Comparative immunogenomics of molluscs

Transcriptome-based network analysis reveals hub immune genes and pathways of hepatopancreas against LPS in Amphioctopus fangsiao

The hepatopancreas is the biggest digestive organ in Amphioctopus fangsiao (A. fangsiao), but also undertakes critical functions like detoxification and immune defense. Generally, pathogenic bacteria or endotoxin from the gut microbiota would be arrested and detoxified in the hepatopancreas, which could be accompanied by the inevitable immune responses. In recent years, studies related to cephalopods immune have been increasing, but the molecular mechanisms associated with the hepatopancreatic immunity are still unclear. In this study, lipopolysaccharide (LPS), a major component of the cell wall of Gram-negative bacteria, was used for imitating bacteria infection to stimulate the hepatopancreas of A. fangsiao. To investigate the immune process happened in A. fangsiao hepatopancreas, we performed transcriptome analysis of hepatopancreas tissue after LPS injection, and identified 2615 and 1943 differentially expressed genes (DEGs) at 6 and 24 h post-injection, respectively. GO and KEGG enrichment analysis showed that these DEGs were mainly involved in immune-related biological processes and signaling pathways, including ECM-receptor interaction signaling pathway, Phagosome signaling pathway, Lysosome signaling pathway, and JAK-STAT signaling pathways. The function relationships between these DEGs were further analyzed through protein-protein interaction (PPI) networks. It was found that Mtor, Mapk14 and Atm were the three top interacting DEGs under LPS stimulation. Finally, 15 hub genes involving multiple KEGG signaling pathways and PPI relationships were selected for qRT-PCR validation. In this study, for the first time we explored the molecular mechanisms associated with hepatopancreatic immunity in A. fangsiao using a PPI networks approach, and provided new insights for understanding hepatopancreatic immunity in A. fangsiao.

Potential threats of microplastics and pathogenic bacteria to the immune system of the mussels Mytilus galloprovincialis

As one of the main components of marine pollution, microplastics (MPs) inevitably enter the mussel aquaculture environment. At the same time, pathogenic bacteria, especially pathogens such as Vibrio, can cause illness outbreaks, leading to large-scale death of mussels. The potential harm of MPs and pathogenic bacteria to bivalve remains unclear. This study designed two experiments (1) mussels (Mytilus galloprovincialis) were exposed to 100 particles/L or 1,000 particles/L polymethyl methacrylate (PMMA, 17.01 ± 6.74 μm) MPs and 1 × 107 CFU/mL Vibrio parahaemolyticus at the same time (14 days), and (2) mussels were exposed to 100 particles/L or 1,000 particles/L MPs for a long time (30 days) and then exposed to 1 × 107 CFU/mL V. parahaemolyticus to explore the effects of these two stresses on the mussel immune system. The results showed that after the combined exposure of V. parahaemolyticus and MPs, the lysosomal membrane stability of hemocytes decreased, lysozyme activity was inhibited, and hemocytes were induced to produce more lectins and defensins to fight pathogenic invasion. Long-term exposure to MPs caused a large amount of energy consumption in mussels, inhibited most of the functions of humoral immunity, increased the risk of mussel infection with pathogenic bacteria, and negatively affected mussel condition factor, the number of hemocytes, and the number of byssuses. Mussels may allocate more energy to deal with MPs and pathogenic bacterial infections rather than for growth. Above all, MPs exposure can affect mussel immune function or reduce its stress resistance, which in turn has an impact on mollusk farming.

Transcriptome sequencing of the endangered land snail Karaftohelix adamsi from the Island Ulleung: De novo assembly, annotation, valuation of fitness genes and SSR markers

BACKGROUND

The Bradybaenidae snail Karaftohelix adamsi is endemic to Korea, with the species tracked from Island Ulleung in North Gyeongsang Province of South Korea. K. adamsi has been classified under the Endangered Wildlife Class II species of Korea and poses a severe risk of extinction following habitat disturbances. With no available information at the DNA (genome) or mRNA (transcriptome) level for the species, conservation by utilizing informed molecular resources seems difficult.

OBJECTIVE

In this study, we used the Illumina short-read sequencing and Trinity de novo assembly to draft the reference transcriptome of K. adamsi.

RESULTS

After assembly, 13,753 unigenes were obtained of which 10,511 were annotated to public databases (a maximum of 10,165 unigenes found homologs in PANM DB). A total of 6,351, 3,535, 358, and 3,407 unigenes were ascribed to the functional categories under KOG, GO, KEGG, and IPS, respectively. The transcripts such as the HSP 70, aquaporin, TLR, and MAPK, among others, were screened as putative functional resources for adaptation. DNA transposons were found to be thickly populated in comparison to retrotransposons in the assembled unigenes. Further, 2,164 SSRs were screened with the promiscuous presence of dinucleotide repeats such as AC/GT and AG/CT.

