Transcriptome Display During Testicular Differentiation of Channel Catfish (Ictalurus punctatus) as Revealed by RNA-Seq Analysis

Hydin as the Candidate Master Sex Determination Gene in Channel Catfish (Ictalurus punctatus) and Its Epigenetic Regulation

Sex determination is a fascinating area of research. To date, more than 20 master sex determination (SD) genes have been reported from vertebrate animals. With channel catfish (Ictalurus punctatus), much work has been conducted to determine its master SD gene, ranging from genetic linkage mapping, genome-wide association (GWA) analysis, genome sequencing, comparative genome analysis, epigenomic analysis, transcriptome analysis, and functional studies. Here in this mini review, we provide positional, expression, regulatory, and functional evidence supporting hydin (hydrocephalus-inducing protein or HYDIN axonemal central pair apparatus protein-like) as a master SD gene in channel catfish. Hydin is located within the sex determination region (SDR) within a mapped 8.9-Mb non-recombinational segment on chromosome 4 of channel catfish. It is highly expressed in genetic males, but not in genetic females. The alleles of X and Y are highly differentially methylated with the X chromosome being hypermethylated and the Y chromosome hypomethylated. The hypomethylated Y allele of hydin is expressed while the hypermethylated X allele is not expressed. Such allelic expression fits well with the XY sex determination system of channel catfish. Functional analysis using a methylation blocker, 5-aza-dC, demonstrated that demethylation, especially within the SDR, is accompanied with increased expression of hydin, which led to sex reversal of genetic females into phenotypic males. These evidences support the candidacy of hydin as a master SD gene in channel catfish. Future knockout and analysis of affected genes after hydin knockout should provide insights into how hydin functions as a master SD gene.

The potential regulatory role of the non-coding RNAs in regulating the exogenous estrogen-induced feminization in Takifugu rubripes gonad

Estrogen plays a pivotal role in the early stage of sex differentiation in teleost. However, the underlying mechanisms of estrogen-induced feminization process are still needed for further clarification. Here, the comparative analysis of whole-transcriptome RNA sequencing was conducted between 17beta-Estradiol induced feminized XY (E-XY) gonads and control gonads (C) in Takifugu rubripes. A total of 57 miRNAs, 65 lncRNAs, and 4 circRNAs were found to be expressed at lower levels in control-XY (C-XY) than that in control-XX (C-XX), and were up-regulated in XY during E2-induced feminization process. The expression levels of 24 miRNAs, and 55 lncRNAs were higher in C-XY than that in C-XX, and were down-regulated in E2-treated XY. Furthermore, a correlation analysis was performed between miRNA-seq and mRNA-seq data. In C-XX/C-XY, 114 differential expression (DE) miRNAs were predicted to target to 904 differential expression genes (DEGs), while in C-XY/E-XY, 226 DEmiRNAs were predicted to target to 2,048 DEGs. In C-XX/C-XY, and C-XY/E-XY, KEGG pathway enrichment analysis showed that those targeted genes were mainly enriched in MAPK signaling, calcium signaling, steroid hormone biosynthesis and ovarian steroidogenesis pathway. Additionally, the competitive endogenous RNA (ceRNA) regulatory network was constructed by 24 miRNAs, 21 lncRNAs, 4 circRNAs and 5 key sex-related genes. These findings suggested that the expression of critical genes in sex differentiation were altered in E2-treated XY T. rubripes may via the lncRNA-miRNA-mRNA regulation network to facilitate the differentiation and maintenance of ovaries. Our results provide a new insight into the comprehensive understanding of the effects of estrogen signaling pathways on sex differentiation in teleost gonads.

Gonadal transcriptome analysis of sex-biased gene and genome-wide investigation of dmrt gene family in Acanthogobius ommaturus

Acanthogobius ommaturus is one of the largest goby fish, and widely distributed in the Northwestern Pacific as an annual benthic fish. This study aims to report the gonadal transcriptome of A. ommaturus and identify differentially expressed genes (DEGs) between sexes. A total of 5460 (27.94 %) DEGs were detected from genome, with 3301 (16.89 %) biased towards males and 2159 (11.05 %) towards females. Analysis of 76 known vertebrate sex-related genes revealed multiple key genes, including the male-biased genes dmrt1 (Doublesex and Mab-3 related transcription factor 1) and amh (Anti-Mullerian Hormone), and the female-biased genes foxl2 (Forkhead Box L2) and cyp19a1a (Cytochrome P450 Aromatase 19 Subfamily A1). Furthermore, a genome-wide gene family analysis focused on the most significantly differentially expressed male-biased gene, dmrt1, was conducted using the chromosomal-level genome. Six Aodmrt genes were identified and subjected to phylogenetic and protein interaction network analyses. To validate the expression pattern, quantitative real-time PCR (qRT-PCR) was performed and compared with gonadal transcriptome data. The results showed that only dmrt1 exhibited significant male-bias, while the expression levels and sex differences of other dmrt genes in the gonads were inconclusive. Interestingly, the other dmrt genes displayed higher expression levels in other tissues, suggesting currently unknown functions. In conclusion, this study provides valuable genetic information contributing to the understanding of the sex determination mechanism of A. ommaturus and bony fish.

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

Introduction: Plectropomus leopardus, a commercially significant marine fish, is primarily found in the Western Pacific regions and along the coast of Southeast Asia. A thorough analysis of the molecular mechanisms involved in sex differentiation is crucial for gaining a comprehensive understanding of gonadal development and improving sex control breeding. However, the relevant fundamental studies of P. leopardus are relatively lacking. Methods: In this study, a genome-wide association study (GWAS) was conducted to investigate the genetic basis mechanism of sex differentiation and gonadal developmental traits in P. leopardus utilizing about 6,850,000 high-quality single-nucleotide polymorphisms (SNPs) derived from 168 individuals (including 126 females and 42 males) by the genome-wide efficient mixed-model association (GEMMA) algorithm. Results: The results of these single-trait GWASs showed that 46 SNP loci (-log10 p > 7) significantly associated with sex differentiation, and gonadal development traits were distributed in multiple different chromosomes, which suggested the analyzed traits were all complex traits under multi-locus control. A total of 1,838 potential candidate genes were obtained by considering a less-stringent threshold (-log10 p > 6) and ±100 kb regions surrounding the significant genomic loci. Moreover, 31 candidate genes were identified through a comprehensive analysis of significant GWAS peaks, gene ontology (GO) annotations, and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses, including taf7, ddx6, apoeb, sgk1, a2m, usf1, hsd3b7, dll4, xbp1, tet3, esr1, and gli3. These trait-associated genes have been shown to be involved in germline development, male sex differentiation, gonad morphogenesis, hormone receptor binding, oocyte development, male gonad development, steroidogenesis, estrogen-synthetic pathway, etc. Discussion: In the present study, multiple genomic loci of P. leopardus associated with sex differentiation and gonadal development traits were identified for the first time by using GWAS, providing a valuable resource for further research on the molecular genetic mechanism and sex control in P. leopardus. Our results also can contribute to understanding the genetic basis of the sex differentiation mechanism and gonadal development process in grouper fish.

Gonadal transcriptomes reveal sex-biased expression genes associated with sex determination and differentiation in red-tail catfish (Hemibagrus wyckioides)

BACKGROUND

Red-tail catfish (Hemibagrus wyckioides) is an important commercially farmed catfish in southern China. Males of red-tail catfish grow faster than females, suggesting that all-male catfish will produce more significant economic benefits in aquaculture practice. However, little research has been reported on sex determination and gonadal development in red-tail catfish.

RESULTS

In this study, we performed the first transcriptomic analysis of male and female gonads at four developmental stages at 10, 18, 30, and 48 days post hatching (dph) using RNA-seq technology. A total of 23,588 genes were screened in 24 sequenced samples, of which 28, 213, 636, and 1381 differentially expressed genes (DEGs) were detected at four developmental stages, respectively. Seven candidate genes of sex determination and differentiation were further identified. Real-time quantitative PCR (RT-qPCR) further confirmed that anti-Mullerian hormone (amh), growth differentiation factor 6a (gdf6a), testis-specific gene antigen 10 (tsga10), and cytochrome P450 family 17 subfamily A (cyp17a) were highly expressed mainly in the male, while cytochrome P450 family 19 subfamily A polypeptide 1b (cyp19a1b), forkhead box L2 (foxl2), and hydroxysteroid 17-beta dehydrogenase 1 (hsd17b1) were highly expressed in the female. The KEGG pathway enrichment data showed that these identified DEGs were mainly involved in steroid hormone biosynthesis and TGF-β signaling pathways.

CONCLUSIONS

Based on RNA-seq data of gonads at the early developmental stages, seven DEGs shared by the four developmental stages were identified, among which amh and gdf6a may be the male-biased expression genes, while foxl2, cyp19a1b and hsd17b1 may be the female-biased expression genes in red-tail catfish. Our study will provide crucial genetic information for the research on sex control in red-tail catfish, as well as for exploring the evolutionary processes of sex determination mechanisms in fish.

Bioinformatics approaches and big data analytics opportunities in improving fisheries and aquaculture

Aquaculture has witnessed an excellent growth rate during the last two decades and offers huge potential to provide nutritional as well as livelihood security. Genomic research has contributed significantly toward the development of beneficial technologies for aquaculture. The existing high throughput technologies like next-generation technologies generate oceanic data which requires extensive analysis using appropriate tools. Bioinformatics is a rapidly evolving science that involves integrating gene based information and computational technology to produce new knowledge for the benefit of aquaculture. Bioinformatics provides new opportunities as well as challenges for information and data processing in new generation aquaculture. Rapid technical advancements have opened up a world of possibilities for using current genomics to improve aquaculture performance. Understanding the genes that govern economically relevant characteristics, necessitates a significant amount of additional research. The various dimensions of data sources includes next-generation DNA sequencing, protein sequencing, RNA sequencing gene expression profiles, metabolic pathways, molecular markers, and so on. Appropriate bioinformatics tools are developed to mine the biologically relevant and commercially useful results. The purpose of this scoping review is to present various arms of diverse bioinformatics tools with special emphasis on practical translation to the aquaculture industry.

Crosstalk between reproductive and immune systems: the teleostean perspective

The bidirectional interaction between the hypothalamic-pituitary-gonadal (HPG) axis and the immune system plays a crucial role in the adaptation of an organism to its environment, its survival and the continuance of a species. Nonetheless, very little is known about this interaction among teleost, the largest group of extant vertebrates. Fishes being seasonal breeders, their immune system is exposed to seasonally changing levels of HPG hormones. On the contrary, the presence and infiltration of leukocytes, the expression of pattern recognition receptors as well as cytokines in gonads suggest their key role in teleostean gametogenesis as in the case of mammals. Moreover, the modulation of gametogenesis and steroidogenesis by lipopolysaccharide implicates the pathological significance of inflammation on reproduction. Thus, it is important to engage in the understanding of the interaction between these two important physiological systems, not only from a phylogenetic perspective but also due to the importance of fish as an important economic resource. In view of this, the authors have reviewed the crosstalk between the reproductive and immune systems in teleosts and tried to explore the importance of this interaction in their survival and reproductive fitness.

Comparative Transcriptome Analyses Characterize Expression Signatures Among Males, Females, Neo-Males, and Gynogenetic Females in the Yellow Drum (Nibea albiflora)

The yellow drum (Nibea albiflora) is one of the most important marine economic fish in China, and its sexually dimorphic growth makes it preferable for mono-sex culture. Although gynogenesis and neo-male induction techniques have been established, the molecular pathways and regulatory mechanisms of sex determination and maintenance in gynogenetic females and neo-males remains far from fully understood. In this study, the gene expression profiles were investigated in the gonads and brains of wild-type male, wild-type female, neo-male, and gynogenetic female yellow drum using comparative transcriptome analyses. Generally, a total of 52,999 novel transcripts were obtained in RNA-seq, of which 45,651 were isoforms of known protein-coding genes, 1,358 novel protein-coding genes, and 5,990 long non-coding RNAs. We found that the differences between wild-type males and neo-males and between wild-type females and gynogenetic females were relatively small at both the histological and transcriptomic levels, indicating that artificial gynogenesis or hormonal sex reversal may have minimal effects on normal female or male life function, respectively. In the brain, pathways such as “Oocyte meiosis”, “Cell cycle”, and “Riboflavin metabolism” were found to be significantly enriched. In the gonads, pathways such as “Prolactin signaling pathway”, “PPAR signaling pathway”, “Cholesterol metabolism”, and “Jak-STAT signaling pathway” were found to play important roles in maintaining the regular proliferation and differentiation of females and males in yellow drum. In particular, we found that zp4 might be an effective molecular marker to differentiate between gynogenetic and normal females owing to its unique expression pattern. The results of this study may help to elucidate the molecular mechanisms involved in sex maintenance in the gonads and brain and provide basic data for genetic breeding of the yellow drum.

