1. The non-reference allele causes a 161 missense substitution in the seventh exon of ROR2 (ROR2 C1087T , hereafter the Ku2 allele 5 ) in 162 short-beaked pigeons. ROR2 encodes a noncanonical Wnt receptor with well-established roles 163 in cell polarity and motility in multiple embryonic tissues, including the neural crest 34 . In large-164 scale genome-wide association studies, the ROR2 locus has been implicated in normal-range 165 human facial variation, particularly within the central region of the face 8-10 . Furthermore, ROR2 166 is required for normal craniofacial development: in humans, homozygous mutations in ROR2 167 cause autosomal recessive Robinow syndrome, a severe skeletal dysplasia characterized by 168 extensive abnormalities, including a prominent forehead (frontal bossing), wideset eyes 169 (hypertelorism), and a broad, short nose 11,12 . In mice, Ror2 knockout or knock-in of Robinow-170 associated mutations disrupts endochondral bone development and causes profound skeletal 171 abnormalities;The single most significantly-differentiated SNP genome wide (wcF ST =0.88, pF ST =0) is 160 located at scaffold position ScoHet5_445.1:6568443
2. At the amino 180 acid level, the Ku2 allele causes an arginine-to-cysteine transition in the ROR2 extracellular 181 kringle fold, a cysteine-rich, disulfide-bonded domain that is unlikely to tolerate mutations due to 182 its small size and complex folding 40 . The precise number and spacing of cysteine residues in the impact on disulfide bond formation and kringle domain folding;Within the short-beaked group, 98% of pigeons (45/46) were homozygous or hemizygous 175 for the Ku2 allele; only the Chinese Nasal Tuft, a breed that can have a short-or medium-length 176 beak 38 , was heterogyzous