CONCLUSION

The transcriptome-guided discovery of molecular resources in K. adamsi will not only serve as a basis for functional genomics studies but also provide sustainable tools to be utilized for the protection of the species in the wild. Moreover, the development of polymorphic SSRs is valuable for the identification of species from newer habitats and cross-species genotyping.

Transcriptional profiling of Bulinus globosus provides insights into immune gene families in snails supporting the transmission of urogenital schistosomiasis

Schistosomiasis, urogenital and intestinal, afflicts 251 million people worldwide with approximately two-thirds of the patients suffering from the urogenital form of the disease. Freshwater snails of the genus Bulinus (Gastropoda: Planorbidae) serve as obligate intermediate hosts for Schistosoma haematobium, the etiologic agent of human urogenital schistosomiasis. These snails also act as vectors for the transmission of schistosomiasis in livestock and wildlife. Despite their crucial role in human and veterinary medicine, our basic understanding at the molecular level of the entire Bulinus genus, which comprises 37 recognized species, is very limited. In this study, we employed Illumina-based RNA sequencing (RNAseq) to profile the genome-wide transcriptome of Bulinus globosus, one of the most important intermediate hosts for S. haematobium in Africa. A total of 179,221 transcripts (N50 = 1,235) were assembled and the benchmarking universal single-copy orthologs (BUSCO) was estimated to be 97.7%. The analysis revealed a substantial number of transcripts encoding evolutionarily conserved immune-related proteins, particularly C-type lectin (CLECT) domain-containing proteins (n = 316), Toll/Interleukin 1-receptor (TIR)-containing proteins (n = 75), and fibrinogen related domain-containing molecules (FReD) (n = 165). Notably, none of the FReDs are fibrinogen-related proteins (FREPs) (immunoglobulin superfamily (IgSF) + fibrinogen (FBG)). This RNAseq-based transcriptional profile provides new insights into immune capabilities of Bulinus snails, helps provide a framework to explain the complex patterns of compatibility between snails and schistosomes, and improves our overall understanding of comparative immunology.

Triclosan exposure induces immunotoxic impacts by disrupting the immunometabolism, detoxification, and cellular homeostasis in blood clam (Tegillarca granosa)

Omnipresent presence of triclosan (TCS) in aqueous environment puts a potential threat to organisms. However, it's poorly understood about its immunometabolic impacts of marine invertebrates. In present study, we use a representative bivalve blood clam (Tegillarca granosa) as a model, investigating the effects of TCS exposure at 20 and 200 μg/L for 28 days on immunometabolism, detoxification, and cellular homeostasis to explore feasible toxicity mechanisms. Results demonstrated that the clams exposed to TCS resulting in evident immunotoxic impacts on both cellular and humoral immune responses, through shifting metabolic pathways and substances, as well as suppressing the expressions of genes from the immune- and metabolism-related pathways. In addition, significant alterations in contents (or activity) of detoxification enzymes and the expression of key detoxification genes were detected in TCS-exposed clams. Moreover, exposure to TCS also disrupted cellular homeostasis of clams through increasing MDA contents and caspase activities, and promoting activation of the apoptosis-related genes. These findings suggested that TCS might induce immunotoxic impacts by disrupting the immunometabolism, detoxification, and cellular homeostasis.

Immuno-molecular profile for Biomphalaria glabrata/Schistosoma mansoni interaction

The complex innate immune defense of Biomphalaria glabrata, the intermediate host of Schistosoma mansoni, governs the successful development of the intramolluscan stages of the parasite. The interaction between the snail and the parasite involves a complex immune molecular crosstalk between several parasite antigens and the snail immune recognition receptors, evoking different signals and effector molecules. This work seeks to discuss the immune-related molecules that influence compatibility in Biomphalaria glabrata/Schistosoma mansoni interaction and the differential expression of these molecules between resistant and susceptible snails. It also includes the current understanding of the immune molecular determinants that govern the compatibility in sympatric and allopatric interactions, and the expression of these molecules after immune priming and the secondary immune response. Herein, the differences in the immune-related molecules in the interaction of other Biomphalaria species with Schistosoma mansoni compared to the Biomphalaria glabrata model snail are highlighted. Understanding the diverse immune molecular determinants in the snail/schistosome interaction can lead to alternative control strategies for schistosomiasis.

A Structural Analysis of Host-Parasite Interactions in Achatina fulica (Giant African Snail) Infected with Angiostrongylus cantonensis

