Egg yolk environment differentially influences physiological and morphological development of broiler and layer chicken embryos

Influence of sex hormones on the aggressive behavior during peck order establishment and stabilization in meat and egg type chickens

In the poultry industry, broiler and layer strains are genetically selected for different purposes (e.g., high meat-yield and high egg-production). Genetic selection for productivity can have unintended consequences on the behavioral repertoire of the birds, including aggression. Alongside the increasing societal concern regarding the welfare of animal in agriculture, the number of countries that are advocating the prohibition of using battery cages for laying hens has resulted in the transition and adoption of cage-free or free-range systems. Thus, both broiler and layer chickens are housed in large flocks rather than housed individually in cages. Housing birds in groups increases the opportunity for birds to engage in social behaviors, including aggression, that are used to establish social status. Aggressive interactions are associated with the risk of injury and the potential for a subordinate animal to have unmet needs (e.g., access to feed). The aim of this study was to characterize the relationships among aggressive behavior, neurobiology, and hormones during peck order establishment and social hierarchy stabilization of 2 divergently selected strains (meat- and egg-type chicken). Meat-type strains performed more male on male (P < 0.001), male on female (P < 0.0001), and female on female (P < 0.0001) non-reciprocal aggression behavior (NRA) than egg-type strains. Greater serum testosterone and estradiol concentrations in the weeks after the peck order establishment were observed in meat-type birds compared those in egg-type birds for both males and females (all P < 0.05). Greater (P < 0.05) cellular densities of androgen receptors, but not estrogen receptors, were observed in the hypothalamus of meat-type birds compared to egg-type birds. These findings suggest that greater sex hormone concentrations in the meat-type birds may be a consequence of genetic selection for rapid growth resulting in more sex hormones-induced aggressive behavior.

Adiponectin and visfatin expression profile in extra-embryonic annexes and role during embryo development in layer and broiler chickens

Some evidence showed differences between layer and broiler embryo development. We recently showed that two adipokines, adiponectin and visfatin are expressed in the extra embryonic membranes and fluids. However, their role in the embryo development is unknown. Thus, our objectives were 1. to compare the expression of AdipoQ and its receptors AdipoR1 and AdipoR2 and visfatin in extra-embryonic annexes in broiler and layer breeders during the embryo development and 2. to investigate the role of two adipokines in embryo development in both broiler and layer breed after in ovo injection of blocking antibodies against chicken adiponectin or visfatin. We found that adiponectin, AdipoR1, AdipoR2 and visfatin were mainly more expressed in the allantoic that in amniotic membranes. In addition, these expressions increased according the stage of embryo development. We observed a higher expression in layer than in broiler of AdipoQ in allantoic membranes at ED14 and ED18, of AdipoR1 and AdipoR2 in both allantoic and amniotic membranes at ED7 and ED14 and of visfatin only in allantoic membrane from ED7 to ED18. AdipoQ and visfatin were absent in amniotic fluid at ED7 but present at ED14 or ED18 where higher concentrations were detected in layer than in broiler. Interestingly, we showed a strong positive correlation between Adipo and visfatin concentration in amniotic fluid and the body weight of embryo in both breeds. However, after in ovo injection of Adipo antibodies we did not observe any effect on the embryo mortality whereas injection of visfatin antibodies increased in a dose dependent manner the embryo mortality in both breeds. Taken together, Adipo and visfatin are higher expressed in layer than broiler in extra-embryonic membranes and amniotic fluid. Our data suggest also that visfatin could be a main regulator of embryo development.

Phenotypic divergence between broiler and layer chicken lines is regulated at the molecular level during development

BACKGROUND

Understanding the molecular underpinnings of phenotypic variations is critical for enhancing poultry breeding programs. The Brazilian broiler (TT) and laying hen (CC) lines exhibit striking differences in body weight, growth potential, and muscle mass. Our work aimed to compare the global transcriptome of wing and pectoral tissues during the early development (days 2.5 to 3.5) of these chicken lines, unveiling disparities in gene expression and regulation.

RESULTS

Different and bona-fide transcriptomic profiles were identified for the compared lines. A similar number of up- and downregulated differentially expressed genes (DEGs) were identified, considering the broiler line as a reference. Upregulated DEGs displayed an enrichment of protease-encoding genes, whereas downregulated DEGs exhibited a prevalence of receptors and ligands. Gene Ontology analysis revealed that upregulated DEGs were mainly associated with hormone response, mitotic cell cycle, and different metabolic and biosynthetic processes. In contrast, downregulated DEGs were primarily linked to communication, signal transduction, cell differentiation, and nervous system development. Regulatory networks were constructed for the mitotic cell cycle and cell differentiation biological processes, as their contrasting roles may impact the development of distinct postnatal traits. Within the mitotic cell cycle network, key upregulated DEGs included CCND1 and HSP90, with central regulators being NF-κB subunits (RELA and REL) and NFATC2. The cell differentiation network comprises numerous DEGs encoding transcription factors (e.g., HOX genes), receptors, ligands, and histones, while the main regulatory hubs are CREB, AR and epigenetic modifiers. Clustering analyses highlighted PIK3CD as a central player within the differentiation network.

CONCLUSIONS

Our study revealed distinct developmental transcriptomes between Brazilian broiler and layer lines. The gene expression profile of broiler embryos seems to favour increased cell proliferation and delayed differentiation, which may contribute to the subsequent enlargement of pectoral tissues during foetal and postnatal development. Our findings pave the way for future functional studies and improvement of targeted traits of economic interest in poultry.

Expression of chemerin and its receptors in extra-embryonic annexes and role of chemerin and its GPR1 receptor in embryo development in layer and broiler hens

Intensive genetic selection of broiler breeders and layer hens resulted in differences in the mechanisms of growth and also in cell metabolism during embryogenesis. Previous research has shown that an adipokine named chemerin and one of these receptors, CMKLR1 were potentially involved in broiler embryo development. Here, our objectives were 1) to compare the expression of chemerin and its receptors CMKLR1, GPR1, and CCRL2 and chemerin concentration in extra-embryonic annexes (allantoic and amniotic membranes and fluids and plasma) in broiler and layer fertile eggs during the development (embryonic day (ED) 7, 14, and 18) by RT-qPCR and specific chicken ELISA and 2) to investigate the role of chemerin and one of its receptors GPR1 in embryo development after in ovo injections of neutralizing antibodies against chicken chemerin and GPR1. We found that chemerin expression in amniotic membranes was higher in layer than broiler eggs at ED7 and ED14 whereas the expression of the 3 receptors was higher in layer than broiler in the allantoic membranes at ED14 and ED18. Chemerin concentration was more important in layer than broiler at ED14 and ED18 in amniotic liquid and at all the studied stages in blood plasma. We also showed positive correlation between amniotic chemerin concentration and chemerin amniotic membrane expression, chemerin plasma concentration and embryo body weight in both breeds. Finally, in ovo injection of chicken chemerin and GPR1 neutralizing antibodies increased embryo mortality in both layer and broiler eggs. Taken together, even if chemerin concentration and chemerin system expression in embryonic membranes are mainly higher expressed in layer than in broiler, chemerin potentially through GPR1 could promote embryo development in both breeds.

Stage-specific nutritional management and developmental programming to optimize meat production

Over the past few decades, genetic selection and refined nutritional management have extensively been used to increase the growth rate and lean meat production of livestock. However, the rapid growth rates of modern breeds are often accompanied by a reduction in intramuscular fat deposition and increased occurrences of muscle abnormalities, impairing meat quality and processing functionality. Early stages of animal development set the long-term growth trajectory of offspring. However, due to the seasonal reproductive cycles of ruminant livestock, gestational nutrient deficiencies caused by seasonal variations, frequent droughts, and unfavorable geological locations negatively affect fetal development and their subsequent production efficiency and meat quality. Therefore, enrolling livestock in nutritional intervention strategies during gestation is effective for improving the body composition and meat quality of the offspring at harvest. These crucial early developmental stages include embryonic, fetal, and postnatal stages, which have stage-specific effects on subsequent offspring development, body composition, and meat quality. This review summarizes contemporary research in the embryonic, fetal, and neonatal development, and the impacts of maternal nutrition on the early development and programming effects on the long-term growth performance of livestock. Understanding the developmental and metabolic characteristics of skeletal muscle, adipose, and fibrotic tissues will facilitate the development of stage-specific nutritional management strategies to optimize production efficiency and meat quality.

