An article by Peter Bergman of Sydney, Australia

Gene function in Yellowface Budgerigars

Part 2 - Two kinds of yellow pigment!

The true Yellowfaces

Part 1 of this article focused on the dubious status of the ‘Mutant 1’ Yellowface. The Goldenface and Yellowface Mutant 2 by contrast are both true Yellowface alleles in which the double factor birds are visual Yellowfaces. The breeding pattern of the two varieties is identical, the difference being that the Goldenface has a deeper yellow than the Mutant 2.

In Australia Goldenfaces have erroneously been referred to as Mutant 2 Yellowfaces in various publications, the authors having been largely influenced by the rather superficial treatment of the two varieties in Genetics for Budgerigar Breeders. Consequently both indigenous Mutant 2 Yellowfaces and Goldenfaces have been unwittingly lumped together under the banner ‘Australian Yellowface’, leading Yellowface fanciers to believe they are dealing with a single highly variable variety. Evidently in Australia the Goldenface allele appeared on the scene first and when the Mutant 2 allele came along it was not recognised as a separate variety because the two varieties have the same breeding pattern. One doesn’t have to look long or hard in Sydney pet shops to find Yellowfaces which fit the description of the proper Mutant 2 among even the smallest of pet quality birds.

There has been much debate over whether to classify Yellowfaces as blue birds with yellow pigment added or as green birds with some of the yellow removed. This is really a hollow argument and is more of a reflection of how the human mind works than the natural order of things. Some scientists have described the human mind as a great dichotomising machine. We like to pigeon-hole things in either one category or another such as, black or white, right or wrong, good or evil.

In the case of budgerigars it seems we like to think of Yellowfaces as either green series or blue series. Have a look at the genetic formula for the double factor Goldenface b^gb^g. It can hardly be called a green series bird because it doesn’t actually have the green allele in it. Nor can it be called a blue series bird because it doesn’t have a blue allele in it either.

The double factor Goldenface belongs in a category of its own as an intermediate form being neither fully blue nor fully green. ‘Parblue’ would be the operative word. The same principle applies equally to the double factor Mutant 2 Yellowface b^y2b^y2.

The Goldenface

The double factor Goldenface is the bird which really defines this variety. The golden yellow pigment is found mainly in the cap, face, and mask. The yellow pigment is also found in the short lateral tail feathers to varying degrees. In adult feather traces of yellow pigment can also be found scattered throughout the rest of the body but the body colour remains predominantly blue.

Double factor Goldenfaces produce only (b^g/b^g) enzyme. Unlike the enzymes in the two kinds of blue the Goldenface’s enzyme is not as severely damaged so the ability to produce moderate quantities of yellow pigment is retained.

In addition to the double factor there are two types of single factored Goldenfaces. The more familiar traditional type (b^gb¹ genetically) tends to become quite green and is the type which actually comes to mind when one thinks of a single factor Goldenface. The other kind of single factor Goldenface has the genetic formula b^gb². They tend to resemble double factor Goldenfaces in their distribution of yellow but are noticeably paler. The latter are traditionally thought of as Goldenface - Mutant 1 Yellowface composites. However as was pointed out in Part 1 there is no Mutant 1 Yellowface allele. Therefore there can be no Goldenface - Mutant 1 Yellowface composite. These birds are in reality an alternate form of single factor Goldenface based on the Mutant 2 Blue allele b² instead of the Mutant 1 Blue allele b¹.

Single factor Goldenfaces produce three versions of the enzyme but the total quantity of all three enzymes is no greater than the amount produced by the double factor Goldenface.

Blue 2 Single-factor Goldenfaces

Blue 2 single factor Goldenfaces b^gb² produce Goldenface (b^g/b^g) enzyme which is known to be functional, Mutant 2 Blue (b²/b²) enzyme which is known to be defective, and (b^g/b²) hybrid enzyme. It is difficult to judge whether this particular hybrid enzyme is functional or not. If it is functional then it doesn’t work particularly well and adds little yellow to the final appearance of the bird. The bulk of the yellow pigment would have to be produced by the Goldenface fraction (b^g/b^g). Since the Blue 2 single factor Goldenface produces less functional enzyme than the double factor Goldenface, the Blue 2 single factor bird is a paler shade of yellow than the double factor bird.

The fact that the (b^g/b²) hybrid enzyme seems to contribute little if any yellow to the bird tells us that the b^g and b² protein chains do not compensate for each others defects. This in turn suggests that their respective defects occurred on or about the same location along the protein chain. It therefore follows that the mutations which produced the b^g and b² alleles occurred on or about the same position in the original gene for green. When two different mutations occur in the same site within a gene producing two distinct alleles they are termed ‘homoalleles’. It would require DNA sequencing to determine whether we have a pair of true homoalleles or whether we simply have two alleles differing in two different sites which are in approximately the same position. However since the b^g and b² alleles seem to behave like homoalleles I shall refer to them as such in order to simplify this discussion.