Screening and Validation of p38 MAPK Involved in Ovarian Development of Brachymystax lenok

Brachymystax lenok (lenok) is a rare cold-water fish native to China that is of high meat quality. Its wild population has declined sharply in recent years, and therefore, exploring the molecular mechanisms underlying the development and reproduction of lenoks for the purposes of artificial breeding and genetic improvement is necessary. The lenok comparative transcriptome was analyzed by combining single molecule, real-time, and next generation sequencing (NGS) technology. Differentially expressed genes (DEGs) were identified in five tissues (head kidney, spleen, liver, muscle, and gonad) between immature [300 days post-hatching (dph)] and mature [three years post-hatching (ph)] lenoks. In total, 234,124 and 229,008 full-length non-chimeric reads were obtained from the immature and mature sequencing data, respectively. After NGS correction, 61,405 and 59,372 non-redundant transcripts were obtained for the expression level and pathway enrichment analyses, respectively. Compared with the mature group, 719 genes with significantly increased expression and 1,727 genes with significantly decreased expression in all five tissues were found in the immature group. Furthermore, DEGs and pathways involved in the endocrine system and gonadal development were identified, and p38 mitogen-activated protein kinases (MAPKs) were identified as potentially regulating gonadal development in lenok. Inhibiting the activity of p38 MAPKs resulted in abnormal levels of gonadotropin-releasing hormone, follicle-stimulating hormone, and estradiol, and affected follicular development. The full-length transcriptome data obtained in this study may provide a valuable reference for the study of gene function, gene expression, and evolutionary relationships in B. lenok and may illustrate the basic regulatory mechanism of ovarian development in teleosts.

Identification and Characterization of Dimorphic Expression of Sex-Related Genes in Rock Bream, a Fish With Multiple Sex Chromosomes

The rock bream (Oplegnathus fasciatus) is a typical fish with a unique multiple sex chromosome system. In this study, we investigated the gene expression profiling in the gonads and brains of both males and females using RNA-Seq to identify sex-related genes and pathways. In accordance with the dimorphic expression profiles, combined with Gene ontology and KEGG enrichment analyses, a number of potential genes and pathways associated with sex determination were obtained from transcriptional analysis, especially some sex-biased genes and pathways. Next, we selected 18 candidate genes and analyzed their expression in different tissues and developmental stages. We found that the expression levels of Amh, Dmrt1, Sox9, Dmrtb1, and Nanos2 were significantly higher in the testis than those in the ovary or other tissues, whereas the expression levels of ZP4, Bouncer, RNF208, FoxH1, and TOB were significantly higher in the ovary than those in the testis. Furthermore, the expression levels of these genes in different developmental stages of gonads also showed sexually dimorphic patterns, suggesting that they might play important roles during gonadal development. These genes are useful markers for investigating sex determination and differentiation in rock bream. The findings of this study can provide insights into the molecular mechanisms of sex determination and differentiation in fish with multiple sex chromosome systems.

Clustering of Sex-Biased Genes and Transposable Elements in the Genome of the Medaka Fish Oryzias latipes

Although genes with similar expression patterns are sometimes found in the same genomic regions, almost nothing is known about the relative organization in genomes of genes and transposable elements (TEs), which might influence each other at the regulatory level. In this study, we used transcriptomic data from male and female gonads of the Japanese medaka Oryzias latipes to define sexually biased genes and TEs and analyze their relative genomic localization. We identified 20,588 genes expressed in the adult gonads of O. latipes. Around 39% of these genes are differentially expressed between male and female gonads. We further analyzed the expression of TEs using the program SQuIRE and showed that more TE copies are overexpressed in testis than in ovaries (36% vs. 10%, respectively). We then developed a method to detect genomic regions enriched in testis- or ovary-biased genes. This revealed that sex-biased genes and TEs are not randomly distributed in the genome and a part of them form clusters with the same expression bias. We also found a correlation of expression between TE copies and their closest genes, which increases with decreasing intervening distance. Such a genomic organization suggests either that TEs hijack the regulatory sequences of neighboring sexual genes, allowing their expression in germ line cells and consequently new insertions to be transmitted to the next generation, or that TEs are involved in the regulation of sexual genes, and might therefore through their mobility participate in the rewiring of sex regulatory networks.

Advances in Reproductive Endocrinology and Neuroendocrine Research Using Catfish Models

Catfishes, belonging to the order siluriformes, represent one of the largest groups of freshwater fishes with more than 4000 species and almost 12% of teleostean population. Due to their worldwide distribution and diversity, catfishes are interesting models for ecologists and evolutionary biologists. Incidentally, catfish emerged as an excellent animal model for aquaculture research because of economic importance, availability, disease resistance, adaptability to artificial spawning, handling, culture, high fecundity, hatchability, hypoxia tolerance and their ability to acclimate to laboratory conditions. Reproductive system in catfish is orchestrated by complex network of nervous, endocrine system and environmental factors during gonadal growth as well as recrudescence. Lot of new information on the molecular mechanism of gonadal development have been obtained over several decades which are evident from significant number of scientific publications pertaining to reproductive biology and neuroendocrine research in catfish. This review aims to synthesize key findings and compile highly relevant aspects on how catfish can offer insight into fundamental mechanisms of all the areas of reproduction and its neuroendocrine regulation, from gametogenesis to spawning including seasonal reproductive cycle. In addition, the state-of-knowledge surrounding gonadal development and neuroendocrine control of gonadal sex differentiation in catfish are comprehensively summarized in comparison with other fish models.