The effects of hen's age and egg storage duration on selected growth parameters of turkey embryos

The aim of this study was to determine the effects of hen's age (A) and egg storage duration (T) on selected growth parameters of turkey embryos. At 32, 38, 46, and 51 wk of hen's age, 1,512 eggs laid on one or 2 consecutive days were collected randomly and marked. At each sampling date, the eggs were randomly divided into 4 groups and were stored for various periods of time, that is, 7, 10, 13, and 17 d. All eggs were stored at a temperature of 15°C and relative air humidity of 76%. On d 9, 15, 21, and 24 of incubation, 5 eggs containing live embryos were randomly selected from each group for analysis of the following parameters: relative body weight (RBW) of embryos, relative weight of the yolk sac (RWY), relative weight of unused albumen (RWA). The effects of hen's age and egg storage duration on the RBW of embryos were observed on d 15, 21, and 24 of incubation (P < 0.05). The effects of hen's age and egg storage duration on RWY were noted on all analyzed days of incubation (P < 0.05). Embryos in eggs laid by younger hens (aged 32 and 38 wk) and stored for a shorter period were characterized by a faster rate of albumen utilization than embryos in eggs laid by older hens (aged 46 and 51 wk). The largest amount of unused albumen was found in eggs laid by hens in wk 51 of the laying season (P < 0.05), and stored for 17 d (P < 0.05). In conclusion, numerous interactions (AxT) between selected growth parameters of turkey embryos indicate that the quality of hatching eggs changes with hen's age, affecting their suitability for long-term storage under standard conditions. Therefore, eggs laid by younger breeders should not be stored for longer periods due to undesirable changes in RWY and RWA.

Microbial composition of egg component and its association with hatchability of laying hens

The internal quality of eggs is critical for human consumption and embryonic development. However, microorganisms inside eggs have not been thoroughly investigated for their roles in determining the egg's internal quality. Here, a total of 21 hens were selected from more than 1,000 chickens based on their hatching results and were divided into high- and low-hatchability groups. Then, we collected 72 eggs from these 21 hens to obtain egg whites and yolks, including 54 fresh eggs and 18 eggs after 12 days of incubation. We characterized the microbial composition of egg yolks and whites, the microbial change along incubation, and differences in microbial abundance between the high- and low-hatchability groups. The results indicated that egg whites are not sterile. Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes were the dominant phyla in egg yolk and white. There was a large difference in the microbial composition between egg whites and yolks, and this difference increased after 12 days of incubation. Egg whites have lower microbial diversity than egg yolks owing to the presence of antibacterial substances such as lysozyme in the egg white. After a 12-day incubation, the microbial diversity decreased in egg whites but increased slightly in egg yolks. Meanwhile, the microbes in egg white can migrate to egg yolk during incubation. Additionally, Genus Muribaculaceae was identified as a biomarker in egg yolks incubated for 12 days and was more often detected in healthy groups. On the contrary, more genus Rothia were found in the fresh egg yolk of the low hatchability groups and was considered to have low virulence. These findings shed light on the composition and differences in microbiota between egg yolks and whites and may open new avenues for studying embryonic development in chickens.

Nature vs. Nurture: Disentangling the Influence of Inheritance, Incubation Temperature, and Post-Natal Care on Offspring Heart Rate and Metabolism in Zebra Finches

A historic debate in biology is the question of nature vs. nurture. Although it is now known that most traits are a product of both heredity (“nature”) and the environment (“nurture”), these two driving forces of trait development are rarely examined together. In birds, one important aspect of the early developmental environment is egg incubation temperature. Small changes (<1°C) in incubation temperature can have large effects on a wide-array of offspring traits. One important trait is metabolism, because it is related to life-history traits and strategies, organismal performance, and energetic and behavioral strategies. Although it has been shown that embryonic and post-hatch metabolism are related to egg incubation temperature, little is known about how this may vary as a function of genetic differences or post-hatching environmental conditions. Here, we investigated this question in zebra finches (Taeniopygia guttata). We experimentally incubated eggs at two different temperatures: 37.5°C (control), which is optimal for this species and 36.3°C (low), which is suboptimal. We first measured embryonic heart rate as a proxy of embryonic metabolic rate. Then, at hatch, we cross-fostered nestlings to differentiate genetic and pre-hatching factors from post-hatching environmental conditions. When offspring were 30 days-old, we measured their resting metabolic rate (RMR; within the thermoneutral zone) and thermoregulatory metabolic rate (TMR; 12°C; birds must actively thermoregulate). We also measured RMR and TMR of all genetic and foster parents. We found that embryonic heart rate was greater in eggs incubated at the control temperature than those at the low temperature. Further, embryonic heart rate was positively related to genetic father RMR, suggesting that it is both heritable and affected by the pre-natal environment. In addition, we found that post-hatch metabolic rates were positively related to genetic parent metabolic rate, and interactively related to incubation temperature and foster mother metabolic rate. Altogether, this suggests that metabolism and the energetic cost of thermoregulation can be influenced by genetics, the pre-natal environment, and the post-natal environment. Our study sheds light on how environmental changes and parental care may affect avian physiology, as well as which traits may be susceptible to natural selection.

Layers, broiler chickens and their F1 cross develop distinctly different caecal microbial communities when hatched and reared together

Aim

To compare the caecal microbiota of layer, broiler, and intermediate F1 layer × broiler cross birds with the hypothesis that significant differences in caecal microbial composition would persist between the three groups when host and environmental interactions were minimized.

Methods and Results

Caecal contents were characterized using 16S rRNA for males of broiler (n = 12), layer (n = 12) and F1 layer × broiler cross (n = 9) birds that were hatched and reared under the same conditions. The microbial community structure differed significantly between the three groups of birds at phylum, genus and OTU levels, with clear separation of the groups observed. Firmicutes was the phylum most represented across samples; however, the high abundance of Proteobacteria in the layer birds at d28 post‐hatch was unexpected, and driven by a higher abundance of E. coli.

Conclusions

The microbiota phylotype between broilers, layers and their F1 cross significantly differed in community structure, diversity and relative abundance in the absence of environmental confounding, which is generally difficult to avoid in microbial studies.

Significance and Impact of Study

The results provide a unique comparison and evidence that there is a strong genetic component driving microbial composition within poultry strains, despite the embryonic development occurring in ovo.

Nutrient sources differ in the fertilised eggs of two divergent broiler lines selected for meat ultimate pH

The pHu+ and pHu− lines, which were selected based on the ultimate pH (pHu) of the breast muscle, represent a unique model to study the genetic and physiological controls of muscle energy store in relation with meat quality in chicken. Indeed, pHu+ and pHu− chicks show differences in protein and energy metabolism soon after hatching, associated with a different ability to use energy sources in the muscle. The present study aimed to assess the extent to which the nutritional environment of the embryo might contribute to the metabolic differences observed between the two lines at hatching. Just before incubation (E0), the egg yolk of pHu+ exhibited a higher lipid percentage compared to the pHu− line (32.9% vs. 27.7%). Although ¹H-NMR spectroscopy showed clear changes in egg yolk composition between E0 and E10, there was no line effect. In contrast, ¹H-NMR analysis performed on amniotic fluid at embryonic day 10 (E10) clearly discriminated the two lines. The amniotic fluid of pHu+ was richer in leucine, isoleucine, 2-oxoisocaproate, citrate and glucose, while choline and inosine were more abundant in the pHu− line. Our results highlight quantitative and qualitative differences in metabolites and nutrients potentially available to developing embryos, which could contribute to metabolic and developmental differences observed after hatching between the pHu+ and pHu− lines.

