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 effect of thermal stress on the X-organ/sinus gland proteome of the estuarine blue crab Callinectes sapidus during the intermolt and premolt stages

Survival of brachyuran crabs is temperature-dependent and thermal stress promotes changes during molting. We aimed to decipher the impact of thermal stresses on the X-organ/sinus gland (XO/SG) complex, a temperature-sensitive neuroendocrine tissue involved in the molting regulation of Callinectes sapidus during the intermolt and premolt phases. We employed a proteogenomic approach using specimens subjected to control (24 °C), cold (19 °C), and heat (29 °C) temperatures. A total of 1463 protein groups with at least two unique peptides were identified and quantified. C. sapidus in the premolt stage exposed to the cold condition exhibited a proteome closely resembling that of the intermolt stage, as evidenced by measurements of circulating ecdysteroid levels. Compared to the intermolt at control temperature, the premolt stage exhibited increased energy metabolism, structural changes in the cuticle mediated by chitin metabolism and glycoproteins, biosynthesis of methyl farnesoate (MF), and elevated tissue levels of molt-inhibiting hormone (MIH) and crustacean hyperglycemic hormone (CHH), indicating lower secretion rates. Heat temperature (29 °C) seems to induce mitochondrial metabolism in the intermolt XO/SG, while cold temperature elicited a delayed molt cycle in the premolt phase, marked by reduced tissue levels of CHH, indicating increased secretion and Y-organ (YO) inhibition, and decreased MF production (reduced YO stimulation). SIGNIFICANCE STATEMENT: Temperature plays a pivotal role in regulating the metabolism, growth, molting, reproduction, and survival of crabs, such as the blue crab (Callinectes sapidus). Despite the blue crab's significance on both economic and ecological realms, there has been a notable lack of molecular information related to this species and therefore a gap in our knowledge of the blue crab's molecular makeup and genetic diversity. This research established a comprehensive proteome landscape to elucidate the molecular and functional changes in the XO/SG complex involved in the molting process of C. sapidus, and how thermal stresses significantly influence biotransformation processes. Utilizing a proteogenomics approach with multi-round homologous database analysis, we have generated a highly accurate protein repertoire with at least two unique peptide of XO/SG tissue proteome. This resource will be invaluable for future molecular analyses of this species. Our findings demonstrate that thermal stresses induced specific modifications in the XO/SG tissue, depending on the molt cycle phase. Temperature-mediated responses influences the biological processes, enhancing the functional morphogenesis and comprehensive metabolic adaptations on molting cycle supported by a relationship between the XO/SG tissue proteome and circulating ecdysteroid levels.

Identification and Characterization of the Y-Organ of Orange Mud Crab Scylla Olivacea

The Y-organs (YOs) synthesize and secrete molting hormones, and thus regulate crustacean growth and reproduction. However, the YOs of the orange mud crab Scylla olivacea is yet to be described due to its minute size and ambiguous feature. This study describes the location, morphological characteristics, histology, and the changes of YOs at different molt stages, and examines in vitro ecdysteroid secreted by the YOs of S. olivacea. Mature male crabs (> 90 mm carapace width, CW) were used to identify the location of the YOs while male juveniles (50-65 mm CW) at three molt stages (intermoult, premoult, postmoult) were dissected for the characterization of 20-Hydroxyecdysone (20E) levels. The yellowish-white, compact, and oval-shaped YOs are located in pairs at the anterior cephalothorax region, beneath the mandibular external adductor muscle, anterior to the branchial chamber space, and around the end of the nerve tract. The YOs index and YO's cell diameter increased from postmoult to premoult; YO's diameter increased only during premoult stage. Under electron microscopy, the YO's surface morphology is rough, bumpy, network-like, and porous, with several lacunar system tubules. High concentrations of haemolymph 20E during premoult correlated with the changes in the characteristics of YOs at this stage. The presence of ecdysteroid in YOs was confirmed by high performance liquid chromatography (HPLC). The characteristics of the YOs of S. olivacea are similar to the YO descriptions of other brachyurans. The successful identification and characterization of YOs will spearhead future research on understanding its role in molt regulation of mud crabs, consequently contributing to the development and progress of crab culture and soft-shell crab industries.

Identification and Evolution Analysis of the Genes Involved in the 20-Hydroxyecdysone Metabolism in the Mud Crab, Scylla paramamosain: A Preliminary Study

BACKGROUND

20-Hydroxyecdysone (20E) is the most ubiquitous ecdysteroid (Ecd) and plays critical roles during the life cycle of arthropods. To elucidate the metabolism pathway of 20E in the economically important species, Scylla paramamosain, we conducted a comprehensive exploration of the genes involved in the 20E metabolism pathway.

METHODS

A comprehensive exploration of genes involved in the 20E metabolism pathway was conducted, including gene annotation, local blast using the Drosophila ortholog as query, and TreeFam ortholog genes identification. Bioinformatics and expression profiling of the identified genes were performed to assess their roles in the 20E metabolism of green mud crabs.

RESULTS

This experiment indicated that, except for CYP306a1 and CYP314a1, all other ortholog genes involved in the Drosophila 20E metabolism can be found in the mud crab, suggesting that the function of these two genes might be replaced by other CYP genes or the "active" Ecd in mud crabs was not the 20E. All genes had the typical features of each gene family, clustered with the specific clade in the phylogenetic trees. In addition, all the identified genes had the highest expression level in the Y-organ, and sex-biased gene expression was observed in these genes.

CONCLUSIONS

This study provided some valuable insights into the metabolism and diversity of ecdysteroids in crustaceans.

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.

Temperature-driven changes in the neuroendocrine axis of the blue crab Callinectes sapidus during the molt cycle

Environmental cues such as temperature induce macroscopic changes in the molting cycle of crustaceans, however, the physiological mechanisms behind these changes remain unclearWe aimed to investigate the regulatory mechanisms in the intermolt and premolt stages of the Callinectes sapidus molt cycle in response to thermal stimuli. The concentration of ecdysteroids and lipids in the hemolymph, and the expression of heat shock proteins (HSPs) and molt key genes were assessed at 19 °C, 24 °C and 29 °C. The premolt animals exhibited a much larger response to the colder temperature than intermolt animals. Ecdysteroids decreased drastically in premolt animals, whereas the expression of their hepatopancreas receptor (CasEcR) increased, possibly compensating for the low hemolymphatic levels at 19 °C. This decrease might be due to increased HSPs and inhibited ecdysteroidogenesis in the Y-organ. In addition, the molting-inhibiting hormone expression in the X-organ/sinus gland (XO/SG) remained constant between temperatures and stages, suggesting it is constitutive in this species. Lipid concentration in the hemolymph, and the expression of CasEcR and CasHSP90 in the XO/SG were influenced by the molting stage, not temperature. On the other hand, the expression of HSPs in the hepatopancreas is the result of the interaction between the two factors evaluated in the study. Our results demonstrated that temperature is an effective modulator of responses related to the molting cycle at the endocrine level and that temperature below the control condition caused a greater effect on the evaluated responses compared to the thermostable condition, especially when the animal was in the premolt stage.

Effects of molting on the expression of ecdysteroid responsive genes in the crustacean molting gland (Y-organ)

Ecdysteroid molting hormones coordinate arthropod growth and development. Binding of 20-hydroxyecdysone (20E) to ecdysteroid receptor EcR/RXR activates a cascade of nuclear receptor transcription factors that mediate tissue responses to hormone. Insect ecdysteroid responsive and Forkhead box class O (FOXO) transcription factor gene sequences were used to extract orthologs from blackback land crab (Gecarcinus lateralis) Y-organ (YO) transcriptome: Gl-Ecdysone Receptor (EcR), Gl-Broad Complex (Br-C), Gl-E74, Gl-Hormone Receptor 3 (HR3), Gl-Hormone Receptor 4 (HR4), Gl-FOXO, and Gl-Fushi tarazu factor-1 (Ftz-f1). Quantitative polymerase chain reaction quantified mRNA levels in tissues from intermolt animals and in YO of animals induced to molt by multiple limb autotomy (MLA) or eyestalk ablation (ESA). Gl-EcR, Gl-Retinoid X Receptor (RXR), Gl-Br-C, Gl-HR3, Gl-HR4, Gl-E74, Gl-E75, Gl-Ftz-f1, and Gl-FOXO were expressed in all 10 tissues, with Gl-Br-C, Gl-E74, Gl-E75, and Gl-HR4 mRNA levels in the YO lower than those in most of the other tissues. In MLA animals, molting had no effect on Gl-Br-C, Gl-E74, and Gl-Ftz-f1 mRNA levels and little effect on Gl-EcR, Gl-E75, and Gl-HR4 mRNA levels. Gl-HR3 and Gl-FOXO mRNA levels were increased during premolt stages, while Gl-RXR mRNA level was highest during intermolt and premolt stages and lowest at postmolt stage. In ESA animals, YO mRNA levels were not correlated with hemolymph ecdysteroid titers. ESA had no effect on Gl-EcR, Gl-E74, Gl-HR3, Gl-HR4, Gl-Ftz-f1, and Gl-FOXO mRNA levels, while Gl-RXR, Gl-Br-C, and Gl-E75 mRNA levels were decreased at 3 days post-ESA. These data suggest that transcriptional up-regulation of Gl-FOXO and Gl-HR3 contributes to increased YO ecdysteroidogenesis during premolt. By contrast, transcriptional regulation of ecdysteroid responsive genes and ecdysteroidogenesis were uncoupled in the YO of ESA animals.

Ontogeny of the Cytochrome P450 Superfamily in the Ornate Spiny Lobster (Panulirus ornatus)

Cytochrome P450s (CYP450s) are a versatile superfamily of enzymes known to undergo rapid evolution. They have important roles across growth and development pathways in crustaceans, although it is difficult to characterise orthologs between species due to their sequence diversity. Conserved CYP450s enzymes in crustaceans are those associated with ecdysteroidogenesis: synthesising and breaking down the active moult hormone, 20-hydroxyecdysone. The complex life cycle of the ornate spiny lobster, Panulirus ornatus, relies on moulting in order to grow and develop. Many of these diverse life stages have been analysed to establish a comprehensive transcriptomic database for this species. The transcripts putatively encoding for CYP450s were mapped using transcriptomic analysis and identified across growth and development stages. With the aid of phylogeny, 28 transcripts of 42 putative P. ornatus CYP450s were annotated, including the well conserved Halloween genes, which are involved in ecdysteroidogenesis. Expression patterns across the life stages determined that only a subset of the CYP450s can be detected in each life stage or tissue. Four Shed transcripts show overlapping expression between metamorphosis and adult tissues, suggesting pleotropic functions of the multiple Shed orthologs within P. ornatus.

Identifying the genes involved in the egg-carrying ovigerous hair development of the female blue crab Callinectes sapidus: transcriptomic and genomic expression analyses

BACKGROUND

Crustacean female sex hormone (CFSH) controls gradually developing adult female-specific morphological features essential for mating and brood care. Specifically, ovigerous hairs are developed during the prepuberty molt cycle of the blue crab Callinectes sapidus that are essential for carrying the eggs until they finish development. Reduced CFSH transcripts by CFSH-dsRNA injections result in fewer and shorter ovigerous hairs than the control. This study aimed to identify the specific genes responsible for ovigerous hair formation using transcriptomic, genomic and expression analyses of the ovigerous setae at three stages: prepuberty at early (OE) and late premolt (OL), and adult (AO) stages.

RESULTS

The de novo Trinity assembly on filtered sequence reads produced 96,684 Trinity genes and 124,128 transcripts with an N50 of 1,615 bp. About 27.3% of the assembled Trinity genes are annotated to the public protein sequence databases (i.e., NR, Swiss-Prot, COG, KEGG, and GO databases). The OE vs. OL, OL vs. AO, and OE vs. AO comparisons resulted in 6,547, 7,793, and 7,481 differentially expressed genes, respectively, at a log2-fold difference. Specifically, the genes involved in the Wnt signaling and cell cycle pathways are positively associated with ovigerous hair development. Moreover, the transcripts of ten cuticle protein genes containing chitin-binding domains are most significantly changed by transcriptomic analysis and RT-qPCR assays, which shows a molt-stage specific, down-up-down mode across the OE-OL-AO stages. Furthermore, the expression of the cuticle genes with the chitin-binding domain, Rebers and Riddiford domain (RR)-1 appears at early premolt, followed by RR-2 at late premolt stage. Mapping these 10 cuticle protein sequences to the C. sapidus genome reveals that two scaffolds with a 549.5Kb region and 35 with a 1.19 Mb region harbor 21 RR1 and 20 RR2 cuticle protein genes, respectively. With these findings, a putative mode of CFSH action in decapod crustaceans is proposed.

CONCLUSIONS

The present study describes a first step in understanding the mechanism underlying ovigerous hair formation in C. sapidus at the molecular level. Overall, demonstrating the first transcriptome analysis of crustacean ovigerous setae, our results may facilitate future studies into the decapod female reproduction belonging to the suborder Pleocyemata.

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.