Evolution, Expression, and Function of Gonadal Somatic Cell-Derived Factor

Fish gonads develop in very diverse ways different from mammalian gonads. This diversity is contributed by species-specific factors. Gonadal somatic cell-derived factor (Gsdf) is one such factor. The gsdf gene exists mostly in teleosts and is absent in many tetrapods, probably as a result of two gene losses during evolution. The gsdf transcript is expressed mainly in gonadal somatic cells, including Sertoli cell in testis and granulosa cells in ovary; however, these gonadal somatic cells can surround many types of germ cells at different developmental stages depending on the fish species. The function of gsdf is also variable. It is involved in germ cell proliferation, testicular formation, ovarian development and even male sex determination. Here, we summarize the common and diverse expression, regulation and functions of gsdf among different fish species with aspect of evolution.

A Chromosome-Level Genome Assembly of the Mandarin Fish (Siniperca chuatsi)

The mandarin fish, Siniperca chuatsi, is an economically important perciform species with widespread aquaculture practices in China. Its special feeding habit, acceptance of only live prey fishes, contributes to its delicious meat. However, little is currently known about related genetic mechanisms. Here, we performed whole-genome sequencing and assembled a 758.78 Mb genome assembly of the mandarin fish, with the scaffold and contig N50 values reaching 2.64 Mb and 46.11 Kb, respectively. Approximately 92.8% of the scaffolds were ordered onto 24 chromosomes (Chrs) with the assistance of a previously established genetic linkage map. The chromosome-level genome contained 19,904 protein-coding genes, of which 19,059 (95.75%) genes were functionally annotated. The special feeding behavior of mandarin fish could be attributable to the interaction of a variety of sense organs (such as vision, smell, and endocrine organs). Through comparative genomics analysis, some interesting results were found. For example, olfactory receptor (OR) genes (especially the beta and delta types) underwent a significant expansion, and endocrinology/vision related npy, spexin, and opsin genes presented various functional mutations. These may contribute to the special feeding habit of the mandarin fish by strengthening the olfactory and visual systems. Meanwhile, previously identified sex-related genes and quantitative trait locis (QTLs) were localized on the Chr14 and Chr17, respectively. 155 toxin proteins were predicted from mandarin fish genome. In summary, the high-quality genome assembly of the mandarin fish provides novel insights into the feeding habit of live prey and offers a valuable genetic resource for the quality improvement of this freshwater fish.

Differential expression analysis and identification of sex-related genes by gonad transcriptome sequencing in estradiol-treated and non-treated Ussuri catfish Pseudobagrus ussuriensis

The Ussuri catfish (Pseudobagrus ussuriensis) has an XX/XY sex determination system but its sex determination gene(s) remain unknown. To better understand the molecular sex determination mechanism, transcriptome analysis was conducted to obtain sex-related gene expression profiles. Transcriptome analyses were made of male and female developing/differentiating gonads by high-throughput RNA sequencing, including gonads from fish given an estradiol-induced sex reversal treatment. A total of 81,569 unigenes were assembled and 39,904 were significantly matched to known unique proteins by comparison with public databases. Twenty specifically expressed and 142 differentially expressed sex-related genes were extracted from annotated data by comparing the treatment groups. These genes are involved in spermatogenesis (e.g., Dnali1, nectin3, klhl10, mybl1, Katnal1, Eno4, Mns1, Spag6, Tsga10, Septin7), oogenesis (e.g., Lagr5, Fmn2, Npm2, zar1, Fbxo5, Fbxo43, Prdx4, Nrip1, Lfng, Atrip), gonadal development/differentiation (e.g., Cxcr4b, Hmgb2, Cftr, Ch25h, brip1, Prdm9, Tdrd1, Star, dmrt1, Tut4, Hsd17b12a, gdf9, dnd, arf1, Spata22), and estradiol response (e.g., Mmp14, Lhcgr, vtg1, vtg2, esr2b, Piwil1, Aifm1, Hsf1, gdf9). Dmrt1 and gdf9 may play an essential role in sex determination in P. ussuriensis. The expression patterns of six random genes were validated by quantitative real-time PCR, which confirmed the reliability and accuracy of the RNA-seq results. These data provide a valuable resource for future studies of gene expression and for understanding the molecular mechanism of sex determination/differentiation and gonadal development/differentiation (including hormone-induced sexual reversal) in Ussuri catfish. This has the potential to assist in producing monosex Ussuri catfish to increase aquacultural productivity.

Deep Transcriptomic Analysis Reveals the Dynamic Developmental Progression during Early Development of Channel Catfish (Ictalurus punctatus)

The transition from fertilized egg to larva in fish is accompanied with various biological processes. We selected seven early developmental stages in channel catfish, Ictalurus punctatus, for transcriptome analysis, and covered 22,635 genes with 590 million high-quality RNA-sequencing (seq) reads. Differential expression analysis between neighboring developmental timepoints revealed significantly enriched biological categories associated with growth, development and morphogenesis, which was most evident at 2 vs. 5 days post fertilization (dpf) and 5 vs. 6 dpf. A gene co-expression network was constructed using the Weighted Gene Co-expression Network Analysis (WGCNA) approach and four critical modules were identified. Among candidate hub genes, GDF10, FOXA2, HCEA and SYCE3 were involved in head formation, egg development and the transverse central element of synaptonemal complexes. CK1, OAZ2, DARS1 and UBE2V2 were mainly associated with regulation of cell cycle, growth, brain development, differentiation and proliferation of enterocytes. IFI44L and ZIP10 were critical for the regulation of immune activity and ion transport. Additionally, TCK1 and TGFB1 were related to phosphate transport and regulating cell proliferation. All these genes play vital roles in embryogenesis and regulation of early development. These results serve as a rich dataset for functional genomic studies. Our work reveals new insights of the underlying mechanisms in channel catfish early development.

