While it seems like the genetics of chicken egg color would be straightforward, this is not the case. Today we will dive into the intricate process of how eggs get their colors. This journey will start with a basic overview of poultry genetics and the process of coloring an egg. Then we’ll get more specific and include a discussion of the pigments, genes, and retroviruses that we know of that determine the color of a chicken’s egg. It is important to note that eggshell color is still a topic of
research and we do not fully understand everything that goes into it. An example of one area of egg appearance that we don’t completely understand is how bloom thickness is determined and its effects on how the egg appears when dry. The bloom is an antimicrobial protective coating on top of the fully developed and pigmented shell. Very thick blooms can cause a light brown egg to appear dusty pink or a chocolate-colored egg to appear purplish. I say that it affects appearance and not color, because when the egg is wet, it appears normally colored. It is only when the egg is dry that the bloom affects how the egg appears. Despite the role it plays in protecting the egg and the appearance of the egg, there is not a lot of information about what factors into bloom thickness. With the knowledge that this paper will not cover every facet of what makes up egg color, I will try to cover the basic mechanisms and processes by which shells are colored.
To understand what goes into making a colored egg, I think it is first important to understand some basic genetic principles of chickens. Many people know that in mammals, an individual with two X chromosomes is a female, and an individual with one X chromosome and one Y chromosome is a male. In birds however, an individual with one Z chromosome and one W chromosome is a female, and an individual with two W chromosomes is a male. Since the female has the two non-homologous sex chromosomes, it is her chromosomes that determine the gender of the offspring, whereas in mammals, it is the male’s chromosomes that determine gender. Chickens also have 38 pairs of autosomal chromosomes for a total of 39 pairs of chromosomes (Poultry Extension Org, 2021). In this paper, we will discuss genes on chromosomes 1, 3, 5, 6, 33, and Z.
When it comes to how chicken eggshells obtain their color, many consumers buy into the myth that environment and nutrition affects color. This leads to the misconception that colored eggs are healthier because the edible part of the egg contains more nutrients. Part of this is because many people have heard that
environment and nutrition affect yolk color, which is true. Yolk color is affected primarily by the number of carotenoids and xanthophyll isomers in the feed, and darker yolks are more nutritious for human consumption (Hammeishøj, 2011). However, while environment and nutrition can slightly change shell tint, it is overall determined by genes and has no impact on the nutritional quality of the egg. Shell color is produced after the egg is completely formed. Deposition of pigmentation occurs in the shell gland from ciliated cells in the epithelium and takes place half an hour to three hours before the egg is laid (Johnson, 2000). This leads into our discussion on the pigments that give eggshells their color.
There are three main pigments that lead to eggshell coloration. These are protoporphyrin-IX, biliverdin-IX and zinc biliverdin chelate (Sparks, 2011). Protoporphyrin-IX results in brown shell color, and biliverdin-IX and zinc biliverdin chelate result in blue and green shell colors (Zhao et al., 2006). If you think you’ve heard the word biliverdin before, you probably have. Biliverdin is one of the bile pigments produced when we bruise and is what gives bruises that initial greenish/blue color before turning a more purplish/red. The two biliverdin pigments create the blue egg base, one of only two base colors. The other base color is white. The difference between these two colors and brown occurs in the process of forming the eggshell. In white eggs, there is no pigment produced. In blue eggs, biliverdin-IX and zinc biliverdin chelate are deposited early in the shell forming process, which infuses the color throughout the entire shell. In brown eggs, they have a white base and later in the process, protoporphyrin-IX is deposited in the shell resulting in a brown topcoat. Green eggs go through the same process as brown eggs, they just start with a blue base. It is different concentrations of protoporphyrin-IX, biliverdin-IX, and zinc biliverdin chelate controlled by eight known genes that take these three pigments and produce many different shades and colors of shells.
