Early development of neuronal hypothalamic thermosensitivity in birds: influence of epigenetic temperature adaptation

Understanding the role of temperature in seasonal timing: Effects on behavioural, physiological and molecular phenotypes

Organisms adapt to daily and seasonal environmental changes to maximise their metabolic and reproductive fitness. For seasonally breeding animals, photoperiod is considered the most robust cue to drive these changes. It, however, does not explain the interannual variations in different seasonal phenotypes. Several studies have repeatedly shown the influence of ambient temperature on the timing of different seasonal physiologies including the timing of migration, reproduction and its associated behaviours, etc. In the present review, we have discussed the effects of changes in ambient temperature on different seasonal events in endotherms with a focus on migratory birds as they have evolved to draw benefits from distinct but largely predictable seasonal patterns of natural resources. We have further discussed the physiological and molecular mechanisms by which temperature affects seasonal timings. The primary brain area involved in detecting temperature changes is the hypothalamic preoptic area. This area receives thermal inputs via sensory neurons in the peripheral ganglia that measure changes in thermoregulatory tissues such as the skin and spinal cord. For the input signals, several thermal sensory TRP (transient receptor potential ion channels) channels have been identified across different classes of vertebrates. These channels are activated at specific thermal ranges. Once perceived, this information should activate an effector function. However, the link between temperature sensation and the effector pathways is not properly understood yet. Here, we have summarised the available information that may help us understand how temperature information is translated into seasonal timing.

Embryonic thermal manipulation of poultry birds: Lucrative and deleterious effects

The major efforts to improve feed conversion, increase the body weight and breast muscle yield of broilers have been focused on feeding and management at the post hatch period. However, incubation temperature is the most significant factor for the egg hatching rate, chick quality, and post hatch performance. Therefore, incubation factors affecting the performance should be taken with necessary precautions. Incubation temperature not only affects the early development of the hatchlings but also has a lasting impact on the characteristics of the chicks, such as final body weight and meat quality traits. This article provides an overview about embryonic thermal manipulation (TM) of domestic fowls and review the lucrative and deleterious effects of embryonic TM on embryo development, muscle growth, thermotolerance acquisition, and immunity.

Heat stress affects breathing and metabolism of chicks incubated at high temperature

Global warming poses serious implications to animal physiology and a gradual increase in ambient temperature affects all living organisms, particularly fast-growing selected species. We recorded ventilation (V̇_E), body temperature (T_B), oxygen consumption (V̇O₂) and respiratory equivalent (V̇_E/V̇O₂) of 14-day-old (14d) male and female chicks at room air conditions, hypercapnia and hypoxia at heat stress (HS, 32 °C). These chicks had previously been exposed to control (CI, 37.5°C) and high (HI, 39°C) temperatures during the first 5 days of incubation. Under resting conditions, acute HS increased V̇_E in HI females but not in HI males. Hypercapnia combined with heat promoted a potentiation of CO₂-hyperventilatory response in HI females when compared with thermoneutral condition, whereas in HI incubated males a hypoventilation under hypercapnia and heat stress was observed compared to the CI group. Hypoxia associated with heat stress increased V̇_E only in HI females. Our data indicates that females are more sensitive to thermal manipulation during incubation and it seems that the thermal embryonic manipulation, at least during the first days of development, does not improve the adaptive response of chicks to heat stress.

Differential expressions of hypothalamic thermosensitive TRP ion channels may underlie the posthatching ontogeny of brain cooling capacity in broiler chickens

Two trials were performed to evaluate the association of hypothalamic abundances of thermosensitive transient receptor potential (TRP) ion channels with thermoregulation in broiler chickens. In trial 1, temporal changes in body temperatures, and hypothalamic expression patterns of TRP channels and thermoregulatory neurotransmitter concentrations were assessed from 3 to 42 d of age. In trial 2, the same variables were compared at 2 age stages between 2 distinct types of birds with high or low rectal temperatures (HRT or LRT). The core-to-brain temperature difference exhibited a rapid increase after hatching, arriving at a steady state in the fourth week (P < 0.01). The hypothalamus saw a progressive decrease of TRPV4 protein expression through 28 d (P < 0.01), followed by a great increase in the abundance of other channels right up to the end (P < 0.05). Compared to LRT birds, a decline in hypothalamic content of TRPV4 (P < 0.05), together with a bigger core-to-brain temperature difference (P < 0.01), was evident in the HRT counterpart at 33 d. In both trials, the core-to-brain and core-to-surface temperature differences were controlled in a synchronous and coordinated manner. These results allow concluding that developmental changes in the thermal sensitivity of hypothalamic neurons, determined by brain cooling capacity, involve a neuro-genomic mechanism, which regulates the ratio between thermosensitive TRP ion channels to attain a lower proportion of TRPV4 in comparison with other channels.

Incubation Temperature and Lighting: Effect on Embryonic Development, Post-Hatch Growth, and Adaptive Response

During incubation, the content of the egg is converted into a chick. This process is controlled by incubation conditions, which must meet the requirements of the chick embryo to obtain the best chick quality and maximum hatchability. Incubation temperature and light are the two main factors influencing embryo development and post-hatch performance. Because chicken embryos are poikilothermic, embryo metabolic development relies on the incubation temperature, which influences the use of egg nutrients and embryo development. Incubation temperature ranging between 37 and 38°C (typically 37.5–37.8°C) optimizes hatchability. However, the temperature inside the egg called “embryo temperature” is not equal to the incubator air temperature. Moreover, embryo temperature is not constant, depending on the balance between embryonic heat production and heat transfer between the eggshell and its environment. Recently, many studies have been conducted on eggshell and/or incubation temperature to meet the needs of the embryo and to understand the embryonic requirements. Numerous studies have also demonstrated that cyclic increases in incubation temperature during the critical period of incubation could induce adaptive responses and increase the thermotolerance of chickens without affecting hatchability. Although the commercial incubation procedure does not have a constant lighting component, light during incubation can modify embryo development, physiology, and post-hatch behavior indicated by lowering stress responses and fearful behavior and improving spatial abilities and cognitive functions of chicken. Light-induced changes may be attributed to hemispheric lateralization and the entrainment of circadian rhythms in the embryo before the hatching. There is also evidence that light affects embryonic melatonin rhythms associated with body temperature regulation. The authors’ preliminary findings suggest that combining light and cyclic higher eggshell temperatures during incubation increases pineal aralkylamine N-acetyltransferase, which is a rate-limiting enzyme for melatonin hormone production. Therefore, combining light and thermal manipulation during the incubation could be a new approach to improve the resistance of broilers to heat stress. This review aims to provide an overview of studies investigating temperature and light manipulations to improve embryonic development, post-hatch growth, and adaptive stress response in chickens.

