Spotting Genetics

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A new Spotting Mutation in the Mongolian Gerbil?

For almost 40 years, a form of Dominant Spotting has been known and bred for in the Mongolian Gerbil (Meriones unguiculatus). The homozygous form is prenatal lethal, the homozygotes die in uteri of anaemia. Heterozygotes also suffer from a very mild form of anaemia but is so slight that they are not impaired by that condition.

The Dominant Spotting, or Sp for short, is bred in many variants today: The colour to white ratio of the fur can vary a great deal, from just a little white spot on the forehead to an almost completely white animal with very few patches of a diluted base colour. Aside from Sp, no other spotting gene has appeared in the Mongolian Gerbil until now

For some years, some breeders from the German-speaking part of Europe started to extend the white markings on the fur of spotted animals. Such animals there are named "Starkschecken" ( above 50% white) and "Superschecken" (above 80% white). They cannot be compared to the English "Mottled" coat colour, as the term refers only to the amount of white present, and not to the distribution of the remaining colour in the fur, which is very important in the Mottled coat colour variety.

It was around 2005 that research showed some extreme white gerbils were appearing in Germany that were previously unrecognized five years ago. These extremely white spotted animals appeared initially in Swiss stock and also in Czech stocks, but are now also present in Austria and Germany. These animals have always had more than 85% white, and many above 90%. The base colour is hard to recognize, as it is very diluted and only expressed at a few bodily parts, mostly ears, back, and tail. These animals will be referred to as "Extreme White", abbreviated to EW in this text.

While outcrossing these EW gerbils to unspotted gerbils of mixed ancestry, some were fancy gerbils, some were descended from wild catches, we got some strange looking pups appearing in litters. They are not spotted, but show extended white markings from four white feet up, to small white patches on the forehead and neck as well as belly spots. Some are reminiscent of Dominant spotted patched gerbils, but are missing the white tail tip. The single neck spot is not at all typical for Dominant Spotting. We call such gerbils "White Paws", and as such are clearly visible in both A- and aa type gerbils.

When two of these "Superschecken" or a "Superschecke" x Whitepaw were crossed, we observed almost depigmented to completely depigmented pups with black eyes in the F1 generation. If they had any remaining colour, it wasn't reminiscent of the typical Spsp - pattern, instead there was a single plain spot, undiluted and intense, with clear, sharp edges. Each of these "Rumpblack" and Pure White pups died between the time shortly before opening of the eyes and weaning, from 2-4 weeks of age. Shortly before dying, all of them had bloated stomachs. One of the pups was euthanised by a vet, who diagnosed intestinal obstruction.

Furthermore it is remarkable that in all of the author's pairings in which EW take part in, beside "Whitepaws" and "Pure White", only normal solids, EW and Collared varieties with little or only few markings on their back are born. No "Starkschecken" (50%-80% amount of white) are born, only Collared or EW. All attempts to reduce the amount of white during breeding the EW proved to be unsuccessful; the great amount of white seems to be very dominant.

The pairing of two Whitepaws brought again a pure white, "rumpblack" aa type pup with undiluted colour around its tail, (In the second case, a Burmese) very similar to the first rumpblack (see the exteme white spotting colour galleries for examples). So any "interference" from Dominant Spotting was not possible in this pairing.

We assume that the Whitepaws could be the heterozygous carriers of a new, and yet to be identified spotting gene in the Mongolian gerbil. Unlike Dominant spotting, the homozygotes are born alive, but die at weaning or earlier of megacolon. So this new spotting appears to be semi-lethal. This pathology - the megacolon as a side effect of an homozygous spotting mutation - has already been described in other species (e.g. Overo in horses).

The EW gerbils seem to be caused by the interaction of two different spotting mutations, the long known Dominant White Spot (Sp), and the new spotting mutation, which is still to be identified. The "real" intensity of the spotting of the authors EW gerbils may be only centred around collared varieties (depends on the actual breeding line, of course!), but combined with the heterozygous new spotting, the amount of white rises dramatically. Such interactions of two spotting mutations have been observed before in Mus musculus.

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

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

Breeders shouldn't cross EW into existing, intact spotted breeding lines, also it should be disregarded to cross in whitepaws, even if they are believed to be mottled carriers (though this is true to a certain extent, that the two mutations seem to interact, and this interaction influences the amount of white a great extent).

Further health problems with EW

The EW gerbils show other health problems too. One third of the author's EW population suffers from deafness and chronically tilted head; these symptoms started to appear on sexual maturity in all affected gerbils. No diseases that could have caused such symptoms could be observed. The most extreme health problems observed were loss of balance and repetitive, circling movements similar to dancing mice (see video below).






In most cases these health problems correlate with a lack of pigmentation of the ears, all of the author's gerbils with white auriculae show deficits of various severity, though these problems were also be observed in a few animals with coloured auriculae but lesser amounts of white in the fur. Breeders are warned not to breed on EW gerbils or any other carriers of the new spotting (Whitepaws). The new mutation is not yet identified, but reminiscent of Ednrb-zero-mutations in many other species. It seems not to be conductive to the sound breeding of healthy Mongolian Gerbils, because only the heterozygous spsp - animals (Whitepaws) seem not to be impaired. Due to the semi-lethal 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 is not recommended, and existing breeds in the experimental state are not allowed to give out any carriers. The true origin of the mutation remains unclear; as written before Pure White, Extreme White and Whitepaws can be tracked down to 2002 in Germany; the Pure Whites could not be bred successfully then (though it was extensively tried), thus supporting the semi-lethality of this mutation. Further danger lies in confusing the Whitepaws with patched gerbils or indeed mottled carriers. During three generations of breeding, the white marks on the author's White pawed gerbils have remarkably increased, so unexperienced breeders may not note the differences between Whitepaw and Patched or a mottled carrier. A well-meant attempt to reduce spotting intensity through breeding in a "patched" gerbil (if it is actually a mistaken Whitepaw) may so lead to Pure-White homozygotes.

Article by Kira Gysel

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