A Review of eggshell pigmentation

A Network of Circular RNA and MicroRNA Sequencing Provides Insights into Pigment Deposition of Changshun Blue Eggshell Chickens

Eggshell color plays important biological roles and attracts the attention of both egg retailers and researchers. However, whether non-coding RNAs are involved in pigment deposition among different eggshell colors remains unknown. In this study, RNA sequencing was used to analyse the uterine gland transcriptome (CircRNA and miRNA) of Changshun chicken blue-shell hens producing four different eggshell color eggs including dark blue PK(DB) and light blue (LB), dark brown and greenish (between blue and pink, DP) and pink (p). We found that miR-192-x, targeting SLC16a7, was expressed in DB, DP, and LB groups compared with the PK group, which indicates that miR-192-x may play a role in the blue eggshell color. KEGG and GO analyses showed that the "metabolic pathways" with targeted genes such BLVRA and HMOX1 were detected in dark and light blue color eggshell chickens, which confirms the different ratios of biliverdin and HO-1 involved in the deposition of blue color. As annotated by connectivity analysis, RASGRF1 and RASGRF2, belonging to the RASGRF family, are involved in the Ras signaling pathway, which plays an important role in cell growth, differentiation, metastasis and apoptosis. Our findings enrich the database of circRNA, miRNAs and genes for chicken uterine tissue, which will be useful in accelerating molecular selection for blue eggshell color layers.

Refine localizations of functional variants affecting eggshell color of Lueyang black-boned chicken in the SLCO1B3

Table eggs with color-uniformity shell are visually attractive for consumers. Lueyang black-boned chicken (LBC) lays colorful eggs, which is undesirable for sale of table eggs, but provides a segregating population for mapping functional variants affecting eggshell color. SLCO1B3 was identified as the causative gene for blue eggs in the Dongxiang and Araucana chickens. The aim of this study is to map functional variants associated with chicken eggshell color in the SLCO1B3. Eggshell color of LBC (n = 383) was measured using the L*a*b color space. SLCO1B3 was resequencing using a subset (n = 30) of 383 samples. Linkage disequilibrium among 139 SNP was analyzed. Association of 16 SNP in the SLCO1B3 and 8 in CPOX, ALAS1, and ABCG2 genes with L*a*b were tested by a polygenic model (LMM) and a polygenic/oligogenic mixed model (BSLMM). Chromatin state annotations were retrieved from the UCSC database. Effect of SLCO1B3 variants distributed in mapping and upstream 1.6-kb regions on promoter activities were analyzed using dual-luciferase reporter assay. One hundred and thirty-nine variants maintained low linkage disequilibrium with 80% of r2 less than 0.226. Fifteen SLCO1B3 variants were significantly associated with a*, of which 1B3_SNP108 was showed the strongest association and the largest effect on a*. In the BSLMM, 1B3_SNP108 alone appeared in the Markov chain Monte Carlo as major variants in 100% of posterior inclusion probability. None of variants in CPOX, ALAS1, and ABCG2 were significantly associated with color indexes except that 2 ALAS1 variants were associated with L*. 1B3_SNP108 distributes in the Intron4 where 6 active enhancers and 1 ATAC island were enriched. However, 1B3_SNP108-containing constructs showed negligible activities in the reporter assay. No significant differences of activities between haplotypes were found for five 5'-deleted promoter constructs. The data recognizes 1B3_SNP108 as a valuable marker for breeding of eggshell color. Functional variants are localized in the region adjacent to the 1B3_SNP108 due to low linkage disequilibrium in the LBC. Our findings extend the role of SLCO1B3 from a causative gene for blue eggs to a major regulator driving continuous variation of LBC eggshell color.

Effects of Enriched-in-Oleuropein Olive Leaf Extract Dietary Supplementation on Egg Quality and Antioxidant Parameters in Laying Hens

The objective of the present study was to evaluate the effects of an olive leaf extract obtained with an up-to-date laboratory method, when supplemented at different levels in laying hens' diets, on egg quality, egg yolk antioxidant parameters, fatty acid content, and liver pathology characteristics. Thus, 96 laying hens of the ISA-Brown breed were allocated to 48 experimental cages with two hens in each cage, resulting in 12 replicates per treatment. Treatments were: T1 (Control: basal diet); T2 (1% olive leaf extract); T3 (2.5% olive leaf extract); T4 (Positive control: 0.1% encapsulated oregano oil). Eggshell weight and thickness were improved in all treatments compared to the control, with T2 being significantly higher till the end of the experiment (p < 0.001). Egg yolk MDA content was lower for the T2 and T4 groups, while total phenol content and Haugh units were greater in the T2. The most improved fatty acid profile was the one of T3 yolks. The α-tocopherol yolk content was higher in all groups compared to T1. No effect was observed on cholesterol content at any treatment. Based on the findings, it can be inferred that the inclusion of olive leaf extract at a concentration of 1% in the diet leads to enhancements in specific egg quality attributes, accompanied by an augmentation of the antioxidant capacity.

Genetics and breeding of a black-bone and blue eggshell chicken line. 3. Visual eggshell color and colorimeter parameters in 3 consecutive generations

The BG line, originated by crossing 2 Chinese indigenous breeds, Dongxiang blue eggshell and Jiangshan black-bone, has been bred for black carcass and blue-greenish eggs. Aiming to study the genetic parameters and selection aspects of these eggshell colors, the 4 colorimeter parameters (L*, a*, b*, SCI = L*-a*-b*) were measured on ∼5 eggs/hen/age (200 d and 300 d) from each hen in 3 generations (G4 = 452, G5 = 508, G6 = 498). Visual eggshell color was classified as either "Light," "Blue," "Green," or "Olive," and data from G4 and G5 indicated that visual eggshell color was more accurately determined by combining the classifications of single representative egg/hen by 4 independent observers. Based on the apparent gradual variation in visual color, the 4 colors were expressed numerically (Light = 1, Blue = 2, Green = 3, Olive = 4) and the averages of the 4 observers (AveObs) were used as quantitative expression of the visual color of each egg. This expression, in the range from Blue to Olive, was highly significantly correlated with L*, b* and SCI. The a* values were also associated with AveObs, but not linearly; AveObs between 2 (Blue) and 3 (Green) had lowest a*, and it increased as AveObs was more Light (<2) or more Olive (>3). The heritability estimates of the colorimeter parameters were mostly very high; those of b* and SCI ranged between 0.7 and 0.8, and those of L* and a* between 0.6 and 0.7, indicating that they can serve as criterions to select for blue and/or green eggshells. The phenotypic and genetic correlations between the colorimeter parameters were highly significant and favorable. It is suggested that effective breeding for blue eggs can be done by selecting hens laying eggs with highest SCI/L* or lowest b* (against green and olive shades), followed by selection for low a* (against light shades). Breeding for green eggs can be done by selecting hens laying eggs with SCI ≈ 75 and/or L* ≈ 80 and/or b* ≈ 12. Breeding for hens that lay either blue or green eggs can be done by selection for low a* values.

Effect of translucency and eggshell color on broiler breeder egg hatchability and hatch chick weight

A successful hatch has a considerable economic impact on all poultry companies. The aim of the current study was to describe the possible effects of shell translucency (T score) and coloration lightness (L* value) on shell thickness, hatchability, and chick weight. A total of 4,320 eggs from 4 commercial Ross 708 breeder flocks (50-55-wk old) were used. Eggs were selected for T score and L* value. A 3-point subjective scoring system was used for T score (1 = low, 2 = medium, 3 = high), and an electronic colorimeter for L* value, sorting the eggs as light (avg. L* = 80.7) or dark (avg. L* = 76.0). Data were analyzed using the GLIMMIX procedure of SAS (V9.4) and Tukey's HSD test was performed to separate means, a significant difference was considered when P ≤ 0.05. Results suggest that the color of the eggshell was related to the egg weight on the day of collection (P = 0.0056) and at transfer (P = 0.0211), in both cases dark eggs were 0.6 g heavier than light eggs. Dark eggs had a 3.8% increased hatchability of egg set (P = 0.0481) and yielded 6 µm thicker shells (P = 0.0019) when compared to light eggs. Regarding translucency, egg weight at transfer was 0.8 g heavier for T score 1 eggs compared to T score 3 (P = 0.0358). The translucency score of 1 had a 6.9% higher hatchability of eggs set (P = 0.0127) and 0.7 g heavier chick weight (P = 0.0385) compared to T score 3. However, T score 1 eggs had shells 28 µm thinner than the T score 2 and 34 µm thinner than T score 3 (P < 0.0001). An interaction effect was observed for eggshell thickness, L* value, and T score, where eggs classified as light with T score 1 had thinner eggshells compared to those that were dark with T score 3 (P = 0.0292). These results suggest that eggshell translucency and coloration lightness can be good noninvasive indicators of eggshell thickness, hatchability, and chick weight in broiler breeder flocks.

The Pigments in Eggshell with Different Color and the Pigment Regulatory Genes Expression in Corresponding Chicken’s Shell Gland

Eggshell colour is the unique appearance and economically valuable trait of eggs, whereas the colour is often short of uniformity, especially in the blue-shelled breeds, hence, their pigment differences and molecular mechanism need clarity. To investigate the relationship between the pigment content of eggshells and related gene expression in the eggshell glands of chickens, four subtypes of blue-shelled eggs ('Olive', 'Green', 'Blue', and 'Light') from the same blue-eggshell chicken line were selected; Hy-Line 'White' and 'Brown'-shelled eggs were used as control groups. The L*, a*, b* values, and protoporphyrin-IX and biliverdin contents in each group of eggshells were measured. In addition, the shell glands of the corresponding hens were collected to detect SLCO1B3 genotype and mRNA expression, and ABCG2 and HMOX1 transcription and protein expression. Eggshell colour L* values were negatively correlated with protoporphyrin-IX, b* values were positively correlated with total pigment content (P < 0.001), and a* values were positively correlated with protoporphyrin-IX (P < 0.001) but negatively with biliverdin. Moreover, all four blue-eggshell subtypes were SLCO1B3 homozygous, with SLCO1B3 mRNA expression in shell glands being significantly higher than in the White and Brown groups. ABCG2 and HMOX1 mRNA expression were highest in the Brown and Green groups, respectively (P < 0.05), and were positively correlated with protoporphyrin-IX (P < 0.001) and biliverdin contents in eggshells, respectively. Western blot and immunohistochemical results demonstrated that the Brown group had the highest ABCG2 expression (P < 0.05), followed by the Green and Olive groups. HMOX1 protein expression was higher in the Olive and Green groups (P < 0.05), and lowest in the White group. This study suggests that ABCG2 and HMOX1 have important regulatory roles in the production and transport of protoporphyrin-IX and biliverdin in blue-shelled chicken eggs, respectively.

Effects of dietary organic or inorganic iron concentrations on productive performance, egg quality, blood measurements, and tissue iron concentrations in aged laying hens

The objective of the current experiment was to investigate the effects of dietary organic or inorganic iron (Fe) concentrations on productive performance, egg quality, blood measurements, and tissue Fe concentrations in aged laying hens. A total of three hundred fifty 60-week-old Hy-Line Brown laying hens were allotted to one of five dietary treatments with seven replicates. Each replicate had 10 consecutive cages. Organic Fe (Fe-Gly) or inorganic Fe (FeSO₄ ) was added to the basal diet at the levels of 100 or 200 mg/kg Fe. Diets were fed on an ad libitum basis for 6 weeks. Results indicated that supplementation of organic or inorganic Fe in diets increased (p < 0.05) eggshell color and feather Fe concentrations compared with no supplementation of Fe in diets. An interaction was found (p < 0.05) between Fe sources and supplemental levels in diets for egg weight, eggshell strength, and Haugh unit. Hens fed diets supplemented with organic Fe had greater (p < 0.05) eggshell color and hematocrit than those fed diets supplemented with inorganic Fe. In conclusion, dietary supplementation of organic Fe increases the eggshell color of aged laying hens. High supplemental levels of organic Fe in diets improve egg weight in aged laying hens.

Properties, Genetics and Innate Immune Function of the Cuticle in Egg-Laying Species

Cleidoic eggs possess very efficient and orchestrated systems to protect the embryo from external microbes until hatch. The cuticle is a proteinaceous layer on the shell surface in many bird and some reptile species. An intact cuticle forms a pore plug to occlude respiratory pores and is an effective physical and chemical barrier against microbial penetration. The interior of the egg is assumed to be normally sterile, while the outer eggshell cuticle hosts microbes. The diversity of the eggshell microbiome is derived from both maternal microbiota and those of the nesting environment. The surface characteristics of the egg, outer moisture layer and the presence of antimicrobial molecules composing the cuticle dictate constituents of the microbial communities on the eggshell surface. The avian cuticle affects eggshell wettability, water vapor conductance and regulates ultraviolet reflectance in various ground-nesting species; moreover, its composition, thickness and degree of coverage are dependent on species, hen age, and physiological stressors. Studies in domestic avian species have demonstrated that changes in the cuticle affect the food safety of eggs with respect to the risk of contamination by bacterial pathogens such as Salmonella and Escherichia coli. Moreover, preventing contamination of internal egg components is crucial to optimize hatching success in bird species. In chickens there is moderate heritability (38%) of cuticle deposition with a potential for genetic improvement. However, much less is known about other bird or reptile cuticles. This review synthesizes current knowledge of eggshell cuticle and provides insight into its evolution in the clade reptilia. The origin, composition and regulation of the eggshell microbiome and the potential function of the cuticle as the first barrier of egg defense are discussed in detail. We evaluate how changes in the cuticle affect the food safety of table eggs and vertical transmission of pathogens in the production chain with respect to the risk of contamination. Thus, this review provides insight into the physiological and microbiological characteristics of eggshell cuticle in relation to its protective function (innate immunity) in egg-laying birds and reptiles.

Low levels of organic compound trace elements improve the eggshell quality, antioxidant capacity, immune function, and mineral deposition of aged laying hens

In the egg production industry, trace elements are required as additional dietary supplements to play vital roles in performance and egg quality. Compared to inorganic microelements (ITs), appropriate dose of organic trace microelements (OTs) are environmentally friendly and sufficient to satisfy the needs of hens. In order to evaluate the extent to which low-dose OTs replace whole ITs, the effects of organic copper, zinc, manganese, and iron compound on the performance, eggshell quality, antioxidant capacity, immune function, and mineral deposition of old laying hens were investigated. A total of 1 080 57-week-old Jing Hong laying hens were assigned to five groups with six replicates of 36 layers each for an 8-week experimental period. The birds were fed either a basal diet (control treatment (CT)) or the basal diet supplemented with commercial levels of inorganic trace elements (IT 100%) or the equivalent organic trace elements at 20%, 30%, and 50% of the inorganic elements (OT 20%, OT 30%, and OT 50%, respectively). Results showed that compared with those in the CT treatment, feeding hens with inorganic or organic microelement diet had significant effects on the eggshell quality, antioxidant capacity, immune function, and mineral deposition of old laying hens (P < 0.05). The eggshell strength and ratio between OT 30%, OT 50%, and IT 100% were similar at weeks 4 and 8, and the eggshell thickness of these groups was also similar at weeks 6 and 8. At week 8, the eggshell colour in OT 50% was darker than that in IT 100%. The mineral content in the eggshells of OT 50% and IT 100% significantly increased (P < 0.001), with no significant difference in effective thickness, mammillary thickness, and mammillary knob width between groups. There were no differences in the malondialdehyde content, total antioxidant capacity, and total superoxide dismutase activity in serum between OT 30%, OT 50%, and IT100%. While the catalase activities, the interleukin-1β, interleukin-10, immunoglobulin G, and immunoglobulin M concentrations in serum were not significantly different between OT 50% and IT 100%. The mineral contents in the faeces of the organic groups were considerably reduced compared with those in IT 100% (P < 0.001). In conclusion, dietary supplementation with 30-50% organic compound microelements has the potential to replace 100% inorganic microelements in the hen industry for improving eggshell quality, mineral deposition in the eggshell, antioxidant capacity, and immune function, and reducing emissions to the environment without negative effects on laying performance.

Insights Into mRNA and Long Non-coding RNA Profiling RNA Sequencing in Uterus of Chickens With Pink and Blue Eggshell Colors

The blue egg is both of biological interest and economic importance for consumers, egg retailers, and scientists. To date, the genetic mechanisms underlying pigment have mainly focused on protein-coding genes. However, the underpinning mechanism of non-coding RNAs on the pigment deposition among different eggshell colors remains unknown. In this study, RNA sequencing was employed to profile the uterine gland transcriptome (lncRNA and mRNA) of 15 Changshun blue eggshell layers, to better understand the genetic mechanisms of deposition of blue eggshell color. Results showed that differentially expressed mRNAs, GO terms, and KEGG pathways among pink-eggshell and blue-eggshell chickens were mainly targeting immune- and transporter-related terms with the SLC family, IgJ, CD family, and MTMR genes. Furthermore, the progesterone-mediated oocyte maturation and cortisol synthesis and secretion pathway with targeted gene PGR and Pbx1 were significantly enriched between blue- and pink-eggshell chickens. Integrating analysis of lncRNA and mRNA profiles predicted 4 and 25 lncRNA–gene pairs by antisense and cis analysis. They were relative to immune, nerve, and lipids and amino acid metabolisms, porphyrin, and chlorophyll metabolism with targeted gene FECH and oxidative phosphorylation and cardiac muscle contraction pathways with targeted gene COX6A1. Within blue-eggshell chickens, the GO terms hindbrain tangential cell migration and phosphatidylinositol monophosphate phosphatase activity with targeted gene Plxna2 and MTRM1 were identified. Integrating analysis of lncRNA and mRNA profiles predicted 8 and 22 lncRNA–gene pairs. Most pathways were mainly enriched on lipid-related metabolisms as found in mRNA sequencing. The lncRNAs did exert similar functions in color formation by modulating pigment disposition and immune- and lipid-related metabolisms. Our results provide a catalog of chicken uterine lncRNAs and genes worthy of further studies to understand their roles in the selection for blue eggshell color layers.

Connecting the dots: avian eggshell pigmentation, female condition and paternal provisioning effort

Differences in avian eggshell pigmentation could be an honest signal of female quality that males use to inform their nestling provisioning effort. We investigated whether among-individual variation in protoporphyrin-based eggshell pigmentation in house wrens (Troglodytes aedon) reflects female fitness-associated traits and whether males use that information. Females laying lighter clutches were older and larger than females laying darker clutches. Nestlings hatching from lighter clutches had greater size-corrected mass on post-hatch day 11, a measure that strongly predicts survival and recruitment to the breeding population. To test whether male provisioning effort responds to clutch pigmentation, we used a reciprocal clutch cross-fostering design, swapping dark with light clutches and light with dark; in controls, we swapped light with light clutches and dark with dark. Shortly before hatching, clutches were returned to their original nest to avoid confounding effects of nestling quality on male provisioning. Contrary to the sexual selection hypothesis, clutch pigmentation had no effect on male provisioning. Males were probably able to observe eggshell pigmentation and thus had information about female quality, but they did not use this information to modulate their nestling provisioning. This may be because of constraints on species-specific reproductive opportunities, or because variation in eggshell protoporphyrin serves other functions.

Pigmentation of White, Brown, and Green Chicken Eggshells Analyzed by Reflectance, Transmittance, and Fluorescence Spectroscopy

We report on the reflectance, transmittance and fluorescence spectra (λ=200-1200 nm) of four types of chicken eggshells (white, brown, light green, dark green) measured in situ without pretreatment and after ablation of 20-100 μm of the outer shell regions. The color pigment protoporphyrin IX (PPIX) is embedded in the protein phase of all four shell types as highly fluorescent monomers, in the white and light green shells additionally as non-fluorescent dimers, and in the brown and dark green shells mainly as non-fluorescent poly-aggregates. The green shell colors are formed from an approximately equimolar mixture of PPIX and biliverdin. The axial distribution of protein and colorpigments were evaluated from the combined reflectances of both the outer and inner shell surfaces, as well as from the transmittances. For the data generation we used the radiative transfer model in the random walk and Kubelka-Munk approaches.

Genetic variation and potential for genetic improvement of cuticle deposition on chicken eggs

Background

The cuticle is an invisible glycosylated protein layer that covers the outside of the eggshell and forms a barrier to the transmission of microorganisms. Cuticle-specific staining and in situ absorbance measurements have been used to quantify cuticle deposition in several pure breeds of chicken. For brown eggs, a pre-stain and a post-stain absorbance measurement is required to correct for intrinsic absorption by the natural pigment. For white eggs, a post-stain absorbance measurement alone is sufficient to estimate cuticle deposition. The objective of the research was to estimate genetic parameters and provide data to promote adoption of the technique to increase cuticle deposition and reduce vertical transmission of microorganisms.

Results

For all pure breeds examined here, i.e. Rhode Island Red, two White Leghorns, White Rock and a broiler breed, the estimate of heritability for cuticle deposition from a meta-analysis was moderately high (0.38 ± 0.04). In the Rhode Island Red breed, the estimate of the genetic correlation between measurements recorded at early and late times during the egg-laying period was ~ 1. There was no negative genetic correlation between cuticle deposition and production traits. Estimates of the genetic correlation of cuticle deposition with shell color ranged from negative values or 0 in brown-egg layers to positive values in white- or tinted-egg layers. Using the intrinsic fluorescence of tryptophan in the cuticle proteins to quantify the amount of cuticle deposition failed because of complex quenching processes. Tryptophan fluorescence intensity at 330 nm was moderately heritable, but there was no evidence of a non-zero genetic correlation with cuticle deposition. This was complicated furthermore by a negative genetic correlation of fluorescence with color in brown eggs, due to the quenching of tryptophan fluorescence by energy transfer to protoporphyrin pigment. We also confirmed that removal of the cuticle increased reflection of ultraviolet wavelengths from the egg.

Conclusions

These results provide additional evidence for the need to incorporate cuticle deposition into breeding programs of egg- and meat-type birds in order to reduce vertical and horizontal transmission of potentially pathogenic organisms and to help improve biosecurity in poultry.

Electronic supplementary material

The online version of this article (10.1186/s12711-019-0467-5) contains supplementary material, which is available to authorized users.

Association Between the Methylation Statuses at CpG Sites in the Promoter Region of the SLCO1B3, RNA Expression and Color Change in Blue Eggshells in Lushi Chickens

The formation mechanism underlying the blue eggshell characteristic has been discovered in birds, and SLCO1B3 is the key gene that regulates the blue eggshell color. Insertion of an endogenous retrovirus, EAV-HP, in the SLCO1B3 5′ flanking region promotes SLCO1B3 expression in the chicken shell gland, and this expression causes bile salts to enter the shell gland, where biliverdin is secreted into the eggshell, forming a blue shell. However, at different laying stages of the same group of chickens, the color of the eggshell can vary widely, and the molecular mechanism underlying the eggshell color change remains unknown. Therefore, to reveal the molecular mechanism of the blue eggshell color variations, we analyzed the change in the eggshell color during the laying period. The results indicated that the eggshell color in Lushi chickens can be divided into three stages: 20–25 weeks for dark blue, 26–45 weeks for medium blue, and 46–60 weeks for light blue. We further investigated the expression and methylation levels of the SLCO1B3 gene at eight different weeks, finding that the relative expression of SLCO1B3 was significantly higher at 25 and 30 weeks than at other laying weeks. Furthermore, the overall methylation rate of the SLCO1B3 gene in Lushi chickens increased gradually with increasing weeks of egg production, as shown by bisulfite sequencing PCR. Pearson correlation analysis showed that methylation of the promoter region of SLCO1B3 was significantly negatively correlated with both SLCO1B3 expression in the shell gland tissue and eggshell color. In addition, we predicted that CpG5 and CpG8 may be key sites for regulating SLCO1B3 gene transcription. Our findings show that as the level of methylation increases, methylation of the CpG5 and CpG8 sites hinders the binding of transcription factors to the promoter, reducing SLCO1B3 expression during the late period and resulting in a lighter eggshell color.

