03 - Chick Check: The Genetics Behind Blue Chicken Eggs
How did biliverdin get into the egg shell? In non-blue egg producing chickens the SLCO1B3 gene and the resulting OATP1B3 protein is mainly present and active only in the liver to transport bile acid and biliverdin from the blood into the liver. But in blue-egg laying chickens, something special happened and it all started with a chicken getting sick. Specifically, a viral infection by the Endogenous Avian Leukosis Retro Virus caused a small element of retro viral sequence called EAV-HP (Endogenous Avian Leukosis Virus- related Retroviral Sequence) to insert itself near the SLCO1B3 gene in the shell gland or uterus of the infected chicken (Figure 2). This viral insertion acts like a powerful “on switch” that turns on SLCO1B3 not just in the liver, but also in the shell gland. When SLCO1B3 is active in the shell gland, it causes biliverdin to now be deposited into the developing eggshell, creating the signature blue color. It’s important to remember that the insertion at this location doesn’t negatively affect the chicken nor does it make the eggs unsafe to eat. It just changes where the pigment biliverdin is expressed. This means that blue eggs are not unsafe or infectious -- they’re just pretty!
DNA
A
Transposon
DNA
B
EAV-HP
Figure 2: PCR validation of EAV-HP insertion in SLCO1B3.
What exactly is this retroviral “On-Switch”? The viral insertion contains promoter and enhancer sequences that act as regulatory DNA elements upstream of the SLCO1B3 gene (Figure 3). These enhancer/promoter sequences are normally used by the virus to produce more viral products, however when these snippets are randomly inserted in front of the chicken SLCO1B3 gene it causes this gene to be expressed in the shell gland (uterus) where it’s normally not active, in addition to its normal expression in the liver.
Simple Retroviral Genome
5’ LTR
3’ LTR
Ψ-SITE
Ψ-SITE
poly A tail
“Gag” gene
“Pol” gene
“Env” gene
cap
Subunits of the viral Env protein:
capsid nucleocapsid Structural proteins of the virus:
Viral enzymes:
surface unit transmembrane region
matrix
reverse transcriptase
protease integrase
Figure 3: The viral RNA contains a cap at its 5’ end and a poly A tail at the 3’ end. A simple retroviral genome contains three genes: the group-specific antigen glycoprotein (gag) which encodes the structural proteins of the virus, the pol gene which encodes the viral enzymes, and the env gene which encodes for the viral Env protein that is composed of two subunits. Ψ-represents a packaging signal.
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