Ameraucana Forum
The Official Ameraucana Forum => Breeding => Topic started by: Mike Gilbert on January 28, 2016, 02:22:20 PM
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We have visited this topic before, but perhaps some of our newer members would be interested in the information found at the following link.
http://www.virology.ws/2013/09/11/a-retrovirus-makes-chicken-eggshells-blue/
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And here is a link to a similar article. Both links are now included in the list of genetic site pinned on the top of the Breeding subforum.
http://www.futurity.org/surprise-virus-caused-blue-chicken-eggs/
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For those who really wanna get into it, click on the hyperlink to the original study. I've made it about halfway thru a couple times before my eyes start to glaze over.
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Wasn't sure where best to pose the question, so thought I'd resurrect this oldie-but-goodie thread ...
Has anyone ever used the coccidiostat nicarbazin with their Ameraucanas? I wouldn't expect it to be used in a breeding flock, but was curious about its effect on eggshell color.
Ode to Red, White, and Blue Eggs
There are many breeds of multicolored hens;
They lay eggs of every hue.
Their uteri filled with pyrrolic pens
Paint the shells red, white, and blue!
So ask not "what came first, chick'n or egg?"
Nor "what can either one do for you?"
Seek life's mystery of porph'rin and bile pigment,
It is bound up in the red, white, and blue!
-S. Schwartz
ETA: Also, more relevant to the articles posted, I thought mention of the difference in expression between homozygotes and heterozygotes of the Asian blue-egged breeds (in Wang, et al.) interesting. The "expression of SCLOB3 was 2 to 3 fold higher in homozygous blue-shelled chickens than in heterozygous blue-shelled ... individuals." Do folks fairly consistently see a difference in phenotype between their OO and Oo birds?
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I bet more people have O/o+ hens floating around in their flock than they think. Since O is linked to pea comb, in normal lines simply selecting birds with the best pea combs should also select for homozygous blue egg. That's why I find the case with Wheatens curious. I hear tons of people hatching out white laying birds from LF Wheatens, and I've heard this was something not uncommon in the bantams for years. They have nice tight pea combs. Was the pea comb-blue egg linkage broken in that variety? Is it actually the o+ allele that is producing white eggs, or are the birds still genetically O/O but have some other pigment blocking gene?
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Having created the original wheaten and blue wheaten bantams, I can tell you I never had much problem with off colored eggs. Never white eggs, but I got some chicks back from Duane Urch one time and ended up with one pullet that laid brown eggs. She was culled. The problem with creating new varieties is that one never knows what others might be crossing in afterwards. And you have no control over their culling practices.
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I bet more people have O/o+ hens floating around in their flock than they think. Since O is linked to pea comb, in normal lines simply selecting birds with the best pea combs should also select for homozygous blue egg. That's why I find the case with Wheatens curious. I hear tons of people hatching out white laying birds from LF Wheatens, and I've heard this was something not uncommon in the bantams for years. They have nice tight pea combs. Was the pea comb-blue egg linkage broken in that variety? Is it actually the o+ allele that is producing white eggs, or are the birds still genetically O/O but have some other pigment blocking gene?
I think the idea of O/o+ floating around is more true with certain varieties and of course lines/strains. Large fowl wheaten is the variety that we hear most about with the problem, unless those eggs are O/O and severely diluted or blocked of pigment genetically, as you mention, to make them appear white. I don't put much stock into the linkage of pea combs and blue eggs, since they don't always go together. My bantam buffs have the best pea combs, yet I've noticed very light...almost white...eggs from some in recent years. We know how well recessive genes can hide for many generations, but with say my LF blacks I generally hatch many hundreds each year and I don't remember seeing any white looking eggs from them.
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I wonder this, too - if some of the paths to loss of pigment are not just that we're losing the retroviral enhancer (O), but are losing its expression. Most evidence points to protoporphyrin being produced in the oviduct and shell gland. Evidence points to the bulk of biliverdin being produced elsewhere and transported to the oviduct and shell gland. I would think there might be a few ways to hink up the works in this process (heme oxygenase is needed to convert heme to biliverdin, estrogen is needed to regulate the enhancer, biliverdin might be needed elsewhere (as an antioxidant), etc.).
Using comb/blue eggshell linkage is only sort of helpful if original stock is pea combed/blue shelled and single combed/non-blue shelled ... and then there is always the possibility (~3%) of recombination.
In any case, I would use caution in working with these light-shelled birds. It can be pretty frustrating.
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It's been awhile since I've looked into egg shell color but the last I read was a couple 2-3yrs ago and it talked about the retrovirus. The thing I do remember was that at the end it said no one was still sure exactly how the blue egg is formed. So maybe there has been found now.