Reproductive viability of paradoxically masculinised Gambusia holbrooki generated following diethylstilbestrol (DES) treatment

Hormonal sex reversal can produce monosex fish stocks and provide insights into their gamity and reproductive physiology. However, paradoxical effects have been reported in several fish species that remain largely ignored as anomalies, particularly those of masculinisation. As a first step, this study examined reproductive viability of paradoxically masculinised Gambusia holbrooki produced following oral administration (20-100 mg/kg feed) of a feminizing hormone diethylstilbestrol (DES). Contrary to expectation, all treatment groups produced 100% male populations. Survival, mating behaviour, gamete production, breeding output as well as expression of anti-Mullerian hormone (amh), ovarian (cyp19a1a) and brain (cyp19a1b) aromatase of masculinised fish were also examined. Survival (≤ 54.1 ± 7.3%) at termination of DES treatment was significantly lower compared with controls (88.6 ± 4.3%) but remained unaffected post treatment. Gonopodium thrusting frequency (33 ± 9.8 per 10 min) was not significantly different to untreated males just as sperm abundance (3.9 ± 1.5 × 10⁸/male) and their motility (88.6 ± 29.1%). Importantly, paradoxically masculinised fish mated with virgin females and produced clutch sizes (22 ± 4) and progeny survival (87.0 ± %) that were comparable to that of untreated males. Masculinised testes showed high amh and low cyp19a1a expression, a pattern resembling those of untreated males. Production of paradoxically sex-reversed males with a capability to produce viable offspring has not been reported previously in this or other fish species. The outcomes support a feed-back regulation of oestrogenic pathways in this viviparous fish and could be useful for ecological applications such as controlling invasive fish populations.

Transcriptome analysis of Clarias magur brain and gonads suggests neuro-endocrine inhibition of milt release from captive GnRH-induced males

Transcriptome analysis of Clarias magur brain and gonads at preparatory, mature, 6 and 16 h post-GnRH injection (hpi) stages yielded 9.5 GB data with 39,738 contigs. Sequences of 45 reproductive genes were identified for the first time in C. magur along with unique and differentially expressed genes. The expression of 20 genes was validated by qRT-PCR. Upregulation of Cyp11A1, Cyp17A1 and FTZF1 genes in the 16hpi testis accompanied by the 17β-HSD3 expression indicates testosterone (T) synthesis in response to LH surge, while reduced expression of CYP11B1 suggests a high T: 11-KT ratio. It is evident by the gene expression analysis that the inhibitory neurotransmitter GABA, altered T: 11-KT, increased testicular bile acids, and oxytocin-like neuropeptide in the male brain, appear to be involved in arresting the pulsatile motion of testicular smooth muscles. The work generates important leads for an effective induced breeding strategy for silurid catfish.

De novo transcriptome assembly and sex-biased gene expression in the gonads of Amur catfish (Silurus asotus)

Amur catfish is extensively distributed and cultured in Asian countries. Despite of economic importance, the genomic information of this species remains limited. A reference transcriptome of Amur catfish was assembled and the sex-biased gene expression in the gonads was characterized using RNA-sequencing. The assembled transcriptome of Amur catfish consisted of 74,840 transcripts. The N50, mean length and max length of transcripts are 1970, 1235 and 16,748 bp. Putative sex-specific transcripts were identified and sex-specific expression of the representative genes was verified by RT-PCR. Differential expression analysis identified 5401 ovary-biased and 5618 testis-biased genes. The ovary-biased genes were mainly enriched in pathways such as RNA transport and ribosome biogenesis in eukaryotes. The testis-biased genes were enriched in calcium signaling and cytokine-cytokine receptor interaction, etc. Our data provide a valuable genomic resource for further investigating the genetic basis of sex determination, sex differentiation and sexual dimorphism of catfish.

Transcriptome Sequencing Reveals the Traits of Spermatogenesis and Testicular Development in Large Yellow Croaker (Larimichthys crocea)

Larimichthys crocea is an economically important marine fish in China. To date, the molecular mechanisms underlying testicular development and spermatogenesis in L. crocea have not been thoroughly elucidated. In this study, we conducted a comparative transcriptome analysis between testes (TES) and pooled multiple tissues (PMT) (liver, spleen, heart, and kidney) from six male individuals. More than 54 million clean reads were yielded from TES and PMT libraries. After mapping to the draft genome of L. crocea, we acquired 25,787 genes from the transcriptome dataset. Expression analyses identified a total of 3853 differentially expressed genes (DEGs), including 2194 testes-biased genes (highly expressed in the TES) and 1659 somatic-biased genes (highly expressed in the PMT). The dataset was further annotated by blasting with multi-databases. Functional genes and enrichment pathways involved in spermatogenesis and testicular development were analyzed, such as the neuroactive ligand-receptor interaction pathway, gonadotropin-releasing hormone (GnRH) and mitogen-activated protein kinase (MAPK) signaling pathways, cell cycle pathway, and dynein, kinesin, myosin, actin, heat shock protein (hsp), synaptonemal complex protein 2 (sycp2), doublesex- and mab-3-related transcription factor 1 (dmrt1), spermatogenesis-associated genes (spata), DEAD-Box Helicases (ddx), tudor domain-containing protein (tdrd), and piwi genes. The candidate genes identified by this study lay the foundation for further studies into the molecular mechanisms underlying testicular development and spermatogenesis in L. crocea.

Characterization of the European Sea Bass (Dicentrarchus labrax) Gonadal Transcriptome During Sexual Development

The European sea bass is one of the most important cultured fish in Europe and has a marked sexual growth dimorphism in favor of females. It is a gonochoristic species with polygenic sex determination, where a combination between still undifferentiated genetic factors and environmental temperature determines sex ratios. The molecular mechanisms responsible for gonadal sex differentiation are still unknown. Here, we sampled fish during the gonadal developmental period (110 to 350 days post fertilization, dpf), and performed a comprehensive transcriptomic study by using a species-specific microarray. This analysis uncovered sex-specific gonadal transcriptomic profiles at each stage of development, identifying larger number of differentially expressed genes in ovaries when compared to testis. The expression patterns of 54 reproduction-related genes were analyzed. We found that hsd17β10 is a reliable marker of early ovarian differentiation. Further, three genes, pdgfb, snx1, and nfy, not previously related to fish sex differentiation, were tightly associated with testis development in the sea bass. Regarding signaling pathways, lysine degradation, bladder cancer, and NOD-like receptor signaling were enriched for ovarian development while eight pathways including basal transcription factors and steroid biosynthesis were enriched for testis development. Analysis of the transcription factor abundance showed an earlier increase in females than in males. Our results show that, although many players in the sex differentiation pathways are conserved among species, there are peculiarities in gene expression worth exploring. The genes identified in this study illustrate the diversity of players involved in fish sex differentiation and can become potential biomarkers for the management of sex ratios in the European sea bass and perhaps other cultured species.

