Abstract

To optimize all-female production in marbled flounder ( Pseudopleuronectes yokohamae ), a species with marked female-biased growth, hormonal induction of pseudomales using 17 α -methyltestosterone (MT) is essential. However, its impact on somatic growth is dose-dependent, and the underlying mechanisms remain poorly understood. This study employed transcriptome sequencing of muscle and liver to decipher the molecular basis of growth regulation in juveniles fed diets containing 0, 0.5, or 2.0 mg/kg MT. Phenotypically, the 0.5 mg/kg dose sustained growth, while the 2.0 mg/kg dose caused significant inhibition, aligning with prior data. RNA-seq revealed a hierarchical transcriptional response. The low dose induced modest changes primarily in metabolic and signaling pathways. Strikingly, the high dose triggered a potent, coordinated activation of core anabolic and proliferative pathways (e.g., cell cycle, ribosome biogenesis, and aminoacyl-tRNA biosynthesis) in both tissues. Crucially, this activation in muscle was paralleled by a pronounced suppression of the systemic growth hormone/insulin-like growth factor (GH/IGF) axis, evidenced by the downregulation of igf1 and ghr . This conflict between activated cellular biosynthesis and repressed endocrine growth signaling provides a direct mechanistic explanation for the high-dose growth inhibition. Tissue specialization was evident: muscle transcriptomes emphasized structural construction, whereas hepatic responses centered on metabolic reprogramming and proliferation. Key gene expression trends were validated by qRT-PCR. Our findings elucidate that the growth-suppressive effect of high-dose MT originates from a maladaptive transcriptional imbalance. They provide a strong molecular rationale for employing the 0.5 mg/kg MT dosage to achieve effective sex reversal without compromising growth in marbled flounder aquaculture.