Effects of hypoxic conditions during the plateau period on pre- and posthatch broiler performance

Adequate ambient temperature and oxygenation are necessary to maintain normal embryonic development of broilers; however, hypoxia challenge during incubation can aid in improving regulatory plasticity and lead to different phenotypes later in life. This study aimed to examine the effects of moderate hypoxia (O2 17%) during the plateau phase on the embryonic physiological parameters and on posthatch performance (growth rate, feed consumption and feed conversion) up to the age of poultry marketing. The study included examined embryos exposed to O2 17% for 12 h per day (h/d) from E16 through E18 (designated as 12H), or O2 17% continuously, from E16 through E17 (designated as 48H) and a standard incubation control group (21% O2). Physiological and morphological parameters of embryos and hatched chicks were measured. Male Chicks from all 3 treatment groups were raised under recommended temperature regime, and body weight, feed intake and FCR were recorded on a weekly basis. The intermittent hypoxia protocol (12H), allowed embryos to properly adapt to the shortage of oxygen, compensate for the gap in body mass that developed following the first exposure window, and hatch with characteristics similar to those of the control embryos. In contrast, while the 48H embryos were able to adapt to the hypoxic stress, the prolonged exposure prevented them from catching up with both control and 12H embryos. Broilers that were subjected to hypoxia showed hatchling body weights and growth rates similar to those of controls, throughout the entire growth phase. During the fifth wk, lower feed consumption was observed in the 12H and 48H groups and became significantly lower than the control chicks in the sixth wk of growth. Following hypoxia exposure, chicks managed to reach normal body weight with less feed, with the 12H group demonstrating lower and more efficient FCR during the last 2 wk of growth. Broiler embryos reacted to plateau-phase hypoxia challenge with numerous physiological and metabolic modifications. The prudent alterations in metabolism and cardiovascular system during exposure to hypoxia and posthatch, resulted in more efficient energy utilization in broilers, which may have a long-lasting enhancing effect on posthatching thermotolerance and sustainability in chicks reared under sub-optimal environmental conditions.

Early-life thermal stress mediates long-term alterations in hypothalamic microglia

Stressful environmental events in early life have long-lasting consequences on later stress responses. We previously showed that heat conditioning of 3-day-old chicks during the critical period of heat-response development leads to heat vulnerability later in life. Here we assessed the role of early-life heat stress on the inflammatory response in the chick anterior hypothalamus (AH), focusing on hypothalamic microglia. We identified the microglial cell population in the chick AH using anti-KUL01 and anti-CD45 antibodies. Specific microglial features were also confirmed by expression of their signature genes. Under normal environmental conditions, hypothalamic microglia isolated from lipopolysaccharide (LPS)-injected chicks displayed a classical activated proinflammatory profile accompanied by a decreased homeostatic signature, demonstrating similarity of immune response with mammalian microglial cells. In accordance with our previous observations, conditioning of 3-day-old chicks under high ambient temperature decreased the number of newborn cells in the AH, among them microglial precursors. Although heat exposure did not affect microglial cell viability, it had a long-term impact on LPS-induced inflammatory response. Exposure to harsh heat led to heat vulnerability, and attenuated recruitment of peripheral monocytes and T cells into the AH following LPS challenge. Moreover, heat conditioning altered microglial reactivity, manifested as suppressed microglial activation in response to LPS. Innate immune memory developed by heat conditioning might underlie suppression of the microglial response to LPS challenge. We describe alterations in genome-wide CpG methylation profile of hypothalamic microglia, demonstrating probable epigenetic involvement in the reprogramming of microglial function, leading to heat-induced inflammatory cross-tolerance.

Heat Stress Responses in Birds: A Review of the Neural Components

Heat stress is one of the major environmental conditions causing significant losses in the poultry industry and having negative impacts on the world's food economy. Heat exposure causes several physiological impairments in birds, including oxidative stress, weight loss, immunosuppression, and dysregulated metabolism. Collectively, these lead not only to decreased production in the meat industry, but also decreases in the number of eggs laid by 20%, and overall loss due to mortality during housing and transit. Mitigation techniques have been discussed in depth, and include changes in air flow and dietary composition, improved building insulation, use of air cooling in livestock buildings (fogging systems, evaporation panels), and genetic alterations. Most commonly observed during heat exposure are reduced food intake and an increase in the stress response. However, very little has been explored regarding heat exposure, food intake and stress, and how the neural circuitry responsible for sensing temperatures mediate these responses. That thermoregulation, food intake, and the stress response are primarily mediated by the hypothalamus make it reasonable to assume that it is the central hub at which these systems interact and coordinately regulate downstream changes in metabolism. Thus, this review discusses the neural circuitry in birds associated with thermoregulation, food intake, and stress response at the level of the hypothalamus, with a focus on how these systems might interact in the presence of heat exposure.

Embryonic Thermal Manipulation Affects Ventilation, Metabolism, Thermal Control and Central Dopamine in Newly Hatched and Juvenile Chicks

The first third of incubation is critical for embryonic development, and environmental changes during this phase can affect the physiology and survival of the embryos. We evaluated the effects of low (LT), control (CT), and high (HT) temperatures during the first 5 days of incubation on ventilation (V._E), body temperature (Tb), oxygen consumption (V.O₂), respiratory equivalent (V._E/V.O₂), and brain monoamines on 3-days-old (3d) and 14-days-old (14d) male and female chickens. The body mass of LT animals of both ages and sexes was higher compared to HT and CT animals (except for 3d males). The heart mass of 14d HT animals was higher than that of CT animals. Thermal manipulation did not affect V._E, V.O₂ or V._E/V.O₂ of 3d animals in normoxia, except for 3d LT males V._E, which was lower than CT. Regarding 14d animals, the HT females showed a decrease in V._E and V.O₂ compared to CT and LT groups, while the HT males displayed a lower V.O₂ compared to CT males, but no changes in V._E/V.O₂. Both sexes of 14d HT chickens presented a greater Tb compared to CT animals. Thermal manipulations increased the dopamine turnover in the brainstem of 3d females. No differences were observed in ventilatory and metabolic parameters in the 3d animals of either sexes, and 14d males under 7% CO₂. The hypercapnic hyperventilation was attenuated in the 14d HT females due to changes in V.O₂, without alterations in V._E. The 14d LT males showed a lower V._E, during hypercapnia, compared to CT, without changes in V.O₂, resulting in an attenuation in V._E/V.O₂. During hypoxia, 3d LT females showed an attenuated hyperventilation, modulated by a higher V.O₂. In 14d LT and HT females, the increase in V._E was greater and the hypometabolic response was attenuated, compared to CT females, which resulted in no change in the V._E/V.O₂. In conclusion, thermal manipulations affect hypercapnia-induced hyperventilation more so than hypoxic challenge, and at both ages, females are more affected by thermal manipulation than males.

