Tuesday, January 28, 2014
The Expression, Suppression and Interactions of Autosomal Pheomelanin (Aph) in the Domestic Fowl
Originally published September 2012 in Exhibition Poultry Magazine
In this article I will outline my observations of the interaction of factors involved in the distribution and suppression of functionally non-sex-linked, visually pheomelanic pigment in the domestic fowl, which I refer to as Autosomal Pheomelanin (Aph). The many effects of the autosomal pheomelanin complex are of extreme importance to the creation of the various color varieties, especially the very dark, mahogany red varieties and the clean-white, silver varieties. In both of those extreme ends of pheomelanic saturation or suppression, we see homozygosity for the required factors, in order to achieve the color goals. To create these extremes of phenotype, there must be an understanding that we are looking at a multi-layered genotype, not just the action of one gene or one locus with two alleles.
To begin, I will state that I strongly suspect that autosomal pheomelanin is present in all domestic fowl, and their precursors, the four species of jungle fowl. I suspect that the autosomal pheomelanic factor is as basic as sex-linked pheomelanin, eumelanin or the e-locus: i.e., all domestic fowl and jungle fowl have these basic factors. Those varieties of domestic fowl and one or two jungle fowl species, which do not express or only partially express autosomal pheomelanin, are the result of a mutation that removes the autosomal pheomelanin; inhibitor of autosomal pheomelanin (Aph^I). This factor seems to be a simple knockout gene, showing partial dominance to Aph in the heterozygous state and total dominance to Aph in the homozygous state. It is probable that this mutation simply stops the developmental chain that forms the autosomal pheomelanin before it can complete its task, thus “knocking-out” the production of autosomal pheomelanin before the process is completed. Thus a heterozygote can express some small level of Aph while the homozygote expresses none at all. I suspect that a wild form/precursor of Aph^I is found in the Gray jungle fowl, though it seems to vary from population to population, as to whether they are pure for the gene. This may be a natural segregation of the trait due to locality origins, or it could indicate some level of hybridization with domestic fowl. The Green jungle fowl may also exhibit some effects of an Aph^I-like factor. However, I feel the Gray jungle fowl is the most likely source for the Aph^I genetic factor in domestic fowl, much as yellow skin likely entered the domestic fowl via hybridization with Gray jungle fowl.
When Aph is not expressed due to Aph^I, those genes which require Aph in order to be expressed in the phenotype, specifically Mahogany (Mh), have no base upon which to express, and in the homozygote for Aph^I, even the Mh/Mh homozygote does not express Mahogany visually. A bird that is Aph^I/aph^i, the heterozygote for the Autosomal Pheomelanin Inhibitor, can express some small amount of mahogany, but very little. This will hold true whether gold (s+) or silver (S) forms of sex-linked pheomelanin are present, though it is much more obvious when dealing with the silver based males (S/S or S/s+). Autosomal pheomelanin can be found in conjunction with s+/s+, S/s+ or S/S males and S or s+ females. As well, Aph^I can be found in conjunction with any of the s-allele combinations. Finally birds of any s-allele combination can be heterozygotes for Autosomal Pheomelanin Inhibitor. Thus can be produced a fairly confusing array of visual expressions. Many of the potential heterozygous expressions and combinations are merely mistaken for, or thought to be, either S/s+ heterozygosity or diluted forms of s+.
I first introduced the term Autosomal pheomelanin from experiments with golden duckwing (the pure silver form of golden S/S males that are creamy or ‘blond’ in tone) and clean-white silver duckwing phoenix. The presence or absence of the salmon breast on the duckwing allele in the presence of S is dependant on the presence or absence of the Inhibitor of Autosomal pheomelanin (Aph^I), and those lines had no red enhancers (specifically Mh), which creates the so-called “autosomal red” visual effect. Autosomal red was originally described in the literature as the pigment causing the red shoulder on some sex-linked silver-based males. It was suggested it might be related to the salmon breast of silver duckwing hens, but nothing further than that. The visual effect being described as “autosomal red” is the interaction of more than one factor, specifically autosomal pheomelanin (Aph), mahogany and possibly (probably) other factors that enhance saturation of Mh on Aph.
