The nature of the Opaline Locus by Terry Martin

The Genetics of Colour in the Budgerigar and other Parrots

This page created February 1999

Different Strokes for Different Birds

The Nature of the Opaline Locus

by Terry Martin BVSc of Brisbane, Australia

The recent article on the Opaline factor by Clive Hesford in these pages has stimulated me to think about this issue and put forward the following ideas.

Descriptions of Opalines

Budgerigars

Clive clearly defines the features of the opaline allele’s effects upon the budgerigar’s pigment distribution. But I would clarify two points. Firstly, yellow ground colour is never displaced by the opaline allele, body colour is increased at the expense of foreground melanin not ground colour. This is why yellow areas become green not blue.

Clive has also said body colour was increased along with the cloudy layer acting as one. I have confirmed my belief with Inte Onsman that the cloudy layer is present in all feathers, regardless of the presence or absence of melanin in the body colour that would make it visible. There is no evidence that the opaline gene has any effect on any structural components of colour. There are no new pigments produced and no alterations in the type of melanin deposited.

The most interesting features are the apparent increases in size of the wing stripe and the observation of the lower feather and afterfeather becoming white. I will return to these features.

Cockatiels and Eastern Rosellas

Clive has described the features of the Opaline Factor in these two species well and I would summarise both simply as reduced melanin and increased psittacin in various ways as outlined. Significantly again, no new pigments are produced and there is no alteration to the type of melanin deposited. I have not kept Red Easterns and only small numbers of Cockatiels so I will not try to add more.

Subsequent to writing this article, Terry tells me that he was recently (March 1999) flicking through his copy of Australian Birdkeeper’s Guide to Rosellas when he came across a marvellous photograph of a Red Eastern Rosella with wings fully spread. In his words it displayed “ ... beautiful big wing stripes.” [Clive H]

Bourke’s Parrots

Clive has outlined the basic changes, which are decreased melanin distribution and redistribution of psittacin pigments. Pink psittacin is spread into areas where normally only yellow psittacin is found. But no new pigments are produced and no alterations occur to the type of melanin deposited.

I can add a few new observations:

Firstly, all Rose Bourke have large underwing stripes, male and female, whereas the wildtype Bourke cock does not. These stripes are visible as a triangular clear patch when the wing is held in the normal position.
Secondly, Rose Bourke chicks are covered in thick white down compared with the grey down of wildtype chicks. This down appears longer in the Rose.
Thirdly, as a result of this article I have finally ‘looked’ at my birds, rather than just seeing them, and have realised that melanin is removed from the centre of body feathers first. Anyone who has bred numbers of Rose Bourke’s will be aware of their variation in appearance. Much of the variation is accounted for by the degree of melanin loss in individual feathers. Melanin loss starts centrally, not unlike that seen in a Pearl Cockatiel, and clears outwards until only a small crescent of melanin is left in the body feathers of the best examples. These birds when viewed casually appear to be totally devoid of melanin in the body colour areas.

Opaline Redrump Parrots

This version of the opaline allele is well established in Australia where I reside. Foreground melanin is not strongly apparent in this species, except on flight feathers. There is some reduction of this melanin and noticeable enlargement of the underwing stripe, which is retained in both sexes whereas only wildtype hens carry it. The triangular clear patch identified by Clive on the wings of Opaline Budgerigars, is clearly visible. Body colour melanin (i.e. background melanin) is greatly affected, with loss of melanin from the head, neck and back regions. Ground colour becomes visible in these areas. The colour variety produces an effect visually the opposite of the Opaline Budgerigar. Psittacin is redistributed, with red pigments being spread over the head and mantle areas. This is variable and has been enhanced via selection. It is most attractive in the Opaline Lutino combination of Redrump Parrot.

The down on chicks follows the same pattern described for Rose Bourkes; changed from grey (in normals) to snow white (in Opalines), and the down appears longer. Overall, as stated with the other species, there are no new or changed pigments, only changes in distribution.

When the the body feathers are closely examined the same pattern of melanin loss described for the Bourke’s is seen. That is, the centres of feathers are cleared leaving a crescent of melanin which varies in size between individuals. The ‘better’ birds appear to have clear yellow feathers when observed casually.

Opaline (Pied) Turquoisine Parrot

This bird has only recently been imported into Australia and I am no yet in a position to comment.

Nature of the Opaline Gene

As has been previously discussed by Clive Hesford and Jim Hayward, the changes described above for each species vary significantly and sometimes appear to be the opposite in action. However I do not feel this is a valid argument for non-alignment of these various forms. Without doubt, in all these various forms only redistribution of pigments is involved and there are no new pigments or alterations to the type of melanin present.

If we consider then; just what is it that controls pigment distribution in each species? The most significant genes must be those that determine the unique appearance of each species. This does not amount to one simple gene but is, rather, a multigenic trait which can be highlighted by considering the appearance of hybrid parrots. If the distribution of pigments were under only simple (a single gene) control, then when species were crossed, we would not get the blending of traits we actually see. Nor would there be the gradual but irregular return to one species’ traits when the hybrid is back-crossed to that species for a number of generations. I think everyone will agree with this statement.

So if pigment distribution is under such complex control in each species, why wouldn’t a distribution gene that is common across species, be individually affected in each species, under the influence of all the species’ specific distribution genes. This is no different to any other gene. Do not cinnamons and blues look slightly different in each species? What we have to identify are the signal traits for the mutant form. In fact the opaline locus may be one that is slightly altered in each species to contribute to the uniqueness of a species, yet be a common gene through ancestry.

