Background vs Foreground melanin
- Could someone explain in detail just what the terms background and
foreground melanin distribution mean? With examples if possible :D
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And the budgie says:
When did my wild oats turn to prunes and All Bran?
- Message text written by INTERNET:Genetics-Psittacine@egroups.com
>>Could someone explain in detail just what the terms background andforeground melanin distribution mean? With examples if possible :D<<
Since I'm probably responsible for first introducing these terms I'll try
to clarify them for you.
The difference is seen to best advantage in the budgerigar where, simply
speaking, the black 'markings' are produced by heavy deposits of melanin
which can be described as foreground melanin for fairly obvious reasons.
Background melanin is deposited more lightly and it is less obvious that it
is present underlying all green (or blue,etc.) areas of a bird.
Constructional blue colouration (green when psittacin is present) would be
all but invisible were it not for this background melanin absorbing all
other wavelengths derived from the scattering of incident light.
Background melanin becomes visible in its own right in the Grey varieties
where feather structure is modified and there is little differential
More technically background melanin is confined to the central regions of
the medullae, which form the inner core of the barbs, where it fulfills its
role of absorbency.
The much heavier foreground melanin deposits (I believe, though Inte could
go into more detail) occur more or less throughout the solid keratin of
which the feather is formed, but certainly so at or near the surface of the
outer sheath or cortex.
More technical names (such as medullary melanin and cortical melanin) could
be used but I believe those used here are very descriptive and more likely
to be understood by ordinary breeders like ourselves.
Usually foreground and background are reduced in tandem in diluted vareties
but, in the budgie anyway, there are varieties in which they vary
independently so there must be a degree of separate genetic control. This
would not be so obvious in other parrot species where there is not usually
so much contrast between the two types of melanin deposits. However, many
do have significantly darker flights and/or tail feathers so it's worth
looking out for in these. I'm not very familiar with PF Lovebirds but I
believe the American Dilute (or is it Suffused?) shows a darker pencilling
around the perimeter of many feathers which indicates the presence of
foreground melanin and maybe a differential reduction in comparison with
the Japanese Dilute. Dirk has something along these lines on his webpages.
Sorry to go on at such length but you did say 'in detail'. ;-)))
- Royan, Clive
I think the two terms are excellent to use when describing changes
we see in various colour morphs. The background melanin is contributing to
body colour, whilst foreground melanin contributes to markings. When we
describe changes in a colour morph, these are the things we are observing,
we cannot actually see the melanin distribution within the feather, although
we can theorise about it. So unless we are describing feather sections
microscopically, using the terms foreground and background melanin and
describing the changes in them is the better way to go, using medullary and
cortical only when the microscope is used to confirm it.
As Clive mentioned, some dilute alleles specifically target melanin
within one region or the other. We had a discussion on this a long way back
with greywing, clearwing and dilute budgies. The terminology is also useful
when investigating the changes caused by the opaline mutation. The action of
this allele in all species seems to target foreground melanin more strongly
than background melanin. And in at least two species (budgies and Bourkes)
it even increase background melanin, whilst reducing foreground melanin.