Categories: Pigmentation phene

Possibly relevant human trait(s) and/or gene(s) (MIM number): 131242 (gene)

Links to MONDO diseases: No links.

Mendelian trait/disorder: yes

Mode of inheritance: Autosomal dominant

Considered a defect: no

Key variant known: yes

Year key variant first reported: 2011

Cross-species summary: also called Silky/Silkie hyperpigmentation, black bone

Species-specific name: Also known as Dermal hyperpigmentation

Species-specific symbol: FM

Inheritance: The mutant allele (*FM) is dominant to the wild-type, recessive (*N) allele. The locus symbol is FM.

Mapping: Dorshorst et al. (2010) were the first to map this locus, reporting its location as "10.3–13.1 Mb on chromosome [GGA]20". This mapping was slightly narrowed by Shinomiya et al. (2011) to "10.2-11.7 Mb of chicken chromosome [GGA]20".

Molecular basis: Shinomiya et al. (2011) reported that the characteristic hyperpigmentation of Silky chickens is due to a 130kb duplication that contains five genes, including "endothelin 3 (EDN3) which [encodes] a potent mitogen for melanoblasts/melanocytes". The duplication of these genes results in substantial overexpression which gives rise to the hyperpigmentation. This discovery was extended by Dorshorst et al. (2011), who showed that the FM mutation actually involves "the duplication of two genomic regions, each larger than 100 kb and separated by 417 kb on wild-type [GGA20] chromosomes". Their analysis of a range of breeds strongly suggested that the mutation giving rise to this double duplication is the cause of FM in all breeds of chickens, and hence is an old mutation, predating the divergence of breeds.

Dharmayanthi et al. (2017): "Like Chinese Silkie, Indonesian Ayam Cemani exhibits fibromelanosis or dermal hyperpigmentation and possesses complex segmental duplications on chromosome 20 that involve the endothelin 3 gene, EDN3. A genomic region, DR1 of 127 kb, together with another region, DR2 of 171 kb, was duplicated by unequal crossing over, accompanied by inversion of one DR2. . . . These genetic arrangements are identical in Cemani and Silkie, indicating a single origin of the genetic cause of Fm. The two DR1s harbor two distinct EDN3 haplotypes in a form of permanent heterozygosity, although they remain allelic in the ancestral Red Jungle Fowl population and some domesticated chicken breeds, with their allelic divergence time being as recent as 0.3 million years ago. In Cemani and Silkie breeds, artificial selection favoring the Fm phenotype has left an unambiguous record for selective sweep that extends in both directions from tandemly duplicated EDN3 loci. This highly homozygous tract is different in length between Cemani and Silkie, reflecting their distinct breeding histories. It is estimated that the Fm phenotype came into existence at least 6600-9100 years ago, prior to domestication of Cemani and Silkie, and that throughout domestication there has been intense artificial selection with strength s > 50% in each breed."

Breeds: Bohuslän - Dals svarthöna, Sweden (Chicken) (VBO_0007792), H'mong, Viet Nam (Chicken) (VBO_0007907), Kedu, Indonesia (Chicken) (VBO_0007320), Silkie (Chicken) (VBO_0000619).
Breeds in which the phene has been documented. For breeds in which a likely causal variant has been documented, see the variant table below

Associated gene: