Phenotype characterization of crossed beaks in Beijing-You chickens based on morphological observation.

The prevalence of crossed beaks ranging from 0.2 to 7.4% was documented in at least 12 chicken strains. Previous studies focused largely on candidate molecules, whereas the morphological observation was missing. This study reported a detailed phenotype and prevalence of crossed beaks based on morphological observation in nine thousand nine hundred 1-day-old female Beijing-You chicks. Affected chicks were classified into 2 categories based on the direction of the mandibular deformation: left and right. Each category was selected to sacrifice for the measurement of length, width, and thickness of the bilateral mandibular ramus (MR). The normal chicks were used as controls. Paraffin section was made for the bilateral MR of a crossed beak and a normal control for histology analysis. A total of 97 out of 9,900 chickens showed beak deformity including 71 crossed beaks (0.72%) and 26 side beaks (0.26%) for which the upper and lower beak were both bent in the same direction. There was no difference in the direction of the bend of the lower beak in crossed beaks (P > 0.05). The incidence of crossed beaks increased quickly from 0 to 56 d and no new incidence after 56 d. The angle of the crossed beaks was below 5° in the first week and had grown more severe with age until 56 d. The mandible structure showed that condyle served as a growth center for the MR extension. The short-side MR of crossed beaks was thicker than normal ones (P < 0.05) and caused the mandible deviated to the same direction. Meanwhile, the short-side MR prevented the occlusion, leading the jugal arch deformity, which in turn resulted in a bent maxillary horizontally. Similarly, chicks with side beaks also had asymmetry in MR length and the deformities of the jugal arch after dissection. In summary, asymmetric growth of bilateral MR induced crossed beaks and side beaks; the mandibular condyle could be an ideal sample for the related molecular mechanism studies underlying this trait.