Early Phenotype Programming in Birds by Temperature and Nutrition: A Mini-Review

Early development is a critical period during which environmental influences can have a significant impact on the health, welfare, robustness and performance of livestock. In oviparous vertebrates, such as birds, embryonic development takes place entirely in the egg. This allows the effects of environmental cues to be studied directly on the developing embryo. Interestingly, beneficial effects have been identified in several studies, leading to innovative procedures to improve the phenotype of the animals in the long term. In this review, we discuss the effects of early temperature and dietary programming strategies that both show promising results, as well as their potential transgenerational effects. The timing, duration and intensity of these procedures are critical to ensure that they produce beneficial effects without affecting animal survival or final product quality. For example, cyclic increases in egg incubation temperature have been shown to improve temperature tolerance and promote muscular growth in chickens or fatty liver production in mule ducks. In ovo feeding has also been successfully used to enhance digestive tract maturation, optimize chick development and growth, and thus obtain higher quality chicks. In addition, changes in the nutritional availability of methyl donors, for example, was shown to influence offspring phenotype. The molecular mechanisms behind early phenotype programming are still under investigation and are probably epigenetic in nature as shown by recent work in chickens.

Effects of Nutritional Restriction during Laying Period of Fat and Lean Line Broiler Breeder Hens on Meat Quality Traits of Offspring

Simple Summary

The meat quality of livestock products is widely appreciated. Maternal nutrition can affect the deposition of nutrients in eggs, and then change the apparent metabolism, development process, and performance of offspring. Our research indicated that meat quality traits are also affected by maternal nutritional level and are related to the nutritional requirements of different genotypes. Some of the effects disappeared at the end of the growth stage. These situations remind poultry producers to consider the impact of feed restrictions on the quality of meat for future generations.

Abstract

The offspring meat quality of hens undergoing a 25% dietary restriction treatment during the laying period were evaluated in fat and lean line breeder. A total of 768 female birds (384/line) were randomly assigned to four groups (12 replicates/group, 16 birds/replicates). Maternal feed restriction (MFR) and normal started at 27 weeks of age. Offspring broilers were fed ad libitum. The offspring meat quality traits and muscle fiber morphology in different periods were measured. At birth, significant interactions were found on breast muscle fiber morphology (p < 0.05). At 28 days, MFR decreased breast water content and increased thigh crude fat content, and significant interactions were observed on breast crude fat and protein contents (p < 0.05). At 56 days, MFR affected morphology of peroneus longus muscle tissue, and significant interactions were found on thigh redness at 48 h and amino acid contents in breast and thigh muscle (p < 0.05). Overall, MRF may lead to offspring birth sarcopenia. Such offspring grow more easily to deposit fat in a nutritious environment, but they will self-regulate adverse symptoms during growth and development. The two lines respond differently to maternal nutritional disturbance due to different nutritional requirements and metabolic patterns.

Maternal dietary supplementation with grape seed extract in reproductive hens increases fertility in females but decreases semen quality in males of the F1 generation

Genetic selection in parental broiler breeders has increased their susceptibility to metabolic disorders and reproductive dysfunction. We have recently shown that maternal dietary grape seed extract (GSE) supplementation in hens improves fertility parameters, egg quality, oxidative stress in different tissues and the quality of F1 chicks. Here, we analysed the growth and fertility (both female and male) of the F1 generation animals and the quality of their offspring (F2 generation). Eggs issued from hens supplemented with GSE presented lower ROS production than control hens, suggesting a change in the embryonic environment. However, this did not affect the growth nor the body composition of male and female F1s from hatching to adulthood (37 weeks of age). At 37 weeks of age, the biochemistry analysis of the GSE-F1 muscle has revealed an increase in sensitivity to oxidative stress and a slight change in lipid composition. Both male and female F1-GSE groups presented a delay in puberty with a lower testis volume at 30 weeks of age and lower ovary development at 26 weeks of age. Adult GSE-F1 males did not present histological alterations of seminiferous tubules or semen production, but the semen quality was degraded due to higher oxidative stress and DNA-damaged spermatozoa compared with control F1 animals. In adult GSE-F1 females, despite the delay in puberty, the females laid more eggs of better quality (fewer broken eggs and a higher hatching rate). At hatching, the weight of the chicks from GSE-F1 females was reduced, and this effect was stronger in F2 male chicks (F2) compared with F2 control chicks (F2), because of the lower muscle volume. In conclusion, we can raise the hypothesis that maternal dietary GSE supplementation produces eggs with change in embryonic metabolism, which may affect in adulthood the fertility. The data obtained from the F1-GSE group pointed to a sex-specific modification with higher egg quality in females but semen sensitive to stress in males. Finally, male F2 chicks were leaner than control chicks. Thus, maternal dietary grape seed extract (GSE) supplementation in hens may impact on the fertility of the offspring in a sex-specific manner in subsequent generations.

Differential physiological response of slow- and fast-growing broiler lines to hypoxic conditions during chorioallantoic membrane development

Ambient conditions during chicken embryogenesis, such as insufficient oxygen or changes in temperature, are expected to cause permanent phenotypic changes and affect their posthatch performance. Decades of genetic selection for high growth rate resulted with various physiological and morphological changes that can affect the broiler fitness under environmental stress. To evaluate the selection effect on responses to environmental challenge during embryonic development, and the long-term implications, we have used a unique genetic line, that was not selected for over 30 yr (since 1986), as control for the modern commercial genetic line. At embryonic day 5 (E5), broiler embryos from these 2 genetic lines were divided into 2 treatments: 1) control; 2) 15% O2 concentration for 12 h/day from E5 through E12 the embryonic period of chorioallantoic membrane formation. Embryos and hatched chicks were characterized for physiological and morphological parameters. Significant differences in relative embryo weight and yolk consumption were found between the 2 lines. The modern line was characterized by a higher metabolic rate and rapid growth, supported by higher hemoglobin levels and hematocrit concentrations, whereas the 1986 line had slower metabolism, lower levels of hematocrit and hemoglobin, higher oxygen volume per 1 g of embryonic tissue indicating higher oxygen availability. Both lines exhibited changes in heart rate, and blood parameters corresponding to cardiovascular system adaptation after hypoxic exposure, seemingly implemented to increase oxygen-carrying capacity to the embryo tissues. Our finding stand in agreement that the genetic selection for high growth rate that led to higher metabolism without a fit of the cardiovascular system, increased the imbalance between oxygen supply and demand.

In ovo corticosterone administration alters body composition irrespective of arginine supplementation in 35-day-old female chicken meat birds

Context Exposure to maternal hormones can permanently alter an embryo’s developmental trajectory. Maternal mediated effects have significant potential in the chicken meat industry, as breeder hens are feed restricted in a bid to improve performance. Evidence suggests breeder hens are chronically stressed, resulting from periods of prolonged hunger. However, evidence linking embryonic exposure to early-life stress and altered offspring phenotype in meat chickens is lacking. Additionally, methods to alleviate the phenotypic consequences of early-life stress have not been comprehensively explored. Nutritional supplementation with amino acids, such as arginine (Arg), may provide one such option, as Arg reportedly enhances performance characteristics in chicken meat birds. Aims An in ovo study was conducted to investigate whether exposure to in ovo stress altered offspring performance in meat chickens. Additionally, Arg was supplemented post-hatch to alleviate reductions in performance, hypothesised to occur as a result of exposure to corticosterone. Method A total of 400 eggs were divided into two groups and administered a corticosterone (CORT) or control (CON) solution at embryonic Day 11. At hatch, birds were separated into four groups based on in ovo and dietary treatments: CORT-Control, CORT-Arg, CON-Arg and CON-Control. Birds fed supplementary Arg diets received an Arg:lysine inclusion of 125%. Bodyweight (bwt) and feed conversion were recorded weekly. Birds were euthanised at embryonic Day 15, Day 0, 7, 21 (n = 40 birds/time point), 28 and 35 (n = 48 birds/time point) for organ collection. A total of 12 additional female birds were euthanised and subjected to a dual-energy X-ray absorptiometry scan for body composition at Day 35. Results Neither in ovo nor diet treatments influenced bwt, bwt gain, feed conversion or plasma corticosterone at any time point, nor did any in ovo by diet interaction exist. Female birds exposed to CORT exhibited significantly greater fat mass (%bwt; P = 0.007) and reduced lean mass (%bwt; P = 0.026) compared with CON females at Day 35. Supplementary Arg did not influence bird body composition. Conclusions These findings suggest in ovo exposure to CORT may negatively influence body composition of female birds. Implications Understanding the effects of the maternal/in ovo environment may provide a novel approach to further improve carcass quality and flock uniformity.

