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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.
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Dark Eye |
Pearl- orange eye |
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White head |
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Colored head |
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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.
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Head white |
Head colored (non-white) |
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Self |
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Pied |
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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
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