I'm pretty fascinated with eggshell pigments right now so this is what I'm sort of focused on (health issues are keeping me away from the birds at the moment - huge bummer).
I think we all have a priority list as far as trait selection goes and eggshell color is generally pretty low on that list. That said, I don't think it ever hurts to learn more so we are able make more informed decisions - progress is slow enough as it is ... at least for us.
There are a couple of papers from 2013 that talk about the retroviral insertion at
O. The one by Wragg et al. would be more specific to our birds of South American origin. Perhaps my interpretation is incorrect, but I thought they offered some very nice clues as to how our blue eggshells are made. Though further investigation needs to be done - such as to why we are seeing oviduct and shell gland tissue specificity. Based on what we know, they propose estrogen regulation as the likely culprit. We could also use replication of the study and maybe look into other biliverdin sources.
I've been writing down some of my thoughts on this article that I'd like to share. Might not be entirely on track, but I find it all very interesting.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3747184/pdf/pone.0071393.pdfTerms:
*biliverdin = blue-green pigment, sometimes called to oocyanin
*protoporphyrin = red-brown pigment, archaically called oorhodeine
*Blue egg gene =
O for oocyan = SLCO1B3
This gene codes for a protein responsible for transporting small molecules (such as biliverdin) from the bloodstream into an area - normally expressed in the liver.
*HMOX1 = This gene codes for heme oxygenase, the enzyme responsible for turning heme into biliverdin.
*EAV-HP = endogenous avian retrovirus = retrovirus within the bird
Retroviral insertions are pretty common - perhaps up to 10% of the genome ... we have them, too! Most of these insertions are harmless - the virus lacks the region of its genome needed to replicate. They tend to target certain areas of the host genome.
Wragg et al. have mapped the
O gene in South American blue egg laying birds. It has a similar, but different insertion than what was found in the Asiatic blue egg-layers. These insertion events occurred independently and were likely recent - occurring after chickens were domesticated. Humans have been selecting for blue eggs in chickens since they first appeared - the acknowledgement by the authors of the Mapuche people of Chile for preserving these birds is a really nice gesture.
From the article: "We show that [EAV-HP] insertion enhances the expression of the neighbouring solute carrier SLCO1B3 in the shell gland and oviduct of European oocyan chickens and that HMOX1, a previous candidate gene, is not over-expressed."
This research suggests that the sites of biliverdin synthesis are other than primarily in the oviduct and shell gland (as is thought to be the case with protoporphyrin). The retrovirus insertion occurs in the promoter region of our
O gene where we see enhancement of its function. Note "enhancement." I don't think this is as easy as an "opt-out" or "opt-in" - or switching the gene off and on. The gene is already there transporting small molecules (such as biliverdin) into the region - with our famous retroviral insertion, it's just 1) being promoted to a greater extent and 2) expressing tissue-selectivity toward oviduct and shell gland. Also, assuming the source of eggshell biliverdin is through the oxidation of heme and not more of a "ground up" synthesis as is thought to be the case with protoporphyrin, this study found the gene (HMOX1) needed for such biliverdin synthesis is not over-expressed in tissues of blue egg-laying birds of American origin (the brown egg-layers studied actually had greater expression). This is contrary to what had been found previously by a couple of studies by Wang(s) et al. in the Asiatic blue egg-layers with increased expression of HMOX1 in the shell gland. Interesting discrepancy.
The instance of over-expression of this solute carrier gene is specific to reproductive tissues and is likely regulated by estrogen. Thus, changing hormone levels may be one of the factors affecting what we see in fluctuating eggshell color in our layers - increased blue eggshell color in young pullets and reduced blue eggshell color as the laying season progresses.
Also, this solute carrier is likely not terribly specific as to what small molecules it transfers - other bile pigments may go along for the ride. A source to consider, but not likely to be present in quantities that lead to off-color eggshells.
Not from the paper, but other things I've been thinking about:
*Biliverdin acts as an antioxidant. Protoporphyrin acts as a pro-oxidant, making for free oxygen. When this free oxygen combines with biliverdin, the pigment changes color from blue-green to pink. I wonder if this biliverdin derivative (perhaps by different mechanism) might be found in some pink-shelled eggs rather than protoporphyrin exclusively.
*Being an antioxidant, I wonder if the reduction in eggshell color as a hen ages might be due in part to increased competition for its use at other sites over time.
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