Comparative Transcriptome Analysis of Eriocheir sinensis from Wild Habitats in Han River, Korea

Eriocheir sinensis is an euryhaline crab found from East Asia to Europe and North America. This species can live in freshwater and seawater due to the unique physiological characteristics of their life cycle, which allows them to adapt and inhabit different habitats in a wide range of environments. Despite the wealth of studies focusing on adaptation mechanism of E. sinensis to specific environmental factors, the adaptation mechanisms to wild habitats with coexisting environmental factors are not well understood. In this study, we conducted a transcriptome analysis to investigate gene expression differences related to habitat adaptation of E. sinensis from two wild habitats with different environmental factors in the Han River, Korea. A total of 138,261 unigenes were analyzed, of which 228 were analyzed as differentially expressed genes (DEGs) between the two wild habitats. Among 228 DEGs, 110 DEGs were annotated against databases; most DEGs were involved in energy metabolism, immunity, and osmoregulation. Moreover, DEG enrichment analysis showed that upregulated genes were related to biosynthesis, metabolism, and immunity in an habitat representing relatively high salinity whereas downregulated genes were related to ion transport and hypoxia response in habitats with relatively low salinity and dissolved oxygen. The present findings can serve as foundation for future E. sinensis culture or conservation approaches in natural conditions.

MnHR4 Functions during Molting of Macrobrachium nipponense by Regulating 20E Synthesis and Mediating 20E Signaling

HR4, a member of the nuclear receptor family, has been extensively studied in insect molting and development, but reports on crustaceans are still lacking. In the current study, the MnHR4 gene was identified in Macrobrachium nipponense. To further improve the molting molecular mechanism of M. nipponense, this study investigated whether MnHR4 functions during the molting process of M. nipponense. The domain, phylogenetic relationship and 3D structure of MnHR4 were analyzed by bioinformatics. Quantitative real-time PCR (qRT-PCR) analysis showed that MnHR4 was highly expressed in the ovary. In different embryo stages, the highest mRNA expression was observed in the cleavage stage (CS). At different individual stages, the mRNA expression of MnHR4 reached its peak on the fifteenth day after hatching (L15). The in vivo injection of 20-hydroxyecdysone (20E) can effectively promote the expression of the MnHR4 gene, and the silencing of the MnHR4 gene increased the content of 20E in M. nipponense. The regulatory role of MnHR4 in 20E synthesis and 20E signaling was further investigated by RNAi. Finally, the function of the MnHR4 gene in the molting process of M. nipponense was studied by counting the molting frequency. After knocking down MnHR4, the molting frequency of M. nipponense decreased significantly. It was proved that MnHR4 plays a pivotal role in the molting process of M. nipponense.

17β-Estradiol (E2) may be involved in the mode of crustacean female sex hormone (CFSH) action in the blue crab, Callinectes sapidus

17β-estradiol (E2) has been proved to control reproduction, sexual differentiation, and the development of the secondary sexual characteristics of vertebrate females. In decapod crustacean species, crustacean female sex hormone (CFSH), a protein hormone, is required for developing adult-specific ovigerous setae for embryo brooding and gonophores for mating at the blue crab Callinectes sapidus puberty molting. However, it is unclear that whether the mode of CFSH action involves a vertebrate-type sex steroid hormone in crustaceans. To this end, E2 levels were first measured using a competitive ELISA in the hemolymph and the potential CFSH target tissues from both prepuberty and adult females; the presence of E2 was further confirmed with a liquid chromatography tandem mass spectrometry method. Then, the cDNAs of the following genes known to be associated with vertebrate steroidogenic pathways were isolated: StAR-related lipid transfer protein 3 (StAR3); 3β-hydroxysteroid dehydrogenase (3βHSD); two isoforms of 17β-hydroxysteroid dehydrogenase 8 (17βHSD8); and, estradiol-related receptor (ERR). RT-PCR analysis revealed that these genes were widely distributed in the eyestalk ganglia, hepatopancreas, brain, ovary, spermathecae, ovigerous and plumose setae tissues of adult females. The 17βHSD8 transcripts were localized in the follicle cells, the periphery of the nuclear membrane of primary oocytes, and yolk granules of the vitellogenic oocytes using in situ hybridization, and the corresponding protein was detected in the follicle cells and ooplasm of primary oocytes using immunohistochemistry. Furthermore, the adult females injected with CFSH-dsRNA (n = 30 times) had E2 and StAR3 transcripts levels lower in the ovigerous and plumose setae, spermathecae than controls. These results suggested that the mode of CFSH action in C. sapidus might involve E2 in these adult-female-specific tissues.