In my earlier work with Aph/S/e+, I chose at that time to describe the two major “platform” factors as ap and Ap+, or Autosomal pheomelanin and absence of autosomal pheomelanin, designating the later with a plus sing to signify wild type based on evidence that the factor derives from the Gray jungle fowl. However, further analysis of subsequent data shows that this is most likely not the case, but rather, that “ap”, now called Aph, and “Ap+”, now called Inhibitor of Autosomal Pheomelanin (Aph^I) are not alleles at the same locus, but are actually two different factors that may not even be found on the same chromosome.
Now we will look at how these genes interact. To go in either extreme direction, either to dark red or clean-white silver, we need to stack certain factors, which will show the multi-gene nature of these phenotype effects. I will delineate the factors I would suggest are at work here.
Aph and Aph^I - At the very base are the two major “autosomal pheomelanin” expressions. These are Autosomal Pheomelanin (Aph), which as described above is a factor found in all jungle fowl and domestic fowl, and Inhibitor of Autosomal pheomelanin (Aph^I), which as described above is found in a wild type form in the Gray Jungle Fowl and in many color varieties of domestic fowl. My work suggests that these two factors operate as autosomal dominants, and seem to be “co-dominant”, and that while Aph is inherent to all jungle and domestic fowl, Aph^I is a knockout mutation that stops the production of Aph, thus also stopping the interaction genes that depend on Aph for expression (most notably Mh).
The visual effect sometimes called ‘autosomal red’ is the combination of those genes that enhance autosomal pheomelanin, making it deeper in tone and saturation. There is no gene ‘autosomal red’ as such, because this visual effect is the composite of multiple genes. Of these genes, the only one that is known and thus fairly well documented is Mahogany, which is often described as a eumelanic restrictor. However, its primary manifestation is to enhance the pigment saturation in the pheomelanic areas, with the strongest effect occurring on those areas of the body that are saturated with Aph. In other words, Mahogany expresses most strongly and fully in autosomal pheomelanic areas, such as the shoulder or top of the head/outer ring of the hackle, where it shows very little interaction with pheomelanic diluters. However, pheomelanic diluters can have an effect on mahogany when it is saturating sex-linked pheomelanic areas, such as the hackle or saddle.
When Aph and Mh are present on s+ homozygotes, very dark, saturated reds are created such as Rhode Island Reds, BB Red Cubalaya, Dark Brown Leghorns, Speckled Sussex, etc. However, Aph and Mh can occur on S homozygotes and there they produce, as one example, cream-colored hackles/saddles and dark red shoulders in males with similar hackles and very dark red/brown breasts and shoulders in females on the duckwing allele, e+. Aph and/or Aph^I effects and interacts with all the e-alleles, and is not just an expression of the duckwing allele. The interactions of Aph and Aph^I with the e-alleles will be the topic of a forthcoming article, as there is not space in this one for that discussion. Suffice to say, Aph and Aph^I can and do occur on all of the e-alleles, even if they remain unseen due to full melanization, as is the case with black varieties (which, by the way, can have any combination of Aph/Aph^I and S/s+ under their melanization).
On the opposite end of the spectrum from full saturation of Aph (the so-called autosomal red’), what could then be called ‘autosomal silver’ is the combination of genes that help to “whiten” the silver pigment. The most basic is Aph^I, which restricts the expression of Autosomal pheomelanin. I wish to stress the action of Aph^I here. It actively suppresses the expression of Aph and those genes dependant upon Aph for expression even when they are present. As with the autosomal red visual effect, other genes are layered on the Aph^I base to help in the whitening effect by suppressing the expression of any Aph interaction genes, thus allowing full expression of the (S) sex-linked silver allele. Now let’s think about what that means for a moment in order to really get this point. Aph^I and its interaction factors tends to not just be ‘absence’ of Aph, but are factors which seem to actively suppress Aph and its interaction factors even when they are present, creating a stark with to near white plumage in the pheomelanic zones (think of a true, clean silver of any variety – and true, clean white-silver is rare in all varieties calling for it as most are brassy, which means that the entire set of dilution effects are not present and/or Aph^I is heterozygous while the s-allele is homozygous for S).
