Spatiotemporal expression of activin receptor-like kinase-5 and bone morphogenetic protein receptor type II in the ovary of shortfinned eel, Anguilla australis

Cold temperature delays ovarian development of largemouth bass by inhibiting sex hormone release, angiogenesis, apoptosis and autophagy during out-of-season reproduction

Cold temperature is an effective method of achieving out-of-season reproduction and obtaining fry in the autumn. This study investigated the effects of low-temperature (12-16 °C) environment on the out-of-season reproduction of largemouth bass, particularly the delayed effects on ovarian development. During the period of delayed out-of-season reproduction, there was a significant reduction in the levels of serum sex hormones (FSH and LH) and their respective receptors (FSHR and LHCGR). Exposure to cold temperature significantly reduced the expression of gonadal development genes (IGF-1, GDF9, and CDC2) (P<0.05) and diminished the vascular network on the ovarian membrane, as confirmed by angiogenesis-related analyses. In lipid metabolism, AMH mRNA levels decreased overall, while HSD3B, FABP1, APOA1, and APOC2 initially increased before declining. Serum VTG levels decreased gradually with a slight increase post-spawning. These findings suggested that cold temperature delay ovarian development in largemouth bass by impacting sex hormone synthesis, angiogenesis, and lipid deposition. This insight enhances our understanding of out-of-season reproduction and guides the development of more effective reproductive techniques.

Full-length transcriptome analysis of male and female gonads in Japanese Eel (Anguilla japonica)

BACKGROUND

The Japanese eel (Anguilla japonica) holds significant economic value in East Asia, but limitations in understanding its reproductive biology have hindered advancements in artificial breeding techniques. Previous research has primarily focused on conserved sex differentiation genes, offering limited insights into the broader molecular mechanisms driving gonadal development and sexual dimorphism. To address these limitations, this study aims to investigate key genes and pathways involved in gonadal development through a comprehensive transcriptomic analysis of male and female eel gonads.

RESULTS

PacBio Iso-Seq and Illumina RNA-Seq technologies were combined to conduct a full-length transcriptome analysis of male and female Japanese eel gonads at a post-differentiation, pre-maturation stage. A total of 24,661 unigenes were identified in ovaries and 15,023 in testes, along with genomic regulatory elements such as transcription factors, simple sequence repeats, and long non-coding RNAs. Additionally, 1,210 differentially expressed genes were detected. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses revealed significant pathways involved in cell cycle regulation, metabolic processes, apoptosis, and hormone activity. Notably, several reproductive-related genes, including bambi, ccnb1, cdc20, gdf9, prlh, ccdc39, chrebp, tspo, syce3, and ngb, demonstrated significant dimorphic expression in eel gonads.

CONCLUSIONS

This study provides valuable insights into the molecular mechanisms of gonadal differentiation and sexual dimorphism in Japanese eels. The findings expand the genetic resources available for the eel breeding industry and could facilitate the development of improved artificial breeding techniques focused on reproductive development.

Lipoprotein receptors in ovary of eel, Anguilla australis: molecular characterisation of putative vitellogenin receptors

Lipoprotein receptors, including low-density lipoprotein receptor (LDLr) relatives (Lrs) and LDLr-related proteins (Lrps), belong to the LDLr supergene family and participate in diverse physiological functions. In this study, novel sequences of lr and lrp genes expressed in the ovary of the short-finned eel, Anguilla australis, during early gonadal development are presented. The genes encoding the LDLr-like, Lrp1-like, Lrp1b-like, Lrp3, Lrp4-like, Lrp5-like, Lrp6, Lrp10, Lrp11, Lrp12-like, and Lr11-like proteins were found and identified by sequence and structure analysis, in addition to phylogenetic analysis. Genes encoding proteins previously implicated in follicle development and vitellogenin (Vtg) uptake in oviparous vertebrates were also identified, i.e. lr8 (including lr8 + and lr8- variants) and lrp13; their identification was reinforced by conserved synteny with orthologues in other teleost fish. Compared to other lr/lrp genes, the genes encoding Lr8 + , Lr8-, and Lrp13 were highly expressed in ovary during early development, decreasing as oocyte development advanced when induced by hypophysation. Furthermore, lr8 + , lr8-, and lrp13 were dominantly expressed in the ovary when compared with 17 other tissues. Finally, this study successfully detected the expression of both lr8 variants, which showed different expression patterns to those reported in other oviparous vertebrates and provided the first characterisation of Lrp13 in Anguilla sp. We propose that lr8 + , lr8-, and lrp13 encode putative Vtg receptors in anguillid eels.

Expression Patterns and Gonadotropin Regulation of the TGF-β II Receptor (Bmpr2) during Ovarian Development in the Ricefield Eel Monopterus albus

Bmpr2 plays a central role in the regulation of reproductive development in mammals, but its role during ovarian development in fish is still unclear. To ascertain the function of bmpr2 in ovarian development in the ricefield eel, we isolated and characterized the bmpr2 cDNA sequence; the localization of Bmpr2 protein was determined by immunohistochemical staining; and the expression patterns of bmpr2 in ovarian tissue incubated with FSH and hCG in vitro were analyzed. The full-length bmpr2 cDNA was 3311 bp, with 1061 amino acids encoded. Compared to other tissues, bmpr2 was abundantly expressed in the ovary and highly expressed in the early yolk accumulation (EV) stages of the ovary. In addition, a positive signal for Bmpr2 was detected in the cytoplasm of oocytes in primary growth (PG) and EV stages. In vitro, the expression level of gdf9, the ligand of bmpr2, in the 10 ng/mL FSH treatment group was significantly higher after incubation for 4 h than after incubation for different durations. However, bmpr2 expression in the 10 ng/mL FSH treatment group at 2 h, 4 h and 10 h was significantly lower. Importantly, the expression level of bmpr2 and gdf9 in the 100 IU/mL hCG group had similar changes that were significantly decreased at 4 h and 10 h. In summary, Bmpr2 might play a pivotal role in ovarian growth in the ricefield eel, and these results provide a better understanding of the function of bmpr2 in ovarian development and the basic data for further exploration of the regulatory mechanism of gdf9 in oocyte development.

Single-Cell Atlas of the Chinese Tongue Sole (Cynoglossus semilaevis) Ovary Reveals Transcriptional Programs of Oogenesis in Fish

Oogenesis is a highly orchestrated process that depends on regulation by autocrine/paracrine hormones and growth factors. However, many details of 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 (Cynoglossus semilaevis). 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 found that the extensive niche–germline bidirectional communications regulate fish oogenesis, whereas ovulation in fish is accompanied by the coordination of simultaneous and tightly sequential processes across different granulosa cells. 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.