Crossing Colors in Ameraucanas

[Mike Gilbert]

Mike, you mention the O gene.  I've seen that mentioned in the past but I read something last year about some university in England (or Australia or somewhere across the pond) doing like a 4-5yr study and determined that the blue in the egg was caused by a retrovirus.  Oh heck, rather than try to recall I just went and did a Google.  There a real technical report contained in this but this is easier for me to understand:  http://www.nottingham.ac.uk/news/pressreleases/2013/august/unscrambling-the-genetics-of-the-chickens-blue-egg.aspx

So, have you heard about this?  If'n I'm reading this correctly it's the retrovirus that connects itself to the DNA.  What struck me was the part that says, "It's quite remarkable – retroviruses are generally considered to integrate at random locations in the genome, and so the chance of a retrovirus integrating at more or less the same location in two chicken populations is extremely low. Moreover, when appearing in the population, the unusual egg coloration must have attracted the attention of the owners, who must be praised for having selected the trait in subsequent breeding.”   Wouldn't that indicate that there is no "blue egg gene" and that it's the virus causing the blue color from the bile to be extracted and inserted into the egg?

Royce, the retrovirus was just the cause of the genetic mutation.   It is still the O gene that passes on the blue egg trait from one generation to another.
Mutations of genes happen.  Once they are modified or changed they are passed on like any other gene is.  From the text of the original article, "Oocyan is an autosomal dominant trait in chicken resulting from an accumulation of biliverdin in the eggshell, leading to blue/green shelled eggs . . ."    Terms like autosomal dominant refer to genes, not viruses.    At least that is the way I'm understanding it. 

[Tailfeathers]

I hear ya, Mike, about genetic mutations being passed on to subsequent generations once they occur.  That makes sense.  And no doubt you know a whole lot more than me about genetics that I do.  But what's confusing me are a couple of things. 

The first is the narrative that says, "A retrovirus is a virus that, unlike most cellular organisms, carries its genetic blueprint in the form of ribonucleic acid (RNA). It reproduces itself in a host cell using a special enzyme called ‘reverse transcriptase’ which transcribes RNA into deoxyribonucleic acid (DNA). This makes it possible for genetic material from a retrovirus to become permanently incorporated into the DNA of an infected cell."  This doesn't sound like a gene mutation to me.  Am I missing something?

Second is the following statement that says, "In this case, the retrovirus’ effect was to trigger an accumulation of a green-blue bile pigment called biliverdin in the eggshell as the egg develops in the hen."   This sounds like it's the retrovirus that's the "trigger"?

Under the Results and mapping intervals, it says "Variant effect prediction indicates that none of the SNPs results in non-synonymous mutation (Table 1). Structural variant (SV) analyses identify two SVs (c26830 and c129246) common to the oocyan chickens. SV c129246 is located in the PDE3A gene, ~65 kb upstream of the target interval; whilst c26830 is within the target interval, in the intergenic space between the two candidate genes, SLCO1C1 and SLCO1B3."  This sounds to me like the retrovirus is inserting itself between these to genes. 

Then there are these two statements, "Quantitative real-time PCR analysis of European oocyan chicken indicates over-expression of the SLCO1B3 gene (P<0.05) in the shell gland and oviduct. Predicted transcription factor binding sites in the long terminal repeats (LTR) indicate AhR/Ar, a modulator of oestrogen, as a possible promoter/enhancer leading to reproductive tissue-specific over-expression of the SLCO1B3 gene."

And "Based on these results, we used the UB gene as the internal control for normalization of the levels of expression of the four target genes (SLCO1C1, SLCO1B3, HMOX1, and PDE3A). The average relative quantitative (RQ) values are presented in Table S2 together with significance values following Student's t-test. For the qRT-PCR runs, each target gene was analysed in triplicate in each tissue. Significant (P<0.05) over-expression of SLCO1B3 is observed in the shell gland (~19 fold increase) and oviduct (~180 fold increase) of oocyan chickens (Figure 3). The EAV-HP integration is in opposite orientation to its solute carrier neighbours, and the over-expression of SLCO1B3 suggests the retrovirus to be acting as an enhancer insertion. We found no significant association between blue eggs and the over-expression of HMOX1."

What is confusing to me here is that there is no mention of an "O gene" anywhere. 

In the first paragraph of the "Discussion" it says, "The close genomic proximity of the EAV-HP integration to SLCO1B3, its unique presence in oocyan chickens and the tissue-specific over-expression of the solute carrier, known to transport bile salts such as biliverdin, strongly supports the retroviral insertion as the causative mutation of the oocyan phenotype in Mapuche fowl and their modern (European and North American) descendants." 

This seems to me to be saying the retroviral insertion is what's causing the blue color since we know that phenotype is appearance vice genotype. 

My eyes are starting to glass over from all this stuff so I'm gonna end with this and finish reading the paper again tomorrow.  But this paragraph here seems to indicate to me that the retrovirus causing the blue egg is a continuing phenomenon and not a one time thing that just created a mutation that was then passed down. 

"The EAV-HP retrovirus lacks a pol domain, rendering it self-replication defective [20], and so it would likely require an infecting virus acting in trans for it to proliferate. It is entirely possible that an exogenous virus might facilitate the proliferation of EAV-HP within the genome as has been demonstrated previously for the Rous Sarcoma virus (RSV; [21]). The close genomic proximity (23 bp) of the different insertion sites, observed in Mapuche fowl and Dongxiang chicken, suggests a possible integration site preference for EAV-HP, as has been suggested for several retroviruses [22]. Active ongoing insertion and segregation of EAV-HP in chicken populations have been shown with a typical prevalence of 10 to 15 copies per genome [23], [24]. Such EAV-HP genome dynamism is thought to play a recombinant role in the emergence of exogenous avian retrovirus ALV-J due to a uniquely similar env sequence with the ALV-J prototype HPRS-103 [23]. An intact EAV-HP, including pol gene, has previously been identified in G. sonneratii [25], and EAV-HP proviruses with intact pol and env genes have also been found in domestic chicken [23]. Borisenko [26] found the EAV-HP pol gene to be more closely related to that of RSV than of other retroviridae. The expressed EAV-HP transcripts in some lines [23], [27] and pol sequence homology with RSV support therefore the possibility that a helper virus might be contributing to the continued segregation of EAV-HP."

So, with all that being said, what am I missing and where am I going wrong?  PLEASE educate me and maybe some others.  Oh great Master Obi-Wan Kenobi, do enlighten us!!