Pigmentation
of Gallus gallus
By
Brian Reeder
To begin to understand the color forms of our domestic fowl, we should
first look at their major progenitor, the red jungle fowl (Gallus gallus), and
seek to understand the layers of pigments that create the true wildtype, red
duckwing color form. Once we understand how the three forms of melanin come
together to create that color form, then we can move on and begin to see how
the mutations in the domestic fowl redistribute or affect those pigments to
create the many color varieties we know.
The jumping-off point is to understand that there are three distinct
forms of melanin present in the wild type red duckwing pattern of the red
jungle fowl. The classical term for the red jungle fowl pattern would be agouti, (look at the chick down, which
is chipmunk patterned) but that term is not used in the hobby and rarely in the
classic poultry research literature. Instead the terms ‘wildtype’
or ‘duckwing’ are used to refer to
this expression, which is the MC1R gene known as the e-locus allele e+. The
e-locus alleles determine where the three melanins go on the feather areas and
how they layer upon each other in any given area. The three forms of melanin
are eumelanin (black pigment), sex-linked pheomelanin (the well-known z-chromosome,
s-locus genes s+ - sex-linked gold and S – sex-linked silver) and autosomal
pheomelanin, which is a different form of pheomelanin from the sex-linked type
not directly affected by the s-allele mutations.
Autosomal pheomelanin is the least studied, recognized and understood of
the three forms of melanin, yet this form is present with and distributed by
all the e-alleles. It is most prominent and visible on the e-alleles e+ and eWh
in the females, but it is present with all the e-alleles and with both sexes.
There are no e-allele mutations that suppress autosomal pheomelanin, though E
and ER mask this form of pheomelanin under eumelanin (layering).
While I will discuss autosomal pheomelanin as it relates to the red
jungle fowl in this article, it will not be the focus of this article, and will
be dealt with in much greater detail in a forthcoming article. I will
begin here with Autosomal pheomelanin (Aph).
{Please note the change to the abbreviation from my
original published abbreviation of Ap. The designation Ap is already used to
denote an obscure featherless mutation that is not part of any hobby breeds and
thus was a mutation that had never drawn my interest or attention. My thanks to
my friend David Hancox for bringing this to my attention!}
What I will discuss below is a figurative method for understanding the areas of the plumage on both sexes of the duckwing allele e+. This should not be taken to imply that this is meant to describe any developmental processes or melanin studies. This is taken from my own observations of how these pigments seem to be layered into areas. Much of this is observable when other genes remove one pigment or another, allowing the underlying pigments to be seen. For instance when the eumelanin of the tail is suppressed by Db (Dark brown or 'ginger') combined with other eumelanic suppression genes genes on such alleles as eWh or perhaps ER, the pigment that is present in the tail is Aph based pheomelanin with saturation by sex-linked pheomelanin, etc.
Aph is
the key to understanding the red jungle fowl’s coloring, as Aph underlies the coloring
of the entire bird (see image 1).
Image 1
There is much misunderstanding about this factor and many people want to call
it ‘autosomal red’, thinking the vague
references by past researchers to the red shoulder of some sex-linked silver
males was a complete description of this factor. It is not, as the red shoulder
of males is but one aspect of Aph expression and comes about as an interaction
between Aph and mahogany (Mh) and is not the expression of Aph alone. Aph is in
fact not red. It is a warm colored salmon/cinnamon toned pigment. The most
obvious expression of this pigment, without interaction with other genes, is
the breast of unmodified duckwing hens. That salmon breast is the color of
autosomal pheomelanin without other coloring genes interacting with it. Aph is NOT effected in any way by the
sex-linked silver gene (S) as it is autosomal and not the same pigment. They
simply both happen to be forms of a pigment that we loosely call ‘pheomelanin’. This is so important to
understand in order to fully grasp how these three pigments work together to
make the finished phenotype.
Next is the sex-linked pheomelanin (see
image 2) and on the red jungle fowl this is the wildtype form (s+) gold.
Image 2
It
is important to realize that unmodified gold (s+) is not red. It is an orange
tone. To make a red tone from either type of pheomelanin, other genes must
modify each type of pheomelanin. In the red jungle fowl on the e+ e-allele the
sex-linked pheomelanin is distributed in a dimorphic manner, which means that
it manifests differently in the male and the female. On the female, the most
obvious area of sex-linked pheomelanin is the hackle, where s+ layers over Aph
on most of the hackle (just as it does in the male). The area of the head and
hackle with the least expression of s+ is the upper head, the ring of feathers
around the face and the lower edges of the hackle down the front of the neck.
