TORTIE TOMCATS: CHIMERAS
1996 - 2008 Sarah Hartwell

It is possible for 2 fertilised eggs to fuse. If the two fertilized eggs (embryos) are both XY, this gives an XY/XY male chimaera. In an XY/XY chimaera if one X has the O gene and the other does not then some fur will be ginger and some black because different parts of the skin originate from different eggs! It could also be a mix of any other colours found in male cats e.g. a mosaic of grey and black. An XY/XY chimera would be fully fertile.

If an XY egg fuses with an XX egg (a male embryo fuses with a female embryo), it gives a XX/XY chimaera containing some tissues/organs which are genetically female and other tissues which are genetically male. In an XX/XY chimaera, X chromosome inactivtion will occur. The 3 X chromosomes in the animal may contain an assortment of different colour genes (especially if the mother mated several times and the eggs were fertilized by different fathers). The physical appearance and the sexual behaviour of an XX/XY chimaera depends on which structures contain which chromosomes. A good example of this is the now famous Maine Coon Solkatz Pretty Boy Floid who is a mix of red, grey and white - a colour combination impossible in a normal XY male.

Poldhu the fertile blue-cream-and-white male Cornish Rex from the 1950s may also have been an XY/XY chimaera; the main difference between Poldhu and Floid being that Poldhu had cream (dilute) instead of red (non-dilute).

The case of Skipper (Hokkaido, northern Japan) was documented by zoologist Jeremy Angel in the 1980s. Skipper originally came from Sapporo. Angel noted that perhaps one in every one or two hundred tortoiseshells is a tomcat and that these were once prized by superstitious boat owners. In 1981, at one year and three months old, Skipper was described as looking un-tomcat-like. In head shape and general conformation, he more closely resembled a female. His genitalia were unambiguously male. He was calico with clearly defined patches of orange and black and unusually for the Japanese cats of the area, he had a straight tail, not a bobtail. His mother was known to be a longhaired calico, but his father was not known. Skipper's character was described as exceptionally calm and friendly and extremely fastidious in toilet habits. He did not spray and did not show any interest when his mother came into oestrus. When introduced to other cats, including full toms, in an enclosed colony, Skipper was curious, nonchalant and non-aggressive - and again, did not spray.

It was noted that other tomcats treated him as a female and attempted to mount him, This was initially thought to be dominance behaviour towards the newcomer. However, Skipper remained attractive to several of the tomcats and had no objection to being mounted. On some occasions he appeared to enjoy this attention and remained crouched and receptive, even after the other tomcat had dismounted. He also adopted the lordosis position and chirruped to attract his suitors. During summer, he was mounted frequently enough that his neck became callused (common in highly active oestrus females). Skipper's attitude to females was very different. By the time he was almost 2 years old, he still did not spray and still showed no interest in mating with females. On occasion he was aggressive towards females and frequently fought with them. Tomcats do not normally fight with females, particularly with oestrus females although females will fight among themselves. In this respect, Skipper's behaviour was female. He was always the instigator of any fight.

In 1982, Skipper showed signs of male behaviour. He stopped picking fights with females and began to show sexual interest in them. He also began to spray. Compared to a normal tomcat, his spraying and his sexual behaviour were half-hearted. Angel isolated Skipper with an oestrus female. Although not an enthusiastic suitor, Skipper mated her three times, but failed to impregnate her. Angel had intended to mate Skipper with other females to determine whether the cat was fertile, but an epidemic of cat flu claimed Skipper as one of its victims. Angel concluded that either Skipper was a slow starter or that he possessed an additional X chromosome which feminized his behaviour, this being the prevalent theory at that time.

Because, at different times, Skipper showed both female and male behaviour, he was possibly an XX/XY chimaera (external male genitalia but some internal structures, including his brain, being genetically female and causing female behaviour). A post mortem, which might have solved the puzzle, was conducted when Skipper died. Although his condition was assumed to be due to an extra X chromosome (XXY, Klinefelter Syndrome), this does not normally result in female behaviour in phenotypical male. However feminisation was once believed to be the norm and in 1997 a cat owner stated "Most calico toms are born infertile with a single testicle. We are currently in the process of having a calico tom checked to see if he is fertile. If he is he will have some sperm 'banked' before being neutered. He is white, tan/orange tabby patches, and light to dark grey spots (and a few stripes)." The only male calico encountered by his/her vet was a long time ago and very obviously a genetic error, since the cat was sterile and had numerous other problems.

In a comparable case, Rachel E Gibson wrote in 1997 of a male calico which only mated with a very sexually aggressive females and which aligned himself with the females more than the males. The other males treated him like a female so he may not have many male hormones. His behaviour corresponded with the theory of the time that XXY made tortie tomcats feminine in their behaviour. Boo, another calico male, was also believed to be sexually confused because of his extra chromosome. He attempted to mate with both females and males and also tried to nurse kittens or carry them around like a mother cat. Boo continued to spray after being neutered and was described as somewhat fat, not uncommon in XXY cats.

