Further Notes on Spotting Genetics

Contents

• Small litters in spotted x spotted crosses?
• Homozygous and still born pups ?
• Spsp & spsp ?
• Inbreeding to produce mottled gerbils
• A new spotting mutation in the Mongolian gerbil?

Small litters in spotted x spotted crosses?

On a lot of prominent gerbil websites they report that breeding two spotted gerbils together will produce smaller litters because the gerbils that would be SpSp never come to exist in the womb. This information is wrong, and needs amending. Now although the gene is broadly described as pre-natal lethal, this being because many of the homozygous embryos are reabsorbed in the womb, it doesn't result in smaller litters, because the homozygous embryos that are absorbed are simply replaced by new viable embryos. This is why we very often see large litters in spotted gerbils. A spotted to spotted litter should NOT result in smaller litters. If you are experiencing this, look to how closely you are breeding your gerbil lines, as even line breeding can over time reduce the fertility of an animal, and cause fewer and smaller litters, and perhaps think of introducing new blood to these lines would correct this problem.

Homozygous and still born pups?

Some breeders have also noticed an occasional stillborn pup in spotted litters. Now In theory, SpSp gerbils would be black-eyed whites. However, the abnormality in the development of the neural crest that leads to spotting and anaemia in Sp+ gerbils will be far more pronounced in a homozygous individual. Lethal spotting in many domestic species leads to animals that are homozygous for spotting but tend to suffer from deformed skulls and jaws, absent or malformed eyes, deafness, and severe neurological problems as well. Death occurs at some point in development due to inability to eat, nurse or through complications such as megacolon.

Tests on these stillborn pups were conducted around 1978 by Alicia Waring, Timothy Poole, and Timothy Perper on early spotting mutants, where it was noted in their published paper that even though the mutation fits the data for pre-natal lethal, ie a 2 to 1 ratio, there were a number of stillborn young found in SpxSp matings. Because the researchers noticed that both phenotype and breeding data were similar to dominant spotting and steel loci in mice, and that homozygous mice at those mouse loci mentioned produce Black eyed whites, they decided to test skin samples on these stillborn gerbils under the microscope to check the pigment structure of the hair shaft. It was noted that the hair follicles were lacking in pigment granules, which suggested that had they lived they would have been black eyed white gerbils. In the paper it went on to say that the severity of anaemia caused by different spotting mutant alleles determines whether homozygotes die during gestation or live upto a few weeks or even several months. In heterozygous spotted gerbils only very slight anaemia is detected, so slight it doesn't affect health or reproduction. So it seems that homozygous spotted gerbils probably have such severe anaemia that it can cause their demise in the pre-natal condition.

So although we may argue then that Dominant spotting in gerbils isn't really a true dominant but intermediary as we shouldn't be seeing black eyed white stillborn pups, we have to realize that dominant is a relative term as in "X is dominant to Y" and Mendelian genetics is often taught as if genes are simply dominant or recessive. In reality things are far more complex. In the case of dominant spotting it as an incomplete dominant in that Sp is only partially dominant over the wild type. In practice however, this matters little to the breeder, as SpSp is most often pre-natal lethal, so the semi-dominant phenotypes appear to be a normal dominant. This abnormality in homozygous spotted animals (which are usually black eyed white coated animals) is a very common occurrence across a wide range of species, and isn't specific to the Mongolian gerbil.

Spsp & spsp?

Scientist label the gene that cause spotting in gerbils as Sp, and haven't as yet designated a name to the normal wild-type gene that non-spotted gerbils have. The normal wild gerbil is ++ at the spotting locus, and in contrast the spotted gerbil is designated as Sp+. So technically it is incorrect labeling a gerbil as Spsp or spsp. However in practice it is easier, and makes as much sense to just label a spotted gerbil as Sp and leave the locus blank for a non-spotted gerbil.

