Genetics

Cockatielworld! Genetics Seminar by Rick Solis

This work is Copyrighted c 2005-2010 by Rick Solis. No part of this presentation may be reproduced or distributed in any form without permission. Posted 7/11/2010

Normal Melanin Production

There is no such thing as a color mutation! This may seem a very odd way to start this discussion, but as we look into the subject, you will find it is very true. To understand color forms, it helps to understand what constitutes normal feather/color development. All color forms result from faulty genes which prevent a step in the normal process of pigment formation, pigment transport, deposition or feather formation. There is no gene that says, "Paint this bird all white" or "Paint this bird cinnamon" or "Give this bird a red eye". Rather, the color forms we see are the visual consequence of a change in normal tissue and/or feather formation processes.
Melanin production can be subdivided into three major phases. In the skin of our birds, cells are responsible for the production of melanin, the black pigment. These cells are called melanocytes or pigment cells. These cells produce the matrices of the pigment granules at first. One must imagine these matrices as colorless almost invisible granules, in other words, the skeleton of the final melanin granule. These colorless matrices are composed out of at least four different proteins. When one of these proteins is missing, the pigment grain cannot be constructed correctly if at all.
When these uncolored matrices are complete, the first phase of the production process is finished. After that a chemical reaction starts through the action of a certain enzyme. An enzyme is an agent with a specific task and in this case it activates a chemical reaction which color the matrices black (pigment synthesis).
When the whole process runs in a normal way and black melanin granules are produced, the third step follows. This final step is the deposition of the fully developed and colored melanin granules through long hollow tubes called dendrites into the growing feathers.

Summing up, we have three major steps in melanin production.

1. Production of empty matrices by the pigment cell in the skin.
2. Coloring of the transparent matrices black.
3. Transport of the completed, black colored melanin grains to their
destination, the feather follicle, where they are laid down in the growing feathers.

Yellow and Orange: Psittacofulvins

Psittacofulvins represent an unusual class of pigments which are found only in the red, orange, and yellow plumage of parrots.

More is unknown about these pigments than what is known. They are classified as noncarotenoid lipochromes, which means that they are unrelated to carotene, the pigment found in passerine birds like canaries and finches. This means that they are fat-based pigments.

We also know that they are never found circulating in the bloodstream or organs and so are known to be produced in each individual feather follicle and then deposited on the outside surface of the feather.

Feather Structure and Color

Now that we know about the two types of pigment (melanin and psittacofulvins) lets talk about the feathers themselves and how it is that although these are the only two pigments in parrots, we can see such a rich variety of colors.

A good way to understand this is to compare the structure of a feather to a doughnut! If you were to cut a glazed doughnut in half, like a hamburger bun, you would have a hole on the middle, the doughnut itself with many pockets of air and a thin layer of sugar glaze on the outside. The cross-section of a typical feather is similar. Instead of a hole, the center usually holds densely packed grains of melanin. The thin layer of yellow, orange or red psittacofulvins is like the glaze. Between them is a fairly thick layer of air bubble-filled proteins called the spongy or cloudy zone. This middle portion is translucent and can have several layers that each reflect light of particular wavelengths. Most often, when we look at the black melanin through this layer, and there are no psittacofulvins on the outside of the feather, we see it as blue! However when we do have yellow pigment on the outside layer, our eyes combine the blue underneath with the yellow on the surface and we see green. So, this accounts for the green color of most parrots.

The parrots of the Cockatoo family are a little different from most other parrots. Their feathers lack the spongy or cloudy layer! This explains why we see their plumage as black, white and shades in between. This is also why a truly green or ‘Emerald’ cockatiel is an evolutionary impossibility.

LOCI

Now that we have covered the basics of pigments and feather structure, we can go on to talk about the individual mutations that cause the color forms we see in our birds.

It is important to realize that many genes are involved in controlling the many chemical steps, and their proper sequence, involved in the production of the wild-type coloration. When certain ones of these steps fails to happen as a result of a faulty gene, we see the consequence in the color of the plumage.

Genes occur in pairs and reside at a particular place, or address on a particular chromosome. One pair of these are the sex-determining chromosomes. On these particular chromosomes, aside from determining the sex of the bird, are found genes responsible for such things as the proper functioning of the melanocytes which make melanin pigment grains and of the operations controlling the functions of the feather follicle. There are genes located at other places on other chromosomes that control things like the normal placement of the melanocytes in the skin, the formation of the small tubes connecting the melanocytes to the follicles and for the production of the psittacofulvins.

