A novel homeotic mutation in morphology using skeletal specimens in four breeds of Japanese indigenous chickens

Four Japanese indigenous chicken breeds (Tosa-jidori, Ko-shamo, Minohikichabo, and Chabo) indicate morphological diversity, which is supported by their separate phylogenetic positions. These resources have high potential to be a unique model in morphological genetics. Therefore, this study aims to find morphological characteristics using skeletal specimens and computed tomography images at 7 weeks of age and the adult stage. The Tosa-jidori had significantly higher body weight and body length than the others, whereas Chabo showed a significantly lower ratio of neck length to body length than the others. Our observation identified novel mutants (individuals with eight ribs) across the four Japanese indigenous breeds. Significant breed differences were observed in the ratio of wild-type (seven ribs) to mutant (eight ribs) at both 7 weeks of age and the adult stage. The wild-type was predominant in Tosa-jidori and Ko-shamo, whereas the mutant was the majority or half-and-half in Minohikichabo and Chabo. Based on the total number of cervical and thoracic vertebrae, the present eight ribs should represent a novel homeotic mutation. This study mentioned the morphological characteristics of four Japanese indigenous breeds and documented novel variation of rib number within an avian species, the chicken.