Single-cell reconstruction of the Chinese tongue sole (Cynoglossus semilaevis) ovary

Oogenesis is a highly orchestrated process that depends on regulation by autocrine/paracrine hormones and growth factors. The molecular mechanisms that regulate fish oogenesis remain elusive. Here, we performed a single-cell RNA sequencing (scRNA-seq) analysis of the molecular signatures of distinct ovarian cell categories in adult Chinese tongue sole. We characterized the successive stepwise development of three germ cell subtypes. Notably, we identified the cellular composition of fish follicle walls, including four granulosa cell types and one theca cell type, and we proposed important transcription factors (TFs) showing high activity in the regulation of cell identity. Moreover, we constructed a multilineage interactome network of fish oogenesis and validated the conservation of the NOTCH signalling pathway in niche–germline bidirectional communication across vertebrates; however, the TGF-β signalling pathway showed some degree of divergence, ascribed to the variability in the expression pattern of bmp15 in germ cells among species. Further analysis of the specific expression patterns of ovulation-associated genes revealed conserved essential features of ovulation between Chinese tongue sole and mammals. Additionally, a systems biology analysis of the homologous genes shared by Chinese tongue sole and macaques revealed remarkably conserved biological processes in germ cells and granulosa cells across vertebrates. Our results provide key insights into the cell-type-specific mechanisms underlying fish oogenesis at a single-cell resolution, which offers important clues for exploring fish breeding mechanisms and the evolution of vertebrate reproductive systems.