Genome-wide association study and a post replication analysis revealed a promising genomic region and candidate genes for chicken eggshell blueness

The eggshell blueness is an interesting object for chicken genetic studies and blue-shelled chicken industry, especially after the discovery of the causative mutation of chicken blue eggshell. In the present study, genome wide association study (GWAS) was conducted in Chinese Dongxiang blue-shelled chicken underlying four traits of blue eggshell pigments: quantity of biliverdin (QB), quantity of protoporphyrin (QP), quantity of total pigment (QT), and color density trait (CD). A total of 139 individuals were randomly collected for GWAS. We detected two SNPs in genome-wise significance and 35 in suggestive significance, 24 out of the 37 SNP were located either within intron/exon or near 15 genes in a range of ~1.17 Mb on GGA21. For further confirmation of the identified SNP loci by GWAS, the follow-up replication studies were performed in two populations. A total of 146 individuals of the second generation derived from the former GWAS population, as well as 280 individuals from an alternative independent population were employed for genotyping by MALDI-TOF MS in a genotype-phenotype association study. Eighteen SNPs evenly distributed on the GGA21 significant region were successfully genotyped in the two populations, of which 4 and 6 SNP loci were shown significantly associated with QB, QT and QP in the two repeat populations, respectively. Further, the SNPs were narrowed down to a region of ~ 653.819 Kb on GGA21 that harbors five candidate genes: AJAP1, TNFRSF9, C1ORF174, CAMTA1, and CEP104. Shell gland of chickens laying dark and light blue eggshell was chosen for detection of mRNA expression of the five candidate genes. The results showed differential expression levels of these genes in the two groups. The specific function of these genes has not yet been defined clearly in chickens and further in-depth studies are needed to explore the new functional role in chicken eggshell blueness.

Common guillemot (Uria aalge) eggs are not self-cleaning

Birds are arguably the most evolutionarily successful extant vertebrate taxon, in part because of their ability to reproduce in virtually all terrestrial habitats. Common guillemots, Uria aalge, incubate their single egg in an unusual and harsh environment; on exposed cliff ledges, without a nest, and in close proximity to conspecifics. As a consequence, the surface of guillemot eggshells is frequently contaminated with faeces, dirt, water and other detritus, which may impede gas exchange or facilitate microbial infection of the developing embryo. Despite this, guillemot chicks survive incubation and hatch from eggs heavily covered with debris. To establish how guillemot eggs cope with external debris, we tested three hypotheses: (1) contamination by debris does not reduce gas exchange efficacy of the eggshell to a degree that may impede normal embryo development; (2) the guillemot eggshell surface is self-cleaning; (3) shell accessory material (SAM) prevents debris from blocking pores, allowing relatively unrestricted gas diffusion across the eggshell. We showed that natural debris reduces the conductance of gases across the guillemot eggshell by blocking gas exchange pores. Despite this problem, we found no evidence that guillemot eggshells are self-cleaning, but instead showed that the presence of SAM on the eggshell surface largely prevents pore blockages from occurring. Our results demonstrate that SAM is a crucial feature of the eggshell surface in a species with eggs that are frequently in contact with debris, acting to minimise pore blockages and thus ensure a sufficient rate of gas diffusion for embryo development.

Study of formation of green eggshell color in ducks through global gene expression

The green eggshell color produced by ducks is a threshold trait that can be influenced by various factors, such as hereditary, environment and nutrition. The aim of this study was to investigate the genetic regulation of the formation of eggs with green shells in Youxian ducks. We performed integrative analysis of mRNAs and miRNAs expression profiling in the shell gland samples from ducks by RNA-Seq. We found 124 differentially expressed genes that were associated with various pathways, such as the ATP-binding cassette (ABC) transporter and solute carrier supper family pathways. A total of 31 differentially expressed miRNAs were found between ducks laying green eggs and white eggs. KEGG pathway analysis of the predicted miRNA target genes also indicated the functional characteristics of these miRNAs; they were involved in the ABC transporter pathway and the solute carrier (SLC) supper family. Analysis with qRT-PCR was applied to validate the results of global gene expression, which showed a correlation between results obtained by RNA-seq and RT-qPCR. Moreover, a miRNA-mRNA interaction network was established using correlation analysis of differentially expressed mRNA and miRNA. Compared to ducks that lay white eggs, ducks that lay green eggs include six up-regulated miRNAs that had regulatory effects on 35 down-regulated genes, and seven down-regulated miRNAs which influenced 46 up-regulated genes. For example, the ABC transporter pathway could be regulated by expressing gga-miR-144-3p (up-regulated) with ABCG2 (up-regulated) and other miRNAs and genes. This study provides valuable information about mRNA and miRNA regulation in duck shell gland tissues, and provides foundational information for further study on the eggshell color formation and marker-assisted selection for Youxian duck breeding.

Comparative transcriptome analysis provides clues to molecular mechanisms underlying blue-green eggshell color in the Jinding duck (Anas platyrhynchos)

BACKGROUND

In birds, blue-green eggshell color (BGEC) is caused by biliverdin, a bile pigment derived from the degradation of heme and secreted in the eggshell by the shell gland. Functionally, BGEC might promote the paternal investment of males in the nest and eggs. However, little is known about its formation mechanisms. Jinding ducks (Anas platyrhynchos) are an ideal breed for research into the mechanisms, in which major birds lay BGEC eggs with minor individuals laying white eggs. Using this breed, this study aimed to provide insight into the mechanisms via comparative transcriptome analysis.

RESULTS

Blue-shelled ducks (BSD) and white-shelled ducks (WSD) were selected from two populations, forming 4 groups (3 ducks/group): BSD1 and WSD1 from population 1 and BSD2 and WSD2 from population 2. Twelve libraries from shell glands were sequenced using the Illumina RNA-seq platform, generating an average of 41 million clean reads per library, of which 55.9% were mapped to the duck reference genome and assembled into 31,542 transcripts. Expression levels of 11,698 genes were successfully compared between all pairs of 4 groups. Of these, 464 candidate genes were differentially expressed between cross-phenotype groups, but not for between same-phenotype groups. Gene Ontology (GO) annotation showed that 390 candidate genes were annotated with 2234 GO terms. No candidate genes were directly involved in biosynthesis or transport of biliverdin. However, the integral components of membrane, metal ion transport, cholesterol biosynthesis, signal transduction, skeletal system development, and chemotaxis were significantly (P < 0.05) overrepresented by candidate genes.

CONCLUSIONS

This study identified 464 candidate genes associated with duck BGEC, providing valuable information for a better understanding of the mechanisms underlying this trait. Given the involvement of membrane cholesterol contents, ions and ATP levels in modulating the transport activity of bile pigment transporters, the data suggest a potential association between duck BGEC and the transport activity of the related transporters.