Much as in the multigenic ‘autosomal red’ visual effect, when in interaction with Aph^I other genes help to intensify the effect of pheomelanic reduction. There are several genes that interact with (S) silver to create a lighter phenotype. Dilute, cream and others help to suppress pheomelanic pigments, with the greatest effect of these genes being on sex-linked pheomelanin. As an example, this allows one to have a bird homozygous for cream but with a deep mahogany shoulder, as cream interacts with sex-linked pheomelanin, but not with autosomal pheomelanin. However, when cream is in recombination with Aph^I along with S/S, it helps to whiten the silver areas through further diluting them. Likely dilute has a similar effect.
The genetic factors that work with Aph^I are in many ways an undiscovered country. Since Autosomal pheomelanin has been little studied or recognized for the last century, these genes remain to be discovered and understood. The same is true for those red saturating factors, beyond Mahogany, as clearly, all Mahogany expressing lines do not show the same shade. I suspect that some of the recessive black factors interact with Aph and Mahogany to create far darker shades of red, but there may be other genes that do this as well.
Proposed Genes and Definitions
Here I will present a small list of the proposed factors involved in the autosomal pheomelanic complex.
Aph –Autosomal pheomelanin is a proposed autosomal dominant factor found in all chickens and all four species of jungle fowl, which distributes non-sex-linked pheomelanin within the target regions as determined by the e-allele (to be discussed further in a later article).
Aph^I –Inhibitor of Autosomal pheomelanin is a proposed dominant factor, which restricts the distribution of non-sex-linked pheomelanin within the target regions as determined from the e-allele. I suggest that this is a simple knockout gene that turns off the expression of Aph at some point along the developmental chain of pigment formation.
‘Autosomal red’ is the visual result of those factors, which layer onto Aph to make visually dark red plumage areas, most notably mahogany (Mh), but possibly other ‘red intensifiers’ as well. ‘Autosomal red’ is not itself a single gene, but is the visual expression of several genetic factors layered or stacked, interacting, to produce the dark red areas perceived by the naked eye.
Mh – Mahogany is an autosomal dominant gene that interacts with pheomelanin to darken the tone, creating deep red to reddish-brown tones. Mh is the best-known gene in the “autosomal red” complex and is reliant upon the Aph base for expression. On Aph^I homozygotes, even if they are s+/s+ (gold) homozygotes, Mh has little or no effect, due to the suppression on Aph.
‘Autosomal silver’ is the visual result of those factors that interact with Aph^I to make clean white silver plumage by suppressing Aph and its additive factors, when in combination with the s-allele S (silver).
S – sex-linked silver, one of two alleles at the s-locus, the other being the alternate allele s+ or “gold”. Silver produces a dilution effect on the sex-linked pheomelanic areas, producing pigment color ranging from medium yellow/gold to cream to off white to clean white, depending upon interactions with Aph, Aph^I and other dilution factors (such as dilute, cream, etc).
s+ - sex-linked gold, the other allele at the s-locus, which is an orange tone by nature, only becoming darker when combined with Mahogany or other red intensifying factors, or lighter (yellow/gold) when interacting with dilution factors.
It should be noted, that as of this writing, no linkage or pleiotropy has been noted in any of the above-listed genes or factors. Thus, any and all of these factors can segregate and interact, creating a broad range of segregations and phenotypes that requires a separate writing to begin to explain and illustrate. Suffice to say for now that there is a broad range of segregations possible and that they run the gambit from the cleanest, pure, white silver, to the darkest, deep red mahogany, with many possibilities in between. A full description is beyond the scope of this article, but will be the focus of articles to follow.
My intent herein has been to offer an overview of the factors involved in the phenomena of autosomal pheomelanin, and thereby to clarify my limited writings of the past on the subject and express some of my deeper understanding based upon over fifteen years of observation and test mating of these factors.