Clive Hesford’s discussion has highlighted two possible features for possible signal of the opaline locus:

Enlarged white underwing stripe in both sexes
Alteration to down colour from normal pigmentation to white

I will add a third:

Loss of melanin from body colour areas but with traces of melanin being present in the periphery of feathers creating either a spot effect or a crescent of melanin across the top edge.

The pigment distribution change I have described in Bourke’s and Redrumps is strikingly similar in nature to that seen in the Pearl Cockatiel; except that it is taken further. And I am aware that the size of the pearl spots in Cockatiels has been increased by selection until only a small rim of melanin remains around the edge of affected feathers in those individuals often described as Lacewings.

And a fourth:

A pigment distribution gene inherited in a sex-linked recessive fashion.

I believe point four is probably the most important feature and should not be dismissed as coincidence. Colour morphs occur as a result of alterations of pre-existing genes from the normal genome. With the increased breeding of parrots and their colour varieties, we are seeing repeated appearance of action at the same locus in different species. This is to be expected in a related group of species such as parrots as the genes controlling pigment production have been inherited from common ancestry. Therefore, when we are repeatedly finding a sex-linked recessive distribution gene occurring in one group of parrots (Australian Parrots), but never more than one, it is logical to assume they represent a common ancestry for the gene rather than assume the genes are coincidental. When only Budgerigars and Cockatiels existed in opaline forms, it was unclear. But today with the growing number of species appearing, it is only logical to assume a simplistic situation rather than a complex one.

Truth in research

I believe there is enough evidence in what has been discussed to conclude that what we see is a different effect for the same locus in different species. However, there is one way to gain added information without the ultimate method of doing base sequence research in each species.

We need to investigate recombinant frequencies for the various sexlinked loci in each species where they occur and assess how each allele is positioned relatively to others for the different species of parrots. Lutino and cinnamon loci can become the reference points to position the various Opaline forms. This is the only way I can see to resolve the debate beyond question without fully mapping the genome of all species involved.

If you’d like to discuss this article or any other issue with Terry
he may be contacted at: sbankvet@bigpond.com

Clive’s response

to the above article by Terry Martin of Australia

I’m grateful to Terry Martin for reading my article on the subject of Opalines so thoroughly and taking the trouble to respond with his own thoughts on the subject. It’s also pleasing that, although he corrects me on a couple of points, he also presents some new facts which reinforce my ideas and we end up largely agreeing on the status of the various Opaline forms of a number of Australian parakeets and the gene which causes them.

Terry starts out by remarking upon the changes in body and ground colour distribution. Perhaps I was a little careless in my use of words in the original article although, in mitigation, I used the word displaced whilst thinking of the colours as seen and not of the constituent parts of those colours.

More fundamentally, Terry next makes the point that the cloudy layer is present in all feathers regardless of whether or not the background melanin which would render it visible is present. I accept that correction and agree with him that there is no evidence that the opaline gene can initiate the formation of the structural feature we know as the cloudy layer. (I’m rather sad about this as such ideas fed an overactive imagination and had me joyfully casting flights of fancy to the four winds!)

When, a few years ago now, I was breeding budgerigars it was very apparent that there was very great variation in the amount and colour of the down in young chicks, and fairly obvious that these variations correlated with the colour variety into which the chicks ultimately developed. It seemed to me that here was a fruitful avenue for some simple observational research, but I never made a start and cannot even remember now whether (as seems very likely) Opaline chicks had white down. Any breeder of any species could make such observations and extend our knowledge by writing a short report.

Terry’s observations on the underwing stripe are very useful and go a long way toward confirming that this characteristic is present across all species of Australian parakeets which have an Opaline form and might amount to a diagnostic feature. I would welcome information from anyone who can confirm whether of not this feature is present in Pearl/Lacewing Cockatiels, Red Eastern Rosellas (see recent confirmation above), and Opaline/Pied Turquoisines.

An important point brought out in the title of Terry’s article is that the true focal point of our discussions is the opaline locus, and not the gene or allele which might occupy that locus. Almost certainly, as Terry implies, this locus (as well as the ino and cinnamon loci) occupies the same position on the X-chromosome in all the Australian parakeets. Other parrots have the ino and cinnamon loci, again probably in the same relative positions, but there is as yet no evidence in these of an equivalent to the opaline locus.

Although the opaline locus is probably in exactly the same position in the six species having an Opaline form it does not necessarily follow that the alleles, either the opaline or the normal wild-type, occupying that locus are identical in each species. Small changes might have occurred and accumulated in the millennia since these species separated from their common ancestor.

Such allelic variations might account for the differing effects seen in these species. More likely however is that the opaline allele, acting as it does at the colour distribution level, interacts with many other alleles in the various genetic pathways involved. The distinctive appearance of each species shows that their separate colour distribution pathways have accumulated unique changes and are likely therefore to respond differently to the presence of the opaline allele.

Not all the genes on the X-chromosome play any part in the differentiation of the sexes. But, as already brought out in the original article, the opaline gene does seem to get itself involved with at least one of the secondary sexual characteristics. That is the frequent retention, even exaggeration, of the underwing stripe. This might be characterised as a lessening of a trait which distinguishes the male from the female or, to put it another way, causing the mature male to retain a juvenile or female characteristic. There is no indication that this impinges in any way upon the fertility of Opaline males.

Clive Hesford, February 1999