Effects of embryo temperature estimation methodology on the determination of eggshell conductance values in Ross 708 broiler hatching eggs with consideration given to eggshell pigmentation variation

In a preliminary experiment, the influence of pigment color variation on the temperature readings of the shells (EST) of live embryonated Ross 708 broiler hatching eggs was tested. Prior to set, eggshell color in L*, a*, and b* coordinates were measured at the equator of each egg. Eggshell pigmentation was found not to influence EST from zero through 18 d of incubation (DOI). In a subsequent experiment, EST, as well as air cell (ACT) and cloaca (CLT) temperature measurements were used for the calculation of absolute (GH2O) and specific (gH20) eggshell conductance values for these same types of eggs. An infrared thermometer was used to determine EST from zero to 19 DOI, ACT was measured using a transponder from 12 to 19 DOI, and CLT was determined using a transponder at 4:00 PM at 19 DOI. In the 12 to 19 DOI interval, the values for GH2O as well as for gH20 that were calculated using either EST or ACT were significantly correlated (r ≥ 0.99; P < 0.0001). A similar correlation level for both GH2O and gH20 was likewise observed in the 10:00 AM to 10:00 PM time period at 19 DOI when either EST, ACT, or CLT was used. However, in the 12 to 19 DOI interval, calculated GH2O and gH20 values based on ACT were significantly different from those based on EST. In addition, a significant difference in calculated GH2O and gH20 values resulted when ACT was used rather than when EST or CLT was used in the 10:00 AM to 10:00 PM time period at 19 DOI. In both time periods, GH2O and gH20 values calculated using ACT were significantly lower than those derived using the other 2 types of measurements. These findings suggest that although EST was not affected by shell coloration, because ACT more closely reflects embryo body temperature, it should be used to more accurately calculate the GH2O and gH20 of Ross 708 broiler hatching eggs.

Understanding avian egg cuticle formation in the oviduct: a study of its origin and deposition

The cuticle is a unique invisible oviduct secretion that protects avian eggs from bacterial penetration through gas exchange pores. Despite its importance, experimental evidence is lacking for where, when, and what is responsible for its deposition. By using knowledge about the ovulatory cycle and oviposition, we have manipulated cuticle deposition to obtain evidence on these key points. Cuticle deposition was measured using staining and spectrophotometry. Experimental evidence supports the location of cuticle deposition to be the shell gland pouch (uterus), not the vagina, and the time of deposition to be within the final hour before oviposition. Oviposition induced by arginine vasotocin or prostaglandin, the penultimate and ultimate factors for the induction of oviposition, produces an egg with no cuticle; therefore, these factors are not responsible for cuticle secretion. Conversely, oviposition induced by GNRH, which mimics the normal events of ovulation and oviposition, results in a normal cuticle. There is no evidence that cuticle deposition differs at the end of a clutch and, therefore, there is no evidence that the ovulatory surge of progesterone affects cuticle deposition. Overall, the results demonstrate that the cuticle is a specific secretion and is not merely an extension of the organic matrix of the shell. Cuticle deposition was found to be reduced by an environmental stressor, and there is no codependence of the deposition of pigment and cuticle. Defining the basic facts surrounding cuticle deposition will help reduce contamination of hen's eggs and increase understanding of the strategies birds use to protect their eggs.

iTRAQ-Based Quantitative Proteomics Identifies Potential Regulatory Proteins Involved in Chicken Eggshell Brownness

Brown eggs are popular in many countries and consumers regard eggshell brownness as an important indicator of egg quality. However, the potential regulatory proteins and detailed molecular mechanisms regulating eggshell brownness have yet to be clearly defined. In the present study, we performed quantitative proteomics analysis with iTRAQ technology in the shell gland epithelium of hens laying dark and light brown eggs to investigate the candidate proteins and molecular mechanisms underlying variation in chicken eggshell brownness. The results indicated 147 differentially expressed proteins between these two groups, among which 65 and 82 proteins were significantly up-regulated in the light and dark groups, respectively. Functional analysis indicated that in the light group, the down-regulated iron-sulfur cluster assembly protein (Iba57) would decrease the synthesis of protoporphyrin IX; furthermore, the up-regulated protein solute carrier family 25 (mitochondrial carrier; adenine nucleotide translocator), member 5 (SLC25A5) and down-regulated translocator protein (TSPO) would lead to increased amounts of protoporphyrin IX transported into the mitochondria matrix to form heme with iron, which is supplied by ovotransferrin protein (TF). In other words, chickens from the light group produce less protoporphyrin IX, which is mainly used for heme synthesis. Therefore, the exported protoporphyrin IX available for eggshell deposition and brownness is reduced in the light group. The current study provides valuable information to elucidate variation of chicken eggshell brownness, and demonstrates the feasibility and sensitivity of iTRAQ-based quantitative proteomics analysis in providing useful insights into the molecular mechanisms underlying brown eggshell pigmentation.

Haematocrit, eggshell colouration and sexual signaling in the European starling (Sturnus vulgaris)

Background

One hypothesis to explain the blue–green colour of the eggs of many bird species is that it is a sexually-selected signal of the laying female’s quality, which males use to determine their investment. This hypothesis requires that eggshell pigmentation carries a cost or is otherwise linked to female quality. One potential cost is that biliverdin, a haem derivative and the pigment responsible for eggshell colouration, is limiting. To assess this potential cost, we attempted to manipulate haematocrit and haemoglobin in free-living European starlings (Sturnus vulgaris Linnaeus). Upon collecting unmanipulated first clutches, we treated females with phenylhydrazine (PHZ), a haemolytic agent, and measured the blue–green chroma and reproductive performance of replacement clutches. We also investigated whether eggshell colour was associated with haematocrit or haemoglobin levels in unmanipulated first clutches. To test whether eggshell colour might act as a sexual signal, we examined associations between eggshell colour and reproductive performance, as well as the provisioning rate of the male.

Results

PHZ-treatment did not affect eggshell colour in replacement clutches. In unmanipulated first clutches, eggshell colour was not correlated with haematocrit or haemoglobin levels. Eggshell colour was correlated with female mass in unmanipulated first clutches but not replacement clutches. Chicks from eggs with higher eggshell colour had higher haemoglobin levels and longer tarsi just prior to fledging, suggesting that eggshell colour could reflect brood quality. However, eggshell colour was not correlated with the provisioning rate of the male or any other measure of reproductive performance.

Conclusions

We found no evidence to support the hypothesis that the availability of resources required for the synthesis of pigment limits eggshell colour in European starlings, or that eggshell colour is used by males to determine their level of reproductive investment. We found little evidence that eggshell colour is correlated with female or offspring quality in this species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-016-0084-x) contains supplementary material, which is available to authorized users.

