The Gouldian Finch (Erythrura gouldiae) is a truly remarkable avicultural subject. It is viewed by many enthusiasts as one of the icons of modern-day aviculture, and its breathtaking beauty and stunning polymorphism (see head colour inheritance) has made it a firm favourite with bird keepers the world over. The Gouldian Finch has been known in avicultural circles for far more than a century now, and has been successfully kept and bred in captivity for many generations; yet its current standing still makes it highly sought-after and even revered by bird keepers around the globe. For many finch enthusiasts, the Gouldian Finch is one of only a handful of species they ultimately aspire to, due to its striking features, notorious reputation for being a very demanding cage and aviary subject, the rewards of successfully maintaining and breeding the many interesting and prised colour mutations, the accolades that come with the successful management of a good-quality Gouldian Finch stud, as well as the fact that it carries the status of being an endangered bird species in its native Australia.
There is no doubt that captive breeding programmes, and the resultant ongoing domestication of the Gouldian Finch, will continue to play a vital role in the conservation of the species. However, certain provisions govern the practice of good conservation breeding, as highlighted by Evans and Fidler (2005:214), namely to avoid inbreeding [and therefore promote genetic diversity within the species], to establish strains of parent-reared (as opposed to foster-reared by Bengalese Finches) birds in captivity, to exclude colour mutations from the conservation breeding programme, to avoid hybridization between closely-related species, and to make every effort to bolster the image of aviculture as a major contributor towards species conservation. Echoing this sentiment, the Gouldian Finch Society, which was established in South Africa in 1994, listed amongst its set of prime objectives, an understanding of the Gouldian Finch’s colour mutations (and their associated pitfalls), as well as the preservation of the so-called normal bird (i.e. the true wild-type, or purple-breasted, green-backed Gouldian Finch).
Therefore, this series of articles will focus on the genetic principles that underlie the successful breeding of the normal or wild-type bird, as well as the more commonly-encountered colour mutations of the Gouldian Finch in South Africa. Guidelines will be provided to assist both the novice and experienced finch breeder in selecting and mating suitable birds for conservation and colour breeding programmes.
The genetics of colour inheritance in the Gouldian Finch
Colour inheritance in the Gouldian Finch is a fascinating, albeit somewhat daunting subject. The normal, wild-type bird is both extremely colourful, as well as polymorphic (see below), which already sets the scene for a complex interplay between both structural (i.e. blue) and pigment-derived (red, yellow, reddish-brown and black) colours. In addition, altered or mutant genes produce a variety of colour mutations, which add to the many intricacies and complexities of colour inheritance in this bird. Serious Gouldian Finch breeders require a basic understanding of the patterns of single-gene inheritance and how this knowledge translates into practical applications within their bird rooms and aviaries. Therefore, an overview of basic genetic concepts and terminology is provided below.
There are several established colour mutations that currently exist in the Gouldian Finch. All of these well-known mutations, together with the head colour morphs, follow single-gene inheritance patterns, making it possible to anticipate breeding results, and allowing for keen breeders to manipulate their stud-lines and produce ‘coloured’ offspring in a reliable and responsible manner.
Basic genetic principles and the modes of single-gene inheritance
Table 1 highlights a few basic terms used in discussions regarding Gouldian Finch genetics and single-gene inheritance.
Table 1: Basic genetic terminology
Locus The position or location of a specific gene on a chromosome.
Allele When a specific gene can occur in more than one form (for example, if a wild-type gene is labelled ‘A’, and a mutant, recessive gene ‘a’ could be present as an alternative form at the same location, then ‘A’ and ‘a’ are alleles of the same gene).
Example: For the character of head colour, the allele for the red-headed trait shares the same locus on the X-chromosome (also referred to as the Z-chromosome) as the allele for black-headedness (see text).
Genotype The genetic make-up of a living organism, such as a bird, which may very well differ from its physical appearance (see below). Therefore, it refers to a particular pair of alleles that are present at a specific gene locus.
Phenotype The physical appearance or physical characteristics (as well as the biochemical make-up) of a living organism, such as the Gouldian Finch. When an individual finch carries recessive genes in its genetic makeup (i.e. its genotype) that are masked (or hidden) by the presence of dominant genes, then the finch’s phenotype will differ from its genotype and it is also said to be heterozygous (see below).
Example: When an individual Gouldian Finch male carries both purple-breasted and white-breasted alleles, and we know that the former is dominant over the latter, the finch will have the physical appearance of a purple-breasted bird, but the genotype will show him being a carrier of the white-breasted trait as well.
Heterozygous When the two alleles that form a specific gene pair differ from one another (e.g. when one dominant and one recessive gene make up the gene pair), they are said to be heterozygous alleles.
