be placed on determining sex-differences in timing mechanisms and monitoring climate change responses in both sexes, and we discuss how new tools may provide key insights into sex-differences in phenology from all four Tinbergian domains.

GnRH and the photoperiodic control of seasonal reproduction: Delegating the task to kisspeptin and RFRP-3

Synchronization of mammalian breeding activity to the annual change of photoperiod and environmental conditions is of the utmost importance for individual survival and species perpetuation. Subsequent to the early 1960s, when the central role of melatonin in this adaptive process was demonstrated, our comprehension of the mechanisms through which light regulates gonadal activity has increased considerably. The current model for the photoperiodic neuroendocrine system points to pivotal roles for the melatonin-sensitive pars tuberalis (PT) and its seasonally-regulated production of thyroid-stimulating hormone (TSH), as well as for TSH-sensitive hypothalamic tanycytes, radial glia-like cells located in the basal part of the third ventricle. Tanycytes respond to TSH through increased expression of thyroid hormone (TH) deiodinase 2 (Dio2), which leads to heightened production of intrahypothalamic triiodothyronine (T3) during longer days of spring and summer. There is strong evidence that this local, long-day driven, increase in T3 links melatonin input at the PT to gonadotropin-releasing hormone (GnRH) output, to align breeding with the seasons. The mechanism(s) through which T3 impinges upon GnRH remain(s) unclear. However, two distinct neuronal populations of the medio-basal hypothalamus, which express the (Arg)(Phe)-amide peptides kisspeptin and RFamide-related peptide-3, appear to be well-positioned to relay this seasonal T3 message towards GnRH neurons. Here, we summarize our current understanding of the cellular, molecular and neuroendocrine players, which keep track of photoperiod and ultimately govern GnRH output and seasonal breeding.

Photoperiodic Changes in Both Hypothalamus Neurotransmitters and Circulating Gonadal Steroids Metabolomic Profiles in Relation to Seasonal Reproduction in Male Quail

Both hypothalamic neurotransmitters and serum steroid hormones are impacted by photoperiod and have effects on physiology and seasonal reproductive. However, the relationship between circulating gonadal steroids and hypothalamic neurotransmitters underlying different photoperiod is still unclear. To further understand the crosstalk of neurotransmitters and steroids in seasonal reproduction, metabolic changes of 27 neurotransmitters concentrated in hypothalamus tissues and 42 steroids hormones in serum were assessed during two artificial photoperiodic programs. The results showed that photoperiod induce testicular atrophy and recrudescence. In L-to-S groups, significantly decreased levels of testosterone concentration were found in serum (P < 0.001) and increased 11-Dehydrocorticosterone (P < 0.05); Testosterone were almost undetectable at SD_14d. In addition, the hypothalamus exhibited significantly increased arginine and 4-aminobutyric acid (GABA) concentration and decreased serotonin and epinephrine content (P < 0.01 or P < 0.05). Accordingly, serum testosterone and androstenedione became detectable at LD_3d in the S-to-L group and were markedly increase at LD_7d. Furthermore, Serum androstenedione showed a significant increase with long light expose (P < 0.01). Additionally, the hypothalamus exhibited both significantly increased L.Tryptophan and phenylalanine concentration, as well as decreased L-glutamine and L-glutamine.acid content (P < 0.01 or P < 0.05). Serotonin metabolism showed significant differences between L-to-S group and S-to-L group. Furthermore, in the correlation analysis, serum testosterone had a positive correlation with 5-Hydroxyindole-3-acetic acid (5-HIAA), while Androstenedione was significantly negative with L.Tryptophan in L-to-S (P < 0.05). However, in S-to-L group, serum testosterone showed strong negative correlation with both serotonin and 5-HIAA (P < 0.05), but positive correlation with L.Tryptophan (P < 0.01), while Androstenedione was significantly negative correlation with both serotonin (P < 0.05) and L-Glutamine (P < 0.01). Photoperiod also had significant effects on the mRNA expression. We found significant differences in gene expression patterns of both serotonin signaling and steroid biosynthesis, while MAOB, NR5A1, and 3β-HSD showed an opposite tendency between two groups. Taken together, our results revealed that circulating gonadal steroids and hypothalamic neurotransmitters were significantly impact quail’s seasonal reproduction. Circulating gonadal steroids have different effects on neurotransmitter at different photoperiodism, which may coordinately influence the seasonal reproduction of quails.