The eight known genes that control eggshell coloration are the CPOX, SLCO1A2, SLCO1B3 and SLCO1C1 genes located on chromosome 1; the FLVCR gene located on chromosomes 3 and 5; the BCRP ATP-binding cassette half transporter encoded by the ABCG2 gene located on chromosome 6; the HRG1 gene located on chromosome 33; and the FECH gene located on the Z chromosome (Zheng et al., 2014; Zhang et al., 2020).
Before we get into all the shading genes and possibilities, we will look at the single gene that controls base coloring. The coding for the bases is commonly called the oocyan (O) gene and has been mapped to the SLCO1B3 gene, which is part of the solute carrier organic anion transporter family. The O gene is a gene with incomplete dominance that codes for the base color blue. It was originally found in the Mapuche fowl (the ancestor of Araucanas) of Chile and the Dongxiang chickens of China. The expression of the O gene is due to the insertion of the EAV-HP (endogenous avian retrovirus) retrovirus into the chickens’ DNA. The retrovirus is positioned on chromosome 1 next to the SLCO1B3 gene. It is interesting to note that while the retrovirus is found in both descendants of Mapuche fowl and the Dongxiang chickens, it is positioned in slightly different locations. In chickens whose DNA does not contain EAV-HP, the SLCO1B3 gene is not transcribed, and thus, is not expressed. However, in chickens with EAV-HP, the insertion of the virus into the DNA acts as a promoter sequence which allows the gene to be transcribed. The protein that is produced results in the release of the two biliverdin pigments into the eggshell (Racaniello, 2013).
Now that we’ve covered our bases, let’s talk about the seven other known genes that code for the shading of the eggshell. First, it is important to understand the process of heme formation, known as porphyrin synthesis, and the relationship between protoporphyrinogen-IX, protoporphyrin-IX, and heme. Porphyrin synthesis begins in the mitochondria using succinyl coenzyme-A from the citric acid cycle and glycine. These two reactants form 5’aminolevulinic acid (ALA), which then leaves the mitochondria. In the cytosol, porphobilinogen is produced from the ALA, which then goes on to form uroporphyrinogen-III, which then forms coproporphyrinogen-III. Coproporphyrinogen-III is then transported back into the mitochondria to form protoporphyrinogen-IX which leads to the final steps of porphyrin synthesis. Protoporphyrinogen-IX is converted to protoporphyrin-IX. Then ferrous iron is inserted into protoporphyrin-IX using the enzyme ferrochelatase. This creates heme (Ogun et al., 2020). The CPOX (coproporphyrinogen III oxidase) gene codes for production of protoporphyrinogen-IX. This creates a brown
eggshell topcoat. On the other hand, the FECH (ferrochelatase) gene converts porphyrinogen into heme, which results in a lighter color. Since the FECH gene is located on the Z chromosome, expression of it is sex-linked. This means that a hen that has a higher expression of the FECH gene is more likely to produce white layers compared to roosters that have a higher expression of the FECH gene, as demonstrated by the Punnett squares on the last page. The relationship is particularly evident in the middle row with the heterozygous males. Keep in mind that these Punnett squares only show 2 of the FECH gene alleles and only show complete dominance and not partial dominance. This means they are not reliable in predicting whether a bird will have brown eggshell color and are only used as a demonstration of how this gene might impact eggshell color phenotype. This demonstration is backed by a study done in the late 1940’s at the North Carolina State College, which concluded that when it comes to brown coating, females have a higher heritability calculation than males. This study though, did not focus on a single gene, it just looked at heritability based on phenotype. (Blow et al., 1949) Looking at the Punnett squares, while females have a higher heritability calculation than males throughout the offspring results, whether the female offspring’s eggs have a brown coating due to the FECH gene is determined by the male. However, as I stated, these Punnett squares cannot accurately predict whether an offspring’s eggs are coated, because there are several other genes that influence whether an egg has a brown coating and how dark it is. Take for example, the heme transporters BCRP, HRG1, and FLVCR. BCRP (breast cancer resistance protein), an ATP-binding cassette located on ABCG2 results in darker brown eggshells, FLVCR (feline leukemia virus receptor) results in lighter shells, and HRG1 (heme-responsive gene-1) results in white shells. Also associated with brown eggshells are the SLCO1A2 and SLCO1C1 genes which, like the SLCO1B3 gene are part of the solute carrier organic anion transporter family. Expression of these genes may affect how protoporphyrin-IX is taken up by the body (Zheng et al., 2014).