These genetic addresses are referred to at Loci or Locus (singular) in Latin. You can see our English word Location comes from this Latin root word.

In Genetic discussions, the color forms are attributed to the genes at these locations, therefore, we can refer to the action of the Whiteface locus, the Pied locus, the Cinnamon locus, etc.

Now, lets move on to discuss the effects of mutations of gene pairs at various Loci that produce changes to the Wild-type coloration!

Mutations of genes on the Sex-Determining Chromosomes

INO Locus

Inos are birds that lack meaningful amounts of melanin grains in their entire bodies. The condition is caused by the malfunctioning of the

genes on the sex-determining chromosomes that control the production of the protein structure of the melanin granule. Since the

shell of the granule is never completed, the following steps (coloration of it) cannot take place. Basically, there is nothing to color.

When an Ino does have the ability to produce yellow/orange pigments the bird is referred to as a Lutino (from the Latin luteus, meaning yellow).

When an Ino does not have the ability to produce yellow/orange pigments the bird is referred to as an Albino (from the Latin alba, meaning white).

Cinnamon Locus

Cinnamons are birds that look a soft brownish color throughout their plumage, have lighter colored beaks, nails and plum colored eyes, darkening some as they progress in age. This is caused by a mutation of a genes on the sex-linked chromosomes that controls one of the last of many steps in melanin production. This particular step changes the color of the melanin from brown to black. Since it never happens, the bird has the characteristic color throughout. This genetic defect can be seen in many other animal species and so, for example we have Golden Retrievers and Ginger Cats.

Opaline Locus

Also found on the sex-determining chromosomes are the genes that regulate the distribution and deposition of pigments. These genes can be thought of as gatekeepers that allow the placement of pigments on the growing feathers. As an example, many Pearl Cockatiels have yellow edges on the feathers of the mantle and body while the entire tail is predominantly yellow. In the Yellow-sided Green Cheek Conure the melanin deposition is likewise restricted on certain parts of the bird’s body and only yellow can be seen in those areas.

Pewter Locus

The effect of the Pewter locus genes, like in the Cinnamon, affects one of the last stages of melanin production. We do know that while it produces a similarly colored bird, it is a different set of genes responsible for the color form because when bred with a cinnamon, grey Wild-type males result.

Sex-Linked Yellow Cheek Locus

The genes involved are partly responsible for the production of the orange pigments. Whatever the defect in these genes, one of its effects is to render the feather follicle incapable of making orange. This color form is notoriously difficult to breed because these birds frequently do not incubate their eggs consistently. This fact suggests the genetic fault is hormonal in nature. Consequently, it is reasonable to conclude that the production of psittacofulvins is interlinked with the hormonal cycle of birds.

The fact that upon sexual maturity (increased testosterone production in males) the production and distribution of yellow and orange is affected further supports this theory.

Mutations of genes on the NON Sex-Determining Chromosomes

Whiteface (Blue) Locus

This is a good place to stop a moment and talk more about gene pairs that can reside at a locus. Some loci have had more than one mutation happen to genes that can reside there. However, only two genes can occupy the two spots available in any particular place on the chromosome string. The Whiteface (Blue in parrots whose feathers have a spongy or cloudy layer) locus is one of these.

With that in mind, these genes either prevent the production of psittacofulvins altogether or partially reduce the production of one or more of them to varying degrees.

There is a hierarchy or order of dominance among these genes.

The Whiteface genes, when they occur in pairs completely prevent the production of both yellow and orange.

The Creamface genes also can reside at this locus. The effect of this gene is to eliminate all yellow/orange on the body, allow no production of orange in the head but to allow the production of yellow and in limited quantities. Typically, in these birds there is a white mask, a pale yellow cheek spot and a trace of yellow about the nares and crest. Whether there are two CF genes or one CF and one WF present, the bird will appear to be CF.

Next in the order of dominance is the Pastelface. Here, the distribution of yellow/orange pigments is normal, but subdued, hence the reference to a pastel color. It is easy to tell the difference between the Creamface and Pastelface because the former has absolutely no yellow in the body and no orange/peach in the face.