I think it was the 3rd year into my LF WBS that I wound up with a white egg or two. And I know they came outta a couple of BW hens because one of them laid an egg that was so pale blue you had to put it next to something white to see it wasn't white and the other wasn't much better. Over time the one BW went to white as far as I could tell. I initially used them to color test what the roosters would throw.
Egg color has never really been a consideration in my breeding program. Far too many other things to work on that the judges actually look at.
Don't wanna hijack the thread but I did hear recently that someone else wound up with a whole bunch of yellow-legged WBS. And they didn't come from me! So, evidently that's starting to pop out elsewhere. Since it took 8yrs of inbreeding and linebreeding to get it to pop out in my closed flock that tells me that the heterozygosity for the yellow-leg gene is much more prevalent than what it once was.
So here you have a scenario. On one hand you have a bird that lays a white egg. Judges don't look at the egg. On the other hand you have a bird that is carrying the yellow-leg gene but is hidden by slate legs. Judges don't care about hidden genes. Which is of greater importance to work on?
Personally, I am working on getting to a homozygous leg-color flock. Why? Because the yellow is hidden. It takes work to find it and correct it. The white egg is simple. Just don't hatch 'em. And besides, until someone actually figures out how the blue egg is made does it really matter? Just cull the whites. By the time someone figures out how the blues are laid there'll be somebody working on how to get a real Robin Egg Blue that everyone really wants.
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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.pdf (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3747184/pdf/pone.0071393.pdf)
Terms:
*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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Great write-up Holly. Yours is a more thorough hypothesis/explanation for the chemical processes that make up the colored eggshell than any I had read heretofore. I don't know when or how we will ever get absolute answers since there is little economic value associated with these issues, but it makes for a lot of interest and speculation. Until reading your piece, I never gave a shred of credence to the possibility that pink eggs were made from anything other than different hues of brown shell pigmentation. Thanks for taking the time to share your thoughts!
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I'll echo Mike's Thank You to Holly for taking the time to study, share and help better explain what she has learned, but I hope the "health issues" clear up and allow you time to get back to the birds. :)
Even though some genes share the same chromosome, I don't consider that in my breeding. I only consider the genes themselves...individually. Keep in mind the genes that control a trait, determine if it will manifest, it won't or it may be heterozygous...like muffs that are there, but full very full. We don't breed for any heterozygous traits, so I try to breed for birds that are "pure" for any given trait's possibilities. Often times we've heard of the linkage between the pea combs and blue egg shells we desire, but this same linkage can be extended to the pattern gene (Pg)and Melanotic (MI) that breeds find so hard to breed into blue Ameraucanas.
The chart, below, is from Chicken Chromosome Linkages (http://the-coop.org/poultrygenetics/index.php?title=Chicken_Chromosome_Linkages).
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As I understand it, there are close or distant linkages depending on the distance between the two genes on the same chromosome. Scientists map these and record the distances. That is why some linkages are more difficult to break than others. The close ones are the difficult ones to overcome. Holly?
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More than a physical distance, a map unit (or centimorgan) is defined by the chance of a crossover event happening. (It is not a measurement of distance by nucleotides, for example.) It's a recombination probability - 4 units apart means that there's a 4% chance that a crossover event will occur. (Crossover is when, say, an "OP" chromatid from mom breaks between O and P and recombines with an "op" chromatid from dad that also breaks between o and p. The result is offspring that is "Op" on one chromatid and "oP" on the other. They've found that this only happens only about 4% of the time.) In practice, you could have whatever crop up in your first chick from a given cross or the 100th. It gives an idea of chances of something happening, though. Sometimes we test cross for single comb and sell the chicks as barnyard mixes. If folks are looking for blue egg-layers, I'll steer them away from the single-combed chicks. Another example - we come up with some pretty impressive-looking silver laced birds. But I'm pretty sure I'm getting Pg from one side and Ml from the other. Something I'll continue to play with, but not something I'd market to others ... and I know I'll need more than a little patience to get it right and have them breed true. I'm waiting for that 10% chance of a crossover to occur ... knowing when I have it is another issue.
In the case of our Chromosome 1 - Db, Pg, and Ml are so far from O and P that there is practically no linkage. The linkage would be between O and P. Also, to a lesser extent, between Db, Pg, and Ml. Generally speaking, anything over 50 map units is independent assortment - same probability as if they were on separate chromosomes.
If the situation is such that both parents are homozygous for blue eggs and pea comb, recombination - in this context - is not a concern.
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Holly, I'm curious. How do they figure recombination probability if it is not related to physical distance between genes on a chromosome? I always thought the farther apart, the more likely for a given section to break off. Or is it a matter of weak sections on the strands? Is my thought process totally messed up (it would not surprise me)?
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As for the centimorgan...I guess that's coming up now with the dna testing people are doing. I can't remember that in class.
https://genealogy.stackexchange.com/questions/3364/what-is-the-significance-of-shared-centimorgans