Embryonic thermal manipulation of Japanese quail: effects on embryonic development, hatchability, and post-hatch performance

Embryonic thermal manipulation led to several modifications in molecular, physiological, and biochemical parameters which affect pre- and post-hatch growth performance. The current study aims to elucidate the onset and long-term effects of intermittent thermal manipulations (TM) during two-time windows, early/late, of embryogenesis in Japanese quail (Coturnix japonica) on embryonic development, hatchability, muscle histogenesis, and post-hatch growth performance. Four groups were created; quail eggs in the control group were incubated at 37.7 °C and relative humidity (RH) 55%. Three thermally treated groups were incubated intermittently at 41 °C and 65% RH intermittently (3 h/day): early embryogenesis group (EE) was thermally treated during embryonic days (ED) 6-8, late embryogenesis group (LE) was thermally treated during (ED12-ED14), and early and late embryogenesis group (EL) was thermally manipulated in both time windows. Relative embryo weights in EL and EE were significantly lighter than those in LE and Ctrl groups. The hatched chicks were reared under optimal managemental conditions (three replicates per treatment). Average daily feed intake was recorded, and feed conversion ratio (FCR) was calculated. Histological and quantitative analyses of muscle fibers were performed. The results revealed that TM led to significant hypertrophy of quail breast muscle in (EE). Intermittent short-term (3-6 h) thermal manipulation (39-40 °C) protocols during early embryogenesis (ED6-ED8) could be recommended to enhance muscle mass growth and breast muscle yield in the Japanese quail.

Weaning Induces Stress-Dependent DNA Methylation and Transcriptional Changes in Piglet PBMCs

Changes to the epigenome, including those to DNA methylation, have been proposed as mechanisms by which stress can induce long-term physiological changes in livestock species. Pig weaning is associated with dietary and social stress, both of which elicit an immune response and changes to the hypothalamic–pituitary–adrenal (HPA) axis. While differential methylation following stress has been assessed in model organisms, it remains poorly understood how the pig methylome is altered by stressors in production settings. We quantified changes in CpG methylation and transcript abundance in piglet peripheral blood mononuclear cells (PBMCs) following weaning and also assessed differential patterns in pigs exhibiting high and low stress response as measured by cortisol concentration and lesion scores. Blood was collected from nine gilt piglets 24 h before and after weaning, and whole-genome bisulfite sequencing (WGBS) and RNA-sequencing were performed on six and nine animals, respectively, at both time points. We identified 2,674 differentially methylated regions (DMRs) that were enriched within promoters of genes associated with lymphocyte stimulation and transcriptional regulation. Stress groups displayed unique differential methylation and expression patterns associated with activation and suppression of T cell immunity in low and high stress animals, respectively. Differential methylation was strongly associated with differential expression; specifically, upregulated genes were enriched among hypomethylated genes. We observed post-weaning hypermethylation of the glucocorticoid receptor (NR3C1) promoter and a significant decrease in NR3C1 expression (n = 9, p = 6.1 × 10^–3). Our results indicate that weaning-associated stress elicits genome-wide methylation changes associated with differential gene expression, reduced T cell activation, and an altered HPA axis response.

Regulation of gene expression in chickens by heat stress

Abstract

High ambient temperatures are a critical challenge in the poultry industry which is a key producer of the animal-based food. To evaluate heat stress levels, various parameters have been used, including growth rates, blood metabolites, and hormones. The most recent advances have explored expression profiling of genes that may play vital roles under stress. A high ambient temperature adversely affects nutrient uptake and is known to modulate the expression of genes encoding for sodium-dependent glucose transporters, glucose transporters, excitatory amino acid transporters, and fatty acid-binding proteins which are responsible for the absorption of macronutrients in the intestine. Various defensive activities are stimulated to protect the cell of different tissues from the heat-generated stress, including expression of early stress response genes coding for heat shock protein (HSP), c-FOS like protein, brain-derived neurotrophic factor (BDNF), and neuronal nitric oxide synthase (nNOS); antioxidant enzyme genes such as superoxide dismutase (SOD), catalase (CAT), and nicotinamide adenine dinucleotide phosphate oxidase (NOX4); and immune-related genes such as cytokines and toll-like receptors (TLRs). The potential role of HSPs in protecting the cell from stress and their presence in several tissues make them suitable markers to be evaluated under heat stress. BDNF and c-FOS genes expressed in the hypothalamus help cells to adapt to an adverse environment. Heat causes damage to the cell by generating reactive oxygen species (ROS). The NOX4 gene is the inducer of ROS under heat stress, which is in turns controlled by antioxidant enzymes such as SOD and CAT. TLRs are responsible for protecting against pathogenic attacks arising from enhanced membrane permeability, and cytokines help in controlling the pathogen and maintaining homeostasis. Thus, the evaluation of nutrient transporters and defense mechanisms using the latest molecular biology tools has made it possible to shed light on the complex cellular mechanism of heat-stressed chickens. As the impacts of heat stress on the above-mentioned aspects are beyond the extent to which the reduced growth performance could be explained, heat stress has more specific effects on the regulation of these genes than previously thought.

Graphical abstract

Effect of heat exposure on the nutrient transporters, antioxidants, and immune inflammation in chickens. Most of the nutrient transporters were suppressed under heat stress. Increase in the production of reactive oxygen species resulted in enhanced production of antioxidant enzymes. Expression of various proinflammatory cytokines and toll-like receptors were enhanced due to heat stress in chicken.

Endocrinology of thermoregulation in birds in a changing climate

The ability to maintain a (relatively) stable body temperature in a wide range of thermal environments by use of endogenous heat production is a unique feature of endotherms such as birds. Endothermy is acquired and regulated via various endocrine and molecular pathways, and ultimately allows wide aerial, aquatic, and terrestrial distribution in variable environments. However, due to our changing climate, birds are faced with potential new challenges for thermoregulation, such as more frequent extreme weather events, lower predictability of climate, and increasing mean temperature. We provide an overview on thermoregulation in birds and its endocrine and molecular mechanisms, pinpointing gaps in current knowledge and recent developments, focusing especially on non-model species to understand the generality of, and variation in, mechanisms. We highlight plasticity of thermoregulation and underlying endocrine regulation, because thorough understanding of plasticity is key to predicting responses to changing environmental conditions. To this end, we discuss how changing climate is likely to affect avian thermoregulation and associated endocrine traits, and how the interplay between these physiological processes may play a role in facilitating or constraining adaptation to a changing climate. We conclude that while the general patterns of endocrine regulation of thermogenesis are quite well understood, at least in poultry, the molecular and endocrine mechanisms that regulate, e.g. mitochondrial function and plasticity of thermoregulation over different time scales (from transgenerational to daily variation), need to be unveiled. Plasticity may ameliorate climate change effects on thermoregulation to some extent, but the increased frequency of extreme weather events, and associated changes in resource availability, may be beyond the scope and/or speed for plastic responses. This could lead to selection for more tolerant phenotypes, if the underlying physiological traits harbour genetic and individual variation for selection to act on - a key question for future research.