Finally, most people believe that the Goldcheek belongs in this series. The change in these birds appears to be in the tone of the cheek spot and it appears to be dominant to all the other genes of the Blue locus.

It is worth noting here that the genes of this family are not in any way related to the genes on the sex determining chromosomes that cause the SL Yellow Cheeks. The do produce a similar visual effect but the causes are totally different and the genes do not interact with each other.

Fallow Locus

The genes that cause this color form regulate the production of an enzyme called tyrosinase. This enzyme is responsible for the coloring in of the melanin grains produced inside the melanocyte. Due to the very small amounts of tyrosinase, the color remains quite pale and never turns black. This same defect produces the red eyes in these birds. In addition to low levels of tyrosinase, there appears to be an inability to complete the transition of the melanin from brown to blackThe production of psittacofulvins is not at all affected and therefore some Fallows have a strong yellow wash and some others don’t, as in any other cockatiel.

There is another gene that, as in the example of the Whiteface series, can reside on the Fallow locus. It is called a non sex-linked Ino

(although the name is misleading) and at first glance looks almost identical to the sex linked variety. This NSL Ino can be found in Europe and will interact with the regular Fallow gene to produce a very washed out Fallow intermediate in color.

Recessive Silver Locus

This is distinct mutation from Fallow, but is also caused by a deficiency in tyrosinase. However, in these birds, the transition from brown to black in the melanin does take place. The eyes of these birds also are red.

Dominant Mutations

All of the mutations discussed up to this point are recessive to the Wild-type coloration. This means that if, in any bird, there is a mutant gene and a Wild-type gene on any locus, the bird will have Wild-type coloration.

However there are two exceptions to this rule. When one of these genes is present, it will trump the functioning of the Wild-type gene in question.

Dominant Silver Locus

The jury is still out on what exactly causes this color form.

Some geneticists have put forward the theory that this is an autoimmune disease that attacks the melanocyte, reducing it’s ability to produce melanin. Others believe that it is a hormonal condition similar to what produces grey hair in mammals. the case, the production of melanin decreases from birth on. Typically, the edges of the feathers have much more than the centers, simply because the supply of melanin to the follicles diminishes during feather growth. Over successive molts the color of the bird becomes increasingly lighter, and it would appear to be due to the same cause. This color form is referred to as Edged and also as U.K. Silver, because it first appeared there. Interestingly, the head is usually unaffected and some people have called these ‘Black-Headed’ and claimed they were a

distinct rare mutation. More than a few aviculturists have paid large amounts of money for them only to be disappointed. I have never seen a real “Black Head or Black Face” cockatiel in person or in a convincing photograph.

Dominant Yellow Cheek Locus

This other dominant mutation greatly reduces the production of orange psittacofulvins. The cause is unknown at this time, but it is definite that the responsible gene is unrelated to either the Whiteface or the Sex Linked Yellowcheek. The jury is still out whether a cockatiel with two of these genes will produce a more yellow-toned cheek spot than a bird with only one. It is true that there is quite a bit of variety in the reduction of orange.

Mutations with origins in defects of the Nervous System

I have left for last the two mutations that are most interesting to me. Neither affect any production process of either melanin or psittacofulvins and in this they are unique. Birds affected by both of these have perfectly normal melanin grains and the amount of yellows and orange are normal for their family lines and gender. However, both of these color forms have their roots in defects arising in the Nervous System!

ADM (Anti-Dimorphic) Pied Locus

There are several types of Pied mutations but we will only be discussing the kind that occurs in Cockatiels. Pieds are birds with patches of feathers and other body structures such as skin, beaks and nails completely devoid of melanin pigments. This is in contrast to Inos, which although appear to have no black melanin at all, do in fact have stray microscopic black grains of pigment and more so noticeably in the eyes of some specimens as they age. This condition is referred to as Leucism.

Shortly after the embryo starts to develop, at about the third or fourth day of incubation, it resembles a tiny peeled shrimp in many ways. It

has no distinct head at this point but along the top of its body is a structure called the Neural Crest. This will, during the course of incubation develop into the brain and spinal cord and to all the nerves in the birds body. However, before this happens, special cells called melanoblasts migrate from the Neural Crest to predetermined spots all over the embryo’s body. As the embryo develops they turn into melanocytes which eventually make black pigment. If one looks at a melanocyte and a nerve cell side by side they look very much alike. They are somewhat elongated and have long ‘fingers’ called dendrites coming out of them. These dendrites are responsible for transmitting electrical impulses in the nerve cell and for transporting melanin grains in the melanocyte. This is worth remembering as we discuss the Emerald a little later.

