Categories: Pigmentation phene

Single-gene trait/disorder: yes

Mode of inheritance: Autosomal recessive

Disease-related: no

Key variant known: yes

Year key variant first reported: 2021

Cross-species summary: Iris colour, SLC2A11B-related

Species-specific name: pear-eye

Species-specific symbol: tr

Species-specific description: Andrade et al. (2021): "Eye color arises from the deposition of pigments in the pigmented epithelium of the iris. ... Pigeons typically have eyes exhibiting a range of yellow and red hues, which arise from the deposition of pterin pigments in the anterior surface of the iris combined with strong vascularization [Oliphant, 1987]. In addition to the wild-type eye color phenotype, mutant phenotypes such as the pearl-eye arose during domestication [Hollander and Owen, 1939]. Electron microscopy imaging shows that pigment cells in the iris of pearl-eye pigeons are structurally identical to pigment cells in wild-type birds, but lack yellow pterin pigments (the red coloration in both eye color types is due to blood vessels) [Oliphant, 1987]."

Inheritance: Andrade et al. (2021): "Classical genetic studies indicate an autosomal recessive mode of inheritance for pearl-eye [Hollander and Owen, 1939], as presumably birds carrying two copies of the mutant allele (tr/tr) are impaired in their capacity to synthesize or deposit pterins in the iris compared to wild-type birds (Tr/Tr or Tr/tr)."

Mapping: Andrade et al. (2021): "Using a combination of genome-wide association analysis and linkage information in pedigrees, we mapped variation in eye coloration in pigeons to a small genomic region of ~8.5kb. This interval contained a single gene, SLC2A11B, which has been previously implicated in skin pigmentation and chromatophore differentiation in fish."

Molecular basis: Andrade et al. (2021) "used a combination of genetic, genomic and gene expression analyses to reveal the molecular mechanism underlying eye color variation in the domestic pigeon. ... Loss of yellow pigmentation is likely caused by a point mutation [AKCR02000030.1:1,895,934G>A] that introduces a premature STOP codon and leads to lower expression of SLC2A11B through nonsense-mediated mRNA decay." Si et al. (2021) have identified the same variant in a different study: "Using a genome-wide association study (GWAS) in 92 pigeons, we mapped the pearl iris trait to a 9 kb region containing the facilitative glucose transporter gene SLC2A11B. A nonsense mutation (W49X) leading to a premature stop codon in SLC2A11B was identified as the causal variant. Transcriptome analysis suggested that SLC2A11B loss of function may downregulate the xanthophore-differentiation gene CSF1R and the key pteridine biosynthesis gene GCH1, thus resulting in the pearl iris phenotype." Maclary et al. (2021) identified the same nonsense variant in exon 3 of SLC2A11B which was previously reported by Andrade et al. (2021) and Si et al.; 2021) as likely causal variant for pearl iris colour in several pigeon breeds.

Associated gene: