Mutations in Gerbil Coat Colour ~Part 2

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Underwhite Locus (Uw locus)

The first mutation on this locus appeared in a London pet shop in 1975 but unfortunately this line died out.  It then reappeared a couple of years later, succesfully bred, and is now common throughout Europe, but is still regarded as relatively uncommon in the U.S.A and some other parts of the world.

Geneticists first thought that this was the well known chinchilla mutation allied to the C locus.  The Chinchilla mutation produces a grey animal by drastically reducing the yellow pigments in the fur.   Through experimental crosses (Allelic complementation test breeding) it was determined that this wasn't the case and the unknown allele mimicked Chinchilla. It was tentatively designated the 'g' symbol, because it was dissimilar to other grey mutations known at the time.  The effect of the gene dilutes the yellow pigments in the gerbil's coat to a cream colour, and the black pigment are slightly diluted to a slate colour, the net result on an Agouti background is the Grey Agouti coat colour. On non-agouti it produces the Slate coat colour. (Leiper & Robinson, 1985)

The second allele at this locus appeared in 2000. Michelle Inman, an Illinois (USA) breeder began noticing that her gerbil litters were breaking the normal rules of gerbil genetics.  At the time, it was quite confusing when breeding with this particular allele. This was due to the fact that when when homozygous, gerbils that were known to be genetically P-, or dark-eyed, were being born with ruby eyes and cream coloured fur.   Michelle turned to other breeders, gerbil societies, well known gerbil geneticists, and even labs in an attempt to find out what was causing this, but received no valid answers.  After eight years of breeding with this gene, her breeding intuition was telling her the gene was allied to the 'G' locus, however she still had no clear idea which gene could cause these effects.  Close to giving up, she discovered eGerbil, and after carefully studying her breeding records we performed some simple breeding tests to prove her theories.   Allelic breeding studies were conducted and it was found to be allelic to the G locus.  Taking things one step further, we then matched up these two alleles (cream and grey) to the Underwhite locus in the mouse. (Lehman et al., 2000)

To read Michelle's full story about this gene you can visit her website here

 

References

The Underwhite(uw) Locus Acts Autonomously and Reduces the Production of Melanin–Anne L. Lehman, Willys K. Silvers, Neelu Puri, Kazumasa Wakamatsu, Shosuke Ito, and Murray H. Brilliant. -Journal of Investigative Dermatology (2000) 115, 601–606

Gray mutant in the Mongolian gerbil- Leiper, B.D. & Robinson, R. 1985-The Journal of Heredity, 76, 473.

Has a new mutation lead to the identification of the G locus?- E. Cope-(2009)

The Underwhite gene, a new mutation in the Mongolian gerbil. E. Cope (2009)

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Pink-Eyed Dilution Locus (P locus)

In domestic animals the Pink-eyed dilution mutation is a well known and firmly 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 was incorporated into several common laboratory strains of mice during the early part of the last century. (Brilliant et al., 1994) The pink-eyed dilute mutation in the gerbil appeared in 1977 in a North London school. Like most gerbil coat 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. As the name of the mutation suggests, the eye pigment is diluted as well as the fur, and becomes a ruby colour. On a non-agouti 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. (Brilliant et al. 2001; Puri et al. 2000) However when they are in a pH neutral environment the pigment cells produce very little black pigment, but favours the production of yellow pigment, and this is effectively what the pink-eyed dilution allele does; it alters the pH of the pigment cell allowing conditions that favour the production of yellow pigments.(Brilliant 2001)

References

Brilliant, M. H., A Ching. Y. Nakatsu & E.M. Eicher.- The original Pink-Eyed Dilution Mutation (p) arose in Asiatic mice; Implications for the H4 minor histocompatability antigen Myod 1 regulation & the origin of inbred strains. - Genetics 138:203-211 (1994)

Brilliant MH. 2001. The mouse p (pink-eyed dilution) and human P genes, oculocutaneous albinism type 2 (OCA2), and melanosomal pH. Pigment Cell Res.14(2):86-93

Puri N, Gardner JM, Brilliant MH. 2000. Aberrant pH of melanosomes in pink-eyed dilution (p) mutant melanocytes. J Invest Dermatol. 115(4):607-13

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Dominant Spotting Locus (Sp Locus)

Dominant Spotting was the first coat mutation that appeared in the gerbil.  In the U.S.A, gerbils with white markings appeared in a litter at the Peterson Hamstery in the late 1960's. (Silverstein & Silverstein 1976)  By late 1967, it was imported to the U.K., sent from Canada by Frank Lane to Eric Dukes and Tony Jones. The gene is described as being dominant in nature and can occur on any of the known coat colour varieties.

