Characterization of growth differentiation factor 9 and bone morphogenetic factor 15 in Chinese tongue sole (Cynoglossus semilaevis): Sex-biased expression pattern and promoter regulation

Sexual size dimorphism (SSD), characterized by the body size difference in different sexes, has been commonly announced in various species included mammals, birds, reptiles, and fishes. The endocrine factors in the gonads has been regarded to be involved in SSD. Two oocyte secreted factors-growth differentiation factor 9 (gdf9) and bone morphogenetic factor 15 (bmp15) has been shown to be differentially expressed in the gonad of Chinese tongue sole (Cynoglossus semilaevis), a typical marine fish demonstrating female-biased SSD. To figure out their possible roles in fish SSD, gdf9 and bmp15 of C. semilaevis were firstly cloned. The subsequently phylogenetic and structural analysis revealed that gdf9 and bmp15 were clustered with other fish species and both contained TGF-beta domain in the C-terminal. Furthermore, the temporal and spatial expression by qRT-PCR showed that gdf9 and bmp15 displayed the highest expression level in the female gonad. Moreover, the highest levels of gdf9 and bmp15 transcripts were both detected in the 1.5-year-old female gonad. The in situ hybridization and immunofluorescence experiments revealed that their mRNAs and proteins were both located in the oocyte. Based on the methylome data and bisulfite sequencing PCR, the lowest DNA methylation levels for gdf9 was observed in the female gonad, mainly distributed in the upstream and genebody regions. As for bmp15 gene, the methylation level of females in the genebody region, especially the exon 1, was higher than that of males and pseudomale, while the methylation level of females in the downstream was the lowest. Finally, knock-down of gdf9 siRNA in C. semilaevis ovarian cells resulted in the down-regulation of alk4 and tgfbr1, and up-regulation of bmpr2, smad8, and bmp15. Taken together, the female-gonad-biased expression of gdf9 and bmp15 may be partly attributed to their upstream or genebody DNA methylation status. Gdf9 might be involved in reproduction and growth regulation of C. semilaevis by affecting Smad signaling pathway. Further exploration for these two ovarian factors would be helpful to better understand C. semilaevis SSD. (c) 2022 Elsevier Inc. All rights reserved.