In conclusion, the genetics of eggshell colors are anything but simple. While there are only 3 pigments that color the eggs, they are controlled by at least 8 known genes, one of which is only expressed because of a retrovirus. Scientists are still trying to figure out the heritability and the genes involved in the production of eggshell colors, especially when it comes to the brown topcoat.
Eggshell Pigments–from Formation to Deposition. Nicholas H.C. Sparks, 2011. (2021). Avian Biology Research. Retrieved from https://journals.sagepub.com/doi/abs/10.3184/175815511X13228269481875
Hammeishøj, M. (2011). Organic and free-range egg production. Improving The Safety And Quality Of Eggs And Egg Products, 463-486. doi: 10.1533/9780857093912.4.463
JOHNSON, A. (2000). Reproduction in the Female. Sturkie's Avian Physiology, 569-596. doi: 10.1016/b978-012747605-6/50023-7
Ogun, A., Joy, N., & Valentine, M. (2020). Biochemistry, Heme Synthesis. Statpearls Publishing. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK537329/
"POULTRY GENETICS: AN INTRODUCTION – Small And Backyard Poultry". Poultry.Extension.Org, 2021, https://poultry.extension.org/articles/poultry-anatomy/poultry-genetics-an-introduction/. Accessed 24 Mar 2021.
Racaniello, V. (2013). A retrovirus makes chicken eggshells blue. Retrieved 29 March 2021, from https://www.virology.ws/2013/09/11/a-retrovirus-makes-chicken-eggshells-blue/
UCSC Genome Browser Gateway. (2021). Retrieved 24 March 2021, from https://genome.ucsc.edu/cgi-bin/hgGateway
Unscrambling the genetics of the chicken's 'blue' egg. (2021). Retrieved 29 March 2021, from https://www.sciencedaily.com/releases/2013/08/130820083654.htm
Wang, Z., Qu, L., Yao, J., Yang, X., Li, G., & Zhang, Y. et al. (2013). An EAV-HP Insertion in 5′ Flanking Region of SLCO1B3 Causes Blue Eggshell in the Chicken. Plos Genetics, 9(1), e1003183. doi: 10.1371/journal.pgen.1003183
Why are chicken eggs different colors?. (2013). Retrieved 29 March 2021, from https://www.canr.msu.edu/news/why_are_chicken_eggs_different_colors#:~:text=According to Michigan State University,the genetics of the hens.&text=Ameraucana birds have the pigment,being the same blue color.
Zhang, Y., Huang, J., Li, X., Fang, C., & Wang, L. (2020). Identification of Functional Transcriptional Binding Sites within Chicken Abcg2 Gene Promoter and Screening Its Regulators. Genes, 11(2), 186. doi: 10.3390/genes11020186
Zhao, R., Xu, G., Liu, Z., Li, J., & Yang, N. (2006). A study on eggshell pigmentation: biliverdin in blue-shelled chickens. Poultry Science, 85(3), 546-549. doi: 10.1093/ps/85.3.546
Zheng, C., Li, Z., Yang, N., & Ning, Z. (2014). Quantitative expression of candidate genes affecting eggshell color. Animal Science Journal, 85(5), 506-510. doi: 10.1111/asj.12182
To understand what goes into making a colored egg, I think it is first important to understand some basic genetic principles of chickens. Many people know that in mammals, an individual with two X chromosomes is a female, and an individual with one X chromosome and one Y chromosome is a male. In birds however, an individual with one Z chromosome and one W chromosome is a female, and an individual with two W chromosomes is a male. Since the female has the two non-homologous sex chromosomes, it is her chromosomes that determine the gender of the offspring, whereas in mammals, it is the male’s chromosomes that determine gender. Chickens also have 38 pairs of autosomal chromosomes for a total of 39 pairs of chromosomes (Poultry Extension Org, 2021). In this paper, we will discuss genes on chromosomes 1, 3, 5, 6, 33, and Z.
