Role of Halloween genes in ecdysteroids biosynthesis of the swimming crab (Portunus trituberculatus): Implications from RNA interference and eyestalk ablation

Comparative transcriptomic characterization of the ovary in the spawning process of the mud crab Scylla paramamosain

Oviposition is induced upon mating in most insects. Spawning is a physiological process that is fundamental for the reproduction of Scylla paramamosain. However, the molecular mechanisms underlying the spawning process in this species are poorly understood. Herein, comprehensive ovary transcriptomic analysis was conducted at the germinal vesicle breakdown stage (GVBD), spawning stage, 0.5 h post-spawning stage, and 24 h post-spawning stage of S. paramamosain for gene discovery. A total of 67,230 unigenes were generated, and 27,975 (41.61%) unigenes were annotated. Meanwhile, the differentially expressed genes (DEGs) between the different groups were identified, and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis was subsequently conducted. These results suggested that octopamine (OA) and tyramine (TA) could induce oviposition, while dopamine (DA) and serotonin (5-hydroxytryptamine [5-HT]) inhibit oviposition. The 20-hydroxyecdysone (20E) and methyl farnesoate (MF) signal pathways might be positively associated with oviposition. Furthermore, numerous transcripts that encode neuropeptides and their G-protein-coupled receptors (GPCRs), such as CNMamide, RYamide, ecdysis-triggering hormone (ETH), GPA2/GPB5 receptor, and Moody receptor, appear to be differentially expressed during the spawning process. Eleven unigenes were selected for qRT-PCR and the pattern was found to be consistent with the transcriptome expression pattern. Our work is the first spawning-related investigation of S. paramamosain focusing on the ovary at the whole transcriptome level. These findings assist in improving our understanding of spawning regulation in S. paramamosain and provide information for oviposition studies in other crustaceans.

E93 gene in the swimming crab, Portunus trituberculatus: Responsiveness to 20-hydroxyecdysone and methyl farnesoate and role on regulating ecdysteroid synthesis

Ecdysone-induced protein 93 (E93) is a metamorphic determinant involved in crosstalk between 20-hydroxyecdysone (20E) and juvenile hormone (JH) during the insect molting process. The present study identified the E93 gene from the swimming crab, P. trituberculatus, and found it was widely distributed in adult tissues. PtE93 mRNA levels in Y-organ and epidermis fluctuated during the molt cycle, suggesting its involvement in juvenile molting. In vitro and in vivo treatments with 20E led to an induction of PtE93 expression in Y-organ and epidermis, while we found the opposite effect for methyl farnesoate (MF) treatments, a crustacean equivalent of insect JH. We also observed that two genes for ecdysteroid biosynthesis, Spook (Spo) and Shadow (Sad), were suppressed by 20E and induced by MF, showing a negative correlation between PtE93 and ecdysteroid biosynthesis. PtE93 RNA interference (RNAi) induced Spo and Sad expression levels, elevated ecdysteroid content in culture medium, and relieved the 20E inhibitory effect on ecdysteroid synthesis, indicating an inhibitory role of PtE93 on ecdysteroid synthesis. Overall, our results suggest that E93 may be involved in the crosstalk between 20E and MF during crustacean molting, and its presence in Y-organ is closely related to ecdysteroid synthesis.

Effects of molting on the expression of ecdysteroid biosynthesis genes in the Y-organ of the blackback land crab, Gecarcinus lateralis