ADM Pieds genes do not direct the melanoblasts to migrate from the Neural Crest to their intended places in the skin. In a heavy Pied few of them migrate at all. In light Pieds many more do populate the skin.

In addition to this interruption of the melanoblast migration, there is another genetic defect whereby the secondary sexual characteristics involving testosterone do not manifest themselves. In an ADM Pied cock, the black facial pigments do not disappear and so the yellow mask of most adult male cockatiels is not present. The reason for this is very much under debate and no definite cause has been established.

It is worth mentioning at this point the phenomenon of “Pied ticking”. It has been established that the vast majority of Pieds and split Pied parrots of many show yellow areas on the head and nape. However, this is documented in many studies to be a separate dominant mutation. The most famous of these is one involving the Rainbow Lorikeet. Controlled breedings of these birds has been conducted over many generations, breeding Spotted-nape birds together as well as wild-type and Spotted-nape birds. In no instance has a visual Pied been produced from either test group. However, those mated with Wild-type birds all produced Spotted-nape offspring.

The Emerald and the Dilute Locus

My fascination with the Emerald dates back to 1995 when I first saw a picture of one. I knew that it was biologically impossible for there to be a green Cockatiel because none of the Cockatoos have a cloudy layer to reflect blue light. However, the bird in the picture did have a beautiful pale emerald colored hue. I got in touch with Margie Mason the originator of the mutation and she invited me to come out to Texas and have a look at them in person. I flew out the following year to the Texas Bird Breeders and Fanciers show in Temple, met Margie and had a good look at her marvelous birds. In person it was evident that they were not actually green. What caused them to look that way was the odd way the black/grey color was deposited in the feathers. That, combined with the yellow wash on the birds fooled the eye into seeing green. Throwing a twist into the mystery was the fact that an occasional red-eyed bird was being produced among her stock. I had the opportunity to see one of these, bred by Cockatiel Judge Carl Helton in person a couple of years later. It definitely was not a Fallow, Recessive Silver or Lutino.

In late 2005 I contacted Margie again to ask her for some feather samples from her birds with the object of having electron microscopy feather studies done at the Mutavi Research Institute in Holland sponsored by Cockatielworld and the Nederlandse Bond van Vogelliefhebbers (The Dutch National Bird Society). She provided me with samples from what she called Light, Medium and Dark Phase birds and some were sent off to Mutavi. The results that came back a few weeks later were surprising because instead of being anything new in aviculture, the samples were all consistent with the melanin deposition patterns of feathers in their collection from Dilute Budgies. No conclusions besides this were offered by Mutavi. The results opened new avenues of research and possibilities. Are the Light, Medium and Dark Phase Emeralds equivalent to the three color forms seen in Budgies?

In budgies there is a Dilute, a Clearwing and a Greywing. All three can and do reside on the Dilute Locus much as the Whiteface, Creamface, Pastelface and Goldcheek genes can reside on the two spots that make a pair at the Blue Locus.

Regrettably, to date controlled test breedings have not ever taken place to separate the three color forms to see if they will breed true as they do in Budgies.

All the mutations of the Dilute locus (all types of Emeralds reside at the Dilute locus, but not all Emeralds are dilutes just as not all Budgies at the Dilute Locus are dilutes) have one thing in common, namely abnormal dendrites. As previously described a normal melanocyte is a roundish blob with many tiny tubes called dendrites protruding from them. They look like forked lightning bolts. Each little tube helps feed tiny grains of melanin to surrounding feather follicles, which are laid down quite densely. Emeralds are different in that these tubes are either very atrophied or missing altogether. What happens is that the melanocytes produce and produce melanin grains but they have no means to be transported into the feather follicles. The melanocytes get HUGE and rupture, releasing large globs of melanin grains into the skin. Some of them are drawn up into the feather follicle and deposited into the growing feather in a very irregular fashion. What often results is a 'herringbone pattern'. This subcutaneous melanin is what accounts for the very dark beaks and feet in most all specimens.

There is a great opportunity for those that have Emerald cockatiels to do test breedings with the objective of separating the different types.

C 2005-2010 by Rick Solis All Rights Reserved