Chrimeras by Cryberg

Not infrequently a pigeon is observed that has a patch that does not belong with the rest of the bird. For example a blue bird might be bar pattern on one side and check pattern on its other side. Quite often such a patch covers well under half of the bird. In the above example the check pattern might only cover a portion of one wing with the rest of the wing being bar pattern. Such oddities in pigeons are almost always labeled mosaics by the pigeon hobby. A few of them probably are mosaics as probably every bird born is a mosaic by definition for some gene. But, most are not mosaics at all. What they are, are Chimeras.

The definition of a mosaic is quite simple. It is an individual that has developed from a single fertilized egg and that individual has undergone a mutation at some point in development such that as an adult it consists of two populations of cells that differ genetically.¹ All that has happened is some genetic accident happened after the fertilized egg had undergone at least one cell division. The accident could be a simple mutation of some gene, it could be somatic crossing over resulting in a recombination of genes, or it could be a deletion or a transposition of a piece of DNA from one place in the chromosomal structure to a new place. The key thing to recognize is all of these started with one egg fertilized by one sperm forming one single embryo. Lab induced mosaics have been useful in learning about early embryo development. This is particularly true for mutants that would be lethal if the whole organism were homozygous for the mutant. As the average new individual has several mutations at birth that did not come from either parent and also develops new mutations throughout life it is fair to say that probably every single multicellular organism is a mosaic. If the mutation happens in the right mass of cells early enough in life to be incorporated into the ovaries or testes the new mutation may be transmitted to offspring. On the other hand if the mutation does not end up in these tissues it is simply a dead end in animals and will not be transmitted. In plants mosaics are often called sports. Plants can usually be reproduced from a tissue sample by either grafting or by rooting or tissue culture. So mosaics on plants are often a source of new phenotypes as the mutation does not need to be present in the reproductive organs. There is no reason at all to think that the tendency to produce true mosaics could be inherited. In fact if a strong tendency to form true mosaics could be inherited the DNA for that individual would be so unstable that the individual would likely be dead before birth. If nothing else cancer should kill it fast. After all, some types of cancer are simply mosaics.

Chimeras:

Hollander wrote a paper which he mistitled “Sectorial Mosaics in the Domestic Pigeon: 25 More Years” in 1975.² He presents data on some 40 different birds, many that showed patches that differed by two or more mutants from the rest of the bird. In this paper he actually presented the correct interpretation for such animals as being due to chimeras as one possibility. A chimera is simply an individual that consists of two or more populations of cells that originated from different zygotes. Chimeras are actually quite common and in some animals most individuals born are chimeras³. A well understood chimera in cattle involves opposite sex twins. The female will generally be sterile as she will have acquired tissue from her male twin in utero and that male tissue will prevent proper sexual development of the female⁴. These part male, part female chimeras share the same immune systems as can be demonstrated by viability of skin grafts between the male and female. That such females were chimeras has been known since the 1940s when it was very well documented by another of Cole’s students, Ray Owen⁵. Hollander knew about Owen’s work quite well as they actually worked together on a project right at the end of Hollander’s PhD work. Why he seems to have ignored Owen’s work is a puzzle.

More recently it has been shown that in birds sexual expression is not a matter of general hormone levels through the body. Rather, each individual cell has a sexual identity based on the chromosomal content of that cell. In chickens it has been well known for some time that roughly one bird in 5000 exhibits patches where part of the bird has cock plumage and the rest has hen plumage⁶.

Even humans have been identified who have had two, genetically totally different, populations of cells⁷.

In his 1949 paper⁸ on this subject Hollander concluded that he favored an explanation for animals that had two genetically different populations of cells being the result of one egg being fertilized by one sperm forming perfectly normal tissue and a second sperm forming haploid tissue without any fertilization of an egg. This haploid tissue then became incorporated into the embryo. In his 1975 paper he readily admits that “Double fertilization of a binucleate egg, however, could explain most cases as well,” He also goes on to make the incredible comment “It is regrettable that, to date, no karyotype studies have been attempted….it would be interesting to know if haploidy is or is not involved.” In fact it is incredible that he was able to publish this paper without karyotype data. Such data was easy to obtain by that late date. Clearly all the equipment and skills to carry out such a determination were at his ready disposal at Iowa State University. The fact he did not do this simple experiment is telling. It seems to say he did not wish to know that his haploid tissue idea was incorrect. And unfortunately it has laid the seeds for continued confusion on the subject ever since. Some people even seem to think Hollander claimed two sperm fertilized one egg and that somehow lead to what he incorrectly called mosaics. Two sperm plus one egg does happen rarely and results in a triploid embryo which is fatal before hatching. In general, biology has reasonably robust restrictions that prevent a second sperm from entering an egg.

In fact, many of the birds Hollander called mosaics are clearly not mosaics at all. Any that he claimed to be the product of more than a single sperm by definition were not mosaics. They were not considered mosaics by the time of his last paper in 1975 nor even 25 years before then when he had published his earlier paper. What they are known to be are chimeras.

Chimeras have been well studied in the laboratory. Natural chimeras are not particularly uncommon. They are also reasonably easy to make artificially. Artificial chimeras can be made by grafting operations on embryos or by embryo fusion. For instance here is a link to a picture of a chimera made by fusing a sheep and goat embryo:

Making a chimera of a mammal in the laboratory is straightforward. All you need to do is either push two early embryos together in a petri dish and culture under conditions that will allow growth or inject stem cells into an early stage embryo, again in a petri dish. In either case the resulting chimeric embryo is transplanted to a suitable female for gestation. Chimeras of birds are also quite easy to make either by tissue grafting to a developing embryo or injection of stem cells into a developing embryo. With birds the developing embryo is not removed from the egg to perform the grafting. Male chimeras produced by injecting cells from a duck embryo into a developing chicken embryo were even able to produce both viable duck and chicken sperm⁹. Females of the same origin were able to produce both viable duck and chicken eggs.

In a bird the most logical way a natural chimera could happen would be if two eggs were deposited on one yolk. If both eggs got fertilized, each by a single sperm, the result would be two embryos located right next to each other. During development these two embryos could easily fuse and become one individual. This does present the problem as to how two eggs happened to end up deposited on one egg yolk. The easiest way for this to happen is simply for an error in the cell divisions that form an egg to happen. If you recall meiosis to form an egg involves two sequential cell divisions. Here is a link that covers meiosis in pictorial form:

Briefly what happens is the chromosomes are doubled so each cell has two of each chromosome. The original and daughter chromosomes remain bound at the centromere. Crossing over takes place and then the first cell division takes place. One chromosome and its daughter chromosome still attached go to one of the new cells and the homologous pair goes to the other cell. These cells usually differ greatly in size if the final product will be an egg. Almost all cytoplasm goes to one of the new cells while the other gets very little cytoplasm. Thus, in birds one of these cells is male and the other female. This cell division is followed by a second cell division for each of the cells produced in the prior step. In this case the chromosome pairs are pulled apart from the centromere attachment resulting in all haploid cells. Again the cell destined to be the egg gets almost all the cytoplasm while the other cell usually gets little cytoplasm. The net result of this process is four cells in total. One is quite large and contains almost all the cytoplasm that was in the starting tetraploid cell. The other three are tiny by comparison and nearly lack cytoplasm. All three small cells are called polar bodies. The cell that got almost all the cytoplasm also got most of the small structures found in a cell such as mitochondria. Those structures are critical to proper cell function and without them the cell will shortly die. The polar bodies simply do not have critical components that allow survival usually.

A number of people have reported lines of birds that tended to produce such chimeras regularly. In one case a chimera hen pigeon produced eight out of ten young that were also recognizable chimeras in one year¹⁰. Generally they are improperly called mosaics out of habit. It should be quite possible to develop lines of birds that had this tendency. After all, those cell divisions that happen during egg formation are under genetic control. All that has to happen is to change those genes that govern unequal cell division in favor of more equal cell division and you are going to have a line of birds that will produce such freaks. It is worth noting that chimeras in birds are 100% the result of what happens in the female. So it is her genes that will govern how equal, or unequal, those critical cell divisions happen to be. However, in development of a line of birds with the tendency to produce chimeras the father of the hen is just as important as the father is in a Racer, a Roller or a show bird. Mating a son back to a hen that has produced chimeras should be a good mating to capture the trait. It is even possible that different genes are involved in the two different cell divisions so perhaps with selection you could largely avoid making the dual sex chimeras that might have health issues.

1. Strachan, Tom; Read, Andrew P. (1999). "Glossary". Human Molecular Genetics (2nd ed.). New York: Wiley–Liss. ISBN 1-85996-202-5.

2. Hollander, W. F., Sectorial Mosaics in the Domestic Pigeon: 25 More Years, J. Hered 66: 177-202 (1975)

4. Padula AM., The Freemartin Syndrome: An Update, Anim Reprod Sci. 2005 Jun;87(1-2):93-109. Epub 2004 Dec 10.

9. Z. D. Li, H. Deng, C. H. Liu, Y. H. Song, J. Sha, N. Wang and H. Wei, 2002 Poultry Science 81: 1360-1364

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These two photos of the same bird were taken by David M. Longseth at the 2003 NYBS.
The right wing shield is a blue check while the left side is a dark 'dun/brown' color with heavy black flecks or sootiness.
This left side may be almond as the sire was an almond bird.

Chimeras and Other Oddities

Mosaics:

Chimeras: