The ontogenic gonadal transcriptomes provide insights into sex change in the ricefield eel Monopterus albus

Abstract

Background The ricefield eel is a freshwater protogynous hermaphrodite fish and has become an important aquaculture species in China. The sex change of ricefield eel is impeding its aquaculture practice, particularly the large-scale artificial breeding. Many studies including transcriptomes of mixed gonadal samples from different individuals have been aimed to elucidate mechanisms underlying the sex change. However, the key physiological factors involved in the initiation of sex change remain to be identified. Results The present study performed transcriptomic analysis on gonadal samples of different sexual stages obtained through biopsy from the same fish undergoing sex change. A total of 539,764,816 high-quality reads were generated from twelve cDNA libraries of gonadal tissues at female (F), early intersexual (EI), mid-intersexual (MI), and late intersexual (LI) stages of three individual sex-changing fish. Pairwise comparisons between EI and F, MI and EI, and LI and MI identified 886, 319, and 10,767 differentially expressed genes (DEGs), respectively. Realtime quantitative PCR analysis of 12 representative DEGs showed similar expression profiles to those inferred from transcriptome data, suggesting the reliability of RNA-seq data for gene expression analysis. The expression of apoeb, csl2, and enpp2 was dramatically increased and peaked at EI while that of cyp19a1a, wnt4a, fgf16, and foxl2a significantly downregulated from F to EI and remained at very low levels during subsequent development until LI, which suggests that apoeb, csl2, enpp2, cyp19a1a, wnt4a, fgf16, and foxl2a may be closely associated with the initiation of sex change of ricefield eels. Conclusions Collectively, results of the present study confirmed that the down-regulation of female-related genes, such as cyp19a1a, wnt4a, fgf16, and foxl2a, is important for the sex change of ricefield eels. More importantly, some novel genes, including apoeb, csl2, and enpp2, were shown to be expressed with peak values at EI, which are potentially involved in the initiation of sex change. The present transcriptomic data may provide an important research resource for further unraveling the mechanisms underlying the sex change and testicular development in ricefield eels as well as other teleosts.

Genome-wide association study reveals genomic loci of sex differentiation and gonadal development in Plectropomus leopardus

“…As reported, ddx6 (probable ATP-dependent RNA helicase DDX6) belongs to a family of DEAD-box RNA helicases that was highly expressed during various stages of gonad development in the Chinese mitten crab ( Eriocheir sinensis ) ( Li et al, 2015 ). Based on the ontogenic gonadal transcriptomes, the mechanisms of sex change of the ricefield eel ( Monopterus albus ) have been elucidated ( Fan et al, 2022 ), and the expression of apoeb (apolipoprotein Eb) detected was dramatically increased and peaked at the early intersexual stage. Another study has suggested that the expression of apoeb was significantly different between old and young female brains in zebrafish ( Danio rerio ) ( Zhai et al, 2022 ).…”

Meta-analysis of gonadal transcriptome provides novel insights into sex change mechanism across protogynous fishes

“…Through this meta-analysis, we successfully identified sex-related factors including cyp19a1a , foxl2a , hsd17b1 , amh , and gsdf , that had been previously reported to play a role in sex change in various protogynous fish species 10,14,17,19 . Furthermore, our enrichment analysis of downregulated factors highlighted a term related to the “regulation of steroid hormone production.” This finding connects to those of previous reports, which indicate that a rapid decrease in estrogen levels is crucial for female-to-male sex change 6,45,46 .…”

“…These findings suggest that these factors are involved in gonadal transformation, particularly during the initiation of sex changes [16,19]. However, verifying their direct involvement in the sex change process remains challenging.…”

“…In particular, the expression patterns of multiple sex-related genes during sex change within the family Labridae, which serves as a model for protogynous sex change, have been explored [10][11][12][13][14][15] . Recently, nextgeneration sequencing technology has enabled researchers to conduct comprehensive analyses of gene expression, allowing the identification of global gene expression profiles during gonadal transformation in protogynous fish belonging to Labriformes, Perciformes, and Synbranchiformes [16][17][18][19][20] . Thus, these results suggest the involvement of several pivotal genes in the ovary-to-testis transformation in sex-changing fish.…”

“…In particular, upregulation of apolipoprotein Eb ( apoeb ), ectonucleotide pyrophosphatase/phosphodiesterase 2 ( enpp2 ), and l-rhamnose-binding lectin CSL2 ( csl2 ) expression, accompanied by downregulation of galectin-3 ( gal3 ), was observed during the early phase of gonadal sex change in the protogynous Asian swamp eel Monopterus albus . These findings suggest that these factors are involved in gonadal transformation, particularly during the initiation of sex change [16, 19]. However, verifying their direct involvement in the sex change process remains challenging.…”