Emerging Genetic Tools to Investigate Molecular Pathways Related to Heat Stress in Chickens: A Review

Simple Summary

New genomic tools have been used as an instrument in order to assess the molecular pathway involved in heat stress resistance. Local chicken breeds have a better attitude to face heat stress. This review aims to summarize studies linked to chickens, heat stress, and heat shock protein.

Abstract

Chicken products are the most consumed animal-sourced foods at a global level across greatly diverse cultures, traditions, and religions. The consumption of chicken meat has increased rapidly in the past few decades and chicken meat is the main animal protein source in developing countries. Heat stress is one of the environmental factors which decreases the productive performance of poultry and meat quality. Heat stress produces the over-expression of heat shock factors and heat shock proteins in chicken tissues. Heat shock proteins regulate several molecular pathways in cells in response to stress conditions, changing the homeostasis of cells and tissues. These changes can affect the physiology of the tissue and hence the production ability of chickens. Indeed, commercial chicken strains can reach a high production level, but their body metabolism, being comparatively accelerated, has poor thermoregulation. In contrast, native backyard chickens are more adapted to the environments in which they live, with a robustness that allows them to survive and reproduce constantly. In the past few years, new molecular tools have been developed, such as RNA-Seq, Single Nucleotide Polymorphisms (SNPs), and bioinformatics approaches such as Genome-Wide Association Study (GWAS). Based on these genetic tools, many studies have detected the main pathways involved in cellular response mechanisms. In this context, it is necessary to clarify all the genetic and molecular mechanisms involved in heat stress response. Hence, this paper aims to review the ability of the new generation of genetic tools to clarify the molecular pathways associated with heat stress in chickens, offering new perspectives for the use of these findings in the animal breeding field.

Genome-Wide Assessment of DNA Methylation in Chicken Cardiac Tissue Exposed to Different Incubation Temperatures and CO2 Levels

Temperature and CO₂ concentration during incubation have profound effects on broiler chick development, and numerous studies have identified significant effects on hatch heart weight (HW) as a result of differences in these parameters. Early life environment has also been shown to affect broiler performance later in life; it has thus been suggested that epigenetic mechanisms may mediate long-term physiological changes induced by environmental stimuli. DNA methylation is an epigenetic modification that can confer heritable changes in gene expression. Using reduced-representation bisulfite sequencing (RRBS), we assessed DNA methylation patterns in cardiac tissue of 84 broiler hatchlings incubated at two egg shell temperatures (EST; 37.8°C and 38.9°C) and three CO₂ concentrations (0.1%, 0.4%, and 0.8%) from day 8 of incubation onward. We assessed differential methylation between EST treatments and identified 2,175 differentially methylated (DM) CpGs (1,121 hypermethylated, 1,054 hypomethylated at 38.9° vs. 37.8°) in 269 gene promoters and 949 intragenic regions. DM genes (DMGs) were associated with heart developmental processes, including cardiomyocyte proliferation and differentiation. We identified enriched binding motifs among DM loci, including those for transcription factors associated with cell proliferation and heart development among hypomethylated CpGs that suggest increased binding ability at higher EST. We identified 9,823 DM CpGs between at least two CO₂ treatments, with the greatest difference observed between 0.8 and 0.1% CO₂ that disproportionately impacted genes involved in cardiac muscle development and response to low oxygen levels. Using HW measurements from the same chicks, we performed an epigenome-wide association study (EWAS) for HW, and identified 23 significantly associated CpGs, nine of which were also DM between ESTs. We found corresponding differences in transcript abundance between ESTs in three DMGs (ABLIM2, PITX2, and THRSP). Hypomethylation of an exonic CpG in PITX2 at 38.9°C was associated with increased expression, and suggests increased cell proliferation in broiler hatchlings incubated at higher temperatures. Overall, these results identified numerous epigenetic associations between chick incubation factors and heart development that may manifest in long-term differences in animal performance.

Early-Life m6A RNA Demethylation by Fat Mass and Obesity-Associated Protein (FTO) Influences Resilience or Vulnerability to Heat Stress Later in Life

Early life heat stress leads to either resilience or vulnerability to a similar stress later in life. We have previously shown that this tuning of the stress response depends on neural network organization in the preoptic anterior hypothalamus (PO/AH) thermal response center and is regulated by epigenetic mechanisms. Here, we expand our understanding of stress response establishment describing a role for epitranscriptomic regulation of the epigenetic machinery. Specifically, we explore the role of N⁶-methyladenosine (m⁶A) RNA methylation in long-term response to heat stress. Heat conditioning of 3-d-old chicks diminished m⁶A RNA methylation in the hypothalamus, simultaneously with an increase in the mRNA levels of the m⁶A demethylase, fat mass and obesity-associated protein (FTO). Moreover, a week later, methylation of two heat stress-related transcripts, histone 3 lysine 27 (H3K27) methyltransferase, enhancer of zeste homolog 2 (EZH2) and brain-derived neurotrophic factor (BDNF), were downregulated in harsh-heat-conditioned chicks. During heat challenge a week after conditioning, there was a reduction of m⁶A levels in mild-heat-conditioned chicks and an elevation in harsh-heat-conditioned ones. This increase in m⁶A modification was negatively correlated with the expression levels of both BDNF and EZH2. Antisense “knock-down” of FTO caused an elevation of global m⁶A RNA methylation, reduction of EZH2 and BDNF mRNA levels, and decrease in global H3K27 dimethylation as well as dimethyl H3K27 level along BDNF coding region, and, finally, led to heat vulnerability. These findings emphasize the multilevel regulation of gene expression, including both epigenetic and epitranscriptomic regulatory mechanisms, fine-tuning the neural network organization in a response to stress.