When it comes to how chicken eggshells obtain their color, many consumers buy into the myth that environment and nutrition affects color. This leads to the misconception that colored eggs are healthier because the edible part of the egg contains more nutrients. Part of this is because many people have heard that
There are three main pigments that lead to eggshell coloration. These are protoporphyrin-IX, biliverdin-IX and zinc biliverdin chelate (Sparks, 2011). Protoporphyrin-IX results in brown shell color, and biliverdin-IX and zinc biliverdin chelate result in blue and green shell colors (Zhao et al., 2006). If you think you’ve heard the word biliverdin before, you probably have. Biliverdin is one of the bile pigments produced when we bruise and is what gives bruises that initial greenish/blue color before turning a more purplish/red. The two biliverdin pigments create the blue egg base, one of only two base colors. The other base color is white. The difference between these two colors and brown occurs in the process of forming the eggshell. In white eggs, there is no pigment produced. In blue eggs, biliverdin-IX and zinc biliverdin chelate are deposited early in the shell forming process, which infuses the color throughout the entire shell. In brown eggs, they have a white base and later in the process, protoporphyrin-IX is deposited in the shell resulting in a brown topcoat. Green eggs go through the same process as brown eggs, they just start with a blue base. It is different concentrations of protoporphyrin-IX, biliverdin-IX, and zinc biliverdin chelate controlled by eight known genes that take these three pigments and produce many different shades and colors of shells.
The eight known genes that control eggshell coloration are the CPOX, SLCO1A2, SLCO1B3 and SLCO1C1 genes located on chromosome 1; the FLVCR gene located on chromosomes 3 and 5; the BCRP ATP-binding cassette half transporter encoded by the ABCG2 gene located on chromosome 6; the HRG1 gene located on chromosome 33; and the FECH gene located on the Z chromosome (Zheng et al., 2014; Zhang et al., 2020).
Before we get into all the shading genes and possibilities, we will look at the single gene that controls base coloring. The coding for the bases is commonly called the oocyan (O) gene and has been mapped to the SLCO1B3 gene, which is part of the solute carrier organic anion transporter family. The O gene is a gene with incomplete dominance that codes for the base color blue. It was originally found in the Mapuche fowl (the ancestor of Araucanas) of Chile and the Dongxiang chickens of China. The expression of the O gene is due to the insertion of the EAV-HP (endogenous avian retrovirus) retrovirus into the chickens’ DNA. The retrovirus is positioned on chromosome 1 next to the SLCO1B3 gene. It is interesting to note that while the retrovirus is found in both descendants of Mapuche fowl and the Dongxiang chickens, it is positioned in slightly different locations. In chickens whose DNA does not contain EAV-HP, the SLCO1B3 gene is not transcribed, and thus, is not expressed. However, in chickens with EAV-HP, the insertion of the virus into the DNA acts as a promoter sequence which allows the gene to be transcribed. The protein that is produced results in the release of the two biliverdin pigments into the eggshell (Racaniello, 2013).
