Chimera (genetics)
A genetic chimerism or chimera is a single organism composed of cells of different genotypes. Animal chimeras can be produced by the fusion of two embryos. In plants and some animal chimeras, mosaicism involves
distinct types of tissue that originated from the same zygote, but differ due to mutation during ordinary cell division.
Normally, genetic chimerism is not visible on casual inspection; however, it has been detected in the course of proving parentage. More practically, in agronomy, "chimera" indicates a plant or portion of a plant whose tissues are made up of two or more types of cells with different genetic makeup; it can derive from a bud mutation or, more rarely, at the grafting point, from the concrescence of cells of the two bionts; in this case it is commonly referred to as a "graft hybrid", although it is not a hybrid in the genetic sense of "hybrid".
In contrast, an individual where each cell contains genetic material from two organisms of different breeds, varieties, species or genera is called a hybrid.
Another way that chimerism can occur in animals is by organ transplantation, giving one individual tissues that developed from a different genome. For example, transplantation of bone marrow often determines the recipient's ensuing blood type.
Classifications
Natural chimerism
Some level of chimerism occurs naturally in the wild in many animal species, and in some cases may be a required part of their life cycle.Sponges
Chimerism has been found in some species of marine sponges. Four distinct genotypes have been found in a single individual, and there is potential for even greater genetic heterogeneity. Each genotype functions independently in terms of reproduction, but the different intra-organism genotypes behave as a single large individual in terms of ecological responses like growth.In obligates
It has been shown that male yellow crazy ants are obligate chimeras, the first known such case. In this species, the queens have arisen from fertilized eggs with a genotype of RR, the sterile female workers show a RW arrangement, and the males instead of being haploid, as is usually the case for ants, also display a RW genotype, but for them the egg R and the sperm W do not fuse so they develop as a chimera with some cells carrying an R and others carrying a W genome.Artificial chimerism
Artificial chimerism refers to examples of chimerism that are produced by humans, either for research or commercial purposes.Tetragametic chimerism
Tetragametic chimerism is a form of congenital chimerism. This condition occurs through fertilizing two separate ova by two sperm, followed by aggregation of the two at the blastocyst or zygote stages. This results in the development of an organism with intermingled cell lines. Put another way, the chimera is formed from the merging of two nonidentical twins. As such, they can be male, female, or intersex.The tetragametic state has important implications for organ or stem cell transplantation. Chimeras typically have immunologic tolerance to both cell lines.
Microchimerism
Microchimerism is the presence of a small number of cells that are genetically distinct from those of the host individual. Most people are born with a few cells genetically identical to their mothers' and the proportion of these cells goes down in healthy individuals as they get older. People who retain higher numbers of cells genetically identical to their mother's have been observed to have higher rates of some autoimmune diseases, presumably because the immune system is responsible for destroying these cells and a common immune defect prevents it from doing so and also causes autoimmune problems.The higher rates of autoimmune diseases due to the presence of maternally derived cells is why in a 2010 study of a 40-year-old man with scleroderma-like disease, the female cells detected in his blood stream via FISH were thought to be maternally derived. However, his form of microchimerism was found to be due to a vanished twin, and it is unknown whether microchimerism from a vanished twin might predispose individuals to autoimmune diseases as well. Mothers often also have a few cells genetically identical to those of their children, and some people also have some cells genetically identical to those of their siblings.
Germline chimerism
Germline chimerism occurs when the germ cells of an organism are not genetically identical to its own. It has been recently discovered that marmosets can carry the reproductive cells of their twin siblings due to placental fusion during development.Types
Animals
As the organism develops, it can come to possess organs that have different sets of chromosomes. For example, the chimera may have a liver composed of cells with one set of chromosomes and have a kidney composed of cells with a second set of chromosomes. This has occurred in humans, and at one time was thought to be extremely rare although more recent evidence suggests that this is not the case.This is particularly true for the marmoset. Recent research shows most marmosets are chimeras, sharing DNA with their fraternal twins. 95% of marmoset fraternal twins trade blood through chorionic fusions, making them hematopoietic chimeras.
In the budgerigar, due to the many existing plumage colour variations, tetragametic chimeras can be very conspicuous, as the resulting bird will have an obvious split between two colour types often divided bilaterally down the centre. These individuals are known as half-sider budgerigars.
An animal chimera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic. Innate chimeras are formed from at least four parent cells. Each population of cells keeps its own character and the resulting organism is a mixture of tissues. Cases of human chimeras have been documented.
Chimerism in humans
Some consider mosaicism to be a form of chimerism, while others consider them to be distinct. Mosaicism involves a mutation of the genetic material in a cell, giving rise to a subset of cells that are different from the rest. Natural chimerism is the fusion of more than one fertilized zygote in the early stages of prenatal development. It is much rarer than mosaicism.In artificial chimerism, an individual has one cell lineage that was inherited genetically at the time of the formation of the human embryo and the other that was introduced through a procedure, including organ transplantation or blood transfusion. Specific types of transplants that could induce this condition include bone marrow transplants and organ transplants, as the recipient's body essentially works to permanently incorporate the new blood stem cells into it.
Boklage argues that many human 'mosaic' cell lines will be "found to be chimeric if properly tested".
In contrast, a human where each cell contains human genetic material as well as that from another species would be a human–animal hybrid.
While German dermatologist Alfred Blaschko described Blaschko's lines in 1901, the genetic science took until the 1930s to approach a vocabulary for the phenomenon. The term genetic chimera has been used at least since the 1944 article of Belgovskii.
This condition is either innate or it is synthetic, acquired for example through the infusion of allogeneic blood cells during transplantation or transfusion.
In nonidentical twins, innate chimerism occurs by means of blood vessel anastomoses. The likelihood of offspring being a chimera is increased if it is created via in vitro fertilisation. Chimeras can often breed, but the fertility and type of offspring depend on which cell line gave rise to the ovaries or testes; varying degrees of intersex differences may result if one set of cells is genetically female and another genetically male.
On January 22, 2019, the National Society of Genetic Counselors released an article Chimerism Explained: How One Person Can Unknowingly Have Two Sets of DNA, where they state, "where a twin pregnancy evolves into one child, is currently believed to be one of the rarer forms. However, we know that 20 to 30% of singleton pregnancies were originally a twin or a multiple pregnancy".
Most human chimeras will go through life without realizing they are chimeras. The difference in phenotypes may be subtle or completely undetectable. Chimeras may also show, under a certain spectrum of UV light, distinctive marks on the back resembling that of arrow points pointing downward from the shoulders down to the lower back; this is one expression of pigment unevenness called Blaschko's lines.
Another case was that of Karen Keegan, who was also suspected of not being her children's biological mother, after DNA tests on her adult sons for a kidney transplant she needed seemed to show that she was not their mother.