Eggshell color in brown-egg laying hens - a review

The major pigment in eggshells of brown-egg laying hens is protoporphyrin IX, but traces of biliverdin and its zinc chelates are also present. The pigment appears to be synthesized in the shell gland. The protoporphyrin IX synthetic pathway is well defined, but precisely where and how it is synthesized in the shell gland of the brown-egg laying hen is still ambiguous. The pigment is deposited onto all shell layers including the shell membranes, but most of it is concentrated in the outermost layer of the calcareous shell and in the cuticle. Recently, the genes that are involved in pigment synthesis have been identified, but the genetic control of synthesis and deposition of brown pigment in the commercial laying hen is not fully understood. The brown coloration of the shell is an important shell quality parameter and has a positive influence on consumer preference. The extent of pigment deposition is influenced by the housing system, hen age, hen strain, diet, stressors, and certain diseases such as infectious bronchitis. In this article, the physiological and biochemical characteristics of the brown pigment in commercial brown-egg layers are reviewed in relation to its various functions in the poultry industry.

Molecular phylogenetic analysis of Chinese indigenous blue-shelled chickens inferred from whole genomic region of the SLCO1B3 gene

In total, 246 individuals from 8 Chinese indigenous blue- and brown-shelled chicken populations (Yimeng Blue, Wulong Blue, Lindian Blue, Dongxiang Blue, Lushi Blue, Jingmen Blue, Dongxiang Brown, and Lushi Brown) were genotyped for 21 SNP markers from the SLCO1B3 gene to evaluate phylogenetic relationships. As a representative of nonblue-shelled breeds, White Leghorn was included in the study for reference. A high proportion of SNP polymorphism was observed in Chinese chicken populations, ranging from 89% in Jingmen Blue to 100% in most populations, with a mean of 95% across all populations. The White Leghorn breed showed the lowest polymorphism, accounting for 43% of total SNPs. The mean expected heterozygosity varied from 0.11 in Dongxiang Blue to 0.46 in Yimeng Blue. Analysis of molecular variation (AMOVA) for 2 groups of Chinese chickens based on eggshell color type revealed 52% within-group and 43% between-group variations of the total genetic variation. As expected, FST and Reynolds' genetic distance were greatest between White Leghorn and Chinese chicken populations, with average values of 0.40 and 0.55, respectively. The first and second principal coordinates explained approximately 92% of the total variation and supported the clustering of the populations according to their eggshell color type and historical origins. STRUCTURE analysis showed a considerable source of variation among populations for the clustering into blue-shelled and nonblue-shelled chicken populations. The low estimation of genetic differentiation (FST) between Chinese chicken populations is possibly due to a common historical origin and high gene flow. Remarkably similar population classifications were obtained with all methods used in the study. Aligning endogenous avian retroviral (EAV)-HP insertion sequences showed no difference among the blue-shelled chickens.

Enhancing the egg's natural defence against bacterial penetration by increasing cuticle deposition

The cuticle is a proteinaceous layer covering the avian egg and is believed to form a defence to microorganism ingress. In birds that lay eggs in challenging environments, the cuticle is thicker, suggesting evolutionary pressure; however, in poultry, selection pressure for this trait has been removed because of artificial incubation. This study aimed to quantify cuticle deposition and to estimate its genetic parameters and its role on trans-shell penetration of bacteria. Additionally, cuticle proteins were characterised to establish whether alleles for these genes explained variation in deposition. A novel and reliable quantification was achieved using the difference in reflectance of the egg at 650 nm before and after staining with a specific dye. The heritability of this novel measurement was moderate (0.27), and bacteria penetration was dependent on the natural variation in cuticle deposition. Eggs with the best cuticle were never penetrated by bacteria (P < 0.001). The cuticle proteome consisted of six major proteins. A significant association was found between alleles of one of these protein genes, ovocleidin-116 (MEPE), and cuticle deposition (P = 0.015) and also between alleles of estrogen receptor 1 (ESR1) gene and cuticle deposition (P = 0.008). With the heritability observed, genetic selection should be possible to increase cuticle deposition in commercial poultry, so reducing trans-generational transmission of microorganisms and reversing the lack of selection pressure for this trait during recent domestication.

Comparison of HMOX1 expression and enzyme activity in blue-shelled chickens and brown-shelled chickens

Blue egg coloring is attributed to biliverdin derived from the oxidative degradation of heme through catalysis by heme oxygenase (HO). The pigment is secreted into the eggshell by the shell gland. There is uncertainty as to whether the pigment is synthesized in the shell gland or in other tissues. To investigate the site of pigment biosynthesis, the expression of heme oxygenase (decycling) 1 (HMOX1), a gene encoding HO, and HO activity in liver and spleen were compared between blue-shelled chickens (n = 12) and brown-shelled chickens (n = 12). There were no significant differences in HMOX1 expression and HO activity in these tissues between the two groups. Since the liver and spleen, two important sites outside the shell gland where heme is degraded into biliverdin, CO and Fe(2+), did not differ in HO expression and activity we conclude that the pigment is most likely synthesized in the shell gland.

An EAV-HP insertion in 5' Flanking region of SLCO1B3 causes blue eggshell in the chicken

The genetic determination of eggshell coloration has not been determined in birds. Here we report that the blue eggshell is caused by an EAV-HP insertion that promotes the expression of SLCO1B3 gene in the uterus (shell gland) of the oviduct in chicken. In this study, the genetic map location of the blue eggshell gene was refined by linkage analysis in an F₂ chicken population, and four candidate genes within the refined interval were subsequently tested for their expression levels in the shell gland of the uterus from blue-shelled and non-blue-shelled hens. SLCO1B3 gene was found to be the only one expressed in the uterus of blue-shelled hens but not in that of non-blue-shelled hens. Results from a pyrosequencing analysis showed that only the allele of SLCO1B3 from blue-shelled chickens was expressed in the uterus of heterozygous hens (O*LC/O*N). SLCO1B3 gene belongs to the organic anion transporting polypeptide (OATP) family; and the OATPs, functioning as membrane transporters, have been reported for the transportation of amphipathic organic compounds, including bile salt in mammals. We subsequently resequenced the whole genomic region of SLCO1B3 and discovered an EAV-HP insertion in the 5′ flanking region of SLCO1B3. The EAV-HP insertion was found closely associated with blue eggshell phenotype following complete Mendelian segregation. In situ hybridization also demonstrated that the blue eggshell is associated with ectopic expression of SLCO1B3 in shell glands of uterus. Our finding strongly suggests that the EAV-HP insertion is the causative mutation for the blue eggshell phenotype. The insertion was also found in another Chinese blue-shelled breed and an American blue-shelled breed. In addition, we found that the insertion site in the blue-shelled chickens from Araucana is different from that in Chinese breeds, which implied independent integration events in the blue-shelled chickens from the two continents, providing a parallel evolutionary example at the molecular level.

