circRNA Profiling Reveals Regulatory Networks Underlying Gonadal Differentiation in Nile Tilapia (Oreochromis niloticus)

The Nile tilapia (Oreochromis niloticus), a key aquaculture species, displays marked sexual growth dimorphism, with males growing faster than females. This process is governed by intricate interactions between antagonistic regulators, including transcription factors, growth factors, and steroid hormones, operating through sex-specific developmental pathways. While circular RNAs (circRNAs) are known to modulate gene expression by sponging microRNAs (miRNAs), their role in teleost sex differentiation remains poorly understood. To address this gap, we profiled circRNA expression in tilapia gonads by constructing six circRNA libraries from testes and ovaries of 180 days after hatching (dah) fish, followed by high-throughput sequencing. We identified 6564 gonadal circRNAs distributed across all 22 linkage groups, including 226 differentially expressed circRNAs (DECs; 108 testis-biased, 118 ovary-biased). Functional enrichment analysis linked their host genes to critical pathways such as cAMP signaling, cell adhesion molecules, and—notably—sexual differentiation processes (e.g., estrogen signaling, oocyte meiosis, and steroid hormone biosynthesis). Furthermore, we deciphered competing endogenous RNA (ceRNA) networks, uncovering circRNA–miRNA–mRNA interactions targeting germ cell determinants, sex-specific transcription factors, and steroidogenic enzymes. This study provides the first systematic exploration of circRNA involvement in tilapia sex differentiation and gonadal differentiation, offering novel insights into the post-transcriptional regulation of sexual dimorphism. Our findings advance the understanding of circRNA biology in fish and establish a framework for future studies on aquaculture species with similar reproductive strategies.