Inbreeding to produce mottled gerbils

Personally speaking, I have been breeding good mottled coat colours for nearly a decade and have never had a problem with them health wise, and they are perhaps the most placid and well-tempered of all my breeding lines. I do try to stress that the mottled coat colour isn't really a good choice for novice breeders, and that a large number of breeding lines are always needed to produce a good mottled gerbil. I find once a particular breeding line has the desired type of coat i.e. an even mottling throughout the coat, and replacing the white collar with mottling, then new blood should be introduced from solid coloured coat lines. These can be in the form of unrelated solid coat gerbils, or gerbils carrying the normal spotting modifiers. I found out that in my own breeding lines that unlike a collared X collared cross, mottled X mottled, very rarely produced the best mottled gerbils, but mottled X mottled modifier, mottled X unrelated solid colour, and mottled modifier X unrelated solid colour, consistently produced mottled gerbils with exceptional, and very well placed markings. The trick then is keeping the mottling nice and even in future generations. Inbreeding need not be used to achieve these ends, but instead rely on a lot more breeding pairs than you would when attempting to improve a normal coat coloured variety. Inbreeding, if practiced, shouldn't be attempted for more than one or two generations before out-crossing, as quite often a lack of good size, and a reduction of fertility can easily be the result. When mottled gerbil breeding lines are established, the goal shouldn't be for just more and more white, as the unique mottling of the coat will be quickly lost, but rather aim for an even placing of the mottling throughout the coat, with no areas of just pure white. Seeing as the placing of white markings can just as easily be the result of good luck too, inbreeding to fix traits or markings in this coat colour variety can be a moot point, and a better result can be achieved with good line breeding and out-crossing to solid lines to introduce further good traits, such as size, temperament, health, or even a recessive coat colour suspected to have an effect on extending white markings.

A new Spotting Mutation in the Mongolian Gerbil?

Around 2005 gerbils regularly started appearing in Germany, and especially in Swiss and Czech stock, that had white in excess of 85% on their coat. The base colour of these gerbils are hard to recognize, and the colour is expressed in only a few regions of the body (usually ears, back and tail), and appears very diluted. They are referred to as "Extreme white"gerbils. The heterozygous carriers of these gerbils are known as "White paws" and have extremely pronounced bibs and socks and often a faint head and/or neck spot. Unlike Dominant spotting in gerbils, the homozygotes are born alive, but die most often at weaning or earlier from megacolon. The mutation is being described as semi-lethal in nature. The extreme white coat colour seems to be produced by the combination of two spotting genes, the Dominant spot and as yet to be identified new spotting mutation. Even a simple cross with a normal dominant spot gerbil (non-mottled) and the new heterozygous spotting gene will produce extreme white gerbils. Such interactions of two spotting mutations isn't new, and was the method first used by the mouse fancy when combining two genetically different types of spotting genes when developing the black eyed white mouse.

The following genotype is assumed for different patterns of spotting, based on test pairings:

+/+ ++ normal unspotted gerbil
+/+ Sp+ Spotted gerbil with normal Sp - pattern, such as collared or patched
X/+ ++ "Whitepaw"
X/+ Sp+ Extreme White (EW)
XX ++ "Rumpblack", pure white gerbil with single undiluted colour spot ->Semi-lethal
XX Sp+ Completely White -> Semi-lethal
 

Examples of the the rumpblack & black eyed white coat phenotypes, can be seen on the Extreme white spotting page.

Extreme spotting

The Gallery on Dominant spotting can be found here,

Dominant spotting

The extreme white coat colour can be associated with health problems, including deafness, chronically tilted head, and spinning or dancing behaviour similar to dancing mice. These health problems are known to correlate with ear pigmentation.

The gene itself is very reminiscent of Ednrb-zero mutations in other species, and it may turn out that it is feasible only to develop the heterozygous "white paws", and discontinue their breeding lines of Extreme White coat colours. Due to the semilethal nature of the new mutation and because it supports the EW pattern, the "Deutscher Rennmauszuchtverein e.V" (German Gerbil Breeders Club) classifies EW and the heterozygous Whitepaws as a problem breed. Breeding of the new spotting gene isn'tt recommended, and existing examples of this breed in the experimental state are not allowed to give out any carriers of the coat colour.

The true origin of this mutation remains unclear; as mentioned earlier, Pure White, Extreme White and Whitepaws can be tracked down to around 2002 in Germany. My own theory of this possible new gene appearing is that after a second introduction of wild Mongolian gerbils arrived in western Europe around 1995 to the Leibnitz institute for study, many breeders spotted lines were out-crossed to wild types to strengthen their stock. This was needed as many of the spotted varieties had been extensively inbred in an attempt to achieve a mottled type coat colour. Many wild type gerbil species often exhibit recessive spotting or similar, including Shaws, Tristrams, and Sundevall jirds. So it's not unlikely that some form of spotting may have existed within this new population of Mongolian gerbils, which was very different from dominant spotting, and possibly introduced via this route.

Sources

COMPARATIVE GENETICS OF COAT COLOUR IN MAMMALS by A G Searle

White spotting in the Mongolian gerbil. Waring, A.D., Poole, T.W. & Perper, T. 1978. The Journal of Heredity, 69, 347-349.

A new Spotting Mutation in the Mongolian Gerbil? By Kira Gysel

Article by Eddie Cope

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