Another appearance of a dominant spotting gene occurred in 1976, in a colony of random bred gerbils maintained at the Department of Biology, Livingston College, Rutgers University, New Brunswick, New Jersey, and subsequently a report about the gene appeared in the scientific literature. (Waring et al., 1978).  The gene behaved remarkably similarly to the one already in circulation and may have been regarded as a re-mutation by the scientific community.  However, although the gene looked and behaved very similar to the existing Dominant spotting in wide circulation, it could of just as easily been another allele of Dominant spot, or a separate allele altogether, but there was never a subsequent report published to test this theory.

Although described as a dominant gene, it is, in reality, a semi-dominant gene.  In breeding experiments, the data retrieved from  breeding studies showed that the gene didn't fit the 3: 1 ratio expected for a dominant gene. Instead, the results from breeding spotted x spotted gerbils fit a 2:1 ratio, which would be expected if the spotted mutation is pre-natally lethal in the homozygous condition. (Waring et al, 1978) When an embryo receives 2 of these genes, (one from the mother, the other from the father) the SpSp embryos suffer from macrocytic anaemia of such severity that they die before birth. The heterozygotes (Sp+) that live, still suffer from anaemia, but it is so slight that it has no apparent effects on health, fertility or longevity (Waring, et al, 1978; Russell & Bernstein,1966).  In exceptionally rare circumstances, some SpSp individuals appear in litters.  These are usually still-born.  Skin studies on stillborn gerbil pups from the spotted x spotted matings were conducted showing that had the stillborn pups lived they would have been black-eyed whites; the hair follicles in the skin, which were examined microscopically, completely lacked pigment. (Waring et al, 1978)  The heterozygotes that live (Sp+) are often described as being "obligate heterozygotes".  This simply means that these individuals are obliged by necessity to be heterozygous because of the death of the homozygote.

Spotting genes are best dealt with separately from other genes, as it works as a pattern that is overlaid onto the known gerbil colours.  Patterning variants such as collared, mottled and variegated are thought to be achieved by modifying genes that extend the white markings. The exact shape, size and patterning depend on these modifying genes, but it can also be dependent on several 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 been born to spotted parents, it carries minor modifying genes for spotting.  In these gerbils, the white patches present on the paws, and 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 pigments, especially black pigment; if the white markings are extensive, the more the base colour will be diluted. Mottled gerbils tend to be markedly diluted when compared to spotted or collared gerbils and in cases where the white markings are extremely exaggerated, 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 Dominant Spotting locus, or even involve a different locus; however this theory would be very difficult to prove and would 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 these variations in the white patterning that we see when we breed Dominant spot gerbils.

However, while it remains theory in gerbils, this concept has been extensively analysed by geneticists and proven in mice.  In the mouse, the specific spotting modifiers that extend the white markings are known as "K complex" genes.  These minor genes act alongside the spotting gene and recombine freely with each other and have cumulative effects. (Dunn & Charles, 1937; Dunn, 1942; Grüneberg, 1952)

For more detailed information about spotting see,

References

Gerbils All About them - Silverstein and Silverstein- Lippincott, Williams & Wilkins 1976

ALICIA D. WARING, TIMOTHY W. POOLE, & TIMOTHY PERPER- White spotting in the Mongolian gerbil-The Journal of Heredity 69:347-349. 1978.

Dunn, L.C.: Studies on spotting patterns. V. Further analysis of minor spotting genes in the house mouse. Genetics 27: 258-267, 1942.

Dunn, L.C., and Charles, D. R.: Studies on spotting patterns. I. Analysis of quantitative variations in the pied spotting of the house mouse. Genetics 22: 14-42, 1937.

Elizabeth S. Russell and Seldon E. Bernstein-Blood and Blood Formation-In Biology of the Laboratory Mouse. E.L. Green, Ed. Second edition. McGraw-Hill Book Company, New York. 1966.