Effect of heat stress on mitogen-activated protein kinases in the hypothalamic-pituitary-gonadal axis of developing Wenchang chicks

The regulatory roles of mitogen-activated protein kinase (MAPK) signaling pathways on the hypothalamic-pituitary-gonadal axis (HPG axis) of Wenchang chicks under heat stress (HS) were investigated. Additionally, the crosstalk between these signaling pathways was explored. Immunohistochemical experiments and Western blotting were employed to quantify extracellular regulated protein kinases (ERK), c-Jun N-terminal kinases (JNK), and p38MAPK (P38). In female chicks, hypothalamic ERKs were upregulated in Weeks 1 and 2 in the HS group compared with the control group (CK), while JNK and p38 were downregulated (P < 0.05). Pituitary MAPKs were all downregulated in the HS group compared with the CK group in Week 3, but p38 was upregulated in Week 4 (P < 0.01). In the HS group, ovarian MAPKs were all upregulated compared with the CK group during Week 5, whereas ERK was downregulated in Week 6 (P < 0.01). In contrast to the patterns of MAPK expression in female chicks in the HS and CK groups, ERK in male chicks showed a completely opposite pattern in Weeks 1, 2, and 5, while p38 and JNK were downregulated in both female and male chicks under HS during Weeks 2 and 3. In the HS group, pituitary and testis MAPKs showed a pattern opposite to that observed in female chicks under HS in Week 5; MAPKs were all downregulated (P < 0.05). Thus, there are gender differences in the MAPK signaling pathways in the HPG axis in chicks, and these pathways showed plasticity. Early HS can enhance chick growth and development as well as promote developing in the MAPK signaling pathways in the HPG axis. However, after heated brooding was discontinued in chicks, long-term HS obstructed chick development and caused tissue and function injury to the HPG axis.

Early-life epigenetic changes along the corticotropin-releasing hormone (CRH) gene influence resilience or vulnerability to heat stress later in life

Stressful events in early life might lead to stress resilience or vulnerability, depending on an adjustable stress-response set-point, which can be altered during postnatal sensory development and involves epigenetic regulation of corticotropin-releasing hormone (CRH). During the critical developmental period of thermal-control establishment in 3-day-old chicks, heat stress was found to affect both body temperature and expression of CRH in the hypothalamic paraventricular nucleus. Both increased during heat challenge in vulnerable chicks, whereas they decreased in resilient chicks. Our aim was to elucidate the epigenetic mechanism underlying the regulation of stress resilience or vulnerability. Accordingly, DNA CpG methylation (5mC) and hydroxymethylation (5hmC) at the CRH intron, which we found to serve as a repressor element, displayed low 5mc% alongside high 5hmc% in resilient chicks, and high 5mc% with low 5hmc% in vulnerable ones. RE1-silencing transcription factor (REST), which has a binding site on this intron, bound abundantly during acute heat stress and was nearly absent during moderate stress, restricting repression by the repressor element, and thus activating CRH gene transcription. Furthermore, REST assembled into a protein complex with TET3, which bound directly to the CRH gene. Finally, the adjacent histone recruited the histone acetylation enzyme GCN5 to this complex, which increased H3K27ac during harsh, but not moderate heat conditioning. We conclude that an epigenetic mechanism involving both post-translational histone modification and DNA methylation in a regulatory segment of CRH is involved in determining a resilient or vulnerable response to stress later in life.

Insulin receptor localization in the embryonic avian hypothalamus

The hypothalamus is a brain region critical for the homeostatic regulation of appetite and energy expenditure. Hypothalamic neuronal activity that is altered during development can produce permanent physiological changes later in life. For example, circulating hormones such as insulin have been shown to influence hypothalamic neuronal projections, leading to altered metabolism in adult rodents. While insulin signaling in the post-hatch chicken has been shown to mirror that of mammals, the developmental role of insulin in the avian embryonic hypothalamus remains largely unexplored. Here we present the earliest known evidence for insulin receptor (InsR) expression in embryonic avian hypothalamic nuclei governing energy homeostasis. RT-PCR analysis reveals InsR mRNA in E8, E10, and E12 neurons while western blot data demonstrate protein expression in E12 avian whole brain and hypothalamic lysates. Immunohistochemical analysis of avian hypothalamic brain slices demonstrates a shift in InsR localization from paraventricular expression in E8 to a more defined concentration of InsR in developmental regions resembling the ventromedial hypothalamus (VMH) and arcuate nucleus (ARC) in E12 time points. In addition, InsR expression appears in a heterogeneous pattern, suggesting receptor localization to subpopulations of avian hypothalamic neurons as early as E8. With increasing evidence suggesting energy homeostasis pathways may be altered via the gestational environment, it is important to understand how insulin signaling may affect embryogenesis. Our research provides evidence for the earliest known embryonic expression of InsR protein in the avian hypothalamus and may suggest a developmental role for insulin signaling in the early patterning of metabolic pathways in the central nervous system.

PARP Inhibitor Affects Long-term Heat-stress Response via Changes in DNA Methylation

Resilience to stress can be obtained by adjusting the stress-response set point during postnatal sensory development. Recent studies have implemented epigenetic mechanisms to play leading roles in improving resilience. We previously found that better resilience to heat stress in chicks can be achieved by conditioning them to moderate heat stress during their critical developmental period of thermal control establishment, 3 days posthatch. Furthermore, the expression level of corticotropin-releasing hormone (CRH) was found to play a direct role in determining future resilience or vulnerability to heat stress by alterations in its DNA-methylation and demethylation pattern. Here we demonstrate how intraperitoneal injection of poly (ADP-ribose) polymerase (PARP) inhibitor (PARPi) influences the DNA methylation pattern, thereby affecting the long-term heat-stress response. Single PARPi administration, induced a reduction in both 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC), without affecting body temperature. The accumulated effect of three PARPi doses brought about a long-term decrease in 5mC% and 5hmC%. These changes coincided with a reduction in body temperature in non-conditioned chicks, similar to that occurring in moderately conditioned heat-stress-resilient chicks. The observed changes in DNA methylation can be explained by decreased activity of the enzyme DNA methyltransferase as a result of the PARPi injection. Furthermore, evaluation of the DNA-methylation pattern along the CRH intron showed a reduction in 5mC% as a result of PARPi treatment, alongside a reduction in CRH mRNA expression. Thus, PARPi treatment can affect DNA methylation, which can alter hypothalamic-pituitary-adrenal (HPA) axis anchors such as CRH, thereby potentially enhancing long-term resilience to heat stress.