Now that we’ve covered our bases, let’s talk about the seven other known genes that code for the shading of the eggshell. First, it is important to understand the process of heme formation, known as porphyrin synthesis, and the relationship between protoporphyrinogen-IX, protoporphyrin-IX, and heme. Porphyrin synthesis begins in the mitochondria using succinyl coenzyme-A from the citric acid cycle and glycine. These two reactants form 5’aminolevulinic acid (ALA), which then leaves the mitochondria. In the cytosol, porphobilinogen is produced from the ALA, which then goes on to form uroporphyrinogen-III, which then forms coproporphyrinogen-III. Coproporphyrinogen-III is then transported back into the mitochondria to form protoporphyrinogen-IX which leads to the final steps of porphyrin synthesis. Protoporphyrinogen-IX is converted to protoporphyrin-IX. Then ferrous iron is inserted into protoporphyrin-IX using the enzyme ferrochelatase. This creates heme (Ogun et al., 2020). The CPOX (coproporphyrinogen III oxidase) gene codes for production of protoporphyrinogen-IX. This creates a brown
In conclusion, the genetics of eggshell colors are anything but simple. While there are only 3 pigments that color the eggs, they are controlled by at least 8 known genes, one of which is only expressed because of a retrovirus. Scientists are still trying to figure out the heritability and the genes involved in the production of eggshell colors, especially when it comes to the brown topcoat.
References
Blow, W., Bostian, C., & Glazener, E. (1950). The Inheritance of Egg Shell Color. Poultry Science, 29(3), 381-385. doi: 10.3382/ps.0290381Eggshell Pigments–from Formation to Deposition. Nicholas H.C. Sparks, 2011. (2021). Avian Biology Research. Retrieved from https://journals.sagepub.com/doi/abs/10.3184/175815511X13228269481875
Hammeishøj, M. (2011). Organic and free-range egg production. Improving The Safety And Quality Of Eggs And Egg Products, 463-486. doi: 10.1533/9780857093912.4.463
JOHNSON, A. (2000). Reproduction in the Female. Sturkie's Avian Physiology, 569-596. doi: 10.1016/b978-012747605-6/50023-7
Ogun, A., Joy, N., & Valentine, M. (2020). Biochemistry, Heme Synthesis. Statpearls Publishing. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK537329/
"POULTRY GENETICS: AN INTRODUCTION – Small And Backyard Poultry". Poultry.Extension.Org, 2021, https://poultry.extension.org/articles/poultry-anatomy/poultry-genetics-an-introduction/. Accessed 24 Mar 2021.
Racaniello, V. (2013). A retrovirus makes chicken eggshells blue. Retrieved 29 March 2021, from https://www.virology.ws/2013/09/11/a-retrovirus-makes-chicken-eggshells-blue/
UCSC Genome Browser Gateway. (2021). Retrieved 24 March 2021, from https://genome.ucsc.edu/cgi-bin/hgGateway
Unscrambling the genetics of the chicken's 'blue' egg. (2021). Retrieved 29 March 2021, from https://www.sciencedaily.com/releases/2013/08/130820083654.htm
Wang, Z., Qu, L., Yao, J., Yang, X., Li, G., & Zhang, Y. et al. (2013). An EAV-HP Insertion in 5′ Flanking Region of SLCO1B3 Causes Blue Eggshell in the Chicken. Plos Genetics, 9(1), e1003183. doi: 10.1371/journal.pgen.1003183
Why are chicken eggs different colors?. (2013). Retrieved 29 March 2021, from https://www.canr.msu.edu/news/why_are_chicken_eggs_different_colors#:~:text=According to Michigan State University,the genetics of the hens.&text=Ameraucana birds have the pigment,being the same blue color.
Zhang, Y., Huang, J., Li, X., Fang, C., & Wang, L. (2020). Identification of Functional Transcriptional Binding Sites within Chicken Abcg2 Gene Promoter and Screening Its Regulators. Genes, 11(2), 186. doi: 10.3390/genes11020186
Zhao, R., Xu, G., Liu, Z., Li, J., & Yang, N. (2006). A study on eggshell pigmentation: biliverdin in blue-shelled chickens. Poultry Science, 85(3), 546-549. doi: 10.1093/ps/85.3.546
Zheng, C., Li, Z., Yang, N., & Ning, Z. (2014). Quantitative expression of candidate genes affecting eggshell color. Animal Science Journal, 85(5), 506-510. doi: 10.1111/asj.12182