Blue 1 Single-factor Goldenfaces

The traditional single factor Goldenface is the Blue 1 single factor Goldenface b^gb¹. These are the birds which cause fanciers so much frustration because of the spillage of yellow pigment into the body colour which turns the bird green.

Blue 1 single factor Goldenface b^gb¹ produce Goldenface (b^g/b^g) enzyme, Mutant 1 Blue (b¹/b¹) enzyme which is known to be defective, and (b^g/b¹) hybrid enzyme. The Blue 1 single factor Goldenface differs from the Blue 2 single factor mainly in the kind of hybrid enzyme it produces.

In Part 1 it was determined that b¹ and b² are heteroalleles and that the protein chains they produce compensate for each others defects well enough to manufacture the modest quantities of yellow pigment seen in the Creamface. It was determined above that b² and b^g are homoalleles which means that b¹ and b^g are likewise heteroallelic with respect to each other. The b¹ and b^g protein chains compensate for each others defects very well and produce an extremely active hybrid enzyme. It is the (b^g/b¹) hybrid enzyme which is responsible for the over-production of yellow pigment turning the traditional single factor Goldenface green.

Breeding strategy

For decades breeders have been advised to pair single factor Goldenfaces back to common blues and select from those young with the least amount of spillage. The irony of it all is that it is the common allele for blue itself which is instrumental in the overproduction of yellow pigment. Consider the birds with the following genetic formulae b^gb^g, b^gb², and b^gb¹. The first two have the desired distribution of yellow and the last exhibits substantial quantities of yellow spillage. What is instrumental in producing the desired distribution of yellow is not the doubling of the b^g allele but the absence of the b¹ allele, i.e. the inability to produce the active hybrid enzyme (b^g/b¹). The b¹ allele is not the innocent bystander it has been generally assumed to be.

Since (b^g/b¹) hybrid enzyme is part and parcel of the traditional single factor Goldenface it cannot be bred out of the bird. The enzyme and the associated yellow spillage will be a perpetual problem. History has born that out. Goldenfaces look the same as they did sixty years ago. The best thing to do is to avoid the (b^g/b¹) hybrid enzyme in the first place by breeding the alternate single factor Goldenfaces b^gb². However breeders have traditionally been advised against breeding Blue 2 single factor Goldenfaces b^gb² because of the erroneous belief that they are breeding a bird which is not only a double factor Yellowface but a composite between two varieties b^gb^y1.

In a world in which the two blue alleles had been discovered in the reverse order breeding our b^gb² Goldenfaces would be standard practise while our traditional greenish single factor Goldenfaces b^gb¹ would be regarded as the bane of Yellowface breeding. What has made the Goldenface a ‘problem variety’ is that the wrong blue allele was discovered first coupled with the subsequent failure by the hobby to realise that if the double factor Mutant 1 Yellowface ‘looks like’ a blue then it most probably ‘is’ a blue.

Yellowface Mutant 2

The Mutant 2 Yellowface represents another allele in the series which like the Goldenface produces its own unique version of yellow-producing enzyme. It has the same breeding pattern as the Goldenface and the same foibles including spillage of yellow into the body colour due to the overactive enzyme (b^y2/b¹). The only real difference between the Mutant 2 and the Goldenface is that the Mutant 2 enzyme is less efficient at producing yellow pigment than the Goldenface enzyme.

Composites between Mutant 2 Yellowface and Goldenface do exist and likewise produce three versions of the enzyme namely (b^g/b^g) enzyme, (b^y2/b^y2) enzyme, and (b^g/b^y2) hybrid enzyme. All three versions of the enzyme are active but the hybrid enzyme is not overactive. These birds closely resemble the double factor Goldenface. The Mutant 2 Yellowface allele can therefore be regarded as another homoallele of b^g and b².

Two yellow pigments

There is a whole other dimension to budgerigars to which the hobby appears to be oblivious since I haven’t seen anything on this topic discussed in the budgerigar press. The budgerigar doesn’t contain one kind of yellow pigment in its feathers but two kinds.

A German scientist, Otto Völker, discovered in 1936 and 1937 that budgerigars and many other Australian parrots glow in the dark. Herr Völker found that parrots with yellow feathers have spots, stripes, neckrings, and other markings that glow when placed under ultra-violet light. Those of us old enough to remember the blacklight posters of the late 1960’s can construct a mental image of what the phenomenon looks like.

The crests and yellow underwing feathers of the Sulphur-Crested Cockatoos glow brightly as do the yellow areas of the Cockatiel; to name two examples. This phenomenon has not been found in Lorikeets, African Lovebirds, or most South American species. It is thought that parrots see a wider range of colours than humans do, ranging into the ultra-violet spectrum.

