Gerbil Genetics ~ Section 6
The Loci in gerbils
A locus
The first gerbils in captivity descended from 20 pairs that were caught in the 1935 by Dr. C.Kasuga in the basin of the Amur River in Eastern Mongolia. These gerbils had 'agouti' or wild coloured coats. These gerbils would have all been carrying the normal amount of dominant genes at all the loci and can be described as 'AACCDDEEGGPP'. Geneticists would often use '++' to denote the normal wild colour agouti, however our domestic 'Golden agouti' gerbils are more commonly referred to as 'AA' when homozygous, but most Golden Agoutis we come across will most likely carry recessive genes at the various known loci, because since their introduction several mutations have occurred. We know that because of a recessive mutation at the Agouti locus that not only can they now be 'AA', but they can also just as easily be 'Aa' or in the case of the black gerbil (non-agouti) mutation, they can be 'aa'. When we are uncertain of the ancestry of our domestic golden agouti gerbils we tend to use the notation 'A-C-D-E-G-P-' because until we breed the animal we are unsure of which recessive genes that it carries, so will use a dash, dot or an asterisk as shorthand notation where we are unsure of the gene at each particular locus.
It wasn't until 1971, nearly 30 years later after its introduction into captivity that the non-agouti mutation appeared in gerbils. At this time only a few mutations were known, which were the Dominant spotting mutation and the Himalayan mutation.
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C locus
The Chinchilla or the Albino locus as it is sometimes called is responsible for the overall level of colour produced in the gerbil's coat. It effectively controls a crucial piece of the pathway by which pigment is formed within cells. It is mutations occurring on this locus which are responsible for two of the most commonly occurring mutation in animals, the first being that of a white animal with pink eyes or as it is commonly known, the albino, the second common mutation is the chinchilla mutation, this is where the black pigment is produced in the coat and body, but the yellow pigment is removed. This mutation is very variable amongst species, both in the intensity of the black pigment, and also the amount of yellow left in the coat. Both are recessive mutations occurring at the C locus. However neither of them has appeared in gerbils. White gerbils with pink eyes do exist but are the result of a combination of diluting genes and are known as "pseudo albinos" but genes at the C locus are key factors in these combinations.
A true albino gerbil would be 'cc', but as of yet none have appeared.
The was however a report of albino gerbils in this scientific paper, Matsuzaki, T., Yasuda, Y. & Nonaka, S. 1989. The genetics of coat colors in the Mongolian gerbil (Meriones unguiculatus). Experimental Animals, 38, 337-341.
The paper is quite controversial and presents three loci in gerbils for discussion, these being the A-locus, B-locus and C-locus. The gerbil colony under investigation was well established and a total of 1855 gerbils were bred between over a 39 year period, from 1949-1988. During this period they introduced four white coated gerbils that were discovered in a pet shop in Yokohama. These white gerbils were proven to be carrying the black mutation. Further breeding showed that both white and black were recessive traits in the gerbils. The authors of the paper then went on to say that this was the first report of a black coat colour mutation which was incorrect because black gerbils had previously been described by Cramlet et al. In 1974 and were further analysed by Waring et al in 1980. The authors of the paper then also claimed that the white gerbils under investigation were cc or albino and not the DTW acromelanistic gerbils as described previously by Robinson in 1973. The authors of the paper as well as reporting these white gerbils as albinos, also hypothesised on a brown locus that controlled the amount of melanin in the black gerbils they studied, but as we now know, no brown locus has ever appeared in gerbils.
However two mutations do occur on the C locus in gerbils, these being c(h) or Himalayan and c(chm) or Chinchilla Medium. Himalayan is an old mutation in gerbils and first appeared in the 1960's; Chinchilla Medium appeared much later in 1994. It received the name of chinchilla medium because of its close similarity with the mutation of that name in Rabbits when displayed on an Agouti coat colour. It should be noted though that it's old name was "Burmese" as it was instrumental in producing the Burmese coat colour variety in gerbils, however, the effect of this gene is dissimilar from the mutation that causes the Burmese coat in cats. Both of these mutations are recessive in nature and both are acromelanic (acromelanism literally means "coloured ends") and they are both temperature sensitive genes.
In true albino animals the pigment pathway is totally broken and this leads to the animal being unable to produce pigment anywhere in the body, which in turn leads to white fur, pink eyes and translucent nails. In gerbils however, the mechanism which produces the pigments in c(h) and c(chm) type gerbils is very fragile and temperature sensitive, so wherever the temperature is too high, little or no pigments are produced. This is the reason why the colourpoint gerbils have full colour in the cooler regions of the body such as the ears, nose, feet and tails, when compared to the lighter colouring at the warmer regions of the body. The Himalayan mutation is a more pronounced form of this gene, being more severe than c(chm) in its effect, and it is only the tail and very rarely the ears that show any pigmentation.
There is also another factor that can further modify a gerbils coat colour at the C locus, and this is known as Dominance modification. This effect occurs with the recessive genes at the C and P locus. The normal dominance hierarchy of alleles at the C locus are modified when there are two recessive alleles at the P locus ('pp'). Rather than the full colour expression C being completely dominant over the lesser alleles, it becomes incompletely dominant, allowing the c(h) and c(chm) alleles to be expressed. This creates paler forms of the 'A- CCpp' (Argente Golden). These being 'A-Cc(h)pp' (Argente Cream) and 'A-Cc(chm)pp' (Argente fawn or Topaz). The same coat colour effect occurs on a non-agouti (Black) background, where Lilac ('aaCCpp') becomes Dove ('aaCc(h)pp') or Sapphire ('aaCc(chm)pp'). The Sapphire shade is in-between Lilac and Dove in colour.
A similar effect to this also occurs with the C alleles when combined with 'ee', where c(h) and c(chm) effectively lighten the coat producing lighter versions of DEH's and Nutmegs.
Also as a quick aside, A gerbil that carries two copies of either c(h) or c(chm) along with pp, will always be white all over with pink eyes regardless of the other genes carried. However if a gerbil is c(h)c(h)PP or c(h)c(h)Pp (We know the presence of at least one P gene stops one c(h) gene having any effect) a pair of c(h)c(h) genes creates the Dark tailed white gerbil. The presence of the dominant dark eyed genes doesn't produce a dark eyed gerbil as we might expect but then again the pair of c(h) genes doesn't produce a completely white gerbil either.
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D locus
The mutation in this locus occurred recently in 1997 and was allocated the symbol "d" by geneticists. The mutation is recessive in nature, and homozygous gerbils have less intense or diluted fur. The reason for this is that the gene causes the pigment granules to clump in the hair shaft, and as a result of this, light is reflected in a different way off the hair. In gerbils this gene seems to dilute black pigments much more effectively than yellow pigments, and like in other domestic species, such as mice and rabbits, etc, it produces the well known coat colour Blue ('aadd') which is in essence a modified black coat colour variety.
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