Polyadenylation sites and their characteristics in the genome of channel catfish (Ictalurus punctatus) as revealed by using RNA-Seq data

Polyadenylation plays important roles in gene expression regulation in eukaryotes, which typically involves cleavage and poly(A) tail addition at the polyadenylation site (PAS) of the pre-mature mRNA. Many eukaryotic genes contain more than one PASs, termed as alternative polyadenylation (APA). As a crucial post-transcriptional regulation, polyadenylation affects various aspects of RNA metabolism such as mRNA stability, translocation, and translation. However, polyadenylation has been rarely studied in teleosts. Here we conducted polyadenylation analysis in channel catfish, a commercially important aquaculture species around the world. Using RNA-Seq data, we identified 20,320 PASs which were classified into 14,500 clusters by merging adjacent PASs. Most of the PASs were found in 3' UTRs, followed by intron regions based on the annotation of channel catfish reference genome. No apparent difference in PAS distribution was observed between the sense and antisense strand of the channel catfish genome. The sequence analysis of nucleotide composition and motif around PASs yielded a highly similar profile among various organisms, suggesting the conservation and importance of polyadenylation in evolution. Using APA genes with more than two PASs, gene ontology enrichment revealed genes particularly involved in RNA binding. Reactome pathway analysis showed the enrichment of the innate immune system, especially neutrophil degranulation.

De novo transcriptome based on next-generation sequencing reveals candidate genes with sex-specific expression in Arapaima gigas (Schinz, 1822), an ancient Amazonian freshwater fish

Background

The Arapaima (Arapaima gigas) is one of the world's largest freshwater bony fish, and is found in the rivers of the Amazon basin. This species is a potential aquaculture resource, although reproductive management in captivity is limited in particular due to the lack of external sexual dimorphism. In this study, using the 454 Roche platform (pyrosequencing) techniques, we evaluated a major portion of the transcriptome of this important Amazonian species.

Results

Four libraries obtained from the liver and skin tissue of juvenile specimens (representing males and females separately) were sequenced, yielding 5,453,919 high-quality reads. The de novo transcriptome assembly resulted in 175,792 contigs, with 51,057 significant blast hits. A total of 38,586 transcripts were mapped by Gene Ontology using Blast2GO. We identified 20,219 genes in the total transcriptome (9,551 in the liver and 16,818 in the skin). The gene expression analyses indicated 105 genes in the liver and 204 in the skin with differentiated expression profiles, with 95 being over-expressed in the females and 214 in the males. The log2 Fold Change and heatmap based on Reads Per Kilobase per Million mapped reads (RPKM) revealed that the gene expression in the skin is highly differentiated between male and female arapaima, while the levels of expression in the liver are similar between the sexes.

Conclusion

Transcriptome analysis based on pyrosequencing proved to be a reliable tool for the identification of genes with differentiated expression profiles between male and female arapaima. These results provide useful insights into the molecular pathways of sexual dimorphism in this important Amazonian species, and for comparative analyses with other teleosts.

Comparative transcriptome analysis of the swimbladder reveals expression signatures in response to low oxygen stress in channel catfish, Ictalurus punctatus

Channel catfish is the leading aquaculture species in the US, and one of the reasons for its application in aquaculture is its relatively high tolerance against hypoxia. However, hypoxia can still cause huge economic losses to the catfish industry. Studies on hypoxia tolerance, therefore, are important for aquaculture. Fish swimbladder has been considered as an accessory respiration organ surrounded by a dense capillary countercurrent exchange system. In this regard, we conducted RNA-Seq analysis with swimbladder samples of catfish under hypoxic and normal conditions to determine if swimbladder was responsive to low oxygen treatment and to reveal genes, their expression patterns, and pathways involved in hypoxia responses in catfish. A total of 155 differentially expressed genes (DEGs) were identified from swimbladder of adult catfish, whereas a total of 2,127 DEGs were identified from swimbladder of fingerling catfish under hypoxic condition as compared with untreated controls. Subsequent pathway analysis revealed that many DEGs under hypoxia were involved in HIF signaling pathway ( nos2, eno2, camk2d2, prkcb, cdkn1a, eno1, and tfrc), MAPK signaling pathway (voltage-dependent calcium channel subunit genes), PI3K/Akt/mTOR signaling pathway ( itga6, g6pc, and cdkn1a), Ras signaling pathway ( efna3 and ksr2), and signaling by VEGF ( fn1, wasf3, and hspb1) in catfish swimbladder. This study provided insights into regulation of gene expression and their involved gene pathways in catfish swimbladder in response to low oxygen stresses.

Transcriptome display during tilapia sex determination and differentiation as revealed by RNA-Seq analysis

Background

The factors determining sex in teleosts are diverse. Great efforts have been made to characterize the underlying genetic network in various species. However, only seven master sex-determining genes have been identified in teleosts. While the function of a few genes involved in sex determination and differentiation has been studied, we are far from fully understanding how genes interact to coordinate in this process.

Results

To enable systematic insights into fish sexual differentiation, we generated a dynamic co-expression network from tilapia gonadal transcriptomes at 5, 20, 30, 40, 90, and 180 dah (days after hatching), plus 45 and 90 dat (days after treatment) and linked gene expression profiles to both development and sexual differentiation. Transcriptomic profiles of female and male gonads at 5 and 20 dah exhibited high similarities except for a small number of genes that were involved in sex determination, while drastic changes were observed from 90 to 180 dah, with a group of differently expressed genes which were involved in gonadal differentiation and gametogenesis. Weighted gene correlation network analysis identified changes in the expression of Borealin, Gtsf1, tesk1, Zar1, Cdn15, and Rpl that were correlated with the expression of genes previously known to be involved in sex differentiation, such as Foxl2, Cyp19a1a, Gsdf, Dmrt1, and Amh.

