Differences in postembryonic dorsal fin development resulted in phenotypic divergence in two goldfish strains, Red Cap Oranda and Ranchu

Abstract Goldfish (Carassius auratus auratus) are important ornamental fish that have experienced extreme anthropogenic selection and exhibit diverse phenotypes. Loss of the dorsal fin is a key characteristic that distinguishes the Egg-goldfish from the Wen-goldfish. However, the mechanisms underlying the divergence in dorsal fin development are still unknown between the Egg-goldfish and the Wen-goldfish. In this study, we sought to unravel the postembryonic developmental processes of two goldfish strains, Red Cap Oranda, a representative of the Wen-goldfish, and Ranchu, a representative of the Egg-goldfish. We examined the developmental morphology of the dorsal fin in five-month-old goldfish larvae using both Alcian blue-alizarin red staining and X-ray photography. We showed that the developmental processes of the dorsal fin fold were different in Ranchu compared to Red Cap Oranda, despite similarity in their general developmental processes. We categorized the postembryonic development of Ranchu larvae into four types based on the number of the initial residual dorsal fin fold appearing at protruding-mouth stage. The number, size and position of the initial residual dorsal fin fold appeared at hatching affected and eventually determined the phenotypic variations in dorsal fin defects in Ranchu. In addition, we genotyped the eomesa gene to determine whether it would be a candidate gene responsible for the loss of the dorsal fin in Ranchu. The eomesa CDS sequence exhibited no nonsense mutation in Ranchu. In summary, the absence of the dorsal fin fold during postembryonic development eventually resulted in the loss of the dorsal fin in Ranchu. Further investigation is required to determine whether other genes and developmental pathways play important roles in the loss of the dorsal fin in Ranchu.