Researchers have recently located a frame-shift mutation on MLPH that corresponds to the dilution phenotype in 100% of the rabbits studied.
The Dilute Color in Rabbits
In rabbits there is a distinctive phenotype known as dilute that reduces the intensity (dilutes) both black and yellow/red pigment. Black pigment (eumelanin) is diluted to a blue or gray color and yellow/red pigment (pheomelanin) is diluted to a fawn. This results (in the absence of other modifiers and dilutions) in a rabbit that would otherwise be back being blue or gray while a chestnut rabbit is diluted to opal. Although, by studying inheritance, rabbit breeders have long known that dilution is inherited as a recessive, the causative mutation had never been located. Similar mutations occurring in dogs, cats, and mice however, had been located. These known mutations gave researchers a starting point in the search for dilution in rabbits.
About the Gene
The MLPH (Melanophilin) gene is one of three genes that are responsible for the distribution of pigment. Mutations in this gene tend to cause pigment to “clump” along he hair shaft rather than being evenly distributed. This affects how light reflects from the hair thus affecting shade. Because the MLPH gene processes occur early in pigment production, it affects both red and black pigment. All known MLPH mutations are currently recessive in nature. For more on MLPH... Color Across Species Part 2: MLPH
How it was found
In order to study the mutations, researchers created three half-sibling families by crossing a Checked Giant dilute buck (homozygous for the recessive mutation) with three unrelated Checked Giant Does. All three of these does where black in coloring but two were suspected of being heterozygous for the dilute mutation due to their being the result of black x blue crosses. The third doe was suspected to be homozygous for the wild-type allele (D, non-dilute) due to pedigree information. As expected, the first generation (F1) out of the suspected heterozygous does contained both black and dilute offspring. While the first generation cross out of the homozygous wild-type doe were all black. This confirmed that the dilution mutation is indeed inherited as a recessive.
DNA was then sequenced from 18 rabbits of different coloration and breeds including some of the parental study animals and their offspring. An additional 198 animals from 23 different breeds were used to for testing the results. The initial results found an allele that corresponded to the dilute coal color in most but not all of the studied animals. This lead to the determination that the allele was linked to the causative mutation but was not itself the causative mutation. The researchers then resequenced a large portion of the MLPH gene and this time located a frame-shift mutation (g.549853delG) that corresponded to the dilute coat color 100% of the time when homozygous. Some of the studied animals were heterozygous for the frame-shift mutation but these animals did now exhibit the dilute coat phenotype. These results support a recessive pattern of inheritance.
While it is impossible to completely rule out the presence of another mutation causing the dilute coat color in rabbits researchers state “that the presence of other mutations in the MLPH gene causing this phenotype is highly unlikely.” This means that it is highly probable that the mutation that causes the dilute phenotype in rabbits has been located.