Melanism

Under melanism (gr. " blackening " of melas "black" ) is understood in the animal kingdom a dark pigmentation of, inter alia, skin, hair, dandruff by melanins. This can in addition to genetic causes various other reasons have such as increased sun exposure, greater humidity, lower temperature, and other factors ( modification by environmental influences).

• 2.4.1 Horse: Black
• 2.4.2 Black Farbmäuse
• 2.4.3 Black dogs

Deviation from the norm

In the scientific literature as well as in animal husbandry, the term " melanism " often referred to as excess pigmentation and thus massive deposition of melanins and used deviation from the norm. A melanistisches animal ( also called black one ) is colored completely black according to this usage, in contrast to the usual color scheme of its kind. The best known Schwärzlinge are the black panther.

One case is different:

• Abundism: here appear dark new drawing elements.
• Nigrismus: only existing dark drawing elements are larger.
• Skotasmus: here there is complete blackout.

Melanism can be found in the animal kingdom, not only in big cats. It also occurs for example in snakes (eg adder ) or birds as Goldbrüstchen, Tiger finches, finches Rosttäubchen or tape. In birds, the melanism but sometimes seems to be a response to stress, to be unbalanced diet and lack of light, which can be reversed after the next moult. He is also not inherited. In turtles one knows the so-called Altersmelanismus, blackening in the higher age of the animal.

Opposed to melanism appear albinism and leucism, the color pigments are missing for various reasons in the skin or in fur.

Also the Akromelanismus, in which only the limbs are dark in color, is a partial albinism.

Genetic basis

Melanism is usually the result of a mutation of the gene loci that are responsible for skin review. These include the agouti locus (A) and the extension locus (E). Genetically completely brown or yellow animals without black color pigments the opposite of melanism.

Extension locus (E)

There is a dominant allele ED of the extension locus (E ) by the affected animals are independent of the agouti locus completely dark ( melanism ). Due to the recessive allele e, fur is all over the body reddish yellow, so that the Agoutilocus can not produce more black coloration. In between there are some alleles that allow for the influence of the agouti locus.

Agouti locus (A)

Mutations of the Agouti locus, the black can be magnified in the coat pattern ( Nigrismus ) to, in extreme cases, the entire body is black ( Skotasmus ), so it creates " melanism " to varying degrees. By Agoutilocus induced melanism is a recessive trait.

Other gene loci

Since the color genetics of different species is not yet fully understood, it is likely that still can play a role other gene loci in the development of melanism.

K locus

Black ( KB) > brindle ( kbr ) > yellow ( ky ): when a dog K- locus on canine chromosome 16 It has been shown that neither the agouti nor the Extension locus corresponds and so far has three known alleles.

The K locus encoding beta -defensin 103 ( CBD103 ). Its gene product binds to the melanocortin receptor 1, which is encoded by the Extensionlocus and has a strong effect on the switching of eumelanin to pheomelanin.

Examples of the genetics of black animals in different breeds of domestic animals

In the explanation of melanism has the difficulty that traditionally only Schwärzlinge of wild animals were referred to as melanistic that but was not previously researched mainly to domestic animals, such as the color genetics work, so that our knowledge about the origin of black paint mainly refers to Pets.

Horse: Black

A black horse, which is a black horse, is different from a wild -colored horse by the following mutations:

• In the extension locus (E) has the same dominant allele like a wild horse at least once (EE or Ee ) - a horse that is homozygous for the fox coloring (ee ) can not be darkened to a black horse.
• In the agouti locus ( A), it is homozygous for the allele of the gene that spreads the eumelanin throughout the body. Wild type: AA, Aa looks like wild type, aa is black
• In the Dun locus (D) which is typical for wild -colored horses lightening of the body to fawn falls away.

For Arabs, there is also a dominant allele for black color, which is not, however, assign the Extensionlocus.

Black Farbmäuse

Color black mice may arise in several ways.

Black dogs

At the house dog there on the K locus, which is located on the canine chromosome 16, a gene for dominant black color, which is abbreviated as KB. Thus, the affected dogs are black, it is sufficient if the allele KB occurs only once. So the genotypes KBKB, KBkbr and KBky lead to blackening. It is for example found in black labrador retrievers, black poodles and Newfoundlands.

Additionally, there is a recessive allele for black color ( a) at the agouti locus. Affected animals are only black when the allele on both chromosomes of a Agoutilocus present, ie aa. The gene is epistatic to the K locus, that is, animals with the combination aa on the Agoutilocus remain independent of the K locus always black.

The allele recessive yellow ( e) of the Extension locus brightens both types of black color on a bright red gold. It happens mostly in Labrador Retriever, Irish Setter and Samoyeds. Apart from that both genes can obscure all alleles of Extensionlocus to black for black coloration.