Mosaicism is the status of having more than one genetic population of cells in a single individual which developed from a single fertilized egg. This condition can be caused by a spontaneous mutation in a single cell in the early embryo that leads to the mutation being multiplied every time there is another cell division as the embryo develops. It can also be caused by less than robust DNA replication during cell division (mitosis) so that some genes become inactive or left behind. Mosaicism occurs easily due to the millions of cell divisions and possibilities for replication errors that occur during the development of an embryo. In fact, 70-90% of complex organisms (maybe all?) are affected by mosaicism.
Chimerism is a different thing entirely. In the case of a chimera, a single organism is created from the separate fertilizations of two eggs which accidentally fuse together in the early cell division phase of development. The two balls of cells join forces and make a single organism that has two distinct lines of genes. Chimeras cannot be a common occurrence in birds as it would require a double yolked doubly fertilized egg along with the random possibility that the locations of the fertilization nucleus on the two yolks were adjacent to one another.
Mosaicism can involve any type of chromosome and its genetic apparatus. Some genetic changes are visible and others are invisible. An example of visible mosaicism could be something as simple as a large birthmark where a single genetic change results in a population of skin cells that looks different from all of the rest. An invisible mosaic might have two or more lines of cells with genetic changes that result in slight variations in the molecular structure of the enzymes produced by the different cell lines. In most cases mosaicism does not lead to disabilities but it sometimes can.
I once had an American Show Racer that was both Ash Red and wild-type in feather color. I knew the parents genetics so it was clear that the cells in the bird that made Ash Red had to be genetically male and those that produced wild–type coloration had to be female. So this bird was a visible mosaic in feather color expression and a visible mosaic in sex since the colors were sex-linked. I didn’t know whether it would act like a girl or a boy and I didn’t know whether or not it would be fertile. The sex chromosomes determine the gender of the bird, of course, so it mattered whether or not the embryonic cells that turned into the brain were male or female, it mattered which developed into the adrenal glands, and it mattered which developed into gonads (ovaries or testes). The color pattern gave me a clue as to what to expect as the bird was mostly Ash Red but had a blue neck and wing on one side. I assumed it would “be” a male because the feathers over the head (where the brain is) and the back (where the gonads develop) were visibly Ash Red. I thought, therefore, that the underlying brain and gonads would all be male. In this case I was correct. The bird was a fertile cock. Interestingly, half of his offspring also demonstrated smaller expressions of mosaicism, suggesting that there was a change in his DNA that weakened the perfection of the DNA replication process during cell mitosis.
I once thought that mosaicism was likely not a heritable condition, but I’m now not so sure about that. Heritability is likely related to the original cause. A random mutation is one thing, a disordered DNA replication process is quite another, but I can see how each, once established, could be passed on to future generations.
Seraphim are difficult to assess in regard to mosaicism, as the clues to it require something visual, and since the Seraph Color Gene Complex is not sex-linked there are limitations on mosaic occurrences that could change the appearance of the bird. I once had a Seraph that I thought was a hen by behavior that paired up with a definite cock. However, when it came to mating time the “hen” only wanted to mount the cock and would not mate properly. It also never laid a single egg even though the two went through all the motions of being devoted to one another, setting up a nest, and courtship. I could only conclude that the hen was actually a cock whose brain was feminized. This can happen due to a mutation that affects testosterone cell receptors or due to mosaicism. There was no visible evidence of mosaicism in this case, however, due to the Seraph Color Gene Complex – both birds looked the same – pure white. The clues were in behavior and failure to lay eggs as expected.
Last year I hand-raised a little runt of a baby Seraph that grew into a beautiful adult. As a juvenile it had the usual red Satinette markings, in this case lacewing/spot-tail. I didn’t study each feather, but it turns out that one of the tail feathers was not recessive red. When the bird molted into adult pure white feathers she had a single perfectly marked tail feather in blue with a perfect white spot. Why did this happen? Sometimes mosaicism can happen in a very tiny patch of skin – in this case in a single feather follicle. The cells in that follicle lost their functioning Seraph Color Gene Complex for some reason during embryonic development.
A year later I paired this hen with the single spotted tail feather with a perfectly marked (pure white) Seraph cock. Their third baby is a wonderful specimen but has a large patch of brown mosaicism on his right wing shield. Did this happen simply by chance or did the mother pass some weakness in DNA replication on to this son of hers? Here he is below, still a baby, but showing a brown lacewing pattern in the shield of his right wing where the white-sides and recessive red genes inherent in his genetics failed to express. He has an entire segment of a wing that is missing the genetic machinery of the Seraph Color Gene Complex. I didn’t notice this when he was a baby as the drab markings didn’t register as being different from the surrounding recessive red feathers he had at the time. Now it is obvious. the bird to his lower right is the same age and has nearly complete replacement of red feathers in the wing shield with white as expected. The juvenile red tail feathers have yet to be replaced in that bird as seen in the photo; that is NOT mosaicism.
The cock this boy’s mother is with has sired at least thirty offspring with another perfect white Seraph hen, none of which demonstrated color mosaicism. If it happens again with this hen and cock it will be necessary to take her out of the breeding pool. I think the DNA replication weaknesses that can result in color mosaicism are possibly heritable. Add in that Seraphim are a massive complex of recessive traits and inbreeding and one can begin to see how careful you must be to avoid avoid genetic pitfalls that can ruin the color of an entire line of birds. As it is, I plan to use this mosaic Seraph in an experimental breeding program just to see what happens. I haven’t read anything anywhere about the heritabiliy of mosaicism in pigeons. This is a question that needs an answer.
The first Seraph mosaic I ever had was in 2009. I didn’t understand what he was. He was a beautiful and stunning bird and I should have kept him. I sent him to California to a fellow near Sacramento. A year later he called me to tell me that the colored feathers contaminating the birds left wind did not disappear at the second year molt. It was only then that I realized the colored feathers were brown, not recessive red, and would never disappear. At the time I advised him to breed the bird as standard opinion was that mosaicism wasn’t inherited, but rather just a genetic accident. I don’t know what ever happened in his loft after that as I lost touch. I wish I still had that bird though. He was not a show winner as he defied the Standard, but he was special. (Note that the tail feathers in the youngster above are a combination of new adult white feathers and old short recessive red feathers – the tail has not been affected by mosaicism, only the right wing shield. The tail will molt out correctly as white. I’ll add photos later that show what this bird looks like in the end.)
It’s a shame when one has a bird that cannot be shown or sold due to a trait like mosaicism. The few Seraphim I’ve had with mosaicism have otherwise been fabulous examples of what a Seraph should be with otherwise beautiful stance and form.
I would be interested in hearing from any Seraph breeders about any experience they may have had with this condition in Seraphim.