Somatic mosaicism and chimerism aren't restricted to red or red-and-white cats; it's just that we are more geared up to looking for elusive fertile tortoiseshell males and tend to overlook the fact that some apparently normal tortoiseshell females may be due to genetic abnormalities. Mosaics which differ from the usual patterns are also noticeable. Also in Japan in the 1980s, there was an instance of a beautiful grey-black-white fertile male (Panda). Like Pretty Boy Floid, Panda (born 1978) was probably an XY/XY chimera (2 fused male embryos) as he had a mixture of dilute (grey) and non-dilute (black) in his coat. His father was a solid grey cat (Mo) and his mother was Aya (tabby-and-white). The colony into which he was born was inbred with more then 70% of the cats carrying Mo's genes. In addition to his odd colouration, Panda was born with a deformed tongue and could not suckle, so was hand-reared.

Some parts of Panda's body also appear to display a higher degree of white than others. The effects of the white spotting gene are discussed in Beautiful Bicolours - Tuxedo and Magpie Cats.This cat had normal male behaviour and was "very fertile". He would have bred as either a black or a blue (grey) cat depending on which embryo's cells hade formed his testicular tissue. The pelt diagram, based on a number of photographs (therefore not 100% accurate), gives some idea of his mosaicism.

In many cats, these forms of mosaicism will never be noticed because they naturally have black somewhere in their coloration; when the black is quite unexpected and defies logic then mosaicism is assumed.

Until recently it was believed that chimaeras were normally only produced in the laboratory and usually involved chimaeras made from different, but closely related, species. The geep is an interspecies chimaera mixing a sheep and a goat. Unlike a sheep/goat hybrid, the sheep/goat chimaera did not look like something halfway between the two species. The sheep and goat tissues retained their individual appearance, creating an animal with hairy goat-like legs and a woolly sheep-like body. Rat/mouse chimaeras have also been made. Increasing use of genetic tests on anomalous animals (e.g. those with genetically impossible colours, ambiguous gender) is revealing a surprising number of natural chimeras in the cat population and may account for more tortoiseshell males than previously thought. For example, chimerism is found in cattle. When a cow has twins, they almost always share a circulatory system. Blood stem cells from one embryo end up in the other embryo and both twins end up as chimaeras (microchimeras).

Chimeric semi-identical twins (reported in Nature in March 2007) are identical on their mother's side, but non-identical on their father's side. This happens when 2 sperm fertilise a single egg, which then divides to form two embryos. Each sperm contributes genes to each offspring. If one sperm is X and the other is Y, the 2 offspring may be chimeras with a mix of XX and XY cells. It is generally believed that double-fertilised eggs rarely survive, but it is also possible that chimeras resulting from double-fertilisation go unnoticed unless they have ambiguous genitalia or exhibit mosaicism. Normally, twins are classified as fraternal (2 separate eggs fertilised by 2 sperm) or identical (mono-zygotic - single fertilised egg develops into 2 identical offspring; essentially natural cloning).

Possibly the most famous tortoiseshell male cat is Solkatz Pretty Boy Floid, a pedigree Maine Coon, born Nov 1996 in Bremerton, Washington, USA. Floid is a triple genetic anomaly: calico male, mixed dilute and non-dilute colours and fertile! Floid was initially thought to be a dilute calico male (blue, cream and white) which in itself is unusual enough. However, the cream portions of his coat turned out to be red i.e a non-dilute colour. This is also very unusual as the dilution gene which gave him the blue (grey) colour should have converted all red portions into cream. Finally, Floid is fertile. Floid is a chimaera - the result of two fertilized eggs merging into a single embryo. On a genetic level, different portions of his coat (of his whole body in fact) actually belong to different cats. Although a very nice Maine Coon and the sire of some excellent offspring (none of which are dilute, therefore his testicular tissue is from non-dilute-carrying cells which also formed the red patches), Floid himself was unable to win titles because there are not colour classes for tortoiseshell male cats.

On 23 Feb 2008, Lorna Bowles was catching and neutering some abandoned cats including several kittens. One of the male kittens, Prince, is a black-and-white with small grey patches on both hind legs. Lorna has provided these photos.

In mammals, chimaeras are usually only identified when there is a mismatch between the chromosomal sex (XX or XY) and the genitalia. This can come about through investigation of an illness or through visual clues such as tortoiseshell coats in male cats.

In an experiment done with white mouse embryos, cancerous mouse cells were injected into the embryo. During foetal growth the cancer was somehow tamed and the effect of the cancerous cells was to produce black patches in the otherwise white mice (uninheritable since they were cell mutations, not inherited through genetics).

Chimaeric cattle are not at all rare. When a cow has non-identical twins, the circulatory systems of the two embryos almost always joins together. Blood stem cells from one embryo end up in the other embryo. Both animals end up as chimaeras (microchimaeras). Where one is female and the other is male, the female embryo is exposed to hormones from the male. This causes the female embryo to be masculinised. These are called freemartins. The look female and have infantile external genitalia with underdeveloped internal sex organs, but are infertile. Freemartins occur in other species, but less often. It is a a peculiarity of cattle that the blood supplies of non-identical twin embryos join together.