Homozygous When the alleles that form a specific gene pair are identical (i.e. two of the same dominant or recessive genes make up the gene pair), they are said to be homozygous alleles.
Finches inherit their physical characteristics (and biochemical make-up) in the form of chromosomes from their parents. Chromosomes consist of genes, which in turn, are comprised of DNA (deoxyribonucleic acid). There are two important types of chromosomes, namely a certain number of autosomal ones, and a pair of sex chromosomes (that determine whether a certain individual is male or female). Humans, for example, carry 22 pairs of autosomal chromosomes in addition to their one pair of sex chromosomes, which are designated XX in women and XY in men (giving a total of 23 pairs, or 46 chromosomes). The exact number of chromosomes in many bird species, however, still needs to be determined, although Gouldian Finches carry 14 chromosomes, or seven pairs, according to Vriends (1991:68). Nevertheless, one noticeable difference lies in their sex chromosomes, which differ from humans and other mammals in the sense that males carry two X-chromosomes, while females carry the XY-combination. In birds, though, these chromosomes are more accurately designated ZZ and ZW respectively, under the so-called Z-W classification system.
Whenever a specific physical trait (i.e. as part of a noticeable character or feature), or a new mutation, is brought about by one or more variations at the level of a single gene pair, it becomes both predictable and possible to manipulate with a basic understanding of the laws that govern single-gene inheritance. For all practical purposes, Gouldian Finch enthusiasts will encounter (albeit to varying degrees) only three different modes of single-gene inheritance in their stud-lines, which may be used and manipulated to produce the three head colour morphs, the white-breasted and lilac-breasted varieties, the so-called European yellow-backed (or sex-linked pastel-green) variety, the blue-backed variety, and various combinations of these colour mutations. These three modes of single-gene inheritance are (1) autosomal recessive inheritance, (2) sex-linked recessive inheritance, and (3) incomplete, sex-linked dominance (also referred to as sex-linked co-dominance by several authors).
Autosomal recessive inheritance
As opposed to autosomal dominant inheritance, bringing about the desired phenotype in this mode of single-gene inheritance requires both alleles at a specific gene locus to carry the recessive trait in question (when a specific trait follows an autosomal dominant inheritance pattern, only one of the alleles at a specific locus is required to carry the trait, to bring about the phenotype in question). The major difference between autosomal dominant and autosomal recessive, single-gene inheritance is illustrated in Figure 1 and 2.
Yellow-headed colour morphs, as well as the white-breasted and blue-backed varieties are examples of Gouldian Finch colour mutations that are inherited via the autosomal recessive mode.
Refer to Figure 3 for additional information and a further example.
Sex-linked recessive inheritance
The best example of sex-linked inheritance has to be the actual sex or gender determination in a bird or mammal species. As already mentioned, human males are characterised by the presence of the XY-chromosome combination that determines their gender, whilst females carry the XX-combination, for example. The obvious question in terms of sex determination then becomes: what are the chances of a couple expecting either a baby boy or a baby girl, or of a newly-hatched Gouldian Finch nestling being a male or a female bird? The answer is a very simple 50:50 chance (per single pregnancy in humans and per fertilised egg in birds). Refer to Figure 4(A) for a more detailed explanation.
The same principles that are illustrated in Figure 4(A) apply to birds; however, because males carry the ZZ-combination, and females the ZW-combination, the female ‘determines’ the sex of her offspring, as opposed to the human male with his XY-chromosome combination. Refer to Figure 4(B).
Note that all of the percentages quoted in the text represent the actual odds (or likelihood) of specific allelic combinations occurring in certain birds. However, similar to a game of chance, for the odds to even out, so to speak would be comparable to flipping a coin or rolling a pair of dice. In the case of the coin, the odds of flipping ‘heads’ over ‘tails’ is also a perfect 50:50 – although it could very well happen that one ends up flipping two, three, four, or more ‘tails’ in a row, before ‘heads’ will appear for the first time, and vice versa. This is very similar to the actual situation in finches, for example, where one could obtain four or five fertilized eggs from a single clutch, which all turn out to be males. Then again, could it have been possible for a single pair of Gouldian Finches to produce a hundred chicks in any particular season, the likelihood would have been much higher for the breeder to end up with 50 male and 50 female offspring.
In the Gouldian Finch, the very popular pastel-green, or European yellow-backed mutation follows a sex-linked inheritance pattern with incomplete dominance, whereas the inheritance pattern of the so-called dark factor mutation follows incomplete autosomal dominance. Incomplete dominance implies that a third, intermediary phenotype is created, due to the fact that there are two opposing dominant genes that both contribute to the newly-combined (third) phenotype in equal proportions. Therefore, the various genotypes and phenotypes are proportionately equal in their representation. Incomplete dominance may also be referred to as co-dominance, although the latter may have a different meaning in certain biochemical, horticultural, and other examples. Incomplete dominance will be discussed in more detail in Part 3.
