Comments on

“ The Halfsider ”

by Dr. John Pilkington

This is a most interesting and challenging article which I did not see upon its publication in the May 1997 issue of The Budgerigar (bi-monthly magazine of the Budgerigar Society, UK). Nor do I know whether its provoked any reaction or discussion at that time. The article puts forward arguments in favour of an entirely new explanation for the phenomenon of the half-sider budgerigars which appear rarely but with some regularity. In doing so it sets in motion new trains of thought into previously uncharted territory. However, we have to bear in mind that the lines travelled are connected at present by no more than informed speculation.

The gist of the article is that Dr. Pilkington does not believe the half-sider to be caused by the loss of the green allele (of a genetically green/blue bird) from one of the two cells produced by the division of the original fertilised cell; that cell, now containing only the blue allele, going on by continuing divisions to produce one half of the bird’s body. He theorizes instead that, because the nervous system is strongly differentiated in the two halves of the body, there is likely to be a direct connection between this separation and the loss of yellow pigmentation in one half of the body of half-siders. It is suggested that an injury, perhaps to one cell only, on one side of the delicate tissues of the neural crest at a critical stage in the development of the embryo would lead to impaired function of all descendant cells in the nervous system. Because much of the biochemical pathway which produces adrenalin for the nervous system is also shared in the production of phaeomelanin, which he proposes is the pigment responsible for the yellow ground colour of the budgerigar, he contends that there are strong grounds for proposing that loss of yellow pigmentation to one half of the body is due to such an injury to the nervous system in the embryonic stage.

I have no intention of dismissing this hypothesis out of hand, but I do believe a critical look at some of the points made by Dr. Pilkington can only help us understand the situation more clearly and show that his proposals fall far from being the last words on the subject. For convenience, these points are considered in the order they appear in the article. Since this is an extremely complex subject, and much still remains to be discovered, I only comment where I feel able to offer a constructive alternative view.

Not so obvious flaws.

I do think Dr. Pilkington does something of an injustice to Prof. T G Taylor, or perhaps more accurately his co-author Trevor Daniels; for it is the latter, I believe, who was the more of a genetic theorist. It is true that, for whatever reasons, the theory ascribed to him in the article was the preferred option. But other possibilities were also mentioned; though all supposed either the mutation or the loss in one way or the other of the green allele of a genetically green/blue bird, leaving the blue allele/s to take the active role. The point made concerning the viability of a bird lacking one of the chromosomes carrying the blue locus in the cells of one half of the body was also considered. A possibility not considered was the loss of just a fragment of one chromosome, that part carrying the green/blue locus, by a genetic accident known as a deletion.

Three specific and numbered points are made in the article outlining “obvious flaws” in the generally, though not unanimously, accepted theory. I will paraphrase each of these points before seeing whether I can wriggle my way round it in some plausible way:

1. “Could a budgerigar remain viable after the loss of a chromosome and all the genes it carries?” We can only suppose that this depends upon the number and nature of the lost genes; and remember, too, that the cell still contains the full complement of the matching genes on the remaining matching chromosome. If none of this remaining set of genes were lethal recessives there is a chance that the bird would be viable. Furthermore; although various articles state categorically that the budgerigar possesses thirty-this or thirty-that chromosomes the truth appears to be that the total number of chromosomes has not been definitively established, although a figure of plus or minus 58 may be accepted (see external article on the Birdcraft pages - Mutations in Aviculture). The difficulty arises from the fact that many of the chromosomes are extremely small (microchromosomes) which prove difficult to count accurately and presumably carry relatively few genes. If the green/blue locus were on such a chromosome the chances of viability would be greatly increased.

2. “The first cell divisions create the blastula and do not differentiate immediately into future body parts.” I confess to not being very familiar with the stages of development of the embryo but not for one instant do I believe that the blastula (see external article on Kimball’s Biology Pages - Embryonic Development: Getting Started) is a haphazard group, a mere bundle, of cells with no internal organisation. Mother nature does not work this way. All around us we see her shapes, even in the most lowly organisms; wonderful and intricate and almost infinitely varied. Organisation is evident right from the start in the way the cells sequentially divide and multiply. How can such a body (the blastula) not have a definite form and spatial identity; having a right side and a left side, a top and a bottom, a front and a rear. That there is little or no differentiation into more specialised cell types in these early stages is evidence that a groundwork is being established upon which, at the right time and the right places, that diversification and specialisation will begin.

3. “We should see half-siders of many different sorts.” Here it has to be assumed that the gene causing these other variations, as suggested by Trevor Daniels, must lie on chromosomes where loss of one of the pair would indeed lead to loss of viability. The apparent ability of embryos to survive the loss of a chromosome (or part of a chromosome) containing the green/blue locus being an exception which proves the rule.

Whatever the truth in this instance there is no doubt that mosaicism, of which the half-sider is but one extreme, is a well known and in some cases well-researched and proven genetic phenomenon. A frequently quoted case concerns the ubiquitous fruit-fly (Drosophila melanogaster) where an individual was found in which one half of the body was female and the other half male. Here an XX insect which would normally develop into a female had lost one X-chromosome in one of the daughter cells from the first cell division. The half of the body deriving from that cell developed with male characteristics. Note that in these insects the Y-chromosome does not determine maleness; it is the number of X-chromosomes present as a proportion of the total which determines sex.

Assuming this genetic model, the area(s) of tissue affected would only occur on one side of the body of an organism except in the very unlikely event of the same genetic accident occurring in the formation of a founder cell in the other half of the body.

A different form of mosaicism involves the random inactivation of one of the X-chromosomes resulting in a much finer mosaic effect over the whole body. In birds only cocks would normally show this phenomenon, which would necessarily be a sex-linked characteristic determined at local level. This is the mechanism said to produce the tortoiseshell cat.

Artificially induced mosaicism is used by scientists as a tool to determine whether a particular gene is required for the normal function of cells in different parts of an organism (see external Biology Lab notes from Wright State University - Analysis of genetic mosaics).

A doubtful premise

“In the absence of evidence to the contrary, I would suggest ............ that the yellow pigment found in the cortex of budgerigar feathers belongs to the phaeomelanin group.”

What can I say about this? To assert so categorically, simply on the alleged lack of evidence to the contrary, a proposition which flies in the face of accepted thinking, not only within the budgerigar world but also the wider scientific community, is daring in the extreme. Others have had such flashes of inspiration before and finally been vindicated; but I fear this will not be so of this assertion.

This is not the place for a full discussion of colour in the budgerigar, but it is necessary to summarise conventional wisdom as regards both melanin pigmentation and the yellow pigmentation referred to above before leaping to its defence.

Melanin distribution in the budgerigar can best be appreciated by visualising the Grey colour variety, which has lost the ability to show either blue or yellow (and hence green) colouration. The black areas (known as the markings) have the same distribution as in the normal light green, or wild-type, and are caused by heavy melanin deposits in the cortex of the feathers. The grey areas, which largely coincide with the body colour area (green in the wild-type), are caused by lighter deposits of melanin within the barbs of the feathers in a region known as the medulla. I have long called these two types of deposits foreground and background melanin respectively, as an aid to describing the effects of budgerigar colour genes. This is not to imply that the two types are necessarily different either chemically or physically. The description background melanin derives from the fact that it is not directly visible in most varieties and yet forms the essential background against which blue colour (green when modified by the presence of yellow) produced by the action of the cloudy layer can be seen.

Where melanin is present it takes the form of granules which have a definite shape, probably reflecting their molecular structure, and large enough to be seen quite readily under the microscope. The situation seems so straightforward that there is little discussion on the type of melanin found in the budgerigar. Most would probably agree that it is of the eumelanin type and that no phaeomelanin is present.

The yellow in the budgerigar is usually referred to as ground colour and it is true that its precise nature has not been established. In more technical discussions it is customary to refer to it as psittacin yellow, since the same pigment is assumed to be responsible for the yellow ground colour seen in many other parrots (Psittaciformes). The distribution of psittacin yellow is best seen in the Lutino variety where it is evident that some areas, such as the tail and flights, are not so strongly pigmented as the body.

Where psittacin yellow is present in the feathers, it is diffused throughout the cortex and separate particles of pigment cannot be seen under the microscope; it appears to be “dissolved” in the keratin of which feathers are formed.

Flaws by another name.

We are now in a position to consider a few points which I shall not make the mistake of calling obvious flaws:

1. It is acknowledged that many animals and other birds are coloured by both eumelanins and phaeomelanins and that the latter produce browns and lighter hues verging on yellow. However, is there any precedent in either of these groups for a phaeomelanin yellow as bright and intense and pure as the yellow seen in the budgerigar? I think not.

2. Both eumelanin and phaeomelanin are seen in the form of granules; of varying shape and form, but nevertheless visible under the microscope. As already pointed out psittacin yellow is dispersed very finely in the keratin of the feathers and if it does exist as particles these must be very small indeed; a different order of magnitude entirely. This hardly suggests any close relationship between the two types of pigmentation.

3. What about the Lutino? It is generally acknowledged that the ino allele prevents or suppresses the formation of any melanin in the feathers or body tissue of a bird in which it is expressed. So, in the Lutino budgerigar there is neither foreground or background melanin in the feathers, nor any melanin in the body tissues. But, as its very name asserts, yellow psittacin pigmentation is quite unaffected. Indeed, we see yellow where it was previously obscured by the markings.

Furthermore, Dr. Pilkington himself tells us that eumelanin and phaeomelanin share the same biochemical pathway except for the last few stages. Having made this point, with which most of us might agree, it is difficult to see how he reconciles it with the suppression of one type of melanin but not the other. For the usual explanation for the occurrence of albino animals, and in the case of birds also the lutino, is that they do not possess any of the enzyme tyrosinase and therefore cannot even start to synthesize melanin.

However, Dr. Pilkington might take some comfort from the fact that in reality the situation is far more complex than inferred in his article. In species which we know do contain both eumelanin and phaeomelanin, colour varieties occur which show that these two types of melanin can be under separate genetic control. There are dark-eyed clear varieties (true Whites and Yellows) which show that melanin can be produced in body tissue but not in the feathers. And there are varieties in which the pattern of distribution of melanin is significantly altered either in the feathers alone (e.g. Spangle, Clearbody, Opaline in the budgerigar), or in both the feathers and body tissue (e.g. Pieds in many species).

The biochemical pathways which ultimately produce colour in particular areas or parts of the body are so entwined and dependent one upon the other that we are never likely to unlock more than a handful of their secrets.

4. The budgerigar, like all parrots, has inherited its basic colour producing mechanisms from the ancestral parrot. In all present day parrots there are changes, greater or lesser, from this ancient template; and yet similarities remain. A broadly similar range of mutant colour varieties tend to occur in those species which become popular with breeders, and it can be assumed that very much the same genes and biochemical pathways are responsible. Superimposed upon, or entwined with, the network of biochemical pathways which control the production of colour is another, more species specific, layer controlling the distribution of those colours and imparting to each species its own unique appearance.

Many parrots as well as, or instead of, producing yellow psittacin also produce red and/or orange pigmentation. (These oranges may be due to a mixture of red and yellow pigments rather than a discrete orange pigment.) Where a mutant colour gene brings about a dilution or reduction of yellow psittacin, as for example in the Parblues, there tends to be an accompanying dilution of red or orange pigmentation where this is present. For this reason we assume there is a close relationship between these bright yellow/orange/red pigments which we refer to collectively as psittacins. A relationship similar to that we assume between eumelanins and phaeomlanins. Are we to believe that all these pigments are related and that melanin not only produces the sombre blacks, browns, and ochre yellows; but also the brilliant reds, oranges, and pure yellow?

As an aside it should be pointed out here that, although just one locus (the green/blue) has so far been found which affects the production of yellow psittacin pigment, this certainly does not show that this is the only locus on the relevant biochemical pathway. (The Yellowface varieties of the budgerigar, and most probably the various Parblues in other parrots, are caused by alleles of the blue gene and occupy the same locus.)

Melanins have many functions

The exact chemical structures of the various melanin pigments are still not known although, as described by Dr. Pilkington, most of those found in animals are indolic polymers with eumelanins thought to be formed in the manner described in his article. Even less is known about the nature and synthesis of the phaeomelanins. The classification of melanin into these two broad types relies not on any defining difference in chemical structure but, rather, on the colours they produce; differences in their solubility in acids and alkalis; and the slight inflorescence of phaeomelanin in ultra-violet light. Also, from the point of view of the interested aviculturalist, what can be learned from the way they are affected by quite a large number of different genes; sometimes in tandem, sometimes separately.

The almost universal distribution of melanins in the animal kingdom suggests that they must fulfil some fundamental function(s). At a very early stage in evolution melanin must have served an essential role in protecting organisms from the harmful effects of excess radiation from the Sun. Once mother nature had learned how to produce these pigments she would have deployed this ability for a widening range of purposes, sometimes in ways essential to the survival of her creations and sometimes in an almost frivolous manner. Such wanton and capricious behaviour must be the despair of those looking for an orderly and progressive development to some ultimate perfect state of life or being.

In the budgerigar we may infer a number of other purposes for melanin besides protection against radiation. Most obviously, it plays a major role in the colouration of the bird; which provides camouflage against predators in its typical environment, and serves to identify it and its sex to other budgerigars. It has also been supposed to add strength and rigidity to the flight and tail feathers as well as protecting against abrasion. More certainly, lack of melanin in the eye would lead to impaired vision in bright lighting conditions.

Despite this, melanin does not seem to play any vital role in bodily functions since those birds expressing the ino gene (Lutinos and Albinos) are perfectly viable in the artificial environment of the aviary. They also behave quite normally, show no signs of impairment to the nervous system, are recognised by their companions as budgerigars and as mates, and live out a normal lifespan.

In the natural environment it would undoubtedly be quite a different story. The striking appearance of a Lutino budgerigar would make it stand out to a predator as a relatively stable target amidst the confusion of a disturbed flock and, to add to its difficulties, its impaired vision in harsh sunlight would be a handicap in evading capture. In short, the occasional Lutino born in the wild would be likely to have a reduced lifespan and little opportunity to pass on its genes by breeding. Similar remarks might apply to a lesser degree where melanin is merely reduced or diluted.

What conclusions can we draw?

Although I have stressed the complex interactions and dependencies in the network of biochemical pathways which produce and maintain the budgerigar, the fact remains that melanin is not essential to its viability. There seems to be no evidence of any connection between melanin production, or non-production, and any vital body functions. Specifically, budgerigars lacking melanin show no impairment to the nervous system. If adrenalin does indeed share part of its biochemical pathway with that for melanin the blocks produced by the ino gene, and other genes which dilute or redistribute melanin, must in some way be by-passed.

Add to this my deep scepticism about yellow psittacin pigment being a phaeomelanin and I have to conclude that Dr. Pilkington's theory, however interesting it may be, just does not pass reasonable scrutiny.

Finally; during the writing of this piece it occurred to me that the techniques, popularly known as genetic fingerprinting, which might resolve this whole issue probably already exist. Tissue extracted from the feather shaft can be examined, commercially as well as in University laboratories, to determine the sex of the bird from which it is obtained. Further developments of this or other similar techniques could be, may already have been, used to identify particular colour genes and their alleles. In this way it could be determined whether tissue taken from a blue feather of a half-sider differed genetically to tissue taken from a green feather on the other half of the same bird.

Copyright: Clive Hesford, November 1997