Molecular and epigenetic regulation of seasonal reproduction in Terai tree frog (Polypedates teraiensis)

Seasonal breeders predominantly use photoperiod as the predictable environmental cue to time their reproduction. Terai tree frogs are long-day seasonal breeders, but the molecular mechanism is unknown. We tested the role of different photoperiodic conditions on expression levels of candidate genes involved in seasonal reproduction and epigenetic regulation. Four experiments were performed. In experiment 1, frogs were exposed to long (LD: 16L:8D) or short photoperiod (SD: 8L:16D). In experiment 2, animals were procured at four different phases of breeding, i.e., during April (emergence just after hibernation), June (breeding phase), August (post-breeding), and October (just before hibernation). In experiments 3 and 4, frogs were exposed to equinox photoperiod but different (10, 100, or 500 lx) light intensities (exp. 3) or wavelength (red: 640 nm, green: 540 nm, blue: 450 nm or white; exp. 4). After 2 weeks, animals were euthanized, and their brain was harvested. mRNA levels of transcripts involved in photoperiodic transduction (Eya3 and Opn5), reproduction (Tshß, GnRH, Dio2, and Dio3), and epigenetics regulation (Dnmt1, Dnmt3a, Hdac1, Hdac3, and Tet2) were measured. Results show that LD promotes the upregulation of Eya3, Opn5, Tshß, GnRH, and Dio2. Differential expression of Opn5 during LD and SD suggests its involvement in light perception. Dio3 levels were upregulated in SD (exp.1) and during the post-breeding phase (exp. 2). These results employ the limited role of light intensity and spectrum in reproduction. This is the first study showing molecular machinery involved in the amphibian system's seasonal reproduction and epigenetic regulation.

In vitro effects of polychlorinated biphenyls and their hydroxylated metabolites on the synthesis and metabolism of iodothyronines in the chicken (Gallus domesticus) thyroid gland

To assess the effect of polychlorinated biphenyls (PCBs) and their hydroxylated metabolites (OH-PCBs) on thyroid hormone [TH: thyroxine (T₄) and triiodothyronine (T₃)] secretion, the concentrations of iodothyronine deiodinases (DIO1, DIO2, DIO3), and mRNA expression of genes involved in TH synthesis (TSHR, NIS, TPO, TG), metabolism (DIO1, DIO2, DIO3), and transport (OATP1C1, MCT8, MCT10, LAT1), chicken thyroid explants were incubated in medium supplemented with TSH (250 mU/ml), PCB118, PCB153, 4-OH-PCB107, and 3-OH-PCB153 (0.5 × 10^-8 M), and TSH together with each PCB and OH-PCB. The results of the in vitro experiment revealed that, except for 4-OH-PCB107, all applied PCBs and OH-PCBs inhibited basal and TSH-stimulated T₄ secretion. Moreover, they increased basal and reduced TSH-stimulated T₃ secretion. PCBs and OH-PCBs decreased the TSH-stimulated TSHR expression. Following PCB and OH-PCB exposure, significant changes in mRNA expression of NIS, TPO, and TG were observed. PCBs and OH-PCBs affected DIO1 and DIO3 transcript levels and protein abundances of each DIO. Furthermore, PCB-dependent effects on OATP1C1, MCT8, and MCT10 mRNA expression were found. In conclusion, both PCB118 and PCB153 and their OH-PCBs affect TH synthesis and deiodination processes in the chicken thyroid gland and influence TH transport across the thyrocyte membrane. In addition, the effects of PCBs and OH-PCBs depended mainly on the type of PCB congener and the exposure time. These results indicate that not only parental PCBs but also OH-PCBs are hazardous for the thyroid gland and may disrupt its endocrine function. Further studies are necessary to explain a mechanism of PCB and OH-PCB action in the avian thyroid gland.

Hypothalamic expression of GnRH-I and GnIH in the Eurasian tree sparrow over a single long day

Seasonal reproductive cycles of most birds are regulated by photoperiod via neuroendocrine control. The present study aims to investigate the role of a single long day in triggering hypothalamic expressions of GnRH-I and GnIH in the Eurasian tree sparrow (Passer montanus). Sparrows were divided into two groups (n = 24 each) and pre-treated under short days (9L: 15D) for 4 days. On the fifth day, one group was exposed to long day (14L: 10D), while other was continued under short day for another 1 day. Birds of both the groups were sacrificed and perfused on fifth day at different time points, i.e., ZT 14, ZT 16 and ZT 18 and the expressions of GnRH-I and GnIH mRNAs and peptides were studied using real-time PCR and immunohistochemistry, respectively. In addition, testicular size was measured to know testicular development. Observations revealed that birds exposed to a single long day (14L: 10D) showed an increase in hypothalamic expressions of GnRH-I mRNA and peptide and decrease in levels of GnIH mRNA only at ZT 16 and ZT 18 with no significant change in GnIH peptide. However, no significant change in GnRH-I or GnIH expression was observed at any time point under short day and birds maintained high and low expression levels of GnIH and GnRH-I, respectively. Our results clearly indicate that the photoperiodic response system of sparrow is highly sensitive to light and responds even to single long day. Furthermore, they suggest that the GnRH-I and GnIH are expressed in the hypothalamus of tree sparrow in an anti-phasic manner and switching over of their expression occurs at late hours of exposure of birds to single long day.

