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Understanding the fundamental reproductive biology of a species is the first step toward identifying parameters that are critical for reproduction and for the development of assisted reproductive techniques. Ejaculates were collected from aquarium (n = 24) and in situ (n = 34) sand tiger sharks Carcharias taurus. Volume, pH, osmolarity, sperm concentration, motility, status, morphology, and plasma membrane integrity were assessed for each ejaculate. Semen with the highest proportion of motile sperm was collected between April and June for both in situ and aquarium sand tiger sharks indicating a seasonal reproductive cycle. Overall, 17 of 30 semen samples collected from aquarium sharks from April through June contained motile sperm compared to 29 of 29 of in situ sharks, demonstrating semen quality differences between aquarium and in situ sharks. Sperm motility, status, morphology, and plasma membrane integrity were significantly higher (P < 0.05) for in situ compared to aquarium sand tiger sharks. Testosterone was measured by an enzyme immunoassay validated for the species. Testosterone concentration was seasonal for both aquarium and in situ sharks with highest concentrations measured in spring and lowest in summer. In situ sharks had higher (P < 0.05) testosterone concentration in spring than aquarium sharks. This study demonstrated annual reproduction with spring seasonality for male sand tiger sharks through marked seasonal differences in testosterone and semen production. Lower testosterone and poorer semen quality was observed in aquarium sharks likely contributing to the species' limited reproductive success in aquariums. Summary sentence During mating season, in situ sand tiger sharks Carcharias taurus have higher plasma testosterone and better semen quality than aquarium housed sand tiger sharks impairing reproductive success of aquarium populations.

The first incidence of embryonic diapause in mammals was observed in the roe deer, Capreolus capreolus, in 1854 and confirmed in the early 1900s. Since then scientists have been fascinated by this phenomenon that allows a growing embryo to become arrested for up to 11 months and then reactivate and continue development with no ill effects. The study of diapause has required unraveling basic reproductive processes we now take for granted and has spanned some of the major checkpoints of reproductive biology from the identification of the sex hormones to the hypothalamic–pituitary axis to microRNA and exosomes. This review will describe the history of diapause from its origins to the current day, including its discovery and efforts to elucidate its mechanisms. It will also attempt to highlight the people involved who were instrumental in progressing this field over the last 160 years. The most recent confirmation of mammalian diapause was in the panda in 2009 and there are still multiple mammals where it has been predicted but not yet confirmed. Furthermore, there are many questions still unanswered which ensure that embryonic diapause will continue to be a topic of research for many years to come. Note that there have recently been several extensive reviews covering the recent advances in embryonic diapause, so they will be mentioned only briefly here. For further information refer to Renfree and Shaw 2014; Fenelon et al 2014; Renfree and Fenelon 2017, and references therein. Summary Sentence The history of the discovery of embryonic diapause inmammals, the key breakthroughs, the current understanding, and future directions for the field.

The photoperiod is the predominant environmental factor that governs seasonal reproduction in animals; however, the underlying molecular regulatory mechanism has yet to be fully elucidated. Herein, Yangzhou geese (Anser cygnoides) were selected at the spring equinox (SE), summer solstice (SS), autumn equinox (AE), and winter solstice (WS), and the regulation of seasonal reproduction via the light-driven cyclical secretion of pineal melatonin was investigated. We show that there were seasonal variations in the laying rate and GSI, while the ovarian area decreased 1.5-fold from the SS to the AE. Moreover, not only did the weight and volume of the pineal gland increase with a shortened photoperiod, but the secretory activity was also enhanced. Notably, tissue distribution further revealed seasonal oscillations in melatonin receptors (Mtnrs) in the pineal gland and the hypothalamus–pituitary–gonadal (HPG) axis. The immunohistochemical staining indicated higher Mtnr levels due to the shortened photoperiod. Furthermore, the upregulation of aralkylamine N-acetyltransferase (Aanat) was observed from the SS to the AE, concurrently resulting in a downregulation of the gonadotrophin-releasing hormone (GnRH) and gonadotropins (GtHs). This trend was also evident in the secretion of hormones. These data indicate that melatonin secretion during specific seasons is indicative of alterations in the photoperiod, thereby allowing for insight into the neuroendocrine regulation of reproduction via an intrinsic molecular depiction of external photoperiodic variations.