A pair of Y-organs (YOs) synthesize ecdysteroids that initiate and coordinate molting processes in decapod crustaceans. The YO converts cholesterol to secreted products through a biosynthetic pathway involving a Rieske oxygenase encoded by Neverland (Nvd) and cytochrome P450 monooxygenases encoded by Halloween genes Spook (Spo; Cyp307a1), Phantom (Phm; Cyp306a1), Disembodied (Dib; Cyp302a1), and Shadow (Sad; Cyp315a1). NAD kinase (NADK) and 5-aminolevulinic acid synthase (ALAS) support ecdysteroid synthesis in insects. A 20-hydroxylase, encoded by Shed in decapods and Shade in insects, converts ecdysone to the active hormone 20-hydroxyecdysone (20E). 20E is inactivated by cytochrome P450 26-hydroxylase (Cyp18a1). Contigs encoding these eight proteins were extracted from a Gecarcinus lateralis YO transcriptome and their expression was quantified by quantitative polymerase chain reaction. mRNA levels of Gl-Spo and Gl-Phm were four orders of magnitude higher in YO than those in nine other tissues, while mRNA levels of Gl-NADK and Gl-ALAS were similar in all ten tissues. In G. lateralis induced to molt by multiple leg autotomy, YO mRNA levels of Gl-Nvd, Gl-Spo, Gl-Phm, Gl-NADK, and Gl-ALAS were highest in intermolt and premolt stages and lower in postmolt. Gl-Dib mRNA level was not affected by molt stage. mRNA level of Gl-Sad, which converts 2-deoxyecdysone to ecdysone, was higher in mid- and late premolt stages, when YO ecdysteroidogenic capacity is greatest. Gl-Cyp18a1 mRNA level was highest in intermolt, decreased in premolt stages, and was lowest in postmolt. In animals induced to molt by eyestalk ablation, YO mRNA levels of all eight genes were not correlated with increased hemolymph 20E titers. These results suggest that YO ecdysteroidogenic genes are differentially regulated at transcriptional and translational levels.

Ammonia Stress Disturbs Moult Signaling in Juvenile Swimming Crab Portunus trituberculatus

Ammonia is a significant concern during hatchery culture in brachyuran species, and its accumulation may lead to abortive moulting and large-scale deaths of the early juveniles. To date, the underlying mechanism for ammonia-induced alteration of the moulting process is still unknown. In this study, we aimed to investigate the effects of ammonia on the moulting as well as the potential mechanisms in early juveniles of the swimming crab Portunus trituberculatus, an important aquaculture species in China. We evaluated the survival rate and moulting rate of the juvenile crabs (C2) and analyzed the expression pattern of the genes in key components of molt signaling during a complete moulting cycle under different concentrations of ammonia nitrogen (the control group: <0.1 mg/L; the LA group: 5 mg/L; and the HA group: 20 mg/L). The results showed that: (1) the survival rate in the LA and HA groups was lower than that in the control group at the end of the experiment, and moulting death syndrome (MDS) was only observed in the HA group; (2) the moulting rate was higher in the LA group and lower in the HA group compared to the control group; (3) consistent with the results of the moulting experiment, MIH showed decreased expression, and genes related to ecdysteroid synthesis, ecdysteroid receptors, and responsive effectors exhibited increased expression in the LA group compared to the control group; and (4) although MIH expression was upregulated, increased expression of the genes associated with ecdysteroid synthesis, ecdysteroid receptors and downstream effectors still observed in the HA group. Our results indicated that low levels of ammonia can promote moulting in juvenile swimming crabs by inhibiting the expression of MIH and activating moult signaling, whereas high levels of ammonia inhibit moulting and lead to MDS through impairing moult signaling.

Putative Role of CFSH in the Eyestalk-AG-Testicular Endocrine Axis of the Swimming Crab Portunus trituberculatus

It has been shown in recent studies that the crustacean female sex hormone (CFSH) plays a crucial role in the development of secondary sexual characteristics in Decapoda crustaceans. However, research on the function of CFSH in the eyestalk-AG-testicular endocrine axis has been inadequate. We cloned and identified a homolog of CFSH, PtCFSH, in this study. RT-PCR showed that PtCFSH was mainly expressed in the eyestalk. A long-term injection of dsPtCFSH and recombinant PtCFSH (rPtCFSH) in vivo showed opposite effects on spermatogenesis-related gene expression and histological features in the testis of P. trituberculatus, and was accompanied by changes in AG morphological characteristics and PtIAG expression. In addition, the phosphorylated-MAPK levels and the expression of several IIS pathway genes in the testis was changed accordingly in two treatments, suggesting that PtCFSH may regulate the testicular development via IAG. The hypothesis was further validated by a mixed injection of both dsPtCFSH and dsPtIAG in vivo. The following in vitro studies confirmed the negatively effects of PtCFSH on AG, and revealed that the PtCFSH can also act directly on the testis. Treatment with rPtCFSH reduced the cAMP and cGMP levels as well as the nitric oxide synthetase activity. These findings provide vital clues to the mechanisms of CFSH action in both the eyestalk-AG-testis endocrinal axis and its direct effects on the testis.