The eggshell color of birds is of wide interest, but the molecular basis remained unknown until our discovery, reported here. The blue eggshell is found not only in wild birds but also in domestic fowls. In this study, we identified that blue eggshell in chickens from different geographical regions is caused by a ∼4.2 kb EAV-HP insertion in the 5′ flanking region of SLCO1B3. The EAV-HP insertion in chicken is a derived mutation in domestic chickens. The genetic determination of blue eggshell in other birds requires further investigation. We also found that the EAV-HP insertions in the chickens from China and America were separate integration events, which presents us with a parallel molecular evolution example driven by artificial selection.

Expression and activity analysis reveal that heme oxygenase (decycling) 1 is associated with blue egg formation

Biliverdin is responsible for the coloration of blue eggs and is secreted onto the eggshell by the shell gland. Previous studies confirmed that a significant difference exists in biliverdin content between blue eggs and brown eggs, although the reasons are still unknown. Because the pigment is derived from oxidative degradation of heme catalyzed by heme oxygenase (HO), this study compared heme oxygenase (decycling) 1 (HMOX1), the gene encoding HO expression and HO activity, in the shell glands of the Dongxiang blue-shelled chicken (n = 12) and the Dongxiang brown-shelled chicken (n = 12). Results showed that HMOX1 was highly expressed at the mRNA (1.58-fold; P < 0.05) and protein levels in blue-shelled chickens compared with brown-shelled chickens. At the functional level, blue-shelled chickens also showed 1.40-fold (P < 0.05) higher HO activity than brown-shelled chickens. To explore the reasons for the differential expression of HMOX1, an association study of 6 SNP capturing the majority of HMOX1 variants with the blue egg coloration was performed. Results showed no significant association between SNP and the blue egg coloration in HMOX1 (P > 0.05). Taken together, these results show that blue egg formation is associated with high expression of HMOX1 in the shell gland of Dongxiang blue-shelled chickens, and suggest that differential expression of HMOX1 in the 2 groups of chickens is most likely to arise from an alteration in the trans-acting factor.

Changes in Brown Eggshell Color As the Hen Ages

The color of eggshells from eggs laid by commercial-type Hy-Line brown hens 25 wk of age was studied over a period of 10 mo. Color measurements were made by a color machine vision system and were analyzed using a mixed model to calculate between and within hen variances and to investigate the effect of time on shell color. Hens laid eggs with lighter colored shells as the flock aged, as evidenced by the lightness (L*) values increasing in time. A decrease in pigmentation was associated with a decrease in the amount of redness (a*) in the eggshell. When L* and a* values were corrected for egg weight, the rate of change in the L* and a* values decreased, indicating that size of the egg was a major factor affecting the color of the eggshell. These findings quantified the observations that older hens lay lighter colored eggs due to an increase in egg size associated with no proportionate change in the quantity of pigment deposited over the shell surface. Using a 2-stage sampling analysis and the variances between and within hens, sample sizes required to estimate the color of eggshells within 5% of the true mean were calculated. Accordingly, 11 eggs would need to be collected from each of the 51 hens housed for a study of brown eggshell color using the L*, a*, and b* (yellowness) coordinates.

A study on eggshell pigmentation: biliverdin in blue-shelled chickens

Biliverdin is an important pigment in the eggshell of chickens and other avian species. Determination of the biosynthesis site for biliverdin is essential for understanding the biochemical process and genetic basis of eggshell pigmentation. Either blood or the shell gland could be the biosynthesis site of eggshell biliverdin. A segregation population with full-sib sisters genotyped Oo and oo, which laid blue-shelled eggs and light brown eggs, respectively, was constructed in a native Chinese chicken breed. Ultraviolet spectrophotometry and HPLC were used to determine the biliverdin concentration in eggshells, blood, bile, excreta, and shell gland of both groups of chickens. Biliverdin content was significantly different between egg shells of blue-shelled and brown-shelled chickens (P < 0.01). Blood and bile were tested 3 to 4 h before oviposition, and excreta was tested randomly. Results showed no significant difference in biliverdin concentration in blood, bile, and excreta between the 2 groups. In the shell gland, the biliverdin contents for the blue-shelled and brown-shelled chickens were 8.25 +/- 2.55 and 1.29 +/- 0.12 nmol/g, respectively, which showed a significant difference (P < 0.01). Our results demonstrated that blood is not the biosynthesis site of the shell biliverdin. Biliverdin is most likely synthesized in the shell gland and then deposited onto the eggshell of chickens.

Evolution of bird eggs in the absence of cuckoo parasitism

Historical introductions of species into new habitats can create rare opportunities to test evolutionary hypotheses, such as the role of natural selection in maintaining traits. This study examines two independent introductions of the African village weaverbird (Ploceus cucullatus) to islands where selection on egg appearance traits is expected to differ markedly from that of the source populations. The color and spotting of village weaver eggs in Africa are highly consistent within clutches, but highly variable between individuals. These two features may be an evolutionary response to brood parasitism. In Africa, weavers are parasitized by each other and by the diederik cuckoo (Chrysococcyx caprius), an egg mimic. African village weavers were introduced one century ago to Mauritius, and over two centuries ago to Hispaniola. Both islands are devoid of egg-mimicking brood parasites. In these two populations, between-individual variation and within-clutch consistency in egg appearance have both decreased, as has the incidence of spotting, relative to the source populations in Africa. These reductions are more pronounced on Hispaniola, the earlier introduction. Such changes support the hypothesis that egg appearance in the African village weaver has been maintained by natural selection as a counteradaptation to cuckoo brood parasitism. These results illustrate that the removal of an agent of selection can sometimes bring about rapid evolutionary consequences.