Conclusions

Global gonadal gene expression kinetics during sex determination and differentiation have been extensively profiled in tilapia. These findings provide insights into the genetic framework underlying sex determination and sexual differentiation, and expand our current understanding of developmental pathways during teleost sex determination.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4756-0) contains supplementary material, which is available to authorized users.

The CC and CXC chemokine receptors in channel catfish (Ictalurus punctatus) and their involvement in disease and hypoxia responses

Chemokines are vital regulators of cell mobilization for immune surveillance, inflammation, and development. Chemokines signal through binding to their receptors that are a superfamily of seven-transmembrane domain G-coupled receptors. Recently, a complete repertoire of both CC and CXC chemokines have been identified in channel catfish, but nothing is known about their receptors. In this study, a set of 29 CC chemokine receptor (CCR) genes and 8 CXC chemokine receptor (CXCR) genes were identified and annotated from the channel catfish genome. Extensive phylogenetic and comparative genomic analyses were conducted to annotate these genes, revealing fish-specific CC chemokine receptors, and lineage-specific tandem duplications of chemokine receptors in the teleost genomes. With 29 genes, the channel catfish genome harbors the largest numbers of CC chemokine receptors among all the genomes characterized. Analysis of gene expression after bacterial infections indicated that the chemokine receptors were regulated in a gene-specific manner. Most differentially expressed chemokine receptors were up-regulated after Edwardsiella ictaluri and Flavobacterium columnare infection. Among which, CXCR3 and CXCR4 were observed to participate in immune responses to both bacterial infections, indicating their potential roles in catfish immune activities. In addition, CXCR3.2 was significantly up-regulated in ESC-susceptible fish, and CXCR4b was mildly induced in ESC-resistant fish, further supporting the significant roles of CXCR3 and CXCR4 in catfish immune responses. CXCR4b and CCR9a were both up-regulated not only after bacterial infection, but also after hypoxia stress, providing the linkage between bacterial infection and low oxygen stresses. These results should be valuable for comparative immunological studies and provide insights into their roles in disease and stress responses.

Dimorphic expression of sex-related genes in different gonadal development stages of sterlet, Acipenser ruthenus, a primitive fish species

Molecular mechanism of sex determination and differentiation of sturgeon, a primitive fish species, is extraordinarily important due to the valuable caviar; however, it is still poorly known. The present work aimed to identify the major genes involved in regulating gonadal development of sterlet, a small species of sturgeon, from 13 candidate genes which have been shown to relate to gonadal differentiation and development in other teleost fish. The sex and gonadal development of sterlets were determined by histological observation and levels of sex steroids testosterone (T), 11-ketotestosterone (11-KT), and 17β-estradiol (E2) in serum. Sexually dimorphic gene expressions were investigated. The results revealed that gonadal development were asynchronous in 2-year-old male and female sterlets with the testes in early or mid-spermatogenesis and the ovaries in chromatin nucleolus stage or perinucleolus stage, respectively. The levels of T and E2 were not significantly different between sexes or different gonadal development stages while 11-KT had the higher level in mid-spermatogenesis testis stage. In all the investigated gonadal development stages, gene dmrt1 and hsd11b2 were expressed higher in male whereas foxl2 and cyp19a1 were expressed higher in female. Thus, these genes provided the promising markers for sex identification of sterlet. It was unexpected that dkk1 and dax1 had significantly higher expression in ovarian perinucleolus stage than in ovarian chromatin nucleolus stage and in the testis, suggesting that these two genes had more correlation with ovarian development than with the testis, contrary to the previous reports in other vertebrates. Testicular development-related genes (gsdf and amh) and estrogen receptor genes (era and erb) differentially expressed at different testis or ovary development stages, but their expressions were not absolutely significantly different in male and female, depending on the gonadal development stage. Expression of androgen receptor gene ar or rspo, which was supposed to be related to ovarian development, presented no difference between gonadal development stages investigated in this study whenever in male or female.

Comparative transcriptome analysis reveals conserved branching morphogenesis related genes involved in chamber formation of catfish swimbladder

The swimbladder is an internal gas-filled organ in teleosts. Its major function is to regulate buoyancy. The swimbladder exhibits great variation in size, shape, and number of compartments or chambers among teleosts. However, genomic control of swimbladder variation is unknown. Channel catfish ( Ictalurus punctatus), blue catfish ( Ictalurus furcatus), and their F1 hybrids of female channel catfish × male blue catfish (C × B hybrid catfish) provide a good model in which to investigate the swimbladder morphology, because channel catfish possess a single-chambered swimbladder, whereas blue catfish possess a bichambered swimbladder; C × B hybrid catfish possess a bichambered swimbladder but with a significantly reduced posterior chamber. Here we determined the transcriptional profiles of swimbladder from channel catfish, blue catfish, and C × B hybrid catfish. We examined their transcriptomes at both the fingerling and adult stages. Through comparative transcriptome analysis, ~4,000 differentially expressed genes (DEGs) were identified. Among these DEGs, members of the Wnt signaling pathway ( wnt1, wnt2, nfatc1, rac2), Hedgehog signaling pathway ( shh), and growth factors ( fgf10, igf-1) were identified. As these genes were known to be important for branching morphogenesis of mammalian lung and of mammary glands, their association with budding of the posterior chamber primordium and progressive development of bichambered swimbladder in fish suggest that these branching morphogenesis-related genes and their functions in branching are evolutionarily conserved across a broad spectrum of species.

The chemokinome superfamily: II. The 64 CC chemokines in channel catfish and their involvement in disease and hypoxia responses

Chemokines are a superfamily of structurally related chemotactic cytokines exerting significant roles in regulating cell migration and activation. Based on the arrangement of the first four cysteine residues, they are classified into CC, CXC, C and CX3C subfamilies. In this study, a complete set of 64 CC chemokine ligand (CCL) genes was systematically identified, annotated, and characterized from the channel catfish genome. Extensive phylogenetic and comparative genomic analyses supported their annotations, allowing establishment of their orthologies, revealing fish-specific CC chemokines and the expansion of CC chemokines in the teleost genomes through lineage-specific tandem duplications. With 64 genes, the channel catfish genome harbors the largest numbers of CC chemokines among all the genomes characterized to date, however, they fall into 11 distinct CC chemokine groups. Analysis of gene expression after bacterial infections indicated that the CC chemokines were regulated in a gene-specific and time-dependent manner. While only one member of CCL19 (CCL19a.1) was significantly up-regulated after Edwardsiella ictaluri infection, all CCL19 members (CCL19a.1, CCL19a.2 and CCL19b) were significantly induced after Flavobacterium columnare infection. In addition, CCL19a.1, CCL19a.2 and CCL19b were also drastically up-regulated in ESC-susceptible fish, but not in resistant fish, suggesting potential significant roles of CCL19 in catfish immune responses. High expression levels of certain CC appeared to be correlated with susceptibility to diseases and intolerance to hypoxia.