Molecular identification and putative role of insulin growth factor binding protein-related protein (IGFBP-rp) in the swimming crab Portunus trituberculatus

The insulin-like growth factor/insulin-like polypeptide (IGF/ILP) signaling is vital for growth, physiological metabolism, development, and reproduction. Insulin-like growth factor-binding protein (IGFBP) is involved in the insulin signaling pathway in both vertebrates and invertebrates and is critical for various physiology functions. Herein, we cloned and characterized the full-length cDNA of IGFBP-rp in the swimming crab, Portunus trituberculatus (PtIGFBP-rp). The deduced amino acid sequence of PtIGFBP-rp was found to contain three key domains (insulin-like binding (IB) domain, the kazale-type serine protease inhibitor (KAZAL) domain, and the immunoglobulin-like C2 (IGc2) domain). Results showed that PtIGFBP-rp shared the same expression pattern as P. trituberculatus insulin androgenic gland hormone (PtIAG) transcripts during the embryonic larval, juvenile crab stage and the androgenic gland (AG) developmental cycle. Moreover, PtIGFBP-rp transcripts were also present in high abundance in hepatopancreas, muscle, and androgenic glands. The regulatory relationship between PtIGFBP-rp and PtIAG was investigated by RNA interference and co-localization assays, which showed a co-localization relationship and feedback regulation between them. Bilateral eye stalk ablation (ESA) increased the expression of PtIGFBP-rp in the AG at 7 d after surgery. These results demonstrate the involvement of PtIGFBP-rp in the signaling regulatory network of IAG in P. trituberculatus.

Molecular Characterization of the Cytochrome P450 Epoxidase (CYP15) in the Swimming Crab Portunus trituberculatus and Its Putative Roles in Methyl Farnesoate Metabolism

CYP15, which encodes a microsomal cytochrome P450 enzyme, could be involved in juvenile hormone biosynthesis in insects. In this study, a full-length cDNA of CYP15 was cloned from the swimming crab Portunus trituberculatus. This PtCYP15 amino acid sequence contains six conserved domains, which is a typical feature of the cytochrome P450 family. Phylogenetic tree analysis results showed that PtCYP15 clusters in a single branch of crustacean species, suggesting that CYP15 may be more widely present in crustaceans. The PtCYP15 mRNA has a broad pattern of tissue expression in P. trituberculatus, including high levels of expression in the hepatopancreas of both sexes and in the ovary of female crabs. During ovarian development stages, PtCYP15 mRNA is highly expressed in stages I and II and less so in stages III and IV in the hepatopancreas and the ovary of the female crabs. These expression profiles are opposite those of methyl farnesoate in hemolymph, suggesting that PtCYP15 might be involved in methyl farnesoate metabolism. In vitro studies show that only methyl farnesoate upregulated vitellogenin expression in the hepatopancreas, suggesting that methyl farnesoate might be the equivalent of juvenile hormone III in crustaceans. Methyl farnesoate treatment increased levels of PtCYP15 in explants of the hepatopancreas and ovary, while juvenile hormone III treatment reduced levels of PtCYP15 mRNA in ovary explants, suggesting that PtCYP15 might be involved in degrading methyl farnesoate. Furthermore, PtCYP15 mRNA expression levels were inhibited by adding juvenile hormone III to ovary explants. These findings provide foundational information for future research on methyl farnesoate metabolism in crustaceans.

Sex-Biased Gene Expression and Isoform Profile of Brine Shrimp Artemia franciscana by Transcriptome Analysis

Simple Summary

The brine shrimp Artemia is a promising model organism for ZW sex determination system, but the genes related to sex determination and differentiation of Artemia have not yet been examined in detail. In this study, the first isoform-level transcriptome sequencing was performed on female and male Artemia franciscana. By using PacBio Iso-Seq and Illumina RNA-Seq technologies, we found 39 candidate sex determination genes that showed sex-biased gene expression. The male-biased expressed genes included DMRT1 and Sad genes, which had three and seven isoforms, respectively. Among these, the Sad gene is an ecdysteroid biosynthetic pathway gene associated with arthropod molting and metamorphosis. We propose the importance and the necessity of further research on genes involved in ecdysteroid biosynthesis. These results will contribute to understand sex determination and differentiation of Artemia and other crustaceans having ZW systems.

Abstract

The brine shrimp Artemia has a ZW sex determination system with ZW chromosomes in females and ZZ chromosomes in males. Artemia has been considered a promising model organism for ZW sex-determining systems, but the genes involved in sex determination and differentiation of Artemia have not yet been identified. Here, we conducted transcriptome sequencing of female and male A. franciscana using PacBio Iso-Seq and Illumina RNA-Seq techniques to identify candidate sex determination genes. Among the 42,566 transcripts obtained from Iso-Seq, 23,514 were analyzed. Of these, 2065 (8.8%) were female specific, 2513 (10.7%) were male specific, and 18,936 (80.5%) were co-expressed in females and males. Based on GO enrichment analysis and expression values, we found 10 female-biased and 29 male-biased expressed genes, including DMRT1 and Sad genes showing male-biased expression. Our results showed that DMRT1 has three isoforms with five exons, while Sad has seven isoforms with 2–11 exons. The Sad gene is involved in ecdysteroid signaling related to molting and metamorphosis in arthropods. Further studies on ecdysteroid biosynthetic genes are needed to improve our understanding of Artemia sex determination. This study will provide a valuable resource for sex determination and differentiation studies on Artemia and other crustaceans with ZW systems.

Understanding molt control switches: Transcriptomic and expression analysis of the genes involved in ecdysteroidogenesis and cholesterol uptake pathways in the Y-organ of the blue crab, Callinectes sapidus

The crustacean molting process is regulated by an interplay of hormones produced by the eyestalk ganglia and Y-organs (YO). Molt-inhibiting hormone and crustacean hyperglycemic hormone released by the sinus gland of the eyestalk ganglia (EG) inhibit the synthesis and secretion of ecdysteroid by the YO, hence regulating hemolymph levels during the molt cycle. The purpose of this study is to investigate the ecdysteroidogenesis pathway, specifically genes linked to changes in ecdysteroid levels occurring at early premolt (ePM). To this end, a reference transcriptome based on YO, EG, and hepatopancreas was de novo assembled. Two genes (cholesterol 7-desaturase Neverland and cytochrome p450 307a1-like Spook) involved in ecdysteroidogenesis were identified from the YO transcriptome using sequence comparisons and transcript abundance. Two other candidates, Hormone receptor 4 and probable cytochrome p450 49a1 potentially involved in ecdysteroidogenesis were also identified. Since cholesterol is the ecdysteroid precursor, a putative cholesterol carrier (Apolipoprotein D-like) was also examined to understand if cholesterol uptake coincided with the increase in the ecdysteroid levels at the ePM stage. The expression level changes of the five candidate genes in the YO were compared between intermolt (IM) and induced ePM (iePM) stages using transcriptomic analysis. Expression analysis using qPCR were carried out at IM, iePM, and normal ePM. The increase in Spook and Neverland expression in the YO at the ePM was accompanied by a concomitant rise in ecdysteroid levels. The data obtained from iePM stage were congruent with those obtained from the normal ePM stage of intact control animals. The present findings support the role of Halloween genes in the ecdysteroidogenesis and molt cycle in the blue crab, Callinectes sapidus.

The PEDtracker: An Automatic Staging Approach for Drosophila melanogaster Larvae

The post-embryonal development of arthropod species, including crustaceans and insects, is characterized by ecdysis or molting. This process defines growth stages and is controlled by a conserved neuroendocrine system. Each molting event is divided in several critical time points, such as pre-molt, molt, and post-molt, and leaves the animals in a temporarily highly vulnerable state while their cuticle is re-hardening. The molting events occur in an immediate ecdysis sequence within a specific time window during the development. Each sub-stage takes only a short amount of time, which is generally in the order of minutes. To find these relatively short behavioral events, one needs to follow the entire post-embryonal development over several days. As the manual detection of the ecdysis sequence is time consuming and error prone, we designed a monitoring system to facilitate the continuous observation of the post-embryonal development of the fruit fly Drosophila melanogaster. Under constant environmental conditions we are able to observe the life cycle from the embryonic state to the adult, which takes about 10 days in this species. Specific processing algorithms developed and implemented in Fiji and R allow us to determine unique behavioral events on an individual level—including egg hatching, ecdysis and pupation. In addition, we measured growth rates and activity patterns for individual larvae. Our newly created RPackage PEDtracker can predict critical developmental events and thus offers the possibility to perform automated screens that identify changes in various aspects of larval development. In conclusion, the PEDtracker system presented in this study represents the basis for automated real-time staging and analysis not only for the arthropod development.

Reconstruction of Eriocheir sinensis Y-organ Genome-Scale Metabolic Network and Differential Analysis After Eyestalk Ablation

Genome-scale metabolic network (GSMN) has been proven to be a useful tool for the system analysis of organism metabolism and applied to deeply explore the metabolic functions or mechanisms in many organisms, including model or non-model organisms. However, the systematic studies on the metabolisms of aquatic animals are seldom reported, especially the aquatic crustaceans. In this work, we reconstructed an Eriocheir sinensis Y-organ GSMN based on the transcriptome sequencing of Y-organ, which includes 1,645 reactions, 1,885 unigenes, and 1,524 metabolites distributed in 100 pathways and 11 subsystems. Functional module and centrality analysis of the GSMN show the main metabolic functions of Y-organ. Further analysis of the differentially expressed unigenes in Y-organ after eyestalk ablation reveals that 191 genes in the network were up-regulated and 283 were down-regulated. The unigenes associated with the ecdysone synthetic pathway were all up-regulated, which is consistent with the report on the increasing secretion of ecdysone after eyestalk ablation. Besides, we compared the Y-organ GSMN with that of E. sinensis eyestalk and hepatopancreas, and we analyzed the specific metabolisms in each organ. The specific metabolisms and pathways of these three networks are closely related to their corresponding metabolic functions. The GSMN reconstructed in this work provides a new method and many novel clues for further understanding the physiological function of Y-organ. It also supplies a new platform for the investigation of the interactions among different organs in the growth process of E. sinensis.

Ecdysone inducible gene E75 from black tiger shrimp Penaeus monodon: Characterization and elucidation of its role in molting

Molting is controlled by ecdysteroids, which are synthesized and secreted by the Y-organ in crustaceans. Ecdysone inducible gene, E75, is an early-response gene in the 20-hydroxyecdysone (20E) signaling pathway, with crucial roles in arthropod development. Complementary DNA (cDNA) encoding Penaeus monodon E75 (PmE75) was cloned using RT-PCR and RACE. PmE75 cDNA was 3526 bp long and encoded a 799-amino acid protein. Tissue distribution analysis showed that PmE75 was expressed ubiquitously in selected tissues, and was relatively abundant in the epidermis, muscle, and hepatopancreas. Developmental expression revealed that PmE75 was expressed throughout its life cycle. Silencing PmE75 significantly decreased PmE75 expression. Shrimps injected with PBS and dsGFP started molting on Day 7 and had almost completed molting on Day 9, whereas dsPmE75-injected shrimp presented no signs of molting. These results suggested that PmE75 might be involved in molting. In situ hybridization results support this hypothesis. To explore the role of 20E and eyestalks in the regulation of molting in P. monodon, exogenous 20E injection and eyestalk ablation (ESA) were performed, and showed that 20E can induce the transcription and expression of PmE75 in the hepatopancreas, epidermis, and muscle, which were significantly elevated after ESA. These results provide further insights into our understanding of molting.

Transcriptomic analysis of crustacean molting gland (Y-organ) regulation via the mTOR signaling pathway

The intermolt crustacean Y-organ (YO) maintains a basal state mediated by pulsatile release of molt inhibiting hormone (MIH), a neuropeptide produced in the eyestalk ganglia, inhibiting YO ecdysteroidogenesis. Reduction of MIH results in YO activation and the animal enters premolt. In the crab, Gecarcinus lateralis, molting was induced by eyestalk ablation (ESA). ESA animals were injected with either rapamycin, an mTOR inhibitor, or DMSO vehicle at Day 0. YOs were harvested at 1, 3, and 7 days post-ESA and processed for high throughput RNA sequencing. ESA-induced increases in mRNA levels of mTOR signaling genes (e.g., mTOR, Rheb, TSC1/2, Raptor, Akt, and S6 kinase) declined following rapamycin treatment. In concert with mTOR inhibition, mRNA levels of ecdysteroid biosynthesis genes (e.g., Nvd, Spo, Sad, Dib, and Phm) were decreased and accompanied by a decrease in hemolymph ecdysteroid titer. By contrast, rapamycin increased the mRNA level of FKBP12, the rapamycin-binding protein, as well as the mRNA levels of genes associated with Wnt and insulin-like growth factor signaling pathways. Many MIH and transforming growth factor-β signaling genes were down regulated in ESA animals. These results indicate that mTOR activity either directly or indirectly controls transcription of genes that drive activation of the YO.