Angiostrongylus cantonensis is a nematode parasite that resides in the pulmonary arteries of rodents, serving as its definitive hosts. The life cycle involves several species of non-marine gastropods as intermediate hosts, and the African giant snail Achatina fulica is considered one of the most important around the world. Experimental data concerning A. cantonensis infection in the African giant snail remains notably limited. This helminth causes eosinophilic meningitis or meningoencephalitis in humans, representing an emergent zoonosis in Brazil. Understanding the host-parasite relationship through the application of new tools is crucial, given the complex interaction between zoonosis and the intricate mechanisms involving wild/human hosts, parasite adaptation, and dispersion. The objective of this study was to employ SEM as a novel methodology to understand the structural organization of the host tissue, particularly the granuloma formation. This sheds light on the complex balance between A. fulica and A. cantonensis. Nine three-month-old snails were randomly selected and exposed for 24 h to a concentration of 2000 L1/dose of A. cantonensis. A necropsy was performed 37 days after the infection, and the samples were examined using light and scanning electron microscopy (SEM). The histopathological results revealed third-stage larvae of A. cantonensis associated with granulomas distributed throughout the head-foot mass, mantle, and kidney. Scanning electron microscopy of the histological section surface showed that the granuloma is surrounded by a cluster of spherical particles, which are distributed in the region bordering the larvae. This reveal details of the nematode structure, demonstrating how this methodology can enhance our understanding of the role of granulomas in molluscan tissue. The structural characteristics of granuloma formation in A. fulica suggest it as an excellent invertebrate host for A. cantonensis. This relationship appears to provide protection to the parasite against the host's immune defense system while isolating the snail's tissue from potential exposure to nematode antigens.

A review of the endangered mollusks transcriptome under the threatened species initiative of Korea

Transcriptome studies for conservation of endangered mollusks is a proactive approach towards managing threats and uncertainties facing these species in natural environments. The population of these species is declining due to habitat destruction, illicit wildlife trade, and global climate change. These activities risk the free movement of species across the wild landscape, loss of breeding grounds, and restrictions in displaying the physiological attributes so crucial for faunal welfare. Gastropods face the most negative ecological effects and have been enlisted under Korea's protective species consortium based on their population dynamics in the last few years. Moreover, with the genetic resources restricted for such species, conservation by informed planning is not possible. This review provides insights into the activities under the threatened species initiative of Korea with special reference to the transcriptome assemblies of endangered mollusks. The gastropods such as Ellobium chinense, Aegista chejuensis, Aegista quelpartensis, Incilaria fruhstorferi, Koreanohadra kurodana, Satsuma myomphala, and Clithon retropictus have been represented. Moreover, the transcriptome summary of bivalve Cristaria plicata and Caenogastropoda Charonia lampas sauliae is also discussed. Sequencing, de novo assembly, and annotation identified transcripts or homologs for the species and, based on an understanding of the biochemical and molecular pathways, were ascribed to predictive gene function. Mining for simple sequence repeats from the transcriptome have successfully assisted genetic polymorphism studies. A comparison of the transcriptome scheme of Korean endangered mollusks with the genomic resources of other endangered mollusks have been discussed with homologies and analogies for dictating future research.

Investigating the internal system of defense of Gastropoda Aplysia depilans (Gmelin, 1791): Focus on hemocytes

The internal defense system of mollusks represents an efficient protection against pathogens and parasites, involving several biological immune processes, such as phagocytosis, encapsulation, cytotoxicity, and antigenic recognition of self/non-self. Mollusks possess professional, migratory, and circulating cells that play a key role in the defense of the organism, the hemocytes. Several studies have been performed on hemocytes from different mollusks, but, to date, these cells are still scarcely explored. Different hemocyte populations have been found, according to the presence or absence of granules, size, and the species of mollusks studied. Our study aims to deepen the knowledge of the hemocytes of the gastropod Aplysia depilans using morphological techniques and light and confocal microscopy, testing Toll-like receptor 2, inducible nitric oxide synthetase, and nicotinic acetylcholine receptor alpha 7 subunit. Our results show two hemocyte populations distinguishable by size, and presence/absence of granules in the cytoplasm, strongly positive for the antibodies tested, suggesting for the first time the presence of these receptors on the surface of sea hare hemocytes by immunohistochemistry. These data help in the understanding of the immune system of this gastropod, providing additional data for comprehending the evolution of the defense response in metazoan phylogenesis.

Immune diversity in lophotrochozoans, with a focus on recognition and effector systems

Lophotrochozoa is one of the most species-rich but immunologically poorly explored phyla. Although lack of acquired response in a narrow sense, lophotrochozoans possess various genetic mechanisms that enhance the diversity and specificity of innate immune system. Here, we review the recent advances of comparative immunology studies in lophotrochozoans with focus on immune recognition and effector systems. Haemocytes and coelomocytes are general important yet understudied player. Comparative genomics studies suggest expansion and functional divergence of lophotrochozoan immune reorganization systems is not as "homogeneous and simple" as we thought including the large-scale expansion and molecular divergence of pattern recognition receptors (PRRs) (TLRs, RLRs, lectins, etc.) and signaling adapters (MyD88s etc.), significant domain recombination of immune receptors (RLR, NLRs, lectins, etc.), extensive somatic recombination of fibrinogenrelated proteins (FREPs) in snails. Furthermore, there are repeatedly identified molecular mechanisms that generate immune effector diversity, including high polymorphism of antimicrobial peptides and proteins (AMPs), reactive oxygen and nitrogen species (RONS) and cytokines. Finally, we argue that the next generation omics tools and the recently emerged genome editing technicism will revolutionize our understanding of innate immune system in a comparative immunology perspective.

