Potential role of Methoprene-tolerant (Met) in methyl farnesoate-mediated vitellogenesis in the Chinese mitten crab (Eriocheir sinensis)

The moulting arthropod: a complete genetic toolkit review

Exoskeletons are a defining character of all arthropods that provide physical support for their segmented bodies and appendages as well as protection from the environment and predation. This ubiquitous yet evolutionarily variable feature has been instrumental in facilitating the adoption of a variety of lifestyles and the exploitation of ecological niches across all environments. Throughout the radiation that produced the more than one million described modern species, adaptability afforded by segmentation and exoskeletons has led to a diversity that is unrivalled amongst animals. However, because of the limited extensibility of exoskeleton chitin and cuticle components, they must be periodically shed and replaced with new larger ones, notably to accommodate the growing individuals encased within. Therefore, arthropods grow discontinuously by undergoing periodic moulting events, which follow a series of steps from the preparatory pre-moult phase to ecdysis itself and post-moult maturation of new exoskeletons. Each event represents a particularly vulnerable period in an arthropod's life cycle, so processes must be tightly regulated and meticulously executed to ensure successful transitions for normal growth and development. Decades of research in representative arthropods provide a foundation of understanding of the mechanisms involved. Building on this, studies continue to develop and test hypotheses on the presence and function of molecular components, including neuropeptides, hormones, and receptors, as well as the so-called early, late, and fate genes, across arthropod diversity. Here, we review the literature to develop a comprehensive overview of the status of accumulated knowledge of the genetic toolkit governing arthropod moulting. From biosynthesis and regulation of ecdysteroid and sesquiterpenoid hormones, to factors involved in hormonal stimulation responses and exoskeleton remodelling, we identify commonalities and differences, as well as highlighting major knowledge gaps, across arthropod groups. We examine the available evidence supporting current models of how components operate together to prepare for, execute, and recover from ecdysis, comparing reports from Chelicerata, Myriapoda, Crustacea, and Hexapoda. Evidence is generally highly taxonomically imbalanced, with most reports based on insect study systems. Biases are also evident in research on different moulting phases and processes, with the early triggers and late effectors generally being the least well explored. Our synthesis contrasts knowledge based on reported observations with reasonably plausible assumptions given current taxonomic sampling, and exposes weak assumptions or major gaps that need addressing. Encouragingly, advances in genomics are driving a diversification of tractable study systems by facilitating the cataloguing of putative genetic toolkits in previously under-explored taxa. Analysis of genome and transcriptome data supported by experimental investigations have validated the presence of an "ultra-conserved" core of arthropod genes involved in moulting processes. The molecular machinery has likely evolved with elaborations on this conserved pathway backbone, but more taxonomic exploration is needed to characterise lineage-specific changes and novelties. Furthermore, linking these to transformative innovations in moulting processes across Arthropoda remains hampered by knowledge gaps and hypotheses based on untested assumptions. Promisingly however, emerging from the synthesis is a framework that highlights research avenues from the underlying genetics to the dynamic molecular biology through to the complex physiology of moulting.

Methoprene-Tolerant (Met) Acts as Methyl Farnesoate Receptor to Regulate Larva Metamorphosis in Mud Crab, Scylla paramamosain

The conserved role of juvenile hormone (JH) signals in preventing larvae from precocious metamorphosis has been confirmed in insects. Crustaceans have different metamorphosis types from insects; we previously proved that methyl farnesoate (MF) can prohibit larvae metamorphosis in mud crabs, but the molecular signal of this process still needs to be elucidated. In this study, methoprene-tolerant (Met) of Scylla paramamosain was obtained and characterized, which we named Sp-Met. Sp-Met contains a 3360 bp ORF that encodes 1119 amino acids; the predicted protein sequences of Sp-Met include one bHLH, two PAS domains, one PAC domain, and several long unusual Gln repeats at the C-terminal. AlphaFold2 was used to predict the 3D structure of Sp-Met and the JH binding domain of Met. Furthermore, the binding properties between Sp-Met and MF were analyzed using CD-DOCK2, revealing a putative high affinity between the receptor and ligand. In silico site-directed mutagenesis suggested that insect Mets may have evolved to exhibit a higher affinity for both MF or JH III compared to the Mets of crustaceans. In addition, we found that the expression of Sp-Met was significantly higher in female reproductive tissues than in males but lower in most of the other examined tissues. During larval development, the expression variation in Sp-Met and Sp-Kr-h1 was consistent with the immersion effect of MF. The most interesting finding is that knockdown of Sp-Met blocked the inhibitory effect of MF on metamorphosis in the fifth zoea stage and induced pre-metamorphosis phenotypes in the fourth zoea stage. The knockdown of Sp-Met significantly reduced the expression of Sp-Kr-h1 and two ecdysone signaling genes, Sp-EcR and Sp-E93. However, only the reduction in Sp-Kr-h1 could be rescued by MF treatment. In summary, this study provides the first evidence that MF inhibits crustacean larval metamorphosis through Met and that the MF-Met→Kr-h1 signal pathway is conserved in mud crabs. Additionally, the crosstalk between MF and ecdysteroid signaling may have evolved differently in mud crabs compared to insects.

