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On the genetics and evolution of eye color in domestic pigeons

1. New interest in eye color

After the molecular genetic studies in domestic pigeons mainly dealt with plumage coloration, two studies on eye coloration were presented in quick succession. Chinese scientists have investigated the molecular genetic differences between pearl eyes and orange-red eyes in Racing Homer pigeons (Si Si et al. 2020). The American study by the research group from Utah (Maclary et al. 2021) broadened the scope and also appeals to the dark eye. These are the three colors that have traditionally been differentiated as the basic forms of pigeon colors since the investigations by Hollander / Owens in 1939.

2. Pearl eyes and orange eyes in the Chinese study

146 pearl eyed individuals and 146 orange eyed Racing Homers were examined. In the plumage color, it was all blue-bar in order to rule out any influence on the color of the eyes. In terms of eye color, Racing Homers, even as 'pearl-eyed', generally do not show the distinctive light, often almost white iris that is found in fancy show pigeons. When it comes to racing pigeons, breeders often speak of glass eyes, not pearl eyes. Such an eye was also shown in the pigeons from the experiment (Fig. 1). A light border, a 'pearl eye', runs around the pupil. These kinds of eyes are also common in roller pigeons not bred for exhibitions.

Fig. 1: Racing pigeons with orange-colored eyes and with pearl eye at the right. Source Si Si et al. 2020

With regard to the genetic differences to the wild type, a so-called nonsense or stop mutation W49X of SLC2A11B was found by sequencing the entire genome, which is responsible for the pigment change in the iris of pearl-eyed domestic pigeons. This stop mutation causes the premature termination of the synthesis of the corresponding protein, so that pearl eye occurs. Most of the pearly eyed individuals tested (141 out of 146) were homozygous for this mutation. The individuals with orange eyes were all at least heterozygous for the wild type, which is consistent with the recessive inheritance of pearl eye known to breeders and so also applies to the light form of pearl eye in traditional genetic analyzes (Hollander / Owen 1939).

 

Fig. 2: F1 (middle) from a pearly-eyed high-flying cock and an orange-eyed Racing Homer hen. Authentic picture of a young animal with parents from the own loft. Source: Sell 2019, Fig. 197).

In the analysis by the Chinese research group, five of the pearl-eyed individuals were only heterozygous for the mutation. The influence of non-localized genetic modifications is seen as a possible reason for the deviation from the norm that the trait is only clearly evident in the case of purity. The change in color of the eyes of the pigeon chicks, which are normally black at the beginning, to orange or pearl eye could also have taken place at different speeds. The authors consider it possible that the process of conversion from the black eye of the pigeon chicks to orange or pearl color was not yet completed with five months when the pigeons were classified. The picture of the pear-eyed carrier pigeon attached by the authors (Fig. 1) also allows the possibility of a misclassification, because the pigeons tested are not those with extremely white eyes, as we know them from pigeons in exhibition. These could be more clearly distinguished from orange-eyed individuals. The possible misclassification due to the not yet completed color change of the eye is nevertheless unlikely, since the pearl eye is not a precursor to the orange eye in the normal development of the coloration of the eye. In terms of developmental history, the authors suspect that the mutation to pearl-eye occurred very early. It remains to be seen whether such an exact point in time can be determined, as 5,400 years ago (Si Si 2020, abstract p. 2).

The occurrence of pearl-eyed pigeons in Racing Homers can be explained by the genesis of the Belgian Racing Homer, in which, in addition to owl-pigeons, especially pearl-eyed high-flying pigeons are among the ancestors, which is also reflected in the genetic similarity in the distance measurements for Tipplers, as descendants of these high-fliers, and Racing Homer pigeons shows (Stringham et al. 2012, Sell 2012, 2019 with further documents).

3. Comments on dark eye color in different genetic constellations

With regard to the dark eye color, the authors of the Chinese study correctly write that this was already associated with the white plumage color in the 1939 study. That is correct, but in short it can be misunderstood. Recessive whites, who are the main focus here, have dark eyes (in German ‘Faulaugen’ (rotten eyes), in English bull eyes) like the German breed ‘Mittelhäuser’ in Fig. 3 on the right. Many piebalds with regular color patches with a white head area also have dark eyes, such as the Spot Pigeon in Fig. 3 on the left. The brief comment should not, however, create the impression that the dark eye only exists in connection with white or with white-headed individuals. This is not the case, as shown here in the picture of a color head with dark eyes

Fig. 3: Dark eye at Thuringian Spot Pigeon (left), South German Color Head (center) and at a white ‘Mittelhäuser’ Pigeon (right). Source: Sell 2019, Fig. 194.

There are four possible combinations of colored / white plumage and orange or pearl-colored / dark iris.

 

Dark Eye

Pearl- orange eye

White head

Colored head

 

Fig. 4: Possible combinations of dark eyes on the one hand and pearl and orange eyes on the other hand, and white or colored background on the head. Thuringian whitehead red with dark eye, old Dutch Capuchin ash red with pearl eye, Zurich whitetail with colored head and dark eye, finally a Königsberg colored head blue with pearl eye. From top left to bottom right.

There are dark-eyed pigeons both as self (white or colored) and as piebalds with white or colored heads.

 

 

Head white

Head colored (non-white)

Self

Pied

 

Fig. 5: Dark-eyed pigeons as self and as piebalds with white or colored heads. Brünner Pouter white (photo Layne Gardner), Arabian Trumpeter pale-black, Nuremberg Bagdette heart-pied marking, and South German color head (from top left to bottom right).

Irregularly piebald 'spotted pigeons' (Fleckenschecken) such as Uzbek Tumblers, Racing Homers and Beauty Homer Pigeons have a special status. With these, the eye is orange or pearly colored if the head plumage is completely or predominantly colored. If the eye is in the white area, then it tends to be dark; if it is in the edge area, it is often broken in two colors. It can also be different left and right.

Fig. 6: Uzbek Tumblers with irregular pied marking bred by the author. On the left a cock with a white head area and dark eye, on the right a hen with a still widely colored head area and thus a pearly colored iris. Broken and dark eyes tend to appear as the proportion of white increases. Also, different eye colors, right and left, are possible. Source: Sell 2015, Fig. 559.

A specialty are also the dark-eyed self whites, which regularly are raised from some heterozygous regular piebalds, such as Königsberg Color Heads, the ‘Altstämmer’ breed and Berlin Tumblers, and, less known, from Vogtland Trumpeter Pigeons and some other breeds.

Fig. 7: Complementary colors of the breeding of heterozygous Vogtland White-Headed Trumpeter Pigeons (middle). Left colored (non-white) self, right self white with dark eyes. Source: Sell 2019, Fig. 265.

The term of the ‘Faulaugen’ (rotten eye of bull-eye) is used in the language of breeders on recessive whites and the self whites from heterozygous piebalds. The non-white selfs with dark eyes are referred to as ‘dark eyes’, and in the standards they are also referred to as dark brown. There are also visual differences.

The traditional analysis of inheritance through the evaluation of systematic test pairing had never tried to find a uniform explanation for the different constellations of dark eye color. In breeding, they indicate different bases. Recessive white is usually dealt with in the textbooks on pigeon genetics in the Chapter for plumage coloring. The dark eye there is understood as the eye color correlated with the white plumage color. In relation to the eye color, it is neither recessive nor dominant, but rather overlapping, epistatic, in relation to the orange eye and the pearl eye. Gary Fillmore had shown the epistatic effect in 1992 for non-white self Thai Trumpeter Pigeons and Ice Pigeons and the inheritance behavior was later confirmed in own experiments. For the other pied variants, many research results and theories are presented in the literature (cf. Sell 2012, 2015). For the irregular pied marked (Fleckenschecken), the already mentioned tendency to dark and broken eyes was shown when the white in the head area increases. Initially orange- or pearly-eyed pigeon families can quickly develop into dark-eyed families and dark-eyed whiteheads over the generations.

4. The American Analysis

With regard to the genetic relation of pearl-eye to orange-eyes, the study from Maclary et.al (2021) confirms the recessive character of pearl-eye when crossing an orange-eyed copper Gimpel Pigeon and a pearl-eyed Capuchine (Fig. 8).

Fig. 8 and Fig. 9: Animals and breeds used for the experiment, copper Gimpel with orange eyes, Capuchins with pearl eyes, blue Pomeranian Pouter with a typical white Bavette, colored head and orange eye, Nuremberg Bagdette with a pied-related dark eye on a largely white background. Source: Maclary et al. 2021.

In the F2 there were also some phenotypically dark-eyed young from the Gimpel/Capuchine cross, although none of the parents had a dark eye. Genetically, these young also showed peculiarities in the DNA that were typical of capuchins who did not have the dark eye. However, the relevant area was not located in the DNS where it was suspected in the F2 young from the second mating, which were also classified as dark-eyed. If no initial animal with dark eyes was used in the first pairing and dark-eyed young animals were discovered in F2, the second pairing consisted of a Pomeranian Pouter with orange eyes and a hearted Nuremberg Bagdette with dark eyes.

For the young of the F2 of the 2nd mating (Fig. 9 above), the investigation in comparison to dark-eyed and pearly-eyed individuals from a data set did not provide any clear indication of a gene region that could be responsible for the dark-eyed phenotype.

5. Summary

The authors of the second study did not choose an easy way to uncover relationships in eye coloring. They linked the question with the question of pied-marking, so that in a first approach, not necessarily satisfactory answers to the questions asked were to be expected. Of the four dark-eyed individuals shown in different gene constellations in Fig. 5, a single and perhaps not typical form was used in the study with the Bagdette. Pieds with irregular spotting (Fig. 6) were indirectly addressed in the study because some of these individuals were found in F2. Feeling into the question initially with selfs would certainly have been easier to lay the foundations. It is still important, however, that this investigation is an introduction to the analysis of complex pied markings.

 

Literature

Fillmore, Gary, Eye Color, PGNV&Comments, december 1992, pp. 9-11.

Hollander, W.F., and R.D. Owen, Iris Pigmentation in Domestic Pigeons, Genetica 21, 1939, pp. 408-419.

Maclary, Emily T. et al., Two Genomic Loci Control Three Eye Colors in the Domestic Pigeon (Columba livia), doi: https://doi.org/10.1101/2021.03.11.434326 (preprint)

Sell, Axel, Genetik der Taubenfärbungen, Achim 2015

Sell, Axel, Taubenzucht. Möglichkeiten und Grenzen züchterischer Gestaltung, Achim 2019

Sell, Axel. Pigeon Genetics. Applied Genetics in the Domestic Pigeon, Achim 2012

Si Si, Xiao Xu, Yan Zhuang et. al, The Genetics and Evolution of Eye Color in Domestic Pigeons (Columba livia) doi: https://doi.org/10.1101/2020.10.25.340760 (preprint)

Stringham et al., Divergence, Convergence, and the Ancestry of Feral Populations in the Domestic Rock Pigeons, Currently Biology (2012), doi: 10.1016/j.cub.2011.12.045.

April 2021

AS