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Virus Explains Why Some Chicken Eggs Are Blue

By Lisa Raffensperger
Sep 13, 2013 10:58 PMNov 20, 2019 12:48 AM


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If you haven't lived around chickens you may be surprised to learn that the color palette for eggs extends far beyond the supermarket white and brown. One of the prettier shades is a pastel blue---a genetic alteration, it turns out, that was the result of a virus inserting its DNA in the genomes of those chickens' ancestors. The three kinds of chicken known to lay blue eggs are the Araucana, from Chile, and the Dongxiang and Lushi, both from China. Blue eggshells can also be found in quails, ducks and other wild birds. It was previously known that blue eggshell color is a dominant trait in chickens. That meansthat a chicken with one or two copies of the blue gene will have blue eggs, with two copies producing a deeper blue color. What researchers didn't know was what the specific gene was that was responsible. Now, scientists reported

earlier this year that an endogenous retrovirus is to blame---a chunk of viral DNA that has integrated itself into the chicken's DNA. Hundreds of years ago, the virus inserted its DNA next to a blue-pigment gene that all chickens have, causing the coloration to be turned on. These birds were favored by breeders, and thus began a long lineage of blue egg-layers. What's especially impressive, though, is that this random event happened at least twice. The Virology Blog


The retrovirus insertion has occurred at different positions in the Chilean and Chinese chicken genomes. This observation indicates that the insertion arose independently during breeding of chicken strains several hundred years ago to produce blue egg layers.

So there you have it---naturally occurring blue chicken eggs, no vinegar dye needed---just a smidgen of virus. Image by Julian Berry

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