On the female, the entire back and cushion, much of the shoulder and the wing
also express s+ layered over Aph. On the male, we see the expression of s+ in
the saddles, but not in the shoulder or upper wing (more on this below). The gene s+ is expressed in the main wing
feathers, creating the orange triangle we see when the wing is folded.
Finally, we come to the third main pigment, eumelanin or black pigment (see image 3).
Image 3
Eumelanin expression is
very dimorphic and is most prominent on the male where it covers the breast and
the entire lower body and legs, as well as the tail and sickles and parts of
the wing. On the female, we see much less eumelanic expression where it is most
prominent on the tail and within the wing. However, there is also eumelanic
expression in the center stripe of the hackle and as stippling (small dots expressed
as a mild pattern and perhaps the precursor to pattern gene) across the entire
back, cushion, shoulder and much of the wing.
Now,
we have seen where the three main pigments are distributed and layered in both
sexes, but we are not completely finished, as there are two other factors that
modify these pigments to make the finished product in the wildtype red duckwing.
The first is dilution of the sex-linked pheomelanin and the second is
intensification to red of certain areas of autosomal pheomelanin (this involves
dimorphic expression). We acknowledge both of these factors as modifier genes
in domestic fowl and call one dilute (Di) and the other mahogany (Mh). However,
it is not clear if the two factors in the domestics are exactly the same gene
or alleles of these genes, or if the factors in the red junglefowl are actually
wildtype precursors to the genes we work with in the domestics. It has seemed
to me for some time that my results from numerous test-matings over many years
were suggesting that there was not just one form of dilute and mahogany. I
suspect that the forms of both of these factors seen in the red jungle fowl are
wildtype precursors to the more extreme versions seen in some domestic poultry
varieties. However, I do not have enough conclusive evidence to venture naming
the variations of either of these factors and suggest that much more research
needs to be done on these two factors.
In spite of this lack of clarity as
to how many mutations or alleles there may be of these two factors, I can
conclude certain basic points about them. First, it is important to understand
that dilute has a strong effect on sex-linked pheomelanin but has very little
effect on Aph. Mahogany has little effect on sex-linked pheomelanin unless dilute
is absent, but has a great effect on Aph. In fact, you can say that Aph is the
platform necessary for the expression of mahogany, because if Aph is
suppressed, mahogany does not express in the phenotype (more on this in an upcoming article). It is the combination of Aph
and mahogany that results in the phenotypic effect that past researchers have
called ‘autosomal red’.
Now, let us look at how these two factors come together on the red
jungle fowl to finish the phenotype (see
image 4).
Image 4
Dilute reduces the concentration of sex-linked gold (s+). The
area of greatest dilution is the lower hackle, where sex-linked pheomelanin is
most concentrated in the hackles of both males and females. The upper hackle
has a lesser concentration of s+ and is not diluted to the same extent as the
lower hackle. The back and cushion
of the female show the effect of dilute, though it is less obvious due to the
layering of eumelanin as stippling. On the male, the dilution in the hackle is
nearly identical to the female and the saddles and wing triangle show mild
dilution, though not as much as in the lower hackles. Mahogany layers on
autosomal pheomelanin to create deep, rich red areas. This is very prominent on
the male shoulder, and while less so on the female shoulder, there will be a
mild expression of this effect, obscured somewhat by the stippling and shafting
on the female. On the male, there is a slight expression of mahogany on the rear
edge of the folded wing triangle and at the forward edge of the saddles. On both
sexes there is a strong mahogany expression around the face, on the top of the
head, on the outer edge of the hackles and to a lesser extent on the upper
hackle. The mahogany seen on the wildtype red jungle fowl does not seem to have
a strong effect on the autosomal pheomelanic female breast, unlike some
expressions of mahogany seen in domestic strains.
As can be seen, this wildtype color form is not just the expression of
the e-allele. The e-allele determines where the three pigments go and how they
layer, while two modifier genes then create visual extremes within the two
forms of pheomelanin. It is a very elegant color pattern, designed by natural
selection to create a pattern that is broken up and able to blend more
efficiently with the natural environment. Solid colors are not efficient for
blending into the background, thus we see a complicated layering and shading of
the three forms of melanin, with the two forms of pheomelanin being further
modified into visual extremes to create an array of shades designed to blend
into the background by breaking up the outline of the bird and so help to ensure
survival. From this, we can see that the old way of looking at red duckwing as
simply e+ s+ is very simplistic and inefficient. For those who are comfortable
using the gene abbreviations, wildtype red duckwing would be written as e+/e+
s+/s+ (male) or s+/~ (female) Aph/Aph Di/Di Mh/Mh.