One of the many functions of melatonin in vertebrates is seasonal reproductive timing. Longer nights in winter correspond to an extended duration of melatonin secretion. The purpose of this review is to discuss melatonin synthesis, receptor subtypes, and function in the context of seasonality across vertebrates. We conclude with Tinbergen’s Four Questions to create a comparative framework for future melatonin research in the context of seasonal reproduction.

Background Sheep are short-day breeder, and their reproductvie activity can be activated by short photoperiods, but the regulatory mechanism remains unclear. In this study, hypothalamic tissues were collected from ovariectomized and estradiol-treated (OVX + E 2 ) Sunite ewes exposed to artificial short-day (SP) and long-day (LP) photoperiods. Integrated whole-genome DNA methylation and transcriptomic analyses were performed to elucidate the regulatory mechanisms underlying photoperiod-mediated seasonal reproduction. Results The DNA methylation profiles of the hypothalamus were examined, and whole-genome single-base resolution methylome maps of the sheep hypothalamus were generated across seven time points under various photoperiod treatments. The results revealed that photoperiod-induced changes in genome-wide DNA methylation constitute a dynamic and reversible regulatory process, with a potential transition point occurring approximately 15 days after the switch from SP to LP. Following integrated analysis of differentially methylated regions-related genes (DMRGs) and differentially expressed genes (DEGs), it showed that the DNA methylation levels before the transcription start site (TSS) of the overlapping gene were different, and a negative correlation between gene expression levels and mCG levels in gene promoters was found ( P  < 0.01). Functional analysis of the overlapping genes revealed that pathways involved in synapse development, thyroid hormone signaling, and circadian rhythm were regulated by photoperiod-induced DNA methylation, thereby influencing hypothalamic function in photoperiod-dependent seasonal reproduction. Conclusion This study generated the whole-genome single-base resolution DNA methylome maps of sheep hypothalamus under different photoperiods, and elucidated the regulatory relationship between DNA methylation and photoperiod-dependent seasonal reproduction in sheep hypothalamus tissues. Our findings provided a valuable resource for further research on the underlying mechanisms of the hypothalamic-regulated reproductive seasonality in sheep.

Follicle development in dairy goats is lower after induced estrus during the non-breeding season, reducing conception rates and challenging year-round milk supply. This study investigated follicle development during the breeding and non-breeding seasons and explored molecular mechanisms for variations in the proportions of follicles of different sizes using ovarian RNA-seq and in vitro experiments. Induced estrus during the non-breeding season used a simulated breeding season short photoperiod and male effect methods, while the male effect method was used during the breeding season. This study identified an increase in follicle size during the breeding season and performed RNA-seq on ovaries to explore the underlying causes. The RNA-seq analysis elucidated pathways associated with cellular and hormonal metabolism and identified adenylyl cyclase 5 (ADCY5) as a key differentially expressed gene. In vitro experiments demonstrated that interfering with ADCY5 in ovarian granulosa cells (GCs) reduced steroid synthesis. Conversely, the overexpression of ADCY5 increased steroid synthesis. ADCY5 affects the biological function of GCs and consequently influences follicle development through the cAMP-response element binding protein (CREB) and p38 mitogen-activated protein kinase phosphorylation (MAPK) pathways. Overall, our findings demonstrate that follicle development in dairy goats differs between the breeding and non-breeding seasons and that the differential expression levels of the ADCY5 gene contribute to this discrepancy.