De novo sequencing and comparative analysis of testicular transcriptome from different reproductive phases in freshwater spotted snakehead Channa punctatus

The spotted snakehead Channa punctatus is a seasonally breeding teleost widely distributed in the Indian subcontinent and economically important due to high nutritional value. The declining population of C. punctatus prompted us to focus on genetic regulation of its reproduction. The present study carried out de novo testicular transcriptome sequencing during the four reproductive phases and correlated differential expression of transcripts with various testicular events in C. punctatus. The Illumina paired-end sequencing of testicular transcriptome from resting, preparatory, spawning and postspawning phases generated 41.94, 47.51, 61.81 and 44.45 million reads, and 105526, 105169, 122964 and 106544 transcripts, respectively. Transcripts annotated using Rattus norvegicus reference protein sequences and classified under various subcategories of biological process, molecular function and cellular component showed that the majority of the subcategories had highest number of transcripts during spawning phase. In addition, analysis of transcripts exhibiting differential expression during the four phases revealed an appreciable increase in upregulated transcripts of biological processes such as cell proliferation and differentiation, cytoskeleton organization, response to vitamin A, transcription and translation, regulation of angiogenesis and response to hypoxia during spermatogenically active phases. The study also identified significant differential expression of transcripts relevant to spermatogenesis (mgat3, nqo1, hes2, rgs4, cxcl2, alcam, agmat), steroidogenesis (star, tkt, gipc3), cell proliferation (eef1a2, btg3, pif1, myo16, grik3, trim39, plbd1), cytoskeletal organization (espn, wipf3, cd276), sperm development (klhl10, mast1, hspa1a, slc6a1, ros1, foxj1, hipk1), and sperm transport and motility (hint1, muc13). Analysis of functional annotation and differential expression of testicular transcripts depending on reproductive phases of C. punctatus helped in developing a comprehensive understanding on genetic regulation of spermatogenic and steroidogenic events in seasonally breeding teleosts. Our findings provide the basis for future investigation on the precise role of testicular genes in regulation of seasonal reproduction in male teleosts.

Development of a 690 K SNP array in catfish and its application for genetic mapping and validation of the reference genome sequence

Single nucleotide polymorphisms (SNPs) are capable of providing the highest level of genome coverage for genomic and genetic analysis because of their abundance and relatively even distribution in the genome. Such a capacity, however, cannot be achieved without an efficient genotyping platform such as SNP arrays. In this work, we developed a high-density SNP array with 690,662 unique SNPs (herein 690 K array) that were relatively evenly distributed across the entire genome, and covered 98.6% of the reference genome sequence. Here we also report linkage mapping using the 690 K array, which allowed mapping of over 250,000 SNPs on the linkage map, the highest marker density among all the constructed linkage maps. These markers were mapped to 29 linkage groups (LGs) with 30,591 unique marker positions. This linkage map anchored 1,602 scaffolds of the reference genome sequence to LGs, accounting for over 97% of the total genome assembly. A total of 1,007 previously unmapped scaffolds were placed to LGs, allowing validation and in few instances correction of the reference genome sequence assembly. This linkage map should serve as a valuable resource for various genetic and genomic analyses, especially for GWAS and QTL mapping for genes associated with economically important traits.

Expression of Bcl-2 genes in channel catfish after bacterial infection and hypoxia stress

Bcl-2 proteins are of vital importance in regulation of apoptosis, and are involved in a number of biological processes such as carcinogenesis and immune responses. Bcl-2 genes have been well studied in mammals, while they are not well investigated in teleost fish including channel catfish, the major aquaculture species in the United States. In this study, we identified 34 bcl-2 genes from the channel catfish genome, and verified their identities by conducting phylogenetic and syntenic analyses. The expression profiles of the bcl-2 genes in response to bacterial infections (Edwardsiella ictaluri and Flavobacterium columnare) and hypoxia stress were determined by performing meta-analysis using the existing RNA-Seq datasets. Differential expressions of bcl-2 genes were observed after bacterial infections and hypoxia treatment, including 22 bcl-2 genes after E. ictaluri infection, 22 bcl-2 genes after F. columnare infection, and 19 bcl-2 genes after hypoxia stress. Overall, the expression of the pro-apoptotic bcl-2 genes were repressed after bacterial infection and hypoxia stress, indicating that bcl-2 genes are potentially involved in the stress response by reducing cell apoptosis. Some bcl-2 genes, such as bcl2b, mcl1a, bmf1, and bnip3, showed different expression pattern during the E. ictaluri and F. columnare infection, suggesting the difference in the pathogenicity of diseases. This work presented the first systematic identification and annotation of bcl-2 genes in catfish, providing essential genomic resources for further immune and physiological studies.

De novo Assembly and Analysis of the Chilean Pencil Catfish Trichomycterus areolatus Transcriptome

Trichomycterus areolatus is an endemic species of pencil catfish that inhabits the riffles and rapids of many freshwater ecosystems of Chile. Despite its unique adaptation to Chile's high gradient watersheds and therefore potential application in the investigation of ecosystem integrity and environmental contamination, relatively little is known regarding the molecular biology of this environmental sentinel. Here, we detail the assembly of the Trichomycterus areolatus transcriptome, a molecular resource for the study of this organism and its molecular response to the environment. RNA-Seq reads were obtained by next-generation sequencing with an Illumina® platform and processed using PRINSEQ. The transcriptome assembly was performed using TRINITY assembler. Transcriptome validation was performed by functional characterization with KOG, KEGG, and GO analyses. Additionally, differential expression analysis highlights sex-specific expression patterns, and a list of endocrine and oxidative stress related transcripts are included.