So I got the idea to do this after stumbling across a bunch of viral/farmyard stories this week. I know its currently Tuesday, not Sunday, so, SURPRISE!!! BARNYARD WEEK!
If you ERV readers are anything like me, you have spent countless nights tossing and turning in bed, unable to sleep, unable to stop pondering one of lifes greatest mysteries:
Why are some chickens white?
I finally accepted the fact that this question was ultimately unanswerable, like “What is the meaning of life?” or “Why did anyone ever think Jim Carrey was funny?”. Then I found out that the question actually had an answer. They figured it out like 5 years ago. Its because of an ERV.
Complete association between a retroviral insertion in the tyrosinase gene and the recessive white mutation in chickens.
Now, Im talking about just plain white chickens, here. A recessive phenotype in numerous breeds of chicken. Not chickens that are normally white (dominant white alleles). Not the albino ones with pink eyes, just white chickens with normal pigmented eyes, when lots of their brothers and sisters are brown/black/red.
Albino chickens are white because they have a tiny deletion in their tyrosinase gene. Tyrosinase is an enzyme that helps make melanin pigments like eumelanin and phaeomelanin which color skin/fur/feathers. Deletion = no TYR = no pigment = albino chicken.
Something different is going on in the white chickens. Sure, theyre white, but their eyes are still pigmented normally. It cant be an overall defect in the TYR gene, like the albino chickens.
When scientists investigated the differences between Wild-Type chicken TYR, and white chicken TYR, they didnt see any differences in the TYR messenger RNA. Everything looked the same. So there definitely wasnt anything wrong with the message, like what was going on in the albino chickens.
But they did find a difference between the TYR genes in WT and white chickens… Apparently in white chickens, there is a HUUUUUUUUUUUUUGE insertion in the TYR gene. 7,700 bp virus, Avian Leukosis Virus, plopped its little virus butt into intron 4.
Its in an intron. Introns get chopped out of the mRNA as it moves from the nucleus to the cytoplasm to the ribosomes to make the protein. There is an initial difference between the WT and white chicken TYR mRNA, but that difference goes away, so the TYR protein works fine.
So… if the insertion is in an intron, and the message is fine, and the final protein is fine… Why are the chickens white? And why are their eyes still pigmented just fine?
While the TYR message and protein are fine in white chickens, that big ERV in the intron still comes at a ‘cost’. Not only is it ‘harder’ to make such a large RNA that is just going to go to waste, long-time readers of ERV know that your genome goes to extreme lengths to silence ERVs. Thus, again, even though the ultimate message and protein are fine, white chickens do not make as much TYR mRNA as their WT counterparts, and are therefore white. And while the message can come out fine, sometimes the RNA polymerase just gives up, and the TYR mRNA is made without the last exon, #5, so the mRNA cant make a normal TYR protein.
This suppression is not so severe in the chickens eyes. Why? … They dunno :-/ But they proposed a hypothesis:
Thus, we propose that transcription of the tyrosinase gene is controlled by a different regulation mechanism in the retina than in the skin.
No experimental support for this, but its theoretically a valid hypothesis. Maybe a ‘more aggressive’ set of proteins initiating transcription could overcome the burden of the retrovirus in the intron.
But now we can all sleep soundly at night. We know why some chickens are white.