Mosaic-like colorations in pigeons

Mosaic-like colorations in pigeons

At the time of the early reports on mosaics by Cole and Hollander, it was the name of a phenotype whose possible different causes were speculated about. The terms mosaic, chimera and somatic mutations stood side by side, mosaic as a generic term. 'Somatic Mosaics' is the headline of an article by Cole and Hollander, submitted in 1939 and published in 1940 in the scientific journal 'Genetics'.

Fig. 1: Highflier-hens with mosaic-like contrasting color areas from the own loft at the left and at Rudolf Beneke, at the right.

With 'Bipaternity' in the title of an article in the 'Journal of Heredity' in 1949, Hollander expressed his suspicion about the cause of some mosaic-like colored pigeons, in which other explanatory approaches could not apply due to their potential descent, or only when several unlikely events interacted. With advances in molecular genetics, other explanations came to the fore. The 'mosaic or chimeric effects' summarized by Hollander / Cole in 1940 were divided into chimera and mosaic according to the suspected causes. What they have in common is that they have genetically different cell populations. In the case of chimeras, they originate from the fusion of two or more fertilized egg cells (zygotes); in the case of mosaics, they originate from a single zygote,

https://www.britannica.com/science/chimera-genetics, https://www.embryology.ch/vet/de/kchromaber/klinik02.html

How do we get genetically different cells into a fertilized egg cell? Mutations and deletions are possible. Somatic mutations only set in after the embryo has formed. From then on, the DNA sequence of body cells is changed. These changes can only be reproduced in parts. The change is limited to the body cells and is not inherited. Theoretically, the distinctions are clear, but difficult in practice, as publications from human medicine show (e. g. Boklage 2006). In the case of pigeons, there is usually little evidence of potential parentage.

Fig. 2: Cock with mosaic-like contrasting color areas

In the case of the cock from the own loft shown, it can be narrowed down. The father is a pure, homozygous rubella. The mother is hemizygous, a dilute frosty rubella check. Typical for such hens - with a genetically black basic color - the yellow color in the phenotype.

Fig. 3: Parents of the cock in Fig. 2. Homozygous rubella cock and hemizygous dilute frosty rubella hen.

Outwardly, the young cocks mostly show the plumage coloration expected with this genetic makeup. Compared to most homozygous rubella, heterozygous frosty cocks the color is weakened in a greater degree and thus similar to hemizygous frosty rubella hens (Fig. 5 at the left) and not far away from some homozygous frosty that were classified as heterozygous rubella. Homozygous Frosty Rubella cocks are light silver-gray with translucent bars or checks. At the cock in Fig. 2 we see mosaic-like deviations on one side in several primaries, thumb springs and hand covers. Here he shows the coloring of pure, non-diluted Rubella.

Fig. 4: Grandparents of the cock in Fig. 2 from the mother side: Homozygous Frosty-Rubella cock, black color base, bar pattern, homozygous frosty and rubella, heterozygous dilution, Non-Spread. At the right the blue check grandmother

The potential genetic makeup based on the family tree: The son should have inherited the frosty gene, rubella and the dilution factor on the maternal sex chromosome. On the paternal side, non-frosty, rubella and non-dilution are to be expected on the chromosome at the gene locations mentioned. The majority of the plumage corresponds to the expectations of this genetic makeup. The relatively strong lightening in this cock and the yellowish tinge, especially in the neck area, are probably due to the fact that it is also heterozygous for dilution on the maternal side.

With the known descent one can mentally pursue the potential inheritance processes. In the mosaic-like deviations, the maternal expected frosty gene does not have an effect. The gene for this area of the body seems to be lost in the son's inheritance. It is not due to the father and cannot be explained by another paternity (cross-fertilization) from the breeding group. Another pure-bred rubella cock would be irrelevant. A wild-type bluebird and the pure-bred Frosty Rubella grandfather are also ruled out, as they could not have resulted in the appearance in combination with the mother's genes. Bipaternity as an explanation for appearance falls out for the same reason.

In chimeras, two fertilized egg cells fuse. On the maternal side, the hen can only form the type frosty, rubella, dilution on the sex chromosome. Combined with the genes on the sex chromosome of potential fathers, no zygote for the observed phenotype can result from this. Mosaics arise from a fertilized egg cell, whereby mutations are one possibility of genetically different cells. On the maternal side, Frosty could have mutated back to the wild type. If this occurs after the embryo has formed, only certain body cells are affected (somatic mutations or somatic mosaic). In combination with the unchanged paternal chromosome, this can result in a mosaic pattern, as shown.

Fig. 5: On the effect of Frosty at a Rubella base: At the left a hemizygous frosty-rubella hen, in the centre a hemizygous rubella hen. Source: Sell, Critical Issues in Pigeon Breeding Part III). At the right a homozygous rubella check cock

Other examples of mosaic-like colored pigeons suggest chimera as the cause. Still others are difficult to reconcile with both theories or require a large number of coincidental peculiarities at the same time. For the classic analysis by looking at the phenotypes, there is not only the uncertainty about the ancestry, but also that little or nothing is known about many interactions of color factors. This also applies to the interaction of factors that are not allelically assessed as recessive. They are recessive in mating with the wild type, but cause clear color changes in certain gene constellations even when heterozygous. http://www.taubensell.de/extreme_sexual_dimorphism_in_the.htm

Fig. 6: Dark color patches in a cock with the stipper gene and in an Uzbek Flying Tumbler, 'Tschinny', from the own loft. Tschinnies are self red in the juvenile plumage. Such patches are common with these stains and are also reported from other lofts (Source: Sell, Pigeon Genetics)

In the case of pigeons, there seem to be no meaningful empirical studies on the chromosomal differences between chimeras and mosaics and investigations of the body cells. The relationship between the extent and the distribution of the mosaic areas with the potential cause and the time of mutations in supposed somatic mosaics has probably not yet been examined in pigeons. The general difficulties of differentiation in specific cases are known from human genetics. As Boklage (2006) states, mosaic formation is not a problem in terms of the theoretical definition, but in everyday clinical practice and in diagnostics.

Literature:

Boklage, Charles E. (2006) Embryogenesis of chimera, twins and anterior midline asymmetries, Human Reproduction Vol. 21, No. 3, pp. 579-591.

Hollander, W.F. (1949), Bipaternity in Pigeons, Journal of Heredity, Vol. XI., No. 10, pp.271-277.

Hollander, W.F., und Leon J. Cole (1940), Somatic Mosaics in the Domestic Pigeon, Genetics Vol. 25, pp. 16-40.

http://www.taubensell.de/extreme_sexual_dimorphism_in_the.htm

http://www.taubensell.de/extremer_geschlechtsdimorphismus.htm

https://www.britannica.com/science/chimera-genetics

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

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

Universities of Fribourg, Lausanne and Bern (Switzerland), Online course in embryology for medicine students, developed by the Universities of Fribourg, Lausanne and Bern with the support of Swiss Virtual Campus, (on sight august 11, 2021)