Transcriptomic analysis of the differentiating ovary of the protogynous ricefield eel Monopterus albus

Abstract

BackgroundThe ricefield eel is a protogynous hermaphroditic Synbranchiform species that changes sex naturally from female to male, which offers an interesting model for studying gonadal (particularly ovarian) differentiation in vertebrates. In the present study, transcriptome sequencing of the gonad of ricefield eel larvae was performed to explore the molecular mechanisms underlying the ovarian differentiation and development.ResultsA total of 301,267,988 clean reads were generated from cDNA libraries of gonadal tissues of ricefield eel larvae at 6, 9, 12, and 20 days post hatching (dph), which contained undifferentiated gonads, differentiating ovaries, ovaries with oogonia, and ovaries with meiotic oocytes, respectively. De-novo assembly of all the clean reads generated a total of 265,896 unigenes with a mean size of 720 bp and a N50 of 1107 bp. RT-qPCR analysis of the developmental expression of 13 gonadal development-related functional genes indicated that RNA-seq data are reliable. Transcriptome data suggest that high expression of female development-related genes and low expression of male development-related genes in the early gonads of ricefield eel larvae participate in the cascade of sex differentiation leading to the final female phenotype. The contrasting expression patterns of genes involved in retinoid acid (RA) synthesis and degradation might result in peak production of RA at 12 dph in the gonad of ricefield eel larvae, and induce molecular events responsible for the initiation of meiosis before the meiotic signs could be observed at 20 dph. In addition, only stra6 but not stra8 could be identified in gonadal transcriptome data of ricefield eel larvae, and the expression pattern of stra6 paralleled those of genes involved in RA synthesis, suggesting that stra6 may be a downstream target of RA and play a role in RA metabolism and/or meiotic initiation in the gonad of ricefield eel larvae.ConclusionsThe present study depicted the first large-scale RNA sequencing of the gonad of ricefield eel larvae, and identified many important functional genes, GO terms and KEGG pathways involved in gonadal development and germ cell meiosis. Results of the present study will facilitate future study on the ovarian differentiation of ricefield eels and other teleosts as well.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-017-3953-6) contains supplementary material, which is available to authorized users.

Transcriptomic analysis of female and male gonads in juvenile snakeskin gourami (Trichopodus pectoralis)

“…Our results implyed that these genes might be associated with biological and physiological processes during gonad differentiation. Similar findings were demonstrated in bluehead wrasses (Thalassoma bifasciatum) 9 , southern bluefin tuna (Thunnus maccoyii) 10 , red porgy (Pagrus pagrus) 13 , common pandora (Pagellus erythrinus) 13 , yellow perch (Perca flavescens) 15 , and ricefield eel (Monopterus albus) 12 . Our results identified similar enriched GO terms between ovary and testis, with the highest enriched GO terms being for binding, cellular processes, and metabolic processes.…”

“…In our study, transcriptomic analysis between testis and ovary demonstrated global differential expression of transcripts during sex differentiation in juvenile snakeskin gourami. Our results showed that 9.8% (5,300) of the assembled unigenes had a significant annotation to all databases, which was considerably high when compared with previous reports of ovarian transcriptomic analysis of ricefield eel (Monopterus albus), showing only 3.36% of assembled matching to all databases 12 . Therefore, our transcriptome sequencing and annotating were considered acceptable for further analysis.…”

“…The expressed transcripts were involved in various functional roles, indicating that a number of biological and physiological functions are involved in testis and ovary development. Similarly, high-throughput transcriptomic analysis has been applied to investigate global DEGs between sexes in gonads, brain, liver, muscle, and caudal fin in several teleosts, including gonochoristic and hermaphroditic fish [9][10][11][12][13][14][15][16][17][18] . These high-throughput transcriptome analyses could provide a holistic approach that will be a useful tool for further investigation of various aspects of reproductive biology.…”

“…Furthermore, different sex-determining genes have been shown to exhibit diversity across different fish lineages 7 , and it has been demonstrated and suggested that young Y chromosome drives the sex-biased expression of genes 8 . Currently, to provide a holistic view of the evolution of reproduction biology, genetic studies on sexual system in fish have focused on sex-specific genetic differences and/or sex-biased gene expression, particularly in hermaphroditic and gonochoristic fish, which exhibit strong sexual dimorphism [9][10][11][12][13][14][15][16][17][18] .…”

Comparative transcriptome reveal the potential adaptive evolutionary genes in Andrias davidianus

“…However, because of the large genome of the A. davidianus, evolutionary analysis by comparative genome was hard to carry out. Transcriptome sequencing was a valuable way to obtain large-scale sequences without reference genome [ 21 , 22 ]. Phylogenetic analysis of transcriptome sequence data exhibited high supported tree topologies in many species [ 23 , 24 ].…”