Head colour inheritance
One of this extraordinary little estrildid finch’s many fascinating characteristics, is the fact that it has three naturally-occurring colour morphs (i.e. the three head-colour varieties) that co-exist and interbreed in the wild. This quality is referred to as polymorphism.
These three head colour morphs, as depicted in Figure 5, are the red-headed (RH), black-headed (BH) and yellow-headed (YH) forms. The latter is also commonly, although very much debatable, referred to as the orange-headed form, based on the visual colour range displayed in the adult facemask of these birds, although it is indeed a true yellow colour mutation in genetic terms (to be discussed in more detail in Part 2).
The RH form is now generally accepted to be the original wild-type head colour form; opposing views were held by many authors and enthusiasts in the past, who regarded the BH form as the original wild-type – some still do. The presence of the red facemask is determined by a pair of sex-linked genes (i.e. they occur on the Z-chromosome); the red pigment canthaxanthin is responsible for the magnificent red colour of the face mask.
The BH form is inherited via the sex-linked recessive mode. There is widespread acknowledgement that the red-headed and black-headed alleles share the same locus on the Z-chromosome, as suggested by Southern towards the end of the Second World War already (1945:53-56); annotating the alleles accordingly makes the inheritance pattern a lot easier to understand, as well as allowing for the so-called single-factor RH male to be automatically split for black-headedness. The black pigment eumelanin is responsible for the rich, velvet-black appearance of the facemask in these birds.
The YH form is inherited via the autosomal recessive mode. Note that a BH bird with a yellow-tipped bill (YTB) is both a black-headed and a yellow-headed bird genetically, but the YH is masked by the BH (see Figure 6). This is not the same as a BH that is split for YH (which will have a red-tipped bill). The RTB (red-tipped bill) is the wild-type and is displayed by all RH and BH birds (with the obvious exception of the BH-YTB). All YH birds will display the YTB. Also note, therefore, that the YH can only be expressed in the presence of at least one RH gene, otherwise the bird will be a BH-YTB. A more in-depth explanation of this head colour morph will be provided in Part 2 of this series.
With a basic understanding of the modes of single-gene inheritance that underlie the three head-colour morphs in the Gouldian Finch, a variety of calculations can be made to determine the odds of breeding offspring that display each of the three different head colours. For now, we will focus our attention on the RH and BH forms, since they share a common mode of single-gene inheritance, as well as alleles at the same locus, as previously mentioned:
Red-headed and black-headed males
There are two possible genotypes of the RH male that share the same phenotype. Males with either one or two red-headed alleles will display the splendid red facemask of the red-headed bird. This is due to the fact that the red-headed allele is dominant (being the wild-type allele for head colour) to the recessive black-headed one. Therefore, the heterozygous RH male (i.e. a male with only one allele for red-headedness, with the other allele invariably being one for black-headedness) may also be referred to as a single-factor (SF) RH male, whilst the homozygous bird will be a double-factor (DF) RH male.
Note that the heterozygous, or SF RH male will always be split for black-headedness, because it carries one allele for red-headedness and one for black-headedness at the same locus. On the other hand, a male with two alleles for black-headedness (i.e. a bird that is homozygous for the BH form) will display the black-headed trait and cannot be split for red-headedness.
Red-headed and black-headed females
Due to the fact that females only have one Z-chromosome, they can only carry a single allele for either red-headedness or black-headedness, and can never be split for the other. In this setting, and for all practical purposes, the W-chromosome may be completely disregarded.
To summarise, in terms of the RH and BH colour morphs, there are five different genotypes and only four corresponding phenotypes, namely:
The homozygous (DF) RH male
The heterozygous (SF) RH male, with the same phenotype as the aforementioned male, and that is split for BH
The homozygous BH male
The RH female
The BH female.
Between the five different genotypes listed above, there are six possible combinations in terms of mating the three male genotypes to each of the two females. Three of these breeding combinations are illustrated in Figure 7, 8 and 9 respectively. The reader is encouraged to make use of the principles illustrated in these three figures to calculate the possible results of the other three combinations.
In Part 2 we will continue our discussion of the three head colour morphs, focussing on the YH form, and introduce the two most commonly-encountered breast colour mutations in South Africa.
Please feel free to address any questions, comments, contributions and suggestions to the author at the following email address: firstname.lastname@example.org. Note that a complete list of references will be included in Part 3 of this series. A special word of thanks goes to Mr Marek Buranský of Slovakia, author of the website www.gouldianfinches.eu, who kindly permitted the use of the highly informative photographs from his spectacular collection.
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