Variation in the TGF-β signaling pathway is emerging as an important mechanism by which gonadal sex determination is controlled in teleosts. Here we show that amhy, a Y-specific duplicate of the anti-Müllerian hormone (amh) gene, induces male sex determination in Nile tilapia. amhy is a tandem duplicate located immediately downstream of amhΔ-y on the Y chromosome. The coding sequence of amhy was identical to the X-linked amh (amh) except a missense SNP (C/T) which changes an amino acid (Ser/Leu92) in the N-terminal region. amhy lacks 5608 bp of promoter sequence that is found in the X-linked amh homolog. The amhΔ-y contains several insertions and deletions in the promoter region, and even a 5 bp insertion in exonVI that results in a premature stop codon and thus a truncated protein product lacking the TGF-β binding domain. Both amhy and amhΔ-y expression is restricted to XY gonads from 5 days after hatching (dah) onwards. CRISPR/Cas9 knockout of amhy in XY fish resulted in male to female sex reversal, while mutation of amhΔ-y alone could not. In contrast, overexpression of Amhy in XX fish, using a fosmid transgene that carries the amhy/amhΔ-y haplotype or a vector containing amhy ORF under the control of CMV promoter, resulted in female to male sex reversal, while overexpression of AmhΔ-y alone in XX fish could not. Knockout of the anti-Müllerian hormone receptor type II (amhrII) in XY fish also resulted in 100% complete male to female sex reversal. Taken together, these results strongly suggest that the duplicated amhy with a missense SNP is the candidate sex determining gene and amhy/amhrII signal is essential for male sex determination in Nile tilapia. These findings highlight the conserved roles of TGF-β signaling pathway in fish sex determination.

Studies of gene function in non-model animals have been limited by the approaches available for eliminating gene function. The CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/CRISPR associated) system has recently become a powerful tool for targeted genome editing. Here, we report the use of the CRISPR/Cas9 system to disrupt selected genes, including nanos2, nanos3, dmrt1, and foxl2, with efficiencies as high as 95%. In addition, mutations in dmrt1 and foxl2 induced by CRISPR/Cas9 were efficiently transmitted through the germline to F 1 . Obvious phenotypes were observed in the G0 generation after mutation of germ cell or somatic cell-specific genes. For example, loss of Nanos2 and Nanos3 in XY and XX fish resulted in germ cell-deficient gonads as demonstrated by GFP labeling and Vasa staining, respectively, while masculinization of somatic cells in both XY and XX gonads was demonstrated by Dmrt1 and Cyp11b2 immunohistochemistry and by up-regulation of serum androgen levels. Our data demonstrate that targeted, heritable gene editing can be achieved in tilapia, providing a convenient and effective approach for generating loss-of-function mutants. Furthermore, our study shows the utility of the CRISPR/Cas9 system for genetic engineering in non-model species like tilapia and potentially in many other teleost species. R ECENTLY, a simple and efficient genome editing technology, type II CRISPR/Cas9, has been developed based on the Streptococcus pyogenes clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein (Cas9) adaptive immune system. It requires three components for effective DNA cleavage: the nuclease Cas9, a targeting CRISPR RNA (crRNA), and an additional transactivating crRNA (tracrRNA) (Gasiunas et al. 2012;Jinek et al. 2012;Cho et al. 2013;Cong et al. 2013;Hwang et al. 2013;Mali et al. 2013). Further improvement of the system was achieved by fusing the crRNA and tracrRNA to form a single guide RNA (gRNA) that is sufficient to direct Cas9-mediated target cleavage (Hwang et al. 2013). Importantly, previous studies performed in vitro (Jinek et al. 2012), in bacteria , and in human cells (Cong et al. 2013) have shown that Cas9-mediated cleavage can be abolished by single mismatch at the gRNA-target site interface, particularly in the last 10-12 nucleotides located in the 39 end of the 20-nt gRNA targeting region. Compared to the other two engineered nuclease genome-editing technologies, zinc-finger nucleases (ZFNs) (Urnov et al. 2005;Doyon et al. 2008) and transcription activator-like effector nucleases (TALENs) (Huang et al. 2011;Sander et al. 2011;Tesson et al. 2011), the CRISPR/Cas9 system is substantially less expensive and much easier to program for editing new target sites. This new approach has been widely used for genome engineering in model animals, including Caenorhabditis elegans (Dickinson et al. 2013;Friedland et al. 2013;Tzur et al. 2013), Drosophila (Bassett et al. 2013;Ren et al. 2013;Yu et al. 2013), zebrafish (Chang et al. 2013Hrusc...

Wilms tumor 1 (Wt1) is an essential factor for urogenital system development. Teleosts have two wt1s, named as wt1a and wt1b. In this study, the expression pattern of wt1a and wt1b and their functions on the urogenital system were analyzed by in situ hybridization and CRISPR/Cas9. wt1a was found to be expressed in the glomerulus at 3 dah (days after hatching), earlier than wt1b. wt1a and wt1b were simultaneously expressed in the somatic cells of gonads at 3 dah, while their cell locations were similar, but not identical in adult fish gonads. The wt1a fish displayed pericardial edema and yolk sac edema at 3 dah and subsequently expanded as general body edema at 6 dah, failed to develop glomerulus and died during 6-10 dah, whereas the wt1b fish were phenotypically normal. Immunohistochemical analyses revealed that the germ cell marker Vasa was expressed, while somatic cell genes Cyp19a1a, Amh, Gsdf and Dmrt1 were not expressed in the wt1a gonads at 6 dah. The sex phenotypes of XX and XY in the wt1b fish were not affected. Real-time PCR revealed that the ovarian cyp19a1a expression was up-regulated in XX wt1b fish, compared with XX control at 90 dah. Serum estradiol-17β level was also up-regulated in XX wt1b fish at 90 and 180 dah. The XY wt1b fish had normal serum estradiol-17β and 11-ketotestosterone levels and remained fertile. These results suggest that Wt1a and Wt1b have different functions in the kidneys and gonads of tilapia.