Non-destructive sex-specific monitoring of early embryonic growth rate in light brown broiler eggs using light transmission and its correlation with hatching time and chick weight

1. Monitoring early embryonic growth rate (EGR) has significant economic and animal welfare benefits. This study focuses on monitoring sex-specific early EGR using light transmission, and correlating this with hatching time and chick weight. For broiler eggs in particular, spectral masking of the light brown eggshells needed to be addressed. This was done using longitudinal visible transmission spectroscopy combined with eggshell colour image analysis. 2. Prior to incubation, colour images of eggs were captured followed by daily measurements of transmission spectra of eggs from days one to nine of incubation. The sex of the eggs was subsequently verified 2 d after hatching. 3. To accurately and sensitively determine sex differences in EGR using light transmission, while minimising interference from eggshell colour and thickness, the ratio of longitudinal transmissions was determined to be most effective at 575 and 610 nm. 3. Embryonic growth was detectable from d 3 (72 h) of incubation, 24 h earlier than previously reported lateral transmission measurements. However, at this time, low blood levels meant that no significant sex-differences (P > 0.05) for the mean T575/T610 ratio were detectable. This may have been due, in part, to spectral masking from the light brown eggshells. At d 7, female embryos had a significantly lower (P < 0.05) mean T575/T610 ratio than males. 4. Although the T575/T610 ratio had low correlations with hatching time and hatch-weight of chicks, this could be a good starting point for further non-destructive investigations for such predictions. 5. In conclusion, the methodology had the sensitivity to differentiate sex-specific early EGR in broiler eggs, even with pigmented eggshells, and has the potential to advance precision hatchery management and poultry research.

Chicken embryos can maintain heart rate during hypoxia on day 4 of incubation

Acute exposure to hypoxic conditions is a frequent natural event during the development of bird eggs. However, little is known about the effect of such exposure on the ability of young embryos in which cardiovascular regulation is not yet developed to maintain a normal heart rate (HR). To address this question, we studied the effect of 10-20 min of exposure to moderate or severe acute hypoxia (10% or 5% O₂, respectively) on the HR of day 4 (D4) chicken embryos. In ovo, video recording of the beating embryo heart inside the egg revealed that severe, but not moderate, hypoxia resulted in significant HR changes. The HR response to severe hypoxia consisted of two phases: the first phase, consisting of an initial decrease in HR, was followed by a phase of partial HR recovery. Upon the restoration of normoxia, after an overshoot period of a few minutes, the HR completely recovered to its basal level. In vitro (isolated heart preparation), the first phase of the HR response to severe hypoxia was strengthened (nearly complete heart silencing) compared to that in ovo, and the HR recovery phase was greatly attenuated. Furthermore, neither an overshoot period nor complete HR recovery after hypoxia was observed. Thus, the D4 chicken embryo heart can partially maintain its rhythm during hypoxia in ovo, but not in vitro. Some factors from the egg, such as catecholamines, are likely to be critical for avian embryo responding to hypoxic condition and survival.

Impact of feeding microalgae (Aurantiochytrium limacinum) and co-extruded mixture of full-fat flaxseed as sources of n-3 fatty acids to ISA brown and Shaver white breeders and progeny on pullet skeletal attributes at hatch through to 18 weeks of age

Impact of feeding n-3 fatty acids (FA) to ISA brown and Shaver white breeders and their progeny on bone development in pullets was investigated. Breeders were fed Control (CON); CON + 1% microalgae (DMA: Aurantiochytrium limacinum) as the source of docosahexaenoic acid; and CON + 2.6% of a co-extruded mixture of full-fat flaxseed (FFF) and pulses mixture as source of α-linolenic acid. Test diets (DMA and FFF) were balanced for total n-3 FA and n-6: n-3 FA ratio. Samples of day-old progeny were euthanized for bone mineral content (BMC) and tibia collagen type II. The remaining pullets were fed posthatch treatments as follows: from breeder CON: CON (CON-CON), DMA (CON-DMA), and FFF (CON-FFF), from breeder DMA: CON (DMA-CON) and DMA (DMA-DMA) and from breeder FFF: CON (FFF-CON) and FFF (FFF-FFF). A total of 60 pullets per posthatch diets were reared in cages (12 pullets/cage, n = 5) with free access to feed and water, bled at 6, 12, and 18 wk of age (WOA) for bone turnover markers and necropsied at 18 WOA for tibia and femur samples. Day-old pullets from breeder fed CON had greater BMC (P < 0.001) relative to those from breeders fed other diets. There was strain and diet interaction (P ≤ 0.024) on tibia breaking strength (TBS) and tibia cortical ash concentration at 18 WOA such that diet responses were only observed in Shaver white pullets. In this context, TBS of DMA-DMA and FFF-FFF was greater than for pullets originating from CON breeder, and the cortical ash weight of DMA-DMA and FFF-FFF pullets was 23.8 and 20.2%, respectively, higher than for CON-CON pullets. In conclusions, the strain effects were strong on tibia attributes on 18-week-old pullets. Breeder feeding of n-3 FA was more effective when concomitant with posthatch feeding of n-3 FA in supporting the skeletal strength and cortical bone development in Shaver white pullets. Further investigations are warranted to establish the impact these strategies on skeletal health during laying cycle.

A review on yolk sac utilization in poultry

During incubation, embryonic growth and development are dependent on nutrients deposited in the egg. The content of the yolk can be transferred to the embryo in 2 ways: directly into the intestine via the yolk stalk or through the highly vascularized yolk sac membrane. It has been suggested that, as a result of genetic selection and improved management, the increase in posthatch growth rate and concurrently the increase in metabolic rate of broiler chickens during the last 50 yr has also increased embryonic metabolism. A higher metabolic rate during incubation would imply a lower residual yolk weight and possibly lower energy reserve for the hatchling. This might affect posthatch development and performance. This review examined scientific publications published between 1930 and 2018 to compare residual yolk weight at hatch, metabolic heat production, and yolk utilization throughout incubation. This review aimed to investigate 1) whether or not residual yolk weight and composition has been changed during the 88-yr period considered and 2) which abiotic and biotic factors affect yolk utilization in poultry during incubation and the early posthatch period. It can be concluded that 1) residual yolk weight and the total solid amount of the residual yolk at hatch seem to be decreased in the recent decades. It cannot be concluded whether the (lack of) differences between old and modern strains are due to genetic selection, changed management and incubation conditions, or moment of sampling (immediately after hatch or at pulling). It is remarkable that with the genetic progress and improved management and incubation conditions over the last 88 yr, effects on yolk utilization efficiency and embryonic metabolic heat production are limited; 2) factors specially affecting residual yolk weight at hatch include egg size and incubation temperature, whereas breeder age has more influence on nutrient composition of the residual yolk.

Enriching ISA brown and Shaver white breeder diets with sources of n-3 polyunsaturated fatty acids increased embryonic utilization of docosahexaenoic acid

There is limited information on feeding egg-type chick breeders n-3 polyunsaturated fatty acids (PUFA) and its impact on hatching egg quality and embryonic fatty acid (FA) utilization. We investigated the effects of feeding brown and white egg-type chick breeders diets containing sources of n-3 PUFA on egg composition, apparent embryonic FA utilization, and intestinal FA transporter in hatchlings. Twenty-six-week-old ISA brown and Shaver white breeders were fed either 1) control (CON); 2) CON + 1% of microalgae (DMA, Aurantiochytrium limacinum) fermentation product, as a source of docosahexaenoic acid (DHA); or 3) CON + 2.60% of coextruded full-fat flaxseed and pulse mixture (FFF, 1:1 wt/wt) as a source of α-linolenic acid (ALA). Test diets had similar total n-3 and n-6:n-3 ratio. Eggs were hatched, and residual yolk (RY) samples taken for FA analyses. Apparent embryonic FA utilization was calculated by subtracting concentration of FA in RY from concentration of FA in yolk before incubation. There was an interaction between strains and diets (P < 0.05) on DHA in phospholipid and triglyceride fractions of yolk. Both n-3 PUFA sources increased DHA to a greater extent in Shaver white than in ISA brown. The interactive effect of strains and diets (P = 0.019) on embryonic utilization of ALA was such that DMA and FFF reduced ALA utilization, and this pattern was more prevalent in Shaver white birds than in ISA brown birds. There was no interaction between strains and diets on DHA utilization (P > 0.05). Embryos from hens fed n-3 PUFA sources used less total FA in phospholipid fraction (P < 0.001), and they preferentially used more DHA than CON embryos. Shaver white embryos used more (P < 0.05) ALA and DHA than ISA brown embryos. Although data suggested Shaver white had higher propensity of depositing DHA than ISA brown, irrespective of strain, feeding n-3 PUFA modified embryonic pattern of FA utilization toward utilization of DHA.