Human chimaeras also occur. A boy born in 1994 (and reported in 1998) turned out to be genetically two people. Two fertilised eggs had fused into a single embryo. One egg was XX (female) and the other was XY (male), making some parts of his body genetically female while other parts were genetically male. He was outwardly male, but had a partial womb, fallopian tube and ovarian tissue. Others are less easy to detect, such as a woman who had some XY (male) blood cells. This can happen where non-identical twins share a blood supply in the womb and blood stem cells can migrate from one to the other.

Another human chimaera apparently occurred when an unfertilised egg divided into two. One of the cells was fertilised, the other wasn't. The unfertilised cell contained only the mother's chromosomes, which it duplicated. The fertilisation of the other cell meant the embryo could develop normally and a single XX/XY embryo was formed. Some of the boy's tissues contain only the maternal chromosomes while other tissues contain maternal and paternal chromosomes. A similar scenario occurs when a fertilized egg fuses with an unfertilized egg. In both cases, some tissues contain only maternal DNA and the cause was once believed to be parthenogenesis (the tissues tested were those derived from the unfertilized egg/cell).

In 2003, DNA tests (for kidney transplants) showed that a Boston, USA woman known as "Jane" was not the biological parent of two of her three naturally conceived sons. Her sons underwent blood tests to see if they were compatible kidney donors, but the results showed that two of them did not have any of Jane's DNA, although their father was Jane's husband and they did have his DNA. Initially, doctors thought Jane had undergone fertility treatment using donated eggs or her sister as a surrogate mother. However, Jane's ovaries were found to contained two different sets of genes. One of her sons came from one set of cells, while the other two were derived from the other set of cells. Jane is a tetragametic chimera , one of only 30 reported cases, and is made up of 2 genetically different populations of cells. This occurs when non-identical twin embryos, both female in Jane's case, fuse into a single embryo in her mother's womb. Jane's body contains 2 distinct types of DNA: "her own" cells and "her twin's" cells. Cells from one twin dominate in her blood, but her thyroid gland, mouth and hair carry the other twin's DNA, while cells of both types coexist amicably in her ovaries. Jane's case was reported in The New England Journal of Medicine (Vol 346).

Nature reported a similar case in 1979, when genetic tests suggested that a woman could not be the mother of any of her four children. Again, there were no previous suspicions that the woman was a chimera. In 1978 or 1979 a British tabloid newspaper carried a front page story of a woman who "carried her dead twin in her bloodstream". Although sensationalised, this was another instance of chimerism. The report carried several sensational claims including feeling the lost twin moving within her and blaming it for mood swings! In a case where chimerism is not yet confirmed, doubts were cast on a child's maternity in court during 2003 when a woman sued her partner for child support. Tissue-typing tests have shown that the partner is the father of the child, but that the woman is not the mother. Her case may prove similar to that of Jane.

The incidence of human tetragametic chimerism may be on the increase due to modern fertility treatments that cause women to release multiple eggs or that implant multiple embryos in the hope that some will grow. In 1998, the University of Edinburgh, UK reported a chimeric IVF baby resulting from the fusion of a male and a female embryo (The New England Journal of Medicine, Vol 338). Though outwardly male, the left hand side of his internal reproductive system had developed as female (presumably the child born in 1994 and noted earlier).

Although most of the cases cited above are females, it is possible for a man to be a chimera and for his blood and his testes to contain different DNA, confounding the results of paternity tests on his offspring.

Around 30 similar instances of human chimerism have been reported. Some are identified because they have sought fertility treatment or they required DNA testing or tissue matching. It is not known how common tetragametic chimerism is in the population since there are often no outward signs, especially where same sex embryos have fused (a few might have visible pigmentation mosaicism). One girl was discovered to be a chimera because she had one brown eye and one hazel eye. Others are detected when doctors investigate reproductive problems. A few might demand paternity tests, for example where a child has "impossible" characteristics such as a brown-eyed child born to blue-eyed parents. Many, if not most, chimeras will never discover their unusual origins and will go through life unaware of their condition.

There is also increasing evidence that most people are microchimeras i.e. contain cells from their mothers and older siblings. During pregnancy, the blood of the mother and foetus are kept separate, but the placenta is an imperfect barrier and some cells slip through. Children pick up some cells from their mother and she picks up some foetal cells. 80-90% of women carry their children's cells or DNA in their blood during pregnancy and up to 50% carry them for decades after giving birth. Some of those cells may slip from the mother's bloodstream into the body of the next child. Some cells may even pass from her children into her grandchildren. For some reason, our immune systems tolerate these cells in the blood and it may be an essential part of preventing rejection of the foetus. Some of those maternal cells may become incorporated into the body tissues of the foetus (where they may be associated with autoimmune disease although no firm cause-and-effect has been established).