Categories: Integument (skin) phene

Links to possible relevant human trait(s) and/or gene(s) in OMIM: 610564 (gene)

Mendelian trait/disorder: yes

Mode of inheritance: Autosomal recessive

Disease-related: no

Key variant known: yes

Year key variant first reported: 2014

Species-specific name: Also known as Hookless and Silkiness

Species-specific symbol: h

Species-specific description: The phenotype of this trait was summarised (with photographs) by Feng et al. (2014) as: "In silky-feather chickens beginning with the first pennaceous feathers a clear difference in feather structure is seen as compared to wild-type, with the Silkie chicken maintaining a more downy appearance in the body contour feathers . . . . In the closed pennaceous feather portion, wild-type feathers have hooklets on the distal barbules and forms the vane . . . while silky-feathers lack hooklets . . . . In the afterfeather portion, both wild-type and silky-feather have the barbules structure but no hooklets . . . . The flight feathers and some of the shank feathers can form hooklets in Silkie birds, but fewer than in wild-type chicken . . . . This phenotype has apparently been strongly favored during the development of the Silkie breed due to the beautiful fur-like feathers. The Silkie chickens lose the flight function and do poorly in extreme temperatures because of the lack of closed pennaceous vane."

History: Somes (1990) records this trait being mentioned by Marco Polo in 1298-99 and Lind (1963) cites a mention by Aldrovandi in 1600.

Inheritance: As recounted by Somes (1990), single-locus autosomal recessive inheritance was reported by Davenport (1906), Cunningham (1912), Bonhote (1914), Jones (1921) and Dunn and Jull (1927). The symbol "h" (for hookless) was allocated by Dunn and Jull (1927).

Mapping: As summarised by Dorshorst et al. (2010) " Initial linkage mapping studies suggested the Silkie gene (h) was 43 map units from the naked neck gene (Na) in linkage group III, now known as chromosome 1 (Warren 1938; Hutt 1949). However, it was subsequently shown that Na was located on chromosome 3 suggesting that h was also on this autosome (Pitel et al. 2000)." Dorshorst et al. (2010) confirmed the chromosome to be GGA3, and provided data supporting a location within a 15.7Mb region in which the highest association was with a marker at 69.7Mb. Feng et al. (2014) independently confirmed the above map location via genome-wide linkage analyses in the China Agricultural University Resource Population (CAURP) comprising 31 F0 (12 White Plymouth Rock and 19 Silky), 19 F1 and 229 F2 individuals, first in an analysis involving 125 microsatellites, and then in an analysis of SNP genotypes from the Illumina Chicken 60K SNP Beadchip, the latter pinpoiting the locus to "a 380 kb interval between positions 70,201,106–70,581,126 bp [on chromosome GGA3; RJF genome sequence galGal3, assembled by the Washington University Genome Sequencing Center (WUGSC]". Subsequent identity-by-descent mapping in the same population narrowed the locus to a "56.7-kb interval (70,447,648–70,504,365)". Haplotype analysis and resequencing narrowed it further to 18.9kb (70,468,129–70,487,067 bp).

Molecular basis: Of the 85 SNPs detected by Feng et al. (2014) within the 18.9kb candidate region, only one showed perfect segregation with the silky-feather trait, namely "a C to G transversion at 70,486,623 bp (ss666793747)". Confirmation of this as a causal mutation was reported by the same authors: "We genotyped ss666793747 in 718 birds from 33 populations and found that all 337 silky-feather chickens were homozygous G/G, 341 wild-type chickens were homozygous C/C and 40 known heterozygous birds were G/C (Table 2). These results lead to the hypothesis that ss666793747 is the causal mutation responsible for the silky-feather phenotype." Noting that "The candidate mutation ss666793747 is located between the SOBP and PDSS2 genes, which are two adjacent genes separated by 16.7 kb." and that "The ss666793747 mutation is 103 bp upstream of the initiator codon ATG of PDSS2 and 16.6 kb upstream of SOBP", Feng et al. (2014) designated "the ss666793747 mutation as PDSS2(-103C-G)". Expression analyses enabled Feng et al. (2014) to conclude that "The silky-feather mutation significantly decreases the expression of PDSS2 during feather development in vivo. Consistent with the regulatory effect, the C to G transversion is shown to remarkably reduce PDSS2 promoter activity in vitro".

Associated gene: