Cbx3a/HP1γ Deficiency Disrupts Meiotic Progression and Triggers Germ Cell Apoptosis in Nile Tilapia

Heterochromatin Protein 1γ, encoded by the Cbx3 gene, is a crucial epigenetic regulator that plays an essential role in mammalian meiotic progression. However, the functional divergence and conservation of this protein in teleosts-organisms possessing duplicated Cbx3 paralogs due to whole-genome duplication-remain to be elucidated. Building on previous research, we focused on cbx3a in Nile tilapia (Oreochromis niloticus), a significant aquaculture species and an excellent model for teleost reproductive studies, emphasizing its role in spermatogenesis. Expression analysis revealed that Cbx3a is localized to primordial germ cells and is sustained in spermatogonia, spermatocytes, and spermatids during spermatogenesis. CRISPR/Cas9-mediated knockout of cbx3a demonstrated that Cbx3a deficiency induces germ cell apoptosis, meiotic arrest, and sperm defects, including shortened tails and impaired motility, resulting in profound defects in sperm quantity and quality, strongly implying compromised male fertility. Transcriptomic analysis further identified dysregulated molecular pathways, including cytokine signaling and neuroactive ligand-receptor interactions. This provides novel mechanistic insights into HP1γ-mediated epigenetic regulation of meiosis. Notably, cbx3a mutants exhibited phenotypic bifurcation: a subset showed meiotic defects accompanied by sporadic germ cell apoptosis, whereas others underwent full meiotic arrest with pervasive germ cell apoptosis in adult gonads. Collectively, these findings clarify the essential and conserved role of Cbx3a/HP1γ in Nile tilapia spermatogenesis, thereby advancing the field of vertebrate reproductive epigenetics and providing a valuable theoretical basis for potential applications in reproductive management, such as improving sperm quality.