Grüneberg, H.: The Genetics of the Mouse, 2nd ed., Nijhoff, The Hague, 1952

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Lethal Spotting Locus

This gene first appeared around 2000 in the Czech Republic & Switzerland, and then a little later it migrated to Germany and Austria.  Breeders in these countries noticed that there was a sudden extension of the white markings on their Dominant spotting lines.   However it quickly became apparent that not all of these breeding lines were healthy.  Around 2005, German breeders were referring to these extreme white spotted gerbils as “Superschecken” and the name indicated a very high amount (often in excess of 90%) of de-pigmentation in their fur.


In 2006, Kira Gysel & Eli Wolfmayr conducted extensive breeding studies on this spotted coat variant and isolated the gene that was co-operating with Dominant spotting to produce Extreme White gerbils.  In its heterozygous state (Sls+) it produces only minor spotting on the gerbil; the areas of white markings are the feet, under the chin, and accompanied by odd coloured toenails; where some nails are de-pigmented while others are coloured.  On many examples, but not always, (as there is a relatively wide spectrum of variance) there is also an accompanying spot on the nape of the neck and also a belly spot in addition to the standard white markings mentioned above.   When the gene is in its homozygous (SlsSls) state, it produces a “Rumpblack” gerbil.  As the name suggests, the coat on this gerbil is extensively de-pigmented with only a small amount of pigment left in the rump area.  Later breeding studies (2008) conducted by the Mutation Investigation group showed that these Rumpblacks can also appear with some areas of pigment around the head region.  Unfortunately the lives of these Rumpblacks are very short and most die very young (11-18 days) or around weaning time.  The death is due to toxic megacolon, which is an inability to evacuate the bowel. 


Both the mouse and rat have comparable spotting mutations with similar effects.  White spotting accompanied by megacolon  is usually the result of mutations in either endothelin B or their receptor genes.(Dembowski et al., 2000;Tachibana et al., 2003; Zhu et al., 2004)


In a developing embryo, enteric cells (cells related to the intestine) migrate from the far end of the neural crest to the intestine. The neural crest is a region down the back of the developing embryo which supplies pigment and neural cells that normally migrate all over the body.  These enteric cells are responsible for innervating the colon.  The migration of these cells and also pigment cells depend on the presence of endothelin B and endothelin B receptor genes. These genes regulate the differentiation, proliferation and migration of pigment cells and enteric cells as the embryo is developing.   When these genes mutate and have little or no function it can lead to de-pigmentation and a lack of neural connection to the colon; a condition that is fatal to the animal. (Tsaur et al. 1997)


Lethal spotting in the gerbil bears a very close resemble to Semi-dominant Lethal spotting in the mouse, in both its coat markings and pathological phenotype.  In the mouse, the mutation responsible for these effects can render the endothelin-3 gene non-functional. (Harris et al., 2010)

 

References

Zhu L, Lee HO, Jordan CS, Cantrell VA, Southard-Smith EM, Shin MK. 2004. Spatiotemporal regulation of endothelin receptor-B by SOX10 in neural crest-derived enteric neuron precursors. Nat Genet. 36(7):732-7.

 A new mouse mutation named semidominant lethal spotting is mapped to Chromosome 2-Belinda S Harris, Patricia F Ward Bailey, Roderick T Bronson, Kenneth R Johnson and Muriel Davisson

Dembowski, C.; Hofmann, P.; Koch, T.; Kamrowski-Kruck, H.; Riedesel, H.; Krammer, H.-J.; Kaup, F.-J. and Ehrenreich, H..2000. Phenotype, intestinal morphology, and survival of homozygous and heterozygous endothelin B receptor-deficient (spotting lethal) rats. J. Pediat. Surg. 35(3): 480-488

Tachibana M, Kobayashi Y, Matsushima Y. 2003. Mouse models for four types of Waardenburg syndrome. Pigment Cell Res. 16(5):448-54.

Tsaur ML, Wan YC, Lai FP, Cheng HF. 1997. Expression of B-type endothelin receptor gene during neural development. FEBS Lett. 417(2):208-212

A new spotting mutation in the Mongolian gerbil?  Kira Gysel (2007)

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