Tweets:

This thread may be getting too specialized for general consumption, but
given the impressive level of knowledge in this group, I'll risk boring
some of you in hopes of interesting others, especilly any of you
knowledgable about pigments in birds. Delete if you have a low tolerance
for cell biology.

At 11:16 AM 1/20/99 -0800, Deb Beutler wrote:

>Ornithologists actual recognize four types of albinism :

>Total: <snip> Incomplete: <snip> Inperfect:<snip> Partial

Do you think they could make the names any more similar? They should add
Impartial just for good measure.


>Total: A complete inability to make any melanin pigments. There is no
>melanin in the eyes, skin, or feathers and these structures appear pink
>(eyes and skin) or white (feathers). They may still be able to produce
>carotenoid pigments. He cites a Rose-breasted Grosbeak that had pink eyes
>and legs, white feathers except bright pink breast and under wing coverts.
>
>Incomplete: When the melanin pigments are completely absent from the
>plumage, or iriis, or skin but not all three.

This could be called tissue- or organ- specific albinism to be a little
more informative. It is presumably caused by patterning mutations.

>
>Inperfect: When all of the pigments are reduced ("diluted"), or at least one
>of the pigments is absent, in any or all three areas (my example of the
>female Evening Grosbeak in the WSU collection)

Now I'm confused. Your Evening Grosbeak example sounded exactly like the
Rose-breasted Grosbeak above (no melanin, but carotenoids are present) that
you call total albinism.
In any case, dilution of all pigments is a rather different thing than
absence of one pigment, and it seems odd to lump them. Diluting mutations
in mammals change the way the melanin granules are distributed in the
pigment cells (also called chromatophores) - they become dispersed
throughout the cell instead of being concentrated in one spot. Perhaps the
same mutations would affect the distribution of the carotenoid granules
(vescicles?), since these mutations probably act either through the
cytoskeleton or the Golgi system.

>
>Partial Albinism: when the pigments are reduced, or one or more is absent,
>from the PARTS of any or all three areas (skin, iris or feathers)
>
>So I think the pied mutation would still fit in the partial albinism.
>However, I would hesitate to use the term "pied mutation" until we have
>eliminated the possiblity of a non-genetic cause of the "pied" condition.

Well, fair enough in theory, but I'm not sure I can think of a real example
of a nongenetic causation for this. More on this below.


>Pettingill (pp. 42-45) writes that birds have melanin but he doesn't mention
>phaeomelanin (perhaps this is a mammal only pigment).

This is possible; however, the term "melanin" actually refers to the
whole class of closely related pigments. (I should have called the common
mammalian pigments eumelanin and phaeomelanin.) Thus Hildebrand says
"Melanin is black, brown or red." (Analysis of Vertebrate Structure, 1974).
Pettingill may simply be using "melanin" for the entire class of pigments.


In addition, as you
>mentioned, birds have carotenoids that they get from their diets and modify
>into their feathers. Do mammals have that?

No. Mammals have carotene in their fat (yellow fat, brown fat), but it
doesn't contribute to skin or hair color. Note the general blandness of
mammal colors as a consequence: blacks, grays, browns, dull yellows,
near-reds and whites are about it. Some mammals opportunistically utilize
hemoglobin for display (e.g. uakari monkeys; or to mark estrus in a number
of species), but it's an adaptation of the vascular system, not the
chromatophore system. Blue eyes and the blue face of a mandrill are some of
the few cases of structural colors (Tyndall scattering) in mammals.


In addition, 13 orders of
>birds, inlcuding Strigiformes (owls), have other pigments, porphyrins, that
>create mostly brown, reds and buff. One porphyrin derivitive, turacoverdin,
>is a green pigment found only in the touracos (Musophagidae family).

This is quite interesting. Porphyrins are common (hemoglobin, chlorophyll,
electron transport carriers, etc.), but to my knowledge they are not used
in any systematic way for pigmentation in mammals. Do the 13 orders form a
related group? Is there a distinct class of chromatophores that contain
these pigments in birds or are they more generally distributed? For that
matter, I guess I'm not sure where the carotenoids are located, either.
Some animals have separate chromatophores for melanins and carotenoids; I
imagine this is true in birds, but I don't know.

.... loss of coloration can occur over injuries.
>If the white is replacing a plumage element (black on the wing or a red
>brest patch) it is probably due to a genetic cause. However if it just a
>splotch of color (a big white spot on the back of a robin for example), it
>is less likely that is a mutation and might be an old injury area.

I guess I'm pretty skeptical about losing color to injuries. I will grant
you that scar tissue is less pigmented (non-pigmented?), but if injuries
caused pigment loss to any appreciable degree, we ought to all remember
seeing (or being) people with big white patches in their skin. We ought to
all have seen many dogs and cats with odd patches of white fur over old
injuries. I don't recall ever seeing anything like that. I think I've seen
a few people with blotchy skin, but I don't think it was because of
injuries. The injury seems like it would have to selectively kill off the
pigment cells or cause them to dedifferentiate while leaving the other
epidermal cells intact. It seems implausible in most situations.
As for non-injury environmental causes (other than nutrition for
carotenoids), the pigments in a feather might be extracted or be made to
photo-bleach by exposure to some chemicals, but fortunately such chemicals
are mostly not found in any quantity in nature. If the birds hang around
organic chemistry labs this might be a more plausible explanation.

If I actually did see a robin with a white spot on its back, I would be
less eager to invoke a pied mutation because the back is an unlikely place
for this kind of white patch. However, I've never seen one. Until I do I
will probably continue to choose pied mutations as the most reasonable
working hypothesis for the many cases of partial albinism in the world in
which the white spots are asymmetric and generally ventrally located.

>And thanks for the education.

Likewise.

Paul Talbert