Unexpected behaviour of the cinnamon allele

by Terry Martin BVSc

Colour morphs arising from mutation of the cinnamon locus are one of the most commonly occurring across all species of birds; and even other Orders of animals. They are easily recognised because the mutant cinnamon allele blocks eumelanin production at one of the final steps. In doing so, it prevents conversion of the brown stage of eumelanin production into the normal black eumelanin found in all species. It has no effect on phaeomelanin production, as illustrated by its appearance in finch species with phaeomelanin in their plumage. However, this feature is not seen in parrots, as they do not carry this pigment. It is a sex-linked locus and mutant alleles are always recessive to wildtype. The widespread occurrence of cinnamon colour morphs in the animal kingdom highlights the importance of the cinnamon locus, and its conservation through evolution, as a key component of the genetically determined biochemical pathway leading to normal production of eumelanin.

In isolation, the behaviour of the mutant cinnamon allele (X^cin) is that of a typical sex-linked recessive allele. And there is no direct interaction between the function of this locus and the function of most other common colour loci. This is logical considering that many colour loci control totally disparate aspects of plumage colouration. In fact, even when two different loci control related aspects of colouration, very few interact in any way genetically; with each locus continuing to function irrespective of others.

Gene interaction in Parrots

There are rare examples of gene interaction in parrots, with the most celebrated being the gene interaction between mutant alleles of the dominant pied locus (P) and the recessive pied locus (r). These two partial leucistic colour morphs interact to produce a full leucistic colour morph when present together in the genotype. The phenotypic expression is significantly ‘greater’ than either allele in isolation. Or more precisely, the degree of residual function is dramatically reduced. To understand this perspective, consider that Pied colour morphs have reduced ability to deposit melanin (totally lost in an irregular pattern, but retained in other regions) in the plumage due to failure of melanocyte function in certain regions. Following on, a bird with the genotype P-rr is unable to deposit any melanin in any region of the plumage, which is a greater deviation from wildtype colouration and represents a greater loss of function. Whether one locus has a direct effect on the expression of the second, or whether the gene products of one interacts with the gene products of the second is unknown. Genetically, this interaction could be considered a case of dominant and recessive genes with cumulative effect.

The only other known example of gene interaction involves the mutant allele of the cinnamon locus and one of the mutant alleles (X^ino) of the sex-linked ino locus. In this case, the combination of the two mutant alleles into the genotype (X^cin,ino X^cin,ino or X^cin,ino Y) produces an unexpected phenotype known as Lacewing or simply Cinnamon-ino. The reason this phenotype is unexpected is because the sex-linked ino allele suppresses virtually all melanin production throughout the body and therefore is expected to mask the action of other colour loci involving melanin in their action. The action of dilution, leucistic and most albinistic colour loci are hidden, but not suppressed by the ino allele. They simply have no component in the plumage for their action to be expressed. Structural loci are also masked, although they are still able to alter the feather structure, without the presence of melanin in the feathers the alterations to constructive interference are not detectable.

However, when the cinnamon allele is combined with the ino allele, the resultant Lacewing phenotype has an increased level of melanin deposition above the base for the ino allele. The appearance varies in line with the normal pigment distribution for each species. In Budgerigars, foreground melanin areas that become pale yellow in a Lutino, retain significant amounts of brown melanin although not as dark as that found in Cinnamon birds. Body colour regions appear still to be free from background melanin. In Eastern Rosellas, a similar pattern is seen. In other species without substantial foreground melanin in their Normal phenotype, a light fawn suffusion through the entire body colour is seen, suggesting a small amount of brown melanin deposition in background regions. It could, however, indicate a small suffusion of foreground melanin through the plumage that is normally not noticed.

Identifying cinnamon ino colour morphs in different species

The Budgerigar form was the first to be recognised for what it is genetically, but not until after a long period of ignorance amongst breeders. The tight gene linkage between the cinnamon locus and the sex-linked ino locus created this situation. They lie only three map units apart, which correlates to a recombinant frequency of only 3%. So for many years the Lacewing was viewed as a primary gene mutation from an independent locus. As breeders gained more knowledge in genetics, the true nature of the colour morph was realised and it was then proven to be the cinnamon-ino combination.

Once this lesson was learnt in Budgerigars, breeders of other species set about looking for separate cinnamon-ino phenotypes as well. Likely candidates have been identified in at least four different species, but not all have been investigated in a scientific manner.

In the Eastern Rosella a colour morph has been identified and called Cinnamon-ino by breeders, however it has not been studied with appropriate test matings and many breeders do not even recognise it as being distinct from the base Lutino population. It is similar in appearance to the Lacewing Budgerigar, a basically Lutino colour morph with light fawn suffusion in areas carrying foreground melanin in the wildtype, therefore is a strong contender.

Peachface Lovebird breeders have performed some limited test matings to prove their Lacewing is a cinnamon-ino. Cockatiel breeders have a colour morph fitting the general description, which they have called Cinnamon-ino or Lacewing, but the few test matings documented have thrown doubts on its authenticity.

In Indian Ringnecks, there is a phenotype commonly referred to as ‘Yellow-headed Cinnamon’ (a name which is in dire need of revision), which in some strains breeders have identified as a true cinnamon-ino combination. However this is complicated with alternative genotypes producing an almost identical phenotype.

Other genotypes producing similar phenotypes to the cinnamon ino

In particular, a number of species have a second mutant allele of the sex-linked ino locus, known variously in different species, however it is basically a partial ino allele (Xino^l) which produces the colour morph I prefer to call Lime. The existence of the Lime allele leads to the production of the lutinolime (Xino^l Xino) genotype, which is a heterozygous pairing of these two different alleles and it produces a very similar phenotype to the Lacewing. Of course it can only be produced in cocks because of the sex-linked nature of the locus involved, but it adds significantly to the level of confusion in some species.

The cinnamon-lime combination is a third genotype producing a similar phenotype, although generally a richer colour than the cinnamon-ino.

And in Indian Ringnecks, Deon Smith has reported breeding records for one strain of ‘Yellow-headed Cinnamon’ that suggests the action of a dominant modifier gene altering the gene product of the ino locus. When present in the genotype in conjunction with the ino allele, the phenotype produced has increased melanin resulting in a ‘Lacewing’ phenotype. Interestingly, when combined with a lime allele the modifier reduces melanin production and once again produced a similar phenotype to the ‘Lacewing’.

It is therefore far from clear as to which phenotypes correspond to which genotypes and if differing genotypes are interbred, then the situation becomes very difficult to resolve.

Cinnamon allele interactions in passerine species

In passerine species, the mutant cinnamon allele is also known to interact with mutant alleles of other loci. In the Canary, interaction is known between mutant alleles of the sex-linked ino locus (satinet and agate) and the cinnamon allele. This is very similar to what we see in Parrots, with the cinnamon-ino combination being known incorrectly as 'Sex-linked Isabel' and being incorrectly identified as a primary colour morph.

In the Zebra Finch, the gene interactions increase and become more complex. The Zebra Finch has a well established cinnamon allele producing the colour morph known as Fawn. No true ino allele is known, however there are multiple mutant alleles of a sex-linked locus that is a strong contender for the ino locus. This locus is currently identified as the chestnut-flanked white (cfw) locus. One allele is known as lightback. It retains the greatest level of melanin production and offers little insight into possible gene interaction. However the cfw allele reduces eumelanin quite significantly, producing an almost white body colour, whilst retaining most of the phaeomelanin and eumelanin in the markings.

When Cinnamon is combined to this colour morph, a distinct beige suffusion is added to body colour regions (as well as changing eumelanin markings to shades of brown). It is clear that there is an increase in (brown) eumelanin deposition over the base action for the cfw allele. This is entirely consistent with the interaction seen in parrots between the cinnamon and ino alleles.

But this is not the end of gene interactions for the cinnamon allele in this species. There is an autosomal recessive locus known as isabel with a different form of interaction. The isabel allele produces an albinistic colour morph with no reduction in phaeomelanin, but significant reduction in eumelanin through the whole plumage. The Isabel colour morph appears in a number of different shades. In Europe the dark phase is commonly referred to as ‘Grey Isabel’ and the light phase as ‘Fawn Isabel’. If the genotypes followed this naming pattern, there would be no interaction, with the two mutant alleles producing a combined effect to reduce eumelanin to a greater extent than either separate mutation.

However, many light phase ‘Fawn Isabel’ cocks are only heterozygous for the cinnamon allele, yet are almost indistinguishable from those homozygous for the cinnamon allele. At the same time, no other gene has been implicated in the change of colour between the light phase and dark phase colour morphs. Therefore the popular theory is that the heterozygous cinnamon allele is not inactivated by the presence of the wildtype allele and is still interacting with the isabel locus to produce the light phase. This is a different form of interaction to that between the cinnamon and ino loci, however it indicates that a lone cinnamon allele may not be totally dormant and has the potential for gene interaction with other loci.

Further observations on cinnamon ino in psittacine species

I have performed a limited number of breeding experiments involving the ‘Lacewing’ Cockatiel. Hens produced from ‘Lacewing’ cocks are not all ‘Lacewing’. In fact, in my limited experimentation I have so far produced far more pure Lutino daughters than ‘Lacewing’, but no colours other than these two. And when a ‘Lacewing’ cock was paired to a Cinnamon hen, both wildtype and Cinnamon sons were produced.

These results indicate that the ‘Lacewing’ cocks I used were only heterozygous for the cinnamon allele. My first assessment of these results was that the cinnamon ino theory for this ‘Lacewing’ colour morph was disproven and that the colour morph must be due either a separate allele of the ino locus, or to an unidentified modifier.

I am now reconsidering these results in light of knowledge gained from other species. I now believe that we may have gene interaction occurring between the lone cinnamon allele and the ino locus and that the phenotype produced by these birds is directly resultant from the Ino/cinnamon genotype. Supporting evidence for this theory includes the observation that the cocks used in the matings described above are lighter in colour than the ‘Lacewing’ daughters produced. I suspect a true Cinnamon-ino cock would be darker than the Ino/cinnamon cock. Conclusive evidence would require analysis of extensive breeding results. If proven, this gene interaction would result in the following table of genotypes and corresponding phenotypes:

Other considerations

If proved, this gene interaction would greatly add to the complexity of matings involving combinations of Cinnamon and Lutino, increasing the range of phenotypes produced and the incidence of apparently unexplained results. The interaction needs to be explored in all species, even Budgerigars whose breeders believe they have fully explained and understood the situation.

It also raises the importance of calling these birds Lacewing, rather than, what is traditionally believed to be more correct, Cinnamon-ino. Because a significant percentage of cocks will in fact be Ino/cinnamon rather than Cinnamon-ino. Therefore we are using a name to indicate a phenotype rather than a single genotype.

To investigate these phenotypes, the colour morphs in question need to be test mated to Cinnamon birds to prove the existence of cinnamon alleles in the genotype. Once this is proved, test mating Lacewing hens to Lutino cocks should produce a Lacewing phenotype in the sons, but only Lutino daughters. This mating would prove the Lacewing hens carried an allele for the ino locus, as well as proving the existence of Ino/cinnamon as a Lacewing phenotype.

We must also consider whether gene interaction occurs between other albinistic loci. As albinistic loci control interrelated aspects of melanin metabolism, it is quite reasonable to suspect other gene interactions will be found as more albinistic colour morphs are identified and investigated in Psittacine species.

© Terry Martin, November 2001

Other articles by Terry Martin:

The nature of the Opaline locus

The nature of the Opaline locus - an Update

The Pied (or Olive-Pied) Redrump Parrot

Defining and classifying Fallow colour morphs

Applying a scientific base to our understanding of Parrot colour morphs

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