cdk1 mutation leads to delayed development of testes in zebrafish. A The expression levels of cdk1 gene in different tissues of zebrafish. B Immunofluorescence analysis of Cdk1 localization in zebrafish testes. Scale bar = 10 µm or 5 µm. C Schematic diagram of CRISPR/Cas9 target sites for zebrafish cdk1 gene knockout. D A representative chromatogram of PCR-product sequencing in cdk1+/−. E Anatomical examination of the testes of WT and cdk1+/− zebrafish. Scale bar = 0.5 cm. F Body weight analysis of male WT and cdk1+/− zebrafish (n = 6). G GSI of male WT and cdk1+/− zebrafish (n = 6). H Histological observations of the testes of WT and cdk1+/− zebrafish. Scale bar = 10 µm or 1 µm. I The proportions of testis cells at different stages in WT and cdk1+/− zebrafish, evaluated based on the area ratio. cdk1, cyclin-dependent kinase 1; DAPI, 4,6-diamidino-2-phenylindole; SG, spermatogonia; SC, spermatocytes; SZ, spermatozoa; WT, wild-type zebrafish; cdk1+/−, zebrafish with heterozygous mutation of Cdk1; GSI, gonadosomatic indexes; different lowercase letters indicated significant differences (p < 0.05). *p < 0.05 was considered significant, while **p < 0.01 indicated a very significant difference