Intermuscular bones (IBs) are small, hard-boned spicules located in the muscle tissue that mainly exist in the myosepta of lower teleosts, which hurt the edibleness and economic value of fish. The study of the development of IBs is very important for freshwater aquaculture fish, but the molecular mechanism of its formation and the key regulatory genes remain unclear. In this study, we first constructed two types of zebrafish mutants (the mutants losing IBs and the mutants with partial deletion of IBs) by knocking out bmp6. We then carried out a transcriptomic analysis to reveal the role of bmp6 in the developmental mechanism of IBs; we used the caudal musculoskeletal tissues of these mutants and wild-type zebrafish at three development stages (20, 45, and 60 dph) to perform transcriptomic analysis. The results showed that the deficiency of bmp6 upregulated sik1 and activated the TNF-A signaling via the NF-KB pathway, which inhibited the development of osteoblasts and promoted osteoclast formation, thereby inhibiting the formation of IBs. These results provided insights to understand the role of bmp6 in the development of IBs in zebrafish and are useful for selective breeding of IBs in cyprinids.
Bone morphogenetic protein 7 (BMP7) belongs to the transforming growth factor β (TGF-β) family, which not only induces cartilage and bone formation, but also regulates eye development and melanoma tumorigenesis in mammals. In teleosts, BMP7 differentiates into two subtypes, bmp7a and bmp7b, which have clearly differentiated structures. To fully understand the functional differentiation of bmp7a and bmp7b in fish species, we successfully constructed bmp7a and bmp7b gene deletion mutants in zebrafish using CRISPR/Cas9-mediated gene editing technology. Our results showed that bmp7a mutation caused abnormal development of the embryo’s dorsal-ventral pattern that led to death; bmp7b mutation induced growth inhibition and increased melanin production in the skin and eye of mutants. Histological analysis revealed that melanin in the retina of the eyes in bmp7b mutants increased, and behavioral observation showed that the vision and sensitivity to food of the mutants were reduced. Transcriptome analysis of the skin and eye tissues showed that the expression changes of wnt7ba and gna14 in bmp7b mutants might promote the increase of melanin. Additionally, the eye transcriptome analysis indicated that changes in the structure of the eyes in bmp7b mutants led to defects in phototransduction, and seven DEGs (rgs9a, rgs9b, rcvrn2, guca1d, grk1b, opn1mw4, and gc2) were identified as key candidate genes that affected the photonic response of the eyes. The study revealed the functional differentiation of bmp7a and bmp7b in teleosts and the first report about the inhibitory effect of bmp7b on melanogenesis may provide useful information for the future research on human melanoma-related diseases.