Seasonal variation in serum metabolites of northern European dogs

Background

Metabolic profiling identifies seasonal variance of serum metabolites in humans. Despite the presence of seasonal disease patterns, no studies have assessed whether serum metabolites vary seasonally in dogs.

Hypothesis

There is seasonal variation in the serum metabolite profiles of healthy dogs.

Animals

Eighteen healthy, client‐owned dogs.

Methods

A prospective cohort study. Serum metabolomic profiles were assessed monthly in 18 healthy dogs over a 12‐month period. Metabolic profiling was conducted using a canine‐specific proton nuclear magnetic resonance spectroscopy platform, and the effects of seasonality were studied for 98 metabolites using a cosinor model. Seasonal component was calculated, which describes the seasonal variation of each metabolite.

Results

We found no evidence of seasonal variation in 93 of 98 metabolites. Six metabolites had statistically significant seasonal variance, including cholesterol (mean 249 mg/dL [6.47 mmol/L] with a seasonal component amplitude of 9 mg/dL [0.23 mmol/L]; 95% confidence interval [CI] 6‐13 mg/dL [0.14‐0.33 mmol/L], P < .008), with a peak concentration of 264 mg/dL (6.83 mmol/L) in June and trough concentration of 236 mg/dL (6.12 mmol/L) in December. In contrast, there was a significantly lower concentration of lactate (mean 20 mg/dL [2.27 mmol/L] with a seasonal component amplitude of 4 mg/dL [0.42 mmol/L]; 95% CI 2‐6 mg/dL [0.22‐0.62 mmol/L], P < .001) during the summer months compared to the winter months, with a peak concentration of 26 mg/dL (2.9 mmol/L) in February and trough concentration of 14 mg/dL (1.57 mmol/L) in July.

Conclusions and Clinical Importance

We found no clear evidence that seasonal reference ranges need to be established for serum metabolites of dogs.

Photoperiodic modulation of ovarian metabolic, survival, proliferation and gap junction markers in adult golden hamster, Mesocricetus auratus

Female reproductive physiology is greatly dependent on tight regulation of metabolic and survival factors. Photoperiod regulates female reproductive rhythms but very less information exists explaining whether photoperiod could modulate thyroid hormone homeostasis, metabolic/energy parameters along with survival, proliferation and gap junction proteins in the ovary of a long-day breeder, Mesocricetus auratus. Adult female hamsters were exposed to different photoperiodic regimes i.e., critical photoperiod (CP; 12.5L:11.5D), short photoperiod (SP; 8L:16D) and long photoperiod (LP; 16L:8D) for 12 weeks. LP upregulated thyroidal and gonadal activity as apparent by histoarchitecture, thyroid hormone profile [triiodothyronine (T3), thyroxin (T4) and thyroid stimulating hormone (TSH)], luteinizing hormone (LH), follicle stimulating hormone (FSH), estradiol (E2) and progesterone (P4) levels when compared with SP exposed hamsters. Further, LP increased thyroid hormone receptor-α/deiodinase-2 (TRα/Dio-2), estrogen receptor-α (ERα)/aromatase and insulin receptor/glucose transporter-4 (IR/GLUT-4) expressions in ovary. Interestingly, ovarian sirtuin-1 (SIRT-1) expression was also upregulated under LP condition along with cell proliferation (proliferating cell nuclear antigen or PCNA), survival (B cell lymphoma-2 or Bcl-2) and gap junction (connexin-43) markers when compared to SP exposed hamsters. We also noted elevated levels of circulatory leptin, insulin along with melatonin and its receptor (MT-1) in ovary under SP condition. Thus, we suggest that photoperiod plays a vital role in regulation of thyroid and reproductive hormone homeostasis along with key metabolic and survival markers in the ovary of adult golden hamsters, M. auratus providing further insight into the regulation of female reproductive seasonality in a long-day breeder.

Seasonal differences in hypothalamic thyroid-stimulating hormone β, gonadotropin-releasing hormone-I and deiodinase expression between migrant and resident subspecies of white-crowned sparrow (Zonotrichia leucophrys)

Across taxa, the seasonal transition between non-breeding and breeding states is controlled by localised thyroid hormone signalling in the deep brain via reciprocal switching of deiodinase enzyme expression from type 3 (DIO3) to type 2 (DIO2). This reciprocal switch is considered to be mediated by increasing thyroid-stimulating hormone β (TSHβ) release from the pars tuberalis, which occurs in response to a change in photoperiod. Although well characterised in a handful of model organisms in controlled laboratory settings, this pathway remains largely unexplored in free-living animals under natural environmental conditions. In this comparative gene expression study, we investigated hypothalamic thyroid hormone signalling in two seasonally breeding subspecies of white-crowned sparrow (Zonotrichia leucophrys), across the entirety of their annual cycles. The migratory Gambel's (Z. l. gambelii) and resident Nuttall's (Z. l. nuttalii) subspecies differ with respect to timing of reproduction, as well as life history stage and migratory strategies. Although DIO3 mRNA expression was elevated and DIO2 mRNA expression was reduced in the wintering period in both subspecies, DIO2 peaked in both subspecies prior to the onset of reproduction. However, there was differential timing between subspecies in peak DIO2 expression. Intriguingly, seasonal modulation of TSHβ mRNA was only observed in migrants, where expression was elevated at the start of breeding, consistent with observations from other highly photoperiodic species. There was no correlation between TSHβ, DIO2 and gonadotropin-releasing hormone-I mRNA or reproductive metrics in residents. Based on these observed differences, we discuss potential implications for our understanding of how changes in medial basal hypothalamic gene expression mediates initiation of seasonal reproduction.

Functional differences between TSHR alleles associate with variation in spawning season in Atlantic herring

The underlying molecular mechanisms that determine long day versus short day breeders remain unknown in any organism. Atlantic herring provides a unique opportunity to examine the molecular mechanisms involved in reproduction timing, because both spring and autumn spawners exist within the same species. Although our previous whole genome comparisons revealed a strong association of TSHR alleles with spawning seasons, the functional consequences of these variants remain unknown. Here we examined the functional significance of six candidate TSHR mutations strongly associated with herring reproductive seasonality. We show that the L471M missense mutation in the spring-allele causes enhanced cAMP signaling. The best candidate non-coding mutation is a 5.2 kb retrotransposon insertion upstream of the TSHR transcription start site, near an open chromatin region, which is likely to affect TSHR expression. The insertion occurred prior to the split between Pacific and Atlantic herring and was lost in the autumn-allele. Our study shows that strongly associated coding and non-coding variants at the TSHR locus may both contribute to the regulation of seasonal reproduction in herring.

Junfeng Chen et al. examine potential functional consequences of reproduction timing-associated TSHR alleles segregating in Atlantic herring. By comparing fish that spawn during the spring to those that spawn in the autumn, they find that the spring-allele is correlated with enhanced cAMP signaling and that both coding and non-coding variants in the TSHR locus contribute to seasonal reproduction.

Photoperiodically driven transcriptome-wide changes in the hypothalamus reveal transcriptional differences between physiologically contrasting seasonal life-history states in migratory songbirds

We investigated time course of photoperiodically driven transcriptional responses in physiologically contrasting seasonal life-history states in migratory blackheaded buntings. Birds exhibiting unstimulated winter phenotype (photosensitive state; responsive to photostimulation) under 6-h short days, and regressed summer phenotype (photorefractory state; unresponsiveness to photostimulation) under 16-h long days, were released into an extended light period up to 22 h of the day. Increased tshβ and dio2, and decreased dio3 mRNA levels in hypothalamus, and low prdx4 and high il1β mRNA levels in blood confirmed photoperiodic induction by hour 18 in photosensitive birds. Further, at hours 10, 14, 18 and 22 of light exposure, the comparison of hypothalamus RNA-Seq results revealed transcriptional differences within and between states. Particularly, we found reduced expression at hour 14 of transthyretin and proopiomelanocortin receptor, and increased expression at hour 18 of apolipoprotein A1 and carbon metabolism related genes in the photosensitive state. Similarly, valine, leucine and isoleucine degradation pathway genes and superoxide dismutase 1 were upregulated, and cocaine- and amphetamine-regulated transcript and gastrin-releasing peptide were downregulated in the photosensitive state. These results show life-history-dependent activation of hypothalamic molecular pathways involved in initiation and maintenance of key biological processes as early as on the first long day.