The effect of alternative feeding strategies for broiler breeder pullets: 2. Welfare and performance during lay

Feeding broiler breeders to satiety has negative consequences on their health and reproduction. Alternative feeding strategies during rearing can improve welfare, although their implications during lay are not well understood. The objective was to examine the effect of rearing feeding treatments on the reproductive performance and feeding behavior of broiler breeders under simulated commercial conditions. At 3 wk of age, 1,680 Ross 308 pullets were allocated to 24 pens under 1 of 4 isocaloric treatments: 1) daily control diet; 2) daily alternative diet (40% soybean hulls and 1 to 5% calcium propionate); 3) 4/3 control diet (4 on-feed days, 3 non-consecutive off-feed days per week); and 4) graduated control diet. Feeding frequency of the graduated treatment varied with age and finished on a daily basis. At 23 wk of age, group sizes were adjusted to 40 hens, and 5 mature Yield Plus Males roosters were introduced to each pen. Pens were under the same daily feeding management and same diet during lay. The performance of broiler breeders (growth rate, body weight uniformity, and reproductive performance) was determined until 64 wk of age. At the end of lay, feeding motivation was examined with a feed intake test and a compensatory feeding test. Data were analyzed using linear mixed regression models, with pen nested in the models and age as a repeated measure. The laying rate of hens reared on the graduated treatment decreased slower compared to control hens, resulting in a higher cumulative egg production (178.2 ± 3.8 eggs/hen) than control hens (165.2 ± 3.8 eggs/hen, P < 0.01) by 64 wk of age. Hens reared on non-daily feeding treatments laid lighter eggs with relatively heavier yolks and had higher feed intake at the end of lay than hens fed daily during rearing (P = 0.02). In conclusion, rearing feeding treatments impacted the growth rate and body weight uniformity during lay, feeding motivation at the end of lay, and the laying rate and hatchability depending on hens' age.

Possibilities of early life programming in broiler chickens via intestinal microbiota modulation

The strong selection in search for a higher growth rate in broilers has resulted in adverse effects such as metabolic disorders, low responsiveness of the immune system, and decreased resistance to pathogens. On the other hand, newly hatched chicks rely mostly on innate immune responses until their gut gets colonized with microbiota. In consequence, early access to active substances or bacteria (pre- and post-hatch) is particularly relevant here because in broilers much of the immune system development occurs early in life. Therefore, early stimulation of beneficial microflora is critical, as it affects, to a great extent, the entire life-span of an individual, and also because the nutritional manipulations of the gastrointestinal tract (GIT) microbiome to enhance productivity and health are rather limited by the resilience of the ecosystem once established in the chicken´s gut. Early life or developmental programming is based on the assumption that the development of diseases later in life can be modulated by perturbations or environmental exposures during critical pre- or early post-natal life. Substances such as plant derivatives, Na butyrate, pre- and probiotics, and β-glucans have been shown to induce beneficial microbiological and immunological changes within the GIT, and therefore are potential candidates to be used as tools to manipulate GIT functionality in the young chicken. Accordingly, substances as these might represent promising candidates to study intestinal microbiota/immune system modulation in broilers´ early stages of breeding. In ovo-delivered prebiotics and synbiotics have been shown to have no adverse effect on the development of the immune system in exposed chickens, while being able to affect lymphoid-organs' morphology in chickens. In ovo procedures have also been proposed as means of promoting a healthy microflora in embryonic guts and stimulating maturation of the cellular and humoral immune responses in central and peripheral immune organs, including those in the GIT. The purpose of this presentation is to discuss the potential usefulness of the instruments currently available to induce early life programming in broilers.

The Role of Maternal Thyroid Hormones in Avian Embryonic Development

During avian embryonic development, thyroid hormones (THs) coordinate the expression of a multitude of genes thereby ensuring that the correct sequence of cell proliferation, differentiation and maturation is followed in each tissue and organ. Although THs are needed from the start of development, the embryonic thyroid gland only matures around mid-incubation in precocial birds and around hatching in altricial species. Therefore, maternal THs deposited in the egg yolk play an essential role in embryonic development. They are taken up by the embryo throughout its development, from the first day till hatching, and expression of TH regulators such as distributor proteins, transporters, and deiodinases in the yolk sac membrane provide the tools for selective metabolism and transport starting from this level. TH receptors and regulators of local TH availability are expressed in avian embryos in a dynamic and tissue/cell-specific pattern from the first stages studied, as shown in detail in chicken. Maternal hyperthyroidism via TH supplementation as well as injection of THs into the egg yolk increase TH content in embryonic tissues while induction of maternal hypothyroidism by goitrogen treatment results in a decrease. Both increase and decrease of maternal TH availability were shown to alter gene expression in early chicken embryos. Knockdown of the specific TH transporter monocarboxylate transporter 8 at early stages in chicken cerebellum, optic tectum, or retina allowed to reduce local TH availability, interfering with gene expression and confirming that development of the central nervous system (CNS) is highly dependent on maternal THs. While some of the effects on cell proliferation, migration and differentiation seem to be transient, others result in persistent defects in CNS structure. In addition, a number of studies in both precocial and altricial birds showed that injection of THs into the yolk at the start of incubation influences a number of parameters in posthatch performance and fitness. In conclusion, the data presently available clearly indicate that maternal THs play an important role in avian embryonic development, but how exactly their influence on cellular and molecular processes in the embryo is linked to posthatch fitness needs to be further explored.

Ontogeny of skeletal and cardiac muscle mitochondria oxygen fluxes in two breeds of chicken

From its earliest days of domestication, the domestic chicken (Gallus gallus domesticus) has been selectively bred for specific traits. Decades of genetic selection have resulted in significant dissimilarities in metabolism and growth between breeds, in particular fast-growing broilers and highly productive layers. A chicken develops the capacity to elevate metabolism in response to decreases in ambient temperature upon hatching, including well-developed methods of regulating thermogenesis. However, a differential timing between incipient endothermic capacities of broiler and layer strains exists. Although both broiler and layer chicks show the hallmark rapid attainment of endothermic capacity of precocial birds, endothermic capacity of broilers matures faster than that of layers. Here we characterized changes in morphology and mitochondria physiology during the developmental transition as the animals become endothermic. Changes in body mass occurred at a faster rate in broilers, with hatching embryos showing significant increases over embryonic body mass, while layers did not exhibit significant differences in mass until after hatch. Heart and liver both exhibited rapid growth upon hatching that occurred with little change in body mass in both breeds. Skeletal and cardiac mitochondrial respiration capacity in broilers increased from the embryonic stage through hatching. Oxidative phosphorylation was more tightly coupled to ATP production in broilers than layer muscles during external pipping. By selecting for faster growth and higher meat yield, the physiological transition from ectothermy to endothermy was also affected: differences in whole-animal, tissue, and organelle responses are evident in these two divergent breeds of chicken.

Critical developmental windows for morphology and hematology revealed by intermittent and continuous hypoxic incubation in embryos of quail (Coturnix coturnix)

Hypoxia during embryonic growth in embryos is frequently a powerful determinant of development, but at least in avian embryos the effects appear to show considerable intra- and inter-specific variation. We hypothesized that some of this variation may arise from different protocols that may or may not result in exposure during the embryo’s critical window for hypoxic effects. To test this hypothesis, quail embryos (Coturnix coturnix) in the intact egg were exposed to hypoxia (~15% O₂) during “early” (Day 0 through Day 5, abbreviated as D0-D5), “middle” (D6-D10) or “late” (D11-D15) incubation or for their entire 16–18 day incubation (“continuous hypoxia”) to determine critical windows for viability and growth. Viability, body mass, beak and toe length, heart mass, and hematology (hematocrit and hemoglobin concentration) were measured on D5, D10, D15 and at hatching typically between D16 and D18 Viability rate was ~50–70% immediately following the exposure period in the early, middle and late hypoxic groups, but viability improved in the early and late groups once normoxia was restored. Middle hypoxia groups showed continuing low viability, suggesting a critical period from D6-D10 for embryo viability. The continuous hypoxia group experienced viability reaching <10% after D15. Hypoxia, especially during late and continuous hypoxia, also inhibited growth of body, beak and toe when measured at D15. Full recovery to normal body mass upon hatching occurred in all other groups except for continuous hypoxia. Contrary to previous avian studies, heart mass, hematocrit and hemoglobin concentration were not altered by any hypoxic incubation pattern. Although hypoxia can inhibit embryo viability and organ growth during most incubation periods, the greatest effects result from continuous or middle incubation hypoxic exposure. Hypoxic inhibition of growth can subsequently be “repaired” by catch-up growth if a final period of normoxic development is available. Collectively, these data indicate a critical developmental window for hypoxia susceptibility during the mid-embryonic period of development.

Energetic Effects of Pre-hatch Albumen Removal on Embryonic Development and Early Ontogeny in Gallus gallus

Studies on the yolk and albumen content in bird eggs, and the effects of variations in their relative loads in the phenotype of the birds, have revealed multiple consequences at different levels of biological organization, from biochemical traits to behavior. However, little is known about the effect of albumen variation on energetics performance during development and early ontogeny, despite the fact that variation in energy expenditure may have consequences in terms of fitness for both feral and domestic species. In this work, we evaluated experimentally whether variations in the content of albumen of Gallus gallus eggs could generate differences in metabolic rates during embryonic development. Additionally, we assessed changes in the activity of mitochondrial enzymes (cytochrome c oxidase and citrate synthase) in skeletal muscles and liver. Finally, we evaluated the success of hatching of these embryos and their metabolic rates (MR) post-hatching. The results revealed a significant reduction in MR in the last fifth of embryonic life, and reduced catabolic activities in the skeletal muscle of chicks hatched from albumen-removed eggs. However, the same group demonstrated an increase in catabolic activity in the liver, suggesting the existence of changes in energy allocation between tissues. Besides, we found a decrease in hatching success in the albumen-removed group, suggesting a negative effect of the lower albumen content on eggs, possibly due to lower catabolic activities in skeletal muscle. We also found a compensatory phenomenon in the first week after hatching, i.e., birds from albumen-removed eggs did not show a decrease in MR either at thermoneutral temperatures or at 10°C, compared to the control group. Collectively, our data suggest that a reduction in albumen may generate a trade-off between tissue metabolic activities, and may explain the differences in metabolic rates and hatching success, supporting the immediate adaptive response (IAR) hypothesis.

Incubation relative humidity induces renal morphological and physiological remodeling in the embryo of the chicken (Gallus gallus domesticus)

The metanephric kidneys of the chicken embryo, along with the chorioallantoic membrane, process water and ions to maintain osmoregulatory homeostasis. We hypothesized that changes in relative humidity (RH) and thus osmotic conditions during embryogenesis would alter the developmental trajectory of embryonic kidney function. White leghorn chicken eggs were incubated at one of 25-30% relative humidity, 55-60% relative humidity, and 85-90% relative humidity. Embryos were sampled at days 10, 12, 14, 16, and 18 to examine embryo and kidney mass, glomerular characteristics, body fluid osmolalities, hematological properties, and whole embryo oxygen consumption. Low and especially high RH elevated mortality, which was reflected in a 10-20% lower embryo mass on D18. Low RH altered several glomerular characteristics by day 18, including increased numbers of glomeruli per kidney, increased glomerular perfusion, and increased total glomerular volume, all indicating potentially increased functional kidney capacity. Hematological variables and plasma and amniotic fluid osmolalities remained within normal physiological values. However, the allantoic, amniotic and cloacal fluids had a significant increase in osmolality at most developmental points sampled. Embryonic oxygen consumption increased relative to control at both low and high relative humidities on Day 18, reflecting the increased metabolic costs of osmotic stress. Major differences in both renal structure and performance associated with changes in incubation humidity occurred after establishment of the metanephric kidney and persisted into late development, and likely into the postnatal period. These data indicate that the avian embryo deserves to be further investigated as a promising model for fetal programming of osmoregulatory function, and renal remodeling during osmotic stress.

Environmentally induced return to juvenile-like chemosensitivity in the respiratory control system of adult bullfrog, Lithobates catesbeianus

KEY POINTS

The degree to which developmental programmes or environmental signals determine physiological phenotypes remains a major question in physiology. Vertebrates change environments during development, confounding interpretation of the degree to which development (i.e. permanent processes) or phenotypic plasticity (i.e. reversible processes) produces phenotypes. Tadpoles mainly breathe water for gas exchange and frogs may breathe water or air depending on their environment and are, therefore, exemplary models to differentiate the degree to which life-stage vs. environmental context drives developmental phenotypes associated with neural control of lung breathing. Using isolated brainstem preparations and patch clamp electrophysiology, we demonstrate that adult bullfrogs acclimatized to water-breathing conditions do not exhibit CO₂ and O₂ chemosensitivity of lung breathing, similar to water-breathing tadpoles. Our results establish that phenotypes associated with developmental stage may arise from plasticity per se and suggest that a developmental trajectory coinciding with environmental change obscures origins of stage-dependent physiological phenotypes by masking plasticity.

ABSTRACT

An unanswered question in developmental physiology is to what extent does the environment vs. a genetic programme produce phenotypes? Developing animals inhabit different environments and switch from one to another. Thus a developmental time course overlapping with environmental change confounds interpretations as to whether development (i.e. permanent processes) or phenotypic plasticity (i.e. reversible processes) generates phenotypes. Tadpoles of the American bullfrog, Lithobates catesbeianus, breathe water at early life-stages and minimally use lungs for gas exchange. As adults, bullfrogs rely on lungs for gas exchange, but spend months per year in ice-covered ponds without lung breathing. Aquatic submergence, therefore, removes environmental pressures requiring lung breathing and enables separation of adulthood from environmental factors associated with adulthood that necessitate control of lung ventilation. To test the hypothesis that postmetamorphic respiratory control phenotypes arise through permanent developmental changes vs. reversible environmental signals, we measured respiratory-related nerve discharge in isolated brainstem preparations and action potential firing from CO₂ -sensitive neurons in bullfrogs acclimatized to semi-terrestrial (air-breathing) and aquatic-overwintering (no air-breathing) habitats. We found that aquatic overwintering significantly reduced neuroventilatory responses to CO₂ and O₂ involved in lung breathing. Strikingly, this gas sensitivity profile reflects that of water-breathing tadpoles. We further demonstrated that aquatic overwintering reduced CO₂ -induced firing responses of chemosensitive neurons. In contrast, respiratory rhythm generating processes remained adult-like after submergence. Our results establish that phenotypes associated with life-stage can arise from phenotypic plasticity per se. This provides evidence that developmental time courses coinciding with environmental changes obscure interpretations regarding origins of stage-dependent physiological phenotypes by masking plasticity.

Review: Effects of different growth rates in broiler breeder and layer hens on some productive traits

Genetic selection that has been carried out for several dozen years has led to significant progress in poultry production by improving productive traits and increasing the profitability of broiler breeder and layer hen production. After hatching, broilers and layers differ mainly in feed intake, growth rate, efficiency of nutrient utilization, and development of muscles and adipose tissue. A key role can be played by hormonal mechanisms of appetite control in broilers and layers. The paper discusses the consequences of different growth rates resulting from long-term genetic selection on feed intake, efficiency of nutrient utilization, and development of muscles and adipose tissue, with particular consideration of the hormonal mechanisms of appetite control in broilers and layers. The information presented in this review paper shows that it would be worth comparing these issues in a meta-analysis.

Developmental programming: a new frontier for the poultry industry?

Increasing evidence that the maternal environment influences the programming of developing embryos and fetuses through epigenetic mechanisms has significant potential application in the broiler industry. The broiler breeder hen is subjected to restricted-feeding regimes to maximise egg quantity and quality, but the genetically high-intake potential of these birds makes this regime a stressful one. We propose that this stress is signalled to the developing embryo via changes in yolk composition as an evolutionary adaptation to changing environments, and that exposure to high levels of corticosteroids in ovo is associated with developmental reprogramming, which has effects on the behaviour, health and growth of the progeny. The present paper describes some preliminary results from a series of trials designed to elucidate the relationship between breeder hen diet and egg composition, and the growth, behaviour and immune function of the progeny. We conclude that manipulation of the breeder hen diet is an untapped opportunity to maintain the competitiveness of the chicken meat industry and further, that achieving improved productivity by this means may be compatible with improved animal welfare outcomes for the hen and her progeny.

Differences in egg nutrient availability, development, and nutrient metabolism of broiler and layer embryos

Selection for production traits of broilers and layers leads to physiological differences, which may already be present during incubation. This study aimed to investigate the influence of strain (broiler vs layer) on egg nutrient availability, embryonic development and nutrient metabolism. A total of 480 eggs with an egg weight range of 62.0 to 64.0 g from Lohmann Brown Lite and Ross 308 breeder flocks of 41 or 42 weeks of age were selected in two batches of 120 eggs per batch per strain. For each batch, 30 eggs per strain were used to determine egg composition, including nutrient and energy content, and 90 eggs per strain were separately incubated in one of two climate respiration chambers at an eggshell temperature of 37.8°C. The results showed that broiler eggs had a higher ratio of yolk: albumen with 2.41 g more yolk and 1.48 g less albumen than layers. The yolk energy content of broiler eggs was 46.32 kJ higher than that of layer eggs, whereas total energy content of broiler eggs was 47.85 kJ higher compared to layer eggs. Yolk-free body mass at incubation day 16 and chick weight and length at hatch were higher in broilers compared to layers. Respiration quotient of broiler embryos was higher than layer embryos during incubation day 8 to incubation day 10. A 0.24 g lower residual yolk at the hatch of broiler embryos than for the layer embryos indicated that broiler embryos used more yolk and had a higher energy utilization and energy deposition in yolk-free body mass. Heat production of broiler embryos was higher than that of layer embryos from incubation day 12 to incubation day 18, but efficiency of converting egg energy used by embryos to form yolk-free body mass was similar. In conclusion, broiler and layer embryos have different embryonic development patterns, which affect energy utilization and embryonic heat production. However, the embryos are equal in efficiency of converting the energy used to yolk-free body mass.

Consequences of different growth rates in broiler breeder and layer hens on embryogenesis, metabolism and metabolic rate: A review

Intensive genetic selection of broiler breeders and layer hens for economically important production traits, which has been carried out for almost a century, resulted in considerable differences in the mechanisms of growth and development and, thus, in avian metabolism, both during embryogenesis and after hatching. Selection for meat production (broiler breeders) and eggs (layer hens) led to increased productivity but also brought about metabolic disorders. That intensive genetic selection of broiler breeders and layer hens is effective is seen, for example, in the differences in growth and development, metabolism of the yolk sac, hormones and lipids, gas exchange, and thermogenesis. Due to genetic proximity and different developmental mechanisms in broiler breeders and layer hens, avian embryos and chicks serve as excellent models for fundamental scientific research. This review paper discusses the consequences of different growth rates as a result of long-term genetic selection on embryonic development and metabolic rate of broilers and layers. The evidence presented herein indicates that it would be worth comparing these issues in a meta-analysis.

Epigenetics in comparative biology: why we should pay attention

The past decade has seen an explosion of articles in scientific journals involving non-genetic influences on phenotype through modulation of gene function without changes in gene sequence. The excitement in modern molecular biology surrounding the impact exerted by the environment on development of the phenotype is focused largely on mechanism and has not incorporated questions asked (and answers provided) by early philosophers, biologists, and psychologists. As such, this emergence of epigenetic studies is somewhat "old wine in new bottles" and represents a reformulation of the old debate of preformationism versus epigenesis-one resolved in the 1800s. Indeed, this tendency to always look forward, with minimal concern or regard of what has gone before, has led to the present situation in which "true" epigenetic studies are believed to consist of one of two schools. The first is primarily medically based and views epigenetic mechanisms as pathways for disease (e.g., "the epigenetics of cancer"). The second is primarily from the basic sciences, particularly molecular genetics, and regards epigenetics as a potentially important mechanism for organisms exposed to variable environments across multiple generations. There is, however, a third, and separate, school based on the historical literature and debates and regards epigenetics as more of a perspective than a phenomenon. Against this backdrop, comparative integrative biologists are particularly well-suited to understand epigenetic phenomena as a way for organisms to respond rapidly with modified phenotypes (relative to natural selection) to changes in the environment. Using evolutionary principles, it is also possible to interpret "sunsetting" of modified phenotypes when environmental conditions result in a disappearance of the epigenetic modification of gene regulation. Comparative integrative biologists also recognize epigenetics as a potentially confounding source of variation in their data. Epigenetic modification of phenotype (molecular, cellular, morphological, physiological, and behavioral) can be highly variable depending upon ancestral environmental exposure and can contribute to apparent "random" noise in collected datasets. Thus, future research should go beyond the study of epigenetic mechanisms at the level of the gene and devote additional investigation of epigenetic outcomes at the level of both the individual organism and how it affects the evolution of populations. This review is the first of seven in this special issue of Integrative and Comparative Biology that addresses in detail these and other key topics in the study of epigenetics.

Environmental stressors and the epigenome

Epigenetic modification and transgenerational transfer of phenotype at the individual or population level, particularly in response to environmental change, is at the forefront of biological investigation. The plasticity of this process allows an organism to respond to changes in environmental conditions, potentially conferring a survival advantage. In this review, we discuss epigenetic transgenerational phenomena in the specific context of environmental stressors including hypoxia and environmental toxicants.:

Transgenerational epigenetics: the role of maternal effects in cardiovascular development

Transgenerational epigenetics, the study of non-genetic transfer of information from one generation to the next, has gained much attention in the past few decades due to the fact that, in many instances, epigenetic processes outweigh direct genetic processes in the manifestation of aberrant phenotypes across several generations. Maternal effects, or the influences of maternal environment, phenotype, and/or genotype on offsprings' phenotypes, independently of the offsprings' genotypes, are a subcategory of transgenerational epigenetics. Due to the intimate role of the mother during early development in animals, there is much interest in investigating the means by which maternal effects can shape the individual. Maternal effects are responsible for cellular organization, determination of the body axis, initiation and maturation of organ systems, and physiological performance of a wide variety of species and biological systems. The cardiovascular system is the first to become functional and can significantly influence the development of other organ systems. Thus, it is important to elucidate the role of maternal effects in cardiovascular development, and to understand its impact on adult cardiovascular health. Topics to be addressed include: (1) how and when do maternal effects change the developmental trajectory of the cardiovascular system to permanently alter the adult's cardiovascular phenotype, (2) what molecular mechanisms have been associated with maternally induced cardiovascular phenotypes, and (3) what are the evolutionary implications of maternally mediated changes in cardiovascular phenotype?

Epigenetics as a source of variation in comparative animal physiology – or – Lamarck is lookin' pretty good these days

Considerable variation is inherent both within and between comparative physiological data sets. Known sources for such variation include diet, gender, time of day and season of experiment, among many other factors, but a meta-analysis of physiological studies shows that surprisingly few studies report controlling for these factors. In fact, less than 3% of comparative physiological papers mention epigenetics. However, our understanding of epigenetic influences on physiological processes is growing rapidly, and it is highly likely that epigenetic phenomena are an additional ‘hidden’ source of variation, particularly in wild-caught specimens. Recent studies have shown epigenetic inheritance of commonly studied traits such as metabolic rate (water fleas Daphnia magna; emu, Dromaius novaellandiae), hypoxic tolerance, cardiac performance (zebrafish, Danio rerio), as well as numerous morphological effects. The ecological and evolutionary significance of such epigenetic inheritance is discussed in a comparative physiological context. Finally, against this context of epigenetic inheritance of phenotype, this essay also provides a number of caveats and warnings regarding the interpretation of transgenerational phenotype modification as a true epigenetic phenomenon. Parental effects, sperm storage, multiple paternity and direct gamete exposure can all be confounding factors. Epigenetic inheritance may best be studied in animal models that can be maintained in the laboratory over multiple generations, to yield parental stock that themselves are free of epigenetic effects from the historical experiences of their parents.

Transgenerational epigenetic effects on innate immunity in broilers: an underestimated field to be explored?