I have seen photographs of museum specimens of wild green budgerigars taken under UV light. The part of the bird that glows corresponds precisely to the distribution of yellow in a well-patterned double factor Goldenface; the cap being the brightest area. This includes the stray traces of yellow seen scattered throughout the body on otherwise well-patterned double factor birds. To my mind this is no mere coincidence and it allows us to construct a more precise hypothesis to explain how the Yellowface alleles function.

1. Since the blue alleles block the production of both pigments the two yellow pigments must be chemically related and have steps of their biochemical pathways in common.
   
   
2. Double factor Goldenfaces and Mutant 2 Yellowfaces produce yellow pigment in precisely those areas of the bird where the fluorescent yellow pigment is found. Therefore it is reasonable to conclude that what we see in the aforementioned birds is the fluorescent pigment only. Evidently these two alleles produce versions of the enzyme which function to produce the fluorescent yellow pigment but fail to produce the non-fluorescent yellow.
   
   
3. We may conclude from point 2 that the enzyme produced by a green bird acts on two different substrates. One substrate would be the precursor of the fluorescent yellow pigment and another chemically similar substrate would be the precursor of the non-fluorescent yellow. Furthermore the split in the biochemical pathway which leads to the formation of two discrete substrates, and ultimately two discrete yellow pigments, occurs at a point prior to that governed by the Yellowface and Blue allelic series.
   
   
4. The active hybrid enzymes (b^g/b¹) and (b^y2/b¹) produced by the common single-factor Goldenface and Mutant 2 Yellowface respectively appear to act on both substrates. It is expected that the undesirable yellow spillage seen in these birds consists of the non-fluorescent yellow pigment.
   
   
5. The active hybrid enzyme (b¹/b²) produced by the Creamface is more difficult to judge. Either it produces reduced quantities of non-fluorescent pigment only, or it produces reduced quantities of both types of yellow pigment.
   
   
   

To verify or refute the points above the various genetic types of birds would have to be examined under UV light of the appropriate wavelengths. It would be unethical to expose live birds to UV light. Plucking feathers from strategic areas should suffice or alternately a collection of preserved whole skins could be assembled over time.

One aspect of the two types of yellow pigment appears to be less clear. We can see the limits of the distribution of the fluorescent yellow in double factor Yellowfaces, but it is more difficult to visualise the distribution of the non-fluorescent yellow. Do the feathers of the face, cap, and mask of green series birds contain both types of yellow pigment or does the distribution of the non-fluorescent yellow finish where the fluorescent yellow begins? The answer to this question has a bearing on an interesting theoretical question asked by fellow fancier Andrew Wisniewski:

“Is there a reasonable possibility that a Whitefaced Green variety might one day appear?”

It would be prudent to say that in the long term anything may be possible, even Red budgerigars. However to be more specific, if the feathers of the face of a green bird contains both kinds of yellow pigment then the answer would be no. A hypothetical allele which blocks the formation of the fluorescent yellow whilst permitting the production of the non-fluorescent yellow, would still produce a visually green bird but with a paler than average yellow face.

On the other hand if the two yellow pigments are segregated into discrete zones then the answer is yes. In theory at least, we may one day see an allele which does what is essentially the reverse of the Yellowface Blue alleles.

In summary

• The alleles of the Yellowface and Blue series can be divided into two groups. One group contains the homoalleles (or alleles with mutation sites in approximately the same position) b², b^y2, and b^g. The other group contains only a single member thus far, the common allele for blue, b¹. In the latter, the mutation clearly occurred in a different area of the gene giving it the property of being able to interact in such a way that a series of very active hybrid enzymes is produced.
  
  
• There is no denying that the Mutant 2 Blue b²b² is every bit as much a true blue as the common Mutant 1 Blue b¹b¹ and should be recognised as such. The Creamface may be thought of as ‘the Yellowface formerly known as Mutant 1’.
  
  
• The budgerigar produces two types of yellow pigment. Each allele or combination of alleles in the series codes for enzymes which synthesize these yellow pigments to varying degrees.
  
  
  

Copyright: Peter Bergman (Sydney, Australia)

References:

Völker, Otto. (1936) Ueber den gelben Federfarbstoff des Wellensittichs (Melopsittacus undulatus); Journal für Ornithologie 84, p. 618-630.

Völker, Otto. (1937) Ueber fluoreszierende, gelbe Federpigmente bei Papageien, eine neue Klasse von Federfarbstoffen; Journal für Ornithologie 85, p. 136-146.

Boles, Walter. (1991) Black-light Signature for the Birds?; Australian Natural History, Vol 23 No. 10 p. 752.

Special note:

I would be pleased to have your comments on what I feel is a very important article
CliveHesford@compuserve.com

or you can speak directly to the author via
Andrew_Wisniewski@royalsun.com.au