in addition to altering the phenotypes of gene-modified animals, transgenesis also has the potential to facilitate access to the various mechanisms underlying the development and functioning of specific phenotypes and genes, respectively. Myostatin (MSTN) is implicated in double-muscling when mutated in mammals, indicating that MSTN is a negative regulator of skeletal muscle formation. In order to elucidate the role of an MSTN equivalent in fish muscle formation, we created a transgenic medaka strain that expresses dominant-negative MSTN exclusively in skeletal muscle, d-rR-Tg(OIMA1-C315Y-MSTN-hrGFPII-FLAG). The transgenic fish exhibited increased production of skeletal muscle fibers at the adult stage (hyperplasia), although gross muscle mass was not altered. During embryogenesis, ectopic accumulation and misalignment of muscle fibers, possibly due to muscle-fiber hypertrophy, were observed in the transgenic medaka. Our findings suggest that MSTN function is required for regulating the appropriate growth of skeletal muscle in medaka. Unlike in mammals, MSTN loss-of-function failed to induce double-muscling in medaka. despite the highly conserved nature of MSTN function among taxa. (C) 2009 Elsevier Inc. All rights reserved.