Transgenerational epigenetics is becoming more and more important for understanding the variation of physiological responses of individuals to the environment and the inheritance of these responses based on all mechanisms other than the actual DNA nucleotide sequence. Transgenerational epigenetics is the phenomenon that the information of the environment of (usually) a female animal is translated into memory-like responses preparing the offspring. As a consequence, individuals of the next generation may show different phenotypic traits depending whether their mothers were kept under different environmental conditions. This may result in either positive or negative effects on the next-generation individuals, which is different from individuals from mothers that have been kept in a different environment. Transgenerational epigenetic effects have been proposed and indicated for specific immune (T cell and antibody) responses (especially in mammals, but also in birds) and innate immunity (nonvertebrates), but surprisingly very little is known of transgenerational effects on innate immunity in chickens. Given the short lifespan of the chicken and therefore the likely dependence of chicken on innate immune mechanisms, more attention should be given to this arm of immunity and mechanisms of inheritance including transgenerational effects that can be initiated in the breeder generation. In addition, it is becoming evident that innate immunity also underlies metabolic disorders in broilers. In the current paper, we will argue that although very little is known of transgenerational effects of innate immunity in poultry, more attention should be given to this type of study. We will illustrate examples of transgenerational epigenetics, and finally propose strategies that should reveal the presence of transgenerational epigenetic effects on innate immunity in chickens and strategies to modulate breeder birds such that these effects positively affect innate immunity of broilers. It is suggested that a mismatch between breeder environment and broiler environment may account for unwanted effects of innate immunity in the broiler.

Maternal diet influences offspring feeding behavior and fearfulness in the precocial chicken

Background

In chicken, oils in the maternal diet confer a specific scent to the yolk. Embryos are known to perceive and memorize chemosensory signals of the surrounding environment; however, the potential impact of the maternal diet has not previously been investigated. In the present study, we hypothesized that chicken embryos memorize the chemical signals of the maternal diet and that this perceptual learning may orient subsequent feeding behavior of the hatchlings.

Methodology/Principal Findings

Laying hens were fed standard food enriched with 2% menhaden oil (MH group) or 2% soybean oil (controls). The scent of menhaden was significantly more detected in MH egg yolks than in control yolks by a human panel. We analyzed the development and behavior of offspring towards different types of food, bearing or not bearing the menhaden scent. When chicks were exposed to a 3-min choice test between the familiar food bearing the menhaden scent and the familiar food without menhaden, no effect of treatment was observed. In a 3-min choice test with unfamiliar food (mashed cereals) MH chicks showed a clear positive orientation toward the unfamiliar food bearing the menhaden scent. By contrast, control chicks showed a preference for the non-odorized unfamiliar food. MH chicks expressed higher emotional reactivity level than control chicks as expressed by food neophobia and longer immobility in a restraint test.

Conclusion/Significance

Chicks exposed in ovo to menhaden oil via the maternal diet preferentially oriented their feeding behavior towards food containing menhaden oil, but only when the food was unfamiliar. We propose that oil in the maternal diet engenders maternal effects and contributes to the development of behavioral phenotype in the offspring. In ovo chemosensory learning may have evolved to prepare precocial offspring for their environment. This suggests a common principle of embryonic chemosensory learning across vertebrate taxa.

Metamorphosis in a frog that does not have a tadpole

The evolutionary removal of the tadpole from the frog life history is a very successful strategy, particularly in the tropics. These direct developers form limbs and a frog-like head early in embryogenesis, and they have reduced or lost tadpole-specific structures, like gills, a long, coiled intestine, and tadpole teeth and jaws. Despite the apparently continuous development to the frog morphology, the direct developer, Eleutherodactylus coqui, undergoes a cryptic metamorphosis requiring thyroid hormone. As in Xenopus laevis, there is a stimulation by corticotrophin-releasing factor (CRF) and an upregulation of thyroid hormone receptor β (thrb). In addition to changes in skin and muscle, thyroid hormone stimulates yolk utilization for froglet growth from a novel tissue, the nutritional endoderm. The activities of CRF and corticosterone (CORT) in metamorphosis may provide the basis for the multiple evolutionary origins of direct development in anuran amphibians. Potential roles for maternally supplied thyroid hormone and its receptor and for deiodinases in regulating tissue sensitivity to thyroid hormone should be the subjects of future investigations.

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.

Developmental diversity of amphibians

The current model amphibian, Xenopus laevis, develops rapidly in water to a tadpole which metamorphoses into a frog. Many amphibians deviate from the X. laevis developmental pattern. Among other adaptations, their embryos develop in foam nests on land or in pouches on their mother's back or on a leaf guarded by a parent. The diversity of developmental patterns includes multinucleated oogenesis, lack of RNA localization, huge non-pigmented eggs, and asynchronous, irregular early cleavages. Variations in patterns of gastrulation highlight the modularity of this critical developmental period. Many species have eliminated the larva or tadpole and directly develop to the adult. The wealth of developmental diversity among amphibians coupled with the wealth of mechanistic information from X. laevis permit comparisons that provide deeper insights into developmental processes.

In ovo temperature manipulation differentially influences limb musculoskeletal development in two lines of chick embryos selected for divergent growth rates

Selective breeding has led to diverging phenotypic evolution in layer and broiler chickens through genomic and epigenetic modifications. Here we show that in ovo environmental manipulation differentially influences embryonic limb muscle phenotype in these two breeds. We demonstrate that raising incubation temperature from 37.5 to 38.5°C between embryonic days (ED) 4 and 7 increased motility and body mass in both layer and broiler embryos. In layers, this was accompanied by gastrocnemius muscle hypertrophy, increased fibre and nuclei numbers and a higher nuclei to fibre ratio (ED18), preceded by increased hindlimb Myf5 (ED5–8), Pax7 (ED5–10), BMP4 (ED6–9) and IGF-I (ED9–10, ED18) mRNAs. In broilers, the same temperature treatment led to reduced gastrocnemius cross-sectional area with fewer fibres and nuclei and an unchanged fibre to nuclei ratio (ED18). This was preceded by a delay in the peak of hindlimb Myf5 expression, increased Pax7 (ED5, ED7–10) and BMP4 (ED6–8) but reduced IGF-I (ED8–10) mRNAs. Rather than promoting myogenesis as in layer embryos, the temperature treatment promoted gastrocnemius intramuscular fat deposition in broilers (ED18) preceded by increased hindlimb PPARγ mRNA (ED7–10). The treatment increased tibia/tarsus bone length as well as femur cross-sectional area in both breeds, but femur length and bone to cartilage ratio in the femur and tibia/tarsus were only increased in treated layers (ED18). We conclude that in ovo temperature manipulation differentially affected the molecular regulation of hindlimb myogenic, adipogenic and growth factor expression in broiler and layer embryos, leading to differential changes in muscle phenotype. The underlying interactive mechanisms between genes and the environment need further investigation.

Disentangling prenatal and postnatal maternal genetic effects reveals persistent prenatal effects on offspring growth in mice

Mothers are often the most important determinant of traits expressed by their offspring. These "maternal effects" (MEs) are especially crucial in early development, but can also persist into adulthood. They have been shown to play a role in a diversity of evolutionary and ecological processes, especially when genetically based. Although the importance of MEs is becoming widely appreciated, we know little about their underlying genetic basis. We address the dearth of genetic data by providing a simple approach, using combined genotype information from parents and offspring, to identify "maternal genetic effects" (MGEs) contributing to natural variation in complex traits. Combined with experimental cross-fostering, our approach also allows for the separation of pre- and postnatal MGEs, providing rare insights into prenatal effects. Applying this approach to an experimental mouse population, we identified 13 ME loci affecting body weight, most of which (12/13) exhibited prenatal effects, and nearly half (6/13) exhibiting postnatal effects. MGEs contributed more to variation in body weight than the direct effects of the offsprings' own genotypes until mice reached adulthood, but continued to represent a major component of variation through adulthood. Prenatal effects always contributed more variation than postnatal effects, especially for those effects that persisted into adulthood. These results suggest that MGEs may be an important component of genetic architecture that is generally overlooked in studies focused on direct mapping from genotype to phenotype. Our approach can be used in both experimental and natural populations, providing a widely practicable means of expanding our understanding of MGEs.