Comparative Transcriptome Analysis of Hepatopancreas Reveals Sexual Dimorphic Response to Methyl Farnesoate Injection in Litopenaeus vannamei

Sexually dimorphic traits such as growth and body size are often found in various crustaceans. Methyl farnesoate (MF), the main active form of sesquiterpenoid hormone in crustaceans, plays vital roles in the regulation of their molting and reproduction. However, understanding on the sex differences in their hormonal regulation is limited. Here, we carried out a comprehensive investigation on sexual dimorphic responses to MF in the hepatopancreas of the most dominant aquacultural crustacean-the white-leg shrimp (Litopenaeus vannamei). Through comparative transcriptomic analysis of the main MF target tissue (hepatopancreas) from both female and male L. vannamei, two sets of sex-specific and four sets of sex-dose-specific differentially expressed transcripts (DETs) were identified after different doses of MF injection. Functional analysis of DETs showed that the male-specific DETs were mainly related to sugar and lipid metabolism, of which multiple chitinases were significantly up-regulated. In contrast, the female-specific DETs were mainly related to miRNA processing and immune responses. Further co-expression network analysis revealed 8 sex-specific response modules and 55 key regulatory transcripts, of which several key transcripts of genes related to energy metabolism and immune responses were identified, such as arginine kinase, tropomyosin, elongation of very long chain fatty acids protein 6, thioredoxin reductase, cysteine dioxygenase, lysosomal acid lipase, estradiol 17-beta-dehydrogenase 8, and sodium/potassium-transporting ATPase subunit alpha. Altogether, our study demonstrates the sex differences in the hormonal regulatory networks of L. vannamei, providing new insights into the molecular basis of MF regulatory mechanisms and sex dimorphism in prawn aquaculture.

Ovarian transcriptome and metabolic responses of RNAi-mediated farnesyl pyrophosphate synthase knockdown in Neocaridina denticulata sinensis

Methyl farnesoate (MF) is considered the equivalent of JH in crustaceans and plays an essential role in many crucial physiological processes. It is believed that farnesyl pyrophosphate synthase (FPPS) plays an essential role in the biosynthesis of mevalonate, which is a branch of the JH/MF pathway. The full-length cDNA of FPPS (NdFPPS) from Neocaridina denticulata sinensis was isolated and characterized, and the deduced amino acid of NdFPPS contained a polyprenyl_synt domain. In addition to its ubiquitous tissue expression, NdFPPS was significantly expressed in the ovary. In vivo gene silencing with dsRNA interference was performed to further investigate the function of NdFPPS. An ovarian transcriptomic analysis of dsNdFPPS experimental and control groups was used to compare, annotate, and classify differentially expressed genes (DEGs). A total of 9230 DEGs were identified in the experimental and control groups based on FPKM values, of which 5082 were up-regulated genes and 4148 were down-regulated genes. 761 GO terms and 102 KEGG pathways were enriched for the DEGs. Our results suggest that NdFPPS might play an important role in ovarian development, however, further functional study is needed to elucidate physiological role of NdFPPS in reproduction.

Hepatopancreas transcriptome analyses provide new insights into the molecular regulatory mechanism of fast ovary maturation in Macrobrachium nipponense

Background

Macrobrachium nipponense is an economically and ecologically important freshwater prawn that is widely farmed in China. In contrast to other species of marine shrimp, M. nipponense has a short sexual maturity period, resulting in not only high stocking densities, but also a reduced survival rate and increased risk of hypoxia. Therefore, there is an urgent need to study the molecular mechanisms underlying fast ovary maturation in this species.

