Why do egg shells come in different colors?

You must have noticed that when you buy eggs, there are different colors such as red, white and green. The red eggs have thicker shells than the white and green eggs, and the yolk is not as bright as the white and green eggs. So are there any differences between eggs of different colors? Why are there red, white and green eggs? What determines the color of the eggshell? Let's talk about these things about eggs today.

In fact, there are many factors that affect the color of eggshells, but the most important one is genetic factors. As for the nutritional differences, I have made a video before and briefly described it. Generally speaking, different eggs are definitely different, but the difference is very small. Today, I will mainly solve the problem of eggshell color. This article mainly refers to two articles that study the mechanism of eggshell color determination, so I will briefly talk about it with you.

01. Eggshell color - mainly genetic

Although there are many factors that affect eggshells, such as feeding methods, nutritional levels, age, etc., the most important factor is genetics.

I believe you must have noticed that a certain type of chicken will only produce eggs of a fixed color, while other chickens, no matter how they are treated, it is difficult to produce the corresponding color. This means that breed is critical, and behind this, it shows that genetic factors are at work.

For example, a study of brown eggs found that the heritability of hens was as high as 0.914, which is very high (because most traits are complex and genetics can only solve part of the traits)

02. What’s the deal with the color of eggshells?

There are several colors of eggshells in nature, which are generally divided into three categories, including white shells, brown shells and green shells. So, what factors affect the color of eggshells?

The answer lies in the ingredients in the eggshell. The main ingredients that determine the color of the eggshell include protoporphyrin IX, biliverdin, biliverdin zinc chelate, and a small amount of protoporphyrin IX zinc chelate [1].

The amount of these pigments will affect the color of the eggs. For example, the main pigment of green-shelled eggs is biliverdin, while the main pigment of brown eggshells is protoporphyrin IX.

For example, for brown eggs, the protoporphyrin IX content will affect the depth of their color.

However, these pigments do not appear randomly. The various components of organisms are often controlled by genes, so the synthesis of these pigments is naturally no exception.

For example, the above picture shows heme, which has a porphyrin ring structure. The synthesis pathway involves mitochondria and cytoplasm, which naturally involves genes.

For example, the transcriptome data of brown eggs with different depths show that the expression of eggshell gland genes is also significantly different between dark and light eggshells.

With changes in the transcriptome, there will naturally be changes in the protein level.

By clustering the differentially expressed proteins, we can see that there are differences in multiple clustering angles.

Specifically, it involves secondary metabolic synthesis, transport and decomposition functions, intracellular transport, secretion and vesicle transport functions, etc.

If you combine the transcriptome and the proteome, you see that there are at least two more pathways and several genes that play a role in determining the color of brown eggs.

These are just brown eggs, and the unique gene has not been found yet. If it comes to the differences in other colors, the differences may be even greater.

03. What is the conclusion?

The color of the eggshell is mainly determined by genes, but the specific gene has not yet been determined. It may be controlled by multiple genes and involve quantitative genetic traits, which is more complicated (think about complex factors such as height).

However, this is the truth. If I tell you the definite reason with certainty, it would not be true. After all, the eggshell problem has not been solved yet, and it is impossible to give a unique conclusion.

Everyone should do the same when looking at various conclusions in the future. Don't take them as the standard. In biology, there is currently no truth, let alone any conclusion.

Of course, this doesn't mean that we can be imaginative and imaginative. At least we can see that color is mainly determined by genetic factors, and it happens through some major pathways.

1 Mertens, Kristof, et al. "The transmission color value: A novel egg quality measure for recording shell color used for monitoring the stress and health status of a brown layer flock." Poultry Science89.3 (2010): 609-617.

2 Li Guangqi. Combined analysis of transcriptome and proteome to determine genetic factors affecting eggshell brown color[D]. China Agricultural University, 2015.

3 Bi Huijuan, Li Guangqi, Yang Ning. Research progress on the genetic mechanism of eggshell color[J]. Journal of Animal Science and Veterinary Medicine, 2016, 47(12): 2325-2330.2330.