Gerbil Genetics ~ Section 6 (part 2)
The Loci in gerbils
E locus
The E locus mutations in gerbils are quite interesting. It is named E or "extension" locus as mutations here cause the extension of the yellow pigments or conversely the black pigments in the coat. There are a range of mutations at the E locus in other domestic animal species, and the mutations work alongside the agouti locus to change the length of the yellow banding in the hair shaft. The mutations that are dominant to the wild type or agouti will tend to make the animal completely black whereas recessive mutations at this locus extend the yellow pigment in the hair shaft, sometimes covering all, or almost all of the hair. There also exists a third type of mutation at the E locus that is recessive in nature and quite unusual in its actions and is responsible for harlequin or Japanese brindle coat colours in rabbits and tortoiseshell or brindle coat colours in cavies.
In Gerbils two mutations are known at this time at the E locus, both being recessive in nature. The first mutation 'e' appeared in the U.S.A in the mid 1980's but its appearance here was very poorly documented. Its effect on the Golden Agouti coat brings about a yellow looking gerbil, with light ticking. However because the mutation only changes the way that pigment is produced in the hair shaft and not elsewhere, both the skin and the eyes will retain their dark pigments.
Now we know that 'AAee' effectively produces our Dark-Eyed Honey coat colour, but its effects are quite unusual when displayed on a non-agouti background. In many other animal species 'aaee' produces a yellow coat, the 'ee' effectively masks 'A' and 'a', so 'AAee' and 'aaee' will be virtually the same colour. This effect can be shown in animals such as mice, cavies, Syrian hamsters and even horses. In other species such as the rabbit the 'e' gene is incapable of removing all the extra pigment produced by the non agouti, so we get the sooty yellow coat colour ('aaee'), which leaves black pigment around the muzzle, eyes, ears and stomach. In gerbils a very unusual effect happens and that is 'ee' effectively masks 'aa' when they are juvenile, and the pups are a deep orange/yellow colour, but as the gerbil moults and becomes adult this masking no longer takes place and the result is a nutmeg coat colour. This coat colour looks similar to a very dark Agouti self, i.e. it lacks the white belly.
There is also another unusual effect with the 'e' mutation that has to do with ageing. As I've just explained with the Nutmeg coat colour, these start off as pups as a deep orange/yellow coat colour, but as they moult to adults, their coat colour changes dramatically and long black tips are apparent on the hair shaft which gives the gerbil a very dark ticked appearance. With Dark-Eyed Honeys there are more subtle coat changes occurring, and as the animal ages, the point at which the white bellow shades into the yellow top coat colour gets progressively higher up the sides of the gerbil. Older gerbils can sometimes appear to have a stripe of the original colour, with lighter colour down their flanks.
The second mutation at the E locus in gerbils is 'ef' the 'f' being short for fading. Again this further mutation occured in the U.S.A around 1980 but again its true origins were very poorly documented. Their common name is Schimmel, a German word meaning "to moult" or "to mould" Both 'AAe(f)e(f)' and 'aae(f)e(f)' gerbils are yellow, but with each successive moult the coat colour fades eventually to an off white colour and only pigment remains on the tail and nose regions. The mutation itself is recessive to 'e', but the extension of yellow mutation isn't completely dominant over 'e(f)'. This effectively means that a gerbil that is 'ee(f)' will tend to be a slightly lighter shade than one that is 'ee'.
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G locus
This gene first appeared in a London pet shop in 1975 but unfortunately the line died out, however it re-appeared a couple of years later and is now common throughout Europe, but is still regarded as uncommon in the U.S.A and other parts of the world.
Geneticists first thought that this was the well known chinchilla mutation on the C locus, but through experimental crosses it was determined that this wasn't the case and it is in fact a mimicking gene, producing very similar effects to the chinchilla mutation. It was later designated the 'g' symbol. The effect of the gene drastically reduces the yellow pigments in the gerbil's coat, and the black pigment granules do not form properly, the net result on a Golden Agouti background is our Grey Agouti coat colouring.
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P locus
In domestic animals the Pink-eyed dilution mutation is a well known and very much established mutation. In fancy mice its origins are ancient and are believed to have occurred first in Japanese wild mice (Mus musculus molossinus). The gene itself was incorporated into several common laboratory strains of mice during the early part of the last century. In Mongolian gerbils however this mutation occurred much later and the first pink-eyed dilute mutant occurred in 1977 in a North London school. Like most of the other gerbil mutations it is recessive in nature. On an agouti coat this gene produces the well known Argente Golden coat colour. The black pigment in the coat is greatly reduced and the result is a rich golden colour. The eye pigment is reduced and becomes a ruby colour. On a non-agouti ('aa') coat it produces the Lilac coat colour variety.
The pink-eyed dilution mutation effectively creates a poor cellular environment for pigment synthesis. It is known that an acid environment is needed to produce the correct quantity of pigments within a cell. However when they are in a pH neutral environment the pigment cells produces very little dark pigments, but allow the production of yellow pigments, this is effectively what the P locus mutation does, it alters the pH of the pigment cell allowing conditions that favour the production of yellow pigments.
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For more detailed information about the P locus see here
The Spotting Locus
The Spotting mutation was one of the earliest mutations occurring in gerbils and appeared in the U.S.A in a litter of gerbils at the Peterson Hamstery in the late 1960's. The gene is dominant in nature and can occur in any known coat colour variety.
As mentioned earlier, the spotting gene is best dealt separately from other genes as it works as a pattern that is overlaid onto the known gerbil colours, and patterning variants such as collared, mottled and variegated are thought to be achieved by modifying genes that work on the dominant spotting gene. The exact shape, size and patterning depend on these modifying genes, but it can also depend on many other factors, including luck! This effectively means that no two spotted gerbils will be alike, even with the same genetics they will still have some variation in the white patterning. It also appears that when a gerbil isn't spotted, but has the modifying genes for spotting,(i.e. a solid coated pup from a spotted litter) that any white patches present on the paws or under the chin, such as we can see on self coloured gerbils, will be exaggerated.
One other feature of the spotting gene is that it will dilute the base coat colouring, and the more extensive the white markings are, the more the base colour will be diluted. Collared gerbils tend to be more diluted than spotted animals and in cases where the white markings are extensive, the remaining patches of colour can often be intermingled with smaller patches of lighter and darker shades, this is seen in mottled and variegated coat colours.
It is possible that all these variants could be due to several further mutations at the spotting locus, or even involve a different locus, however this theory would be very difficult to prove and involve an extensive breeding programme with white spotted gerbils to either prove or disprove the theory. My own interpretations of the gene are that multiple mutations are not involved to explain the variations in the coat colours that we see when we breed dominant spot type gerbils.
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For more detailed information about spotting see,
Further Notes On Spotting Genetics