Gaining a comprehensive understanding of all factors influencing fish reproduction in large rivers is vital for proposing improved river discharge management practices and estimating the impact of hydroclimatic changes, thereby supporting fish conservation efforts. However, the role of environmental variables as proximal cues for fish reproduction in tropical regions remains unclear. This study assesses the relative importance of discharge, the monthly rate of discharge change, dissolved oxygen, temperature, electric conductivity, water transparency, and depth in final gonadal preparation of fish in the Madeira River. Over approximately 500 km, we conducted a year-long monthly survey at 11 sampling sites near the mouths of eight tributaries. We determined the reproductive status of each captured fish for both migratory and non-migratory species. An index of reproductive intensity and species beta diversity was used to estimate the synchronism of fish reproduction. We found that the rate of monthly discharge change plays a pivotal role in explaining synchronism in final gonadal maturation. We observed a higher value of reproductive activity for both migratory and non-migratory fish assemblages during periods of significant increases in water flow, despite low overall discharge values. During this period, beta diversity was lower for migratory fish, suggesting a synchronized response among most species, while no clear pattern was observed for non-migratory species. This study provides compelling evidence that a significant and consistent surge in discharge, still during the lower discharge stages of the hydrometric cycle, serves as the primary proximal cue synchronizing the reproduction of rising-water spawner fishes in the Madeira River.

Most vertebrates living outside the tropical zone show robust physiological responses in response to seasonal changes in photoperiod, such as seasonal reproduction, molt, and migration. The highly sophisticated photoperiodic mechanism in Japanese quail has been used to uncover the mechanism of seasonal reproduction. Molecular analysis of quail mediobasal hypothalamus (MBH) revealed that local thyroid hormone activation within the MBH plays a critical role in the photoperiodic response of gonads. This activation is accomplished by two gene switches: thyroid hormone-activating (DIO2) and thyroid hormone-inactivating enzymes (DIO3). Functional genomics studies have shown that long-day induced thyroid-stimulating hormone (TSH) in the pars tuberalis (PT) of the pituitary gland regulates DIO2/3 switching. In birds, light information received directly by deep brain photoreceptors regulates PT TSH. Recent studies demonstrated that Opsin 5-positive cerebrospinal fluid (CSF)-contacting neurons are deep brain photoreceptors that regulate avian seasonal reproduction. Although the involvement of TSH and DIO2/3 in seasonal reproduction has been confirmed in various mammals, the light input pathway that regulates PT TSH in mammals differs from that of birds. In mammals, the eye is the only photoreceptor organ and light information received by the eye is transmitted to the pineal gland through the circadian pacemaker, the suprachiasmatic nucleus. Nocturnal melatonin secretion from the pineal gland indicates the length of night and regulates the PT TSH. In fish, the regulatory machinery for seasonal reproduction, from light input to neuroendocrine output, has been recently demonstrated in the coronet cells of the saccus vasculosus (SV). The SV is unique to fish and coronet cells are CSF-contacting neurons. Here, we discuss the universality and diversity of signal transduction pathways that regulate vertebrate seasonal reproduction.

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.

Seasonal reproduction is a survival strategy by which animals adapt to environmental changes to improve their fitness. Males are often characterized by a significantly reduced testicular volume, indicating that they are in an immature state. Although many hormones, including gonadotropins, have played a role in testicular development and spermatogenesis, research on other hormones is insufficient. The anti-Müllerian hormone (AMH), which is a hormone responsible for inducing the regression of Müllerian ducts involved in male sex differentiation, was discovered in 1953. Disorders in AMH secretion are the main biomarkers of gonadal dysplasia, indicating that it may play a crucial role in reproduction regulation. A recent study has found that the AMH protein is expressed at a high level during the non-breeding period of seasonal reproduction in animals, implying that it may play a role in restricting breeding activities. In this review, we summarize the research progress on the AMH gene expression, regulatory factors of the gene’s expression, and its role in reproductive regulation. Using males as an example, we combined testicular regression and the regulatory pathway of seasonal reproduction and attempted to identify the potential relationship between AMH and seasonal reproduction, to broaden the physiological function of AMH in reproductive suppression, and to provide new ideas for understanding the regulatory pathway of seasonal reproduction.