Abstract
Feathered leg is a trait in domestic chickens that has undergone intense selection by fancy breeders. Previous studies have shown that two major loci controlling feathered leg are located on chromosomes 13 and 15. Here, we present genetic evidence for the identification of candidate causal mutations at these loci. This was accomplished by combining classical linkage mapping using an experimental cross segregating for feathered leg and high-resolution identical-by-descent mapping using whole-genome sequence data from 167 samples of chicken with or without feathered legs. The first predicted causal mutation is a single-base change located 25 kb upstream of the gene for the forelimb-specific transcription factor TBX5 on chromosome 15. The second is a 17.7-kb deletion located ∼200 kb upstream of the gene for the hindlimb-specific transcription factor PITX1 on chromosome 13. These mutations are predicted to activate TBX5 and repress PITX1 expression, respectively. The study reveals a remarkable convergence in the evolution of the feathered-leg phenotype in domestic chickens and domestic pigeons, as this phenotype is caused by noncoding mutations upstream of the same two genes. Furthermore, the PITX1 causal variants are large overlapping deletions, 17.7 kb in chicken and 44 kb in pigeons. The results of the present study are consistent with the previously proposed model for pigeon that feathered leg is caused by reduced PITX1 expression and ectopic expression of TBX5 in hindlimb buds resulting in a shift of limb identity from hindlimb to more forelimb-like identity.
Highlights
The ancestors of chickens had tarsometatarsus and digits that were covered with scales
The study reveals a remarkable convergence in the evolution of the feathered-leg phenotype in domestic chickens and domestic pigeons, as this phenotype is caused by noncoding mutations upstream of the same two genes
SNP Analysis and Linkage Mapping A linkage mapping population consisting of wild-type Houdan and feathered-leg Langshan F0 birds, 25 F1 females, and 222 backcross chickens was established
Summary
The ancestors of chickens had tarsometatarsus and digits that were covered with scales. The transformation of part of those scales into feathers due to genetic changes results in so-called feathered leg, feathered feet, shank feathering, or ptilopody (Bartels 2003). Somes (1992), based on the results of test mating, provided genetic evidence for the presence of two independent dominant mutations, denoted Pti-1 and Pti-2, causing feathered legs in domestic chicken. Breeds with more sparsely feathered legs were supposed to be homozygous mutant at the Pti-1 locus but wild-type (pti-2/pti-2) at the second locus. Breeds with more heavily feathered legs like Sultan and Cochin were thought to carry the dominant mutations at both Pti-1 and Pti-2 (Somes 1992). A recessive allele (pti-3) at a third locus has been suggested to cause leg feathering in the Pavloff chicken (Somes 1990)
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