Do You Like Green Eggs And Ham?
Yes, I like them, Sam-I-Am
White eggs, Brown eggs, Pink ones too
But Tell me, how Do they turn Blue?
(With apologies to Dr. Seuss)
♦ Egg color in birds evolved for obvious reasons of camouflage and recognition, and for less obvious reasons such as thermal regulation, protection against UV light, and even antimicrobial defense. Chicken eggs are commonly white (no pigment), or brown (protoporphyrin). Rare breeds from China and Chile lay blue eggs, colored by the bile pigment biliverdin, a breakdown product of the hemoglobin in red blood cells. Biliverdin is normally excreted by liver cells into the bile. So how does it end up in the egg shell?
♦ Organic anion transporters are proteins that move a large number of compounds- drugs, toxins, hormones and bile pigments, across cell membranes, as part of the liver’s detoxifying day job. Genetic sleuthing mapped the blue color trait to a region of a chicken chromosome. Here was a gene for a transporter protein, SLCO1B3, that could provide blue-green biliverdin to color the shell. But why was the gene inexplicably turned on only in the shell gland of the blue egg laying chicken?
♦ Endogenous retroviruses (ERV) are ancient viruses that inserted randomly into the genomes of prehistoric birds. One such viral fragment inserted right next to the SLCO1B3 gene in blue egg laying chickens, where it behaved like an accidental transcription enhancer, or “on switch”. Because of its sequence, scientists speculate that it mediates estrogen specific regulation, accounting for the high levels of the biliverdin transport protein in the shell gland. Although this story nicely explains our Seussian curiosity about green eggs and ham, it also shows how viruses shape diversity in the living world. For example, an insertion of the avian leukosis virus inside a gene for the enzyme tyrosinase results in white plumage in chickens. Viral insertions can also be incredibly harmful, triggering cancer when they accidentally turn on oncogenes.
Fun Fact: Green Eggs and Ham, published in 1960, is one of the best selling and most beloved children’s books of all time. It has just 50 words, and was written by Dr. Seuss in response to a bet by his publisher.
Photo: Tammy Riojas, Elgin, TX;
H/T to Lorna Salgado for posting the news story that led to this #ScienceSunday post.