This is a picture of the European wild rabbit its coat genotype is "AABBCCDDEEVVWWenenDuDuSaSa". We call this colour Agouti or Chestnut Agouti. The hair shaft on an agouti rabbit has three distinct colour bands. Firstly the slate under colour which extends from the skin the pigment of this band is a result of the presence of the pigment eumelanin. The next band an orange/yellow band is the result of pheomelanin production. Finally the top band comprised of densely expressed eumelanin producing the black tip on the coat.
The "A" locus or agouti locus codes for a short peptide known as the Agouti Signalling Protein (ASIP). This protein is responsible for inducing pheomelanin production by melanocytes. Pheomelanin is yellow orange or red in colour and its production in the melanocytes of the skin gives rise to the yellow band on an other wise black coat as seen in the agouti.
A number of mutations to the A locus have been observed in the domestic rabbit. These and similar mutations have also been observed in the common house mouse, domestic mouse and laboratory mouse, Mus musculus. In fact the agouti gene has been observed in many mammalian species. It has been maped to mouse chromosome 2 and Homo sapien (human) chromosome 20, although in the human no hair banding has ever been reported the agouti gene has been shown to have involvement in conditions such as obesity and diabeties.
What is Agouti and How Does it Work
As already mentioned the agouti gene codes for the expression of ASIP. This peptide works as an antagonist to a receptor on melanocytes (the pigment producing cells). This receptor is known as the melanocortin receptor 1(Mc1r) commonly known as the "E" or extension locus. The binding of ASIP to Mc1r antagonises the effects of Mc1r resulting in the production of pheomelanin. So in order to get the banded hair shaft seen on the agouti coat regulated expression of ASIP is essential.
Regulated expression is required during hair growth. Expression must also be different on the dorsal and ventral regions of the rabbit, allowing for the banding pattern to be absent from the belly colour accounting for the white hair on the ventral side of the rabbit.
Agouti Mutants
The Tan at - No ASIP is expressed on the dorsal regions of the rabbit. This means that the melanocytes will produce only pheomelanin in this region or the coat thus accounting for the black colour.
The Self a - In the self both dorsal and ventral expression of ASIP is absent. Thus eumelanin production is seen on the entire coat of the animal.