Epigenetic Programming Effects of Early Life Stress: A Dual-Activation Hypothesis

Epigenetic processes during early brain development can function as 'developmental switches' that contribute to the stability of long-term effects of early environmental influences by programming central feedback mechanisms of the HPA axis and other neural networks. In this thematic review, we summarize accumulated evidence for a dual-activation of stress-related and sensory networks underlying the epigenetic programming effects of early life stress. We discuss findings indicating epigenetic programming of stress-related genes with impact on HPA axis function, the interaction of epigenetic mechanisms with neural activity in stress-related neural networks, epigenetic effects of glucocorticoid exposure, and the impact of stress on sensory development. Based on these findings, we propose that the combined activation of stress-related neural networks and stressor-specific sensory networks leads to both neural and hormonal priming of the epigenetic machinery, which sensitizes these networks for developmental programming effects. This allows stressor-specific adaptations later in life, but may also lead to functional mal-adaptations, depending on timing and intensity of the stressor. Finally, we discuss methodological and clinical implications of the dual-activation hypothesis. We emphasize that, in addition to modifications in stress-related networks, we need to account for functional modifications in sensory networks and their epigenetic underpinnings to elucidate the long-term effects of early life stress.

Cold tolerance of the Asian tiger mosquito Aedes albopictus and its response to epigenetic alterations

Phenotypic plasticity is considered as one of the key traits responsible for the establishment of populations of the invasive mosquito Aedes albopictus, an important vector of viral and parasitic pathogens. The successful spread of this species to higher altitudes and latitudes may be explained by its ability to rapidly induce a heritable low-temperature phenotype (cold hardiness in eggs). As a result of the low genetic diversity of founder populations, an epigenetic short-term mechanism has been suggested as the driver of this diversification. We investigated if random epigenetic alterations promoted the cold hardiness of Ae. albopictus eggs from a transgenerational study of two epigenetic agents (genistein and vinclozolin). To this end, we evaluated changes in lethal time for 50% of pharate larvae (Lt₅₀) from eggs exposed to -2°C in two subsequent generations that used a new dose-response test design. We detected a significant diversification of the cold hardiness of eggs (up to 64.5%) that was associated with the epigenetic change in the two subsequent offspring generations. An effect size of epigenetically modulated cold hardiness of this magnitude is likely to have an impact on the spatial distribution of this species. Our results provide a framework for further research on epigenetic temperature adaptation of invasive species to better explain and predict their rapid range expansions.

Thermal manipulation of the chicken embryo triggers differential gene expression in response to a later heat challenge

Background

Meat type chickens have limited capacities to cope with high environmental temperatures, this sometimes leading to mortality on farms and subsequent economic losses. A strategy to alleviate this problem is to enhance adaptive capacities to face heat exposure using thermal manipulation (TM) during embryogenesis. This strategy was shown to improve thermotolerance during their life span. The aim of this study was to determine the effects of TM (39.5 °C, 12 h/24 vs 37.8 °C from d7 to d16 of embryogenesis) and of a subsequent heat challenge (32 °C for 5 h) applied on d34 on gene expression in the Pectoralis major muscle (PM). A chicken gene expression microarray (8 × 60 K) was used to compare muscle gene expression profiles of Control (C characterized by relatively high body temperatures, Tb) and TM chickens (characterized by a relatively low Tb) reared at 21 °C and at 32 °C (CHC and TMHC, respectively) in a dye-swap design with four comparisons and 8 broilers per treatment. Real-time quantitative PCR (RT-qPCR) was subsequently performed to validate differential expression in each comparison. Gene ontology, clustering and network building strategies were then used to identify pathways affected by TM and heat challenge.

Results

Among the genes differentially expressed (DE) in the PM (1.5 % of total probes), 28 were found to be differentially expressed between C and TM, 128 between CHC and C, and 759 between TMHC and TM. No DE gene was found between TMHC and CHC broilers. The majority of DE genes analyzed by RT-qPCR were validated. In the TM/C comparison, DE genes were involved in energy metabolism and mitochondrial function, cell proliferation, vascularization and muscle growth; when comparing heat-exposed chickens to their own controls, TM broilers developed more specific pathways than C, especially involving genes related to metabolism, stress response, vascularization, anti-apoptotic and epigenetic processes.

Conclusions

This study improved the understanding of the long-term effects of TM on PM muscle. TM broilers displaying low Tb may have lower metabolic intensity in the muscle, resulting in decreased metabolic heat production, whereas modifications in vascularization may enhance heat loss. These specific changes could in part explain the better adaptation of TM broilers to heat.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2661-y) contains supplementary material, which is available to authorized users.

Developmental expression of Kv1 voltage-gated potassium channels in the avian hypothalamus

Specialized hypothalamic neurons integrate the homeostatic balance between food intake and energy expenditure, processes that may become dysregulated during the development of diabetes, obesity, and other metabolic disorders. Shaker family voltage-gated potassium channels (Kv1) contribute to the maintenance of resting membrane potential, action potential characteristics, and neurotransmitter release in many populations of neurons, although hypothalamic Kv1 channel expression has been largely unexplored. Whole-cell patch clamp recordings from avian hypothalamic brain slices demonstrate a developmental shift in the electrophysiological properties of avian arcuate nucleus neurons, identifying an increase in outward ionic current that corresponds with action potential maturation. Additionally, RT-PCR experiments identified the early expression of Kv1.2, Kv1.3, and Kv1.5 mRNA in the embryonic avian hypothalamus, suggesting that these channels may underlie the electrophysiological changes observed in these neurons. Real-time quantitative PCR analysis on intact microdissections of embryonic hypothalamic tissue revealed a concomitant increase in Kv1.2 and Kv1.5 gene expression at key electrophysiological time points during development. This study is the first to demonstrate hypothalamic mRNA expression of Kv1 channels in developing avian embryos and may suggest a role for voltage-gated ion channel regulation in the physiological patterning of embryonic hypothalamic circuits governing energy homeostasis.

Detection of long-term influence of prenatal temperature stimulation on hypothalamic type-II iodothyronine deiodinase in juvenile female broiler chickens using a novel immunohistochemical amplification protocol

It has been clearly shown that early environmental stimulation may have long-lasting influence on body functions. Because of the strong relationship between thermoregulation and other homeostatic linked physiological parameters, perinatal thermal manipulation will also have an impact on other body functions like reproduction. As a maturation stimulant for later reproductive performance, hypothalamic type-2 iodothyronine deiodinase (Dio2) expression was investigated in 35day old immature female broilers with and without embryonic temperature stimulation. For the first time, human-specific Dio2 primary antibodies combined with additional amplification enabled the immunohistochemical detection of hypothalamic Dio2 protein in birds. The novel protocol includes an additional amplification step involving swine-anti-rabbit/mouse/goat antibodies against both goat anti-Dio2 primary and rabbit anti-goat biotinylated secondary commercial antibodies in the standard diaminobenzidine protocol. However, significant Dio2 expression was exclusively found in perinatally short-term temperature stimulated hens. Caudal but not rostral hypothalamic slices revealed that elevating incubation temperature by 1°C for 2h daily, from day 18 of embryonic development until hatching, induced a statistical significant expression of Dio2 within the subcommisural organ and the median eminence. This ample expression of Dio2 protein within caudal but not rostral hypothalamic slices of embryonic temperature stimulated chickens, leads to the assumption of a novel physiological prospective for embryonic thermal manipulation involving the suppression of thyroid hormone and the boosting of hypothalamic Dio2-induced FSH secretion to considerably advance the age of photoinduced egg production. It could be also of practicable relevance for broiler breeder females, and needs further investigations.

