Myostatin, a secreted growth and differentiation factor (GDF‐8) belongs to transforming growth factor (TGF‐β) superfamily that plays as a negative regulator of skeletal muscle development and growth. Recently, myostatin has been isolated from fish; however, its role in muscle development and growth remains unknown. Here, we present the expression of myostatin during development and the effects of its knock‐down on various genes such as muscle regulatory transcription factors (MRFs), muscle‐specific proteins (MSP), and insulin‐like growth factors (IGFs). The myostatin expression was found to be maternal as it starts in one‐cell stage onward. The reverse transcription‐polymerase chain reaction (RT‐PCR), in situ hybridization, and Southern and Northern blots demonstrated that the myostatin expression is not only restricted to skeletal muscle, but it expressed all the tested tissues. Expression of myostatin was effected by using antisense morpholinos resulted in significant phenotypic difference in stages 18 and 20 hours postfertilization (hpf). To confirm the specificity of myostatin morpholino, furthermore, a rescue experiment was conducted. The length as well as width of somites was increased with almost no gap in between the somites. In addition, it deserves to mention that this is a first animal model that shows changes in the size of the somites. Moreover, analyses of MRFs, MSP, and IGFs in the knock‐down embryos by RT‐PCR revealed the up‐regulation of MyoD, Myogenin, and Mck transcription, whereas IGF‐2 transcription showed mild response with no effect on IGF‐1, Desmin, and Myf5. In situ hybridization showed that there was an increase in the number of somites from 3 to 4 at 13 and 22 hpf. Taken together, these data suggest that myostatin plays a major role during myogenesis, apart from inhibition of proliferation as well as differentiation. Developmental Dynamics 229:847‐856, 2004.

MicroRNA (miRNA) are a class of small, single-stranded, non-coding RNA that regulate mRNA expression at the post-transcriptional level and play important roles in many fundamental biological processes. There is emerging evidence that miRNA are critical regulators of widespread cellular functions, such as differentiation, proliferation, and migration. At present, little is known about miRNA expression profiles related to skeletal muscle growth in aquatic organisms. This study aimed to investigate the phenotypic variation in the body growth of the Nile tilapia (Oreochromis niloticus) and to identify and quantify the differential expression levels of selected growth-related transcriptomic miRNA in the skeletal muscle of this fish. To this end, we performed next-generation sequencing to define the full miRNA transcriptome in muscle tissue from Nile tilapia and to detect differentially expressed miRNA between 2 strains of Nile tilapia. These tilapia strains exhibited significant (P < 0.05) phenotypic variation with respect to growth-related traits (body length and BW), mitochondrial DNA (mtDNA) haplotype diversity, and the differential expression of selected growth-related genes. The results obtained from the transcriptome analysis and real-time quantitative reverse transcription PCR (qRT-PCR) revealed significant differences in miRNA expression between fast-growing and control strains of tilapia. Digital gene expression (DGE) profiling was performed based on the obtained read abundance, and we identified down-regulated miRNA, including let-7j, miR-140, miR-192, miR-204, miR-218a, miR-218b, miR-301c, and miR-460, and up-regulated miRNA, including let-7b, let-7c, miR-133, miR-152, miR-15a, miR-193a, miR-30b, and miR-34, associated with body growth in tilapia. These results were further validated using real-time qRT-PCR and microarray profiling. In summary, the up- and down-regulation of miRNA involved in the GH/IGF-1 axis signaling pathway suggests that the differential expression levels of growth-related miRNA may serve as molecular markers that are predictive of specific functional and diagnostic implications. The obtained data on genetic polymorphisms in miRNA-target interactions are particularly useful for Nile tilapia breeding programs.