Results

Comparative transcriptome analysis was performed using hepatopancreatic tissue from female M. nipponense across five ovarian maturation stages to explore differentially expressed genes and pathways involved in ovarian maturation. In total, 118.01 Gb of data were generated from 15 transcriptomes. Approximately 90.46% of clean reads were mapped from the M. nipponense reference genome. A comprehensive comparative analysis between successive ovarian maturation stages generated 230–5814 differentially expressed genes. Gene Ontology (GO) enrichment was highly concentrated in the “biological process” category in all four comparison groups, and mainly focused on energy synthesis and accumulation, energy decomposition and transport. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment results showed that, among 20 significantly enriched KEGG pathways, nine were involved in the synthesis, degradation, and metabolism of carbohydrates, lipids, and other nutrient intermediates, suggesting that the hepatopancreas has an important role in energy supply during ovarian maturation. Furthermore, the “Insect hormone biosynthesis” pathway was found to have a dominant role in the development of the ovary from immaturity to maturity, supporting the hypothesis that ecdysteroid- and juvenile hormone-signaling pathways have an important role in hepatopancreas regulation of ovarian maturation.

Conclusion

Taken together, this study sheds light on the role of the hepatopancreas in the molecular regulation of ovary maturation in M. nipponense. The present study provided new insights for understanding the mechanisms of reproductive regulation in crustaceans.

Effects and Mechanism of Different Phospholipid Diets on Ovary Development in Female Broodstock Pacific White Shrimp, Litopenaeus vannamei

Research on nutrition and feed development for the broodstock of the Pacific white shrimp, Litopenaeus vannamei, is rare, and a poor broodstock quality is a critical factor restricting the seed supply in shrimp farming. As an essential nutrient for the gonadal development of L. vannamei, one control diet (no phospholipid) and three typical phospholipids (soybean lecithin, egg yolk lecithin, and krill oil) were evaluated in a semipurified diet of 4% phospholipid for a 28-day trial (initial weight 34.7 ± 4.2 g). Dietary phospholipid supplementation significantly promoted the ovarian maturation of female L. vannamei. Compared with soybean lecithin and egg yolk lecithin, krill oil showed the best positive results. Shrimp fed with a diet krill oil has obtained a significantly higher gonadosomatic index, yolk particle deposition, lipid accumulation, and estrogen secretion than from other sources. Ovary lipidomic analysis showed that the krill oil enriched the lipid composition of the ovary. The “glycerophospholipid metabolism” and “sphingolipid metabolism” pathways were significantly varied via topological pathway analysis. Genes and hub genes, with significantly different expression levels, were significantly enriched in the “fatty acid metabolism pathway,” “glycerophospholipid metabolism,” and “arachidonic acid metabolism” pathways by transcriptomic analysis. Correlation analysis of the transcriptome and lipidomics showed that the differential gene “hormone-sensitive lipase-like” (HSL) was positively correlated with various lipids [triglycerides (TG), phosphatidic acid (PA), phosphatidylserine (P), phosphatidylethanolamine (PE), glucosylceramide (GlcCer), phosphatidylglycerol (PG), and phosphatidylinositol (PI)] but was negatively correlated with diacylglycerol (DG), lysophosphatidylethanolamine (LPE), and sphingomyelin (SM). In conclusion, the dietary phospholipids, especially krill oil as a phospholipid source, can promote the development of L. vannamei ovaries by increasing the accumulation of nutrients such as triglycerides and sterols, and the secretion of estrogen or related hormones, such as estradiol and methylfarneside, by affecting the metabolism of glycerol phospholipids and some key fatty acids.

Molecular evidence for farnesoic acid O-methyltransferase (FAMeT) involved in the biosynthesis of vitellogenin in the Chinese mitten crab Eriocheir sinensis

Sesquiterpenoid methyl farnesoate (MF), a crustacean equivalent of insect juvenile hormone (JH III), has essential functions in regulating physiological processes in crustaceans, including reproduction and vitellogenesis. Farnesoic acid O-methyltransferase (FAMeT) is a key rate-limiting enzyme catalyzing the conversion of farnesoic acid (FA) to JH/MF in insects and crustaceans. In this study, a full-length cDNA of EsFAMeT from Eriocheir sinensis was isolated and characterized. The deduced EsFAMeT amino acid sequence indicated there were two conserved Methyltransf-FA domains characteristic of FAMeT family proteins. With use of sequence alignment analysis procedures, there was an indication that FAMeT proteins are highly conserved among crustaceans and FAMeT is more closely related to crustacean FAMeT than to insect FAMeT. Results from quantitative real-time PCR analysis revealed there was ubiquitous EsFAMeT in all tissues examined, with greater abundances of mRNA transcripts in the ovary. The transcription of EsFAMeT indicated there were stage-specific patterns in the hepatopancreas and ovary during ovarian development, with the greatest abundance during ovarian development Stages II and III, respectively. To investigate functions of EsFAMeT in vitellogenin biosynthesis in E. sinensis, RNA interference-mediated gene knockdown was used in vitro and in vivo. Injection of EsFAMeT dsRNA resulted in a marked decrease in EsVg (encoding vitellogenin) transcripts in the ovary and hepatopancreas both in vitro and in vivo. Results from the present study indicated EsFAMeT is involved in vitellogenin biosynthesis in the ovary and hepatopancreas of E. sinensis, providing a new resource to study modulatory effects of the FAMeT family of enzymes in crustacean reproduction.

RNAi silencing of the 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) gene inhibits vitellogenesis in Chinese mitten crab Eriocheir sinensis

The sesquiterpenoid methyl farnesoate (MF), a de-epoxide form of insect juvenile hormone III (JH III), plays an essential role in regulating many crucial physiological processes in crustaceans including vitellogenesis and reproduction. 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) is an important rate-limiting enzyme in the mevalonate pathway, which is critical for the synthesis of JH III and MF. In the present study, a full-length cDNA encoding HMGR (EsHMGR) in Eriocheir sinensis was isolated and characterised. Sequence analysis of EsHMGR revealed that it belongs to Class I HMGR family proteins with HMG-CoA-binding and NADPH-binding domains, both important for HMGR activity. In addition to its ubiquitous tissue expression, expression of EsHMGR was highly specific to the ovary, the main site of Vg synthesis. During ovarian development, EsHMGR expression in ovary displayed a stage-specific pattern, and was correlated with expression of vitellogenin (EsVg) in hepatopancreas, which suggests that EsHMGR possibly involved in vitellogenesis. To further investigate the functional role of EsHMGR in vitellogenin biosynthesis in E. sinensis, RNA interference-mediated gene silencing was carried out both in vitro and in vivo. Quantitative PCR results showed that injection of EsHMGR double-stranded RNA (dsRNA) led to a significant decrease in EsVg expression levels in ovary and hepatopancreas both in vitro and in vivo. Taken together, the results suggest that EsHMGR is involved in vitellogenin biosynthesis in female E. sinensis, which may provide a new resource for HMGR enzymes participating in reproduction in crustaceans.

Identification, characterization and mRNA transcript abundance profiles of the carboxylesterase (CXE5) gene in Eriocheir sinensis suggest that it may play a role in methyl farnesoate degradation

The sesquiterpenoid methyl farnesoate (MF) is a de-epoxidized form of insect juvenile hormone (JH) III in crustaceans, and its precise titer plays important roles in regulating many critical physiological processes, including reproduction and ovarian maturation. Understanding the synthetic and degradation pathways of MF is equally important for determining how to maintain MF titers at appropriate levels and thus for potential applications in crab aquaculture. Although the synthetic pathway of MF has been well established, little is known about MF degradation. Previous research proposed that specific carboxylesterases (CXEs) that degrade MF in crustaceans are conserved from those of JH III. In this study, we identified a novel Es-CXE5 gene from Eriocheir sinensis. The Es-CXE5 protein contains some conserved motifs, including catalytic triad and oxyanion hole, which are characteristics of the biologically active CXE family. The phylogenetic analysis showed that Es-CXE5 belongs to the hormone/semiochemical processing group of the CXE family. Moreover, Tissue and stage-specific expression results suggested that Es-CXE5 expression in hepatopancreas was highest and associated with the hemolymph MF titer. Furthermore, Es-CXE5 mRNA transcripts were detected in both in vitro and in vivo experiments and ESA experiment in the hepatopancreas and ovary. The results of this study showed that Es-CXE5 mRNA abundance in the hepatopancreas was notably induced by MF addition but had no effect on the ovary. Taken together, our results suggest that Es-CXE5 may degrade MF in the hepatopancreas and may thus be involved in ovarian development in E. sinensis.