Cyclic variations in incubation conditions induce adaptive responses to later heat exposure in chickens: a review

Selection programs have enabled broiler chickens to gain muscle mass without similar enlargement of the cardiovascular and respiratory systems that are essential for thermoregulatory efficiency. Meat-type chickens cope with high ambient temperature by reducing feed intake and growth during chronic and moderate heat exposure. In case of acute heat exposure, a dramatic increase in morbidity and mortality can occur. In order to alleviate heat stress in the long term, research has recently focused on early thermal manipulation. Aimed at stimulation of long-term thermotolerance, the thermal manipulation of embryos is a method based on fine tuning of incubation conditions, taking into account the level and duration of increases in temperature and relative humidity during a critical period of embryogenesis. The consequences of thermal manipulation on the performance and meat quality of broiler chickens have been explored to ensure the potential application of this strategy. The physiological basis of the method is the induction of epigenetic and metabolic mechanisms that control body temperature in the long term. Early thermal manipulation can enhance poultry resistance to environmental changes without much effect on growth performance. This review presents the main strategies of early heat exposure and the physiological concepts on which these methods were based. The cellular mechanisms potentially underlying the adaptive response are discussed as well as the potential interest of thermal manipulation of embryos for poultry production.

Exposure of embryos to cyclically cold incubation temperatures durably affects energy metabolism and antioxidant pathways in broiler chickens

Cyclically cold incubation temperatures have been suggested as a means to improve resistance of broiler chickens to ascites; however, the underlying mechanisms are not known. Nine hundred eggs obtained from 48 wk Ross broiler breeders were randomly assigned to 2 incubation treatments: control I eggs were incubated at 37.6°C throughout, whereas for cold I eggs the incubation temperature was reduced by 1°C for 6 h daily from 10 to 18 d of incubation. Thereafter, chickens were reared at standard temperatures or under cold exposure that was associated or not with a postnatal cold acclimation at d 5 posthatch. At hatch, hepatic catalase activity and malondialdehyde content were measured. Serum thyroid hormone and triglyceride concentrations, and muscle expression of several genes involved in the regulation of energy metabolism and oxidative stress were also measured at hatch and 5 and 25 d posthatch. Cold incubation induced modifications in antioxidant pathways with higher catalase activity, but lower expression of avian uncoupling protein 3 at hatch. However, long-term enhancement in the expression of avian uncoupling protein 3 was observed, probably caused by an increase in the expression of the transcription factor peroxisome proliferator activated receptor-γ coactivator-1α. These effects were not systematically associated with an increase in serum triiodothyronine concentrations that were observed only in chickens exposed to both cold incubation and later acclimation at 5 d with cold rearing. Our results suggest that these conditions of cyclically cold incubation resulted in the long-term in changes in antioxidant pathways and energy metabolism, which could enhance the health of chickens reared under cold conditions.

Developmental specificity in skeletal muscle of late-term avian embryos and its potential manipulation

Unlike the mammalian fetus, development of the avian embryo is independent of the maternal uterus and is potentially vulnerable to physiological and environmental stresses close to hatch. In contrast to the fetus of late gestation in mammals, skeletal muscle in avian embryos during final incubation shows differential developmental characteristics: 1) muscle mobilization (also called atrophy) is selectively enhanced in the type II fibers (pectoral muscle) but not in the type I fibers (biceps femoris and semimembranosus muscle), involving activation of ubiquitin-mediated protein degradation and suppression of S6K1-mediated protein translation; 2) the proliferative activity of satellite cells is decreased in the atrophied muscle of late-term embryos but enhanced at the day of hatch, probably preparing for the postnatal growth. The mobilization of muscle may represent an adaptive response of avian embryos to external (environmental) or internal (physiological) changes, considering there are developmental transitions both in hormones and requirements for glycolytic substrates from middle-term to late-term incubation. Although the exact mechanism triggering muscle fiber atrophy is still unknown, nutritional and endocrine changes may be of importance. The atrophied muscle fiber recovers as soon as feed and water are available to the hatchling. In ovo feeding of late-term embryos has been applied to improve the nutritional status and therein enhances muscle development. Similarly, in ovo exposure to higher temperature or green light during the critical period of muscle development are also demonstrated to be potential strategies to promote pre- and posthatch muscle growth.

Epigenetics and phenotypic variability: some interesting insights from birds

Little is known about epigenetic mechanisms in birds with the exception of the phenomenon of dosage compensation of sex chromosomes, although such mechanisms could be involved in the phenotypic variability of birds, as in several livestock species. This paper reviews the literature on epigenetic mechanisms that could contribute significantly to trait variability in birds, and compares the results to the existing knowledge of epigenetic mechanisms in mammals. The main issues addressed in this paper are: (1) Does genomic imprinting exist in birds? (2) How does the embryonic environment influence the adult phenotype in avian species? (3) Does the embryonic environment have an impact on phenotypic variability across several successive generations? The potential for epigenetic studies to improve the performance of individual animals through the implementation of limited changes in breeding conditions or the addition of new parameters in selection models is still an open question.

Thermal manipulation of the embryo modifies the physiology and body composition of broiler chickens reared in floor pens without affecting breast meat processing quality

Selection in broiler chickens has increased muscle mass without similar development of the cardiovascular and respiratory systems, resulting in limited ability to sustain high ambient temperatures. The aim of this study was to determine the long-lasting effects of heat manipulation of the embryo on the physiology, body temperature (Tb), growth rate and meat processing quality of broiler chickens reared in floor pens. Broiler chicken eggs were incubated in control conditions (37.8°C, 56% relative humidity; RH) or exposed to thermal manipulation (TM; 12 h/d, 39.5°C, 65% RH) from d 7 to 16 of embryogenesis. This study was planned in a pedigree design to identify possible heritable characters for further selection of broiler chickens to improve thermotolerance. Thermal manipulation did not affect hatchability but resulted in lower Tb at hatching and until d 28 post-hatch, with associated changes in plasma thyroid hormone concentrations. At d 34, chickens were exposed to a moderate heat challenge (5 h, 32°C). Greater O2 saturation and reduced CO2 partial pressure were observed (P < 0.05) in the venous blood of TM than in that of control chickens, suggesting long-term respiratory adaptation. At slaughter age, TM chickens were 1.4% lighter and exhibited 8% less relative abdominal fat pad than controls. Breast muscle yield was enhanced by TM, especially in females, but without significant change in breast meat characteristics (pH, color, drip loss). Plasma glucose/insulin balance was affected (P < 0.05) by thermal treatments. The heat challenge increased the heterophil/lymphocyte ratio in controls (P < 0.05) but not in TM birds, possibly reflecting a lower stress status in TM chickens. Interestingly, broiler chickens had moderate heritability estimates for the plasma triiodothyronine/thyroxine concentration ratio at d 28 and comb temperature during the heat challenge on d 34 (h(2) > 0.17). In conclusion, TM of the embryo modified the physiology of broilers in the long term as a possible adaptation for heat tolerance, without affecting breast meat quality. This study highlights the value of 2 new heritable characters involved in thermoregulation for further broiler selection.

Effect of manipulation of incubation temperature on fatty acid profiles and antioxidant enzyme activities in meat-type chicken embryos

Eggs (n = 1,800) obtained from Ross broiler breeders at 32 and 48 wk of age were incubated at either a constant temperature of 37.6°C throughout (T1), or the temperature was reduced for 6 h to 36.6°C each day during embryonic age (EA) 10 to 18 (T2). Yolk sac, liver, and brain fatty acid profiles and oxidant and antioxidant status of liver and brain were measured at EA 14, 19, and day of hatch (DOH). Fatty acid profiles of yolk sac, liver, and brain were influenced by age of breeder with significant breeder hen age × incubation temperature interactions. At EA 14, higher levels of 20:4n-6 and 22:6n-3 had been transferred from the yolk sac to T2 embryos from younger than older breeders, whereas for T1 and T2 embryos, yolk sac 20:4n-6 and 22.6n-3 values were similar for older breeders. Accumulation of 20:4n-6 and 22:6n-3 fatty acids in the liver of T1 and T2 embryos from younger breeders was similar; however, T2 embryos from older breeders had higher liver levels of 20:4n-6 and 22:6n-3 than T1 embryos. At EA 19, liver nitric oxide levels were higher for T2 embryos from younger breeders than those from breeders incubated at T1. Brain catalase levels of T2 embryos from younger breeders were higher than those from older breeders at DOH. Thus, changes in fatty acid profiles and catalase and nitric oxide production of brain and liver tissues resulting from 1°C lower incubation temperature from EA 10 to 18 reflect adaptive changes.

Impact of environmental thermal stimulation on activation of hypothalamic neuronal nitric oxide synthase during the prenatal ontogenesis in Muscovy ducks

The aim of the study is to investigate the influence of prenatal temperature stimulation on neuronal NO synthase (nNOS) expression in the anterior hypothalamus of Muscovy duck embryos. Experiments were performed on embryonic day (E) E20, E23, E28, and E33 using histochemistry for identification of the nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) as marker of NOS-containing neurons. Until the experiments, all duck embryos were incubated under standard temperature conditions (37.5°C). During 3 hours before the start of the experiments, one group was incubated at 37.5°C (control group), the second was warm-experienced at 39°C, and the third was cold-experienced at 34°C. In normal and warm-incubated duck embryos, nNOS activity could be first detected on E23. Particularly, after cold stimulation, a significant increase in nNOS activity was found in all embryos investigated even on day 20. Warm stimulation obviously induces the opposite effect, but at later embryonic age (E33). It can be concluded that probably in late-term bird embryos NO acts as a mediator of the neuronal cold pathway in the anterior hypothalamus, which might be improved by prenatal cold stimulation.

Epigenetic control of translation regulation: alterations in histone H3 lysine 9 post-translation modifications are correlated with the expression of the translation initiation factor 2B (Eif2b5) during thermal control establishment

Thermal control set point is regulated by thermosensitive neurons of the preoptic anterior hypothalamus (PO/AH) and completes its development during postnatal critical sensory period. External stimuli, like increase in environmental temperature, influence the neuronal protein repertoire and, ultimately, cell properties via activation or silencing of gene transcription, both of which are regulated by the "histone code.''" Here, we demonstrated an increase in global histone H3 lysine 9 (H3K9) acetylation as well as H3K9 dimethylation in chick PO/AH during heat conditioning at the critical period of sensory development. In contrast to the global profile of H3K9 modifications, acetylation and dimethylation patterns of H3K9 at the promoter of the catalytic subunit of eukaryotic translation initiation factor 2B (Eif2b5) were opposite to each other. During heat conditioning, there was an increase in H3K9 acetylation at the Eif2b5 promoter, simultaneously with decrease in H3K9 dimethylation. These alterations coincided with Eif2b5 mRNA induction. Moreover, exposure to excessive heat during the critical period resulted in long-term effect on both H3K9 tagging at the Eif2b5 promoter and Eif2b5 mRNA expression. These data suggest a role for dynamic H3K9 post-translational modifications in global translation regulation during the thermal control establishment.

Influence of temperature stimulation during the last 4 days of incubation on secondary sex ratio and later performance in male and female broiler chicks

1. In 6 incubation trials a total of 9883 eggs (Ross 308) were incubated from d 1 to 17 under normal incubation conditions (37.2-37.4 degrees C) and then sorted into three hatch incubators (control: 37.2-37.4 degrees C; chronic warm incubation: 38.2-38.4 degrees C, 24 h daily; short-term warm stimulation: 38.2-38.4 degrees C, 2 h daily) in incubation trials 1 and 2 or two hatch incubators (control and short-term warm stimulation) in trials 3-6. 2. The one-day-old chicks were selected by sex and chick quality was analysed in random samples using the Pasgar score. A total of 120 male and 120 female one-day-old chickens from each incubator were used for a 35-d fattening period. 3. Neither chronic nor short-term increase in incubation temperature had a negative effect on hatchability and chick quality. Short-term warm stimulation improved hatchability by more than 1.5% and was associated with a significantly higher proportion of hatched male chicks. 4. In the subsequent broiler growth trial, the mean daily weight gain of the short-term warm stimulated male broiler chicks was significant higher than for the control group, which results in a body weight increase of 2.9%. 5. Feed conversion (feed:gain ratio) of the short-term warm stimulated male and female broilers was significantly lower than in the males and females of the control and chronic warm incubated groups. 6. In conclusion, an incubation temperature profile which includes short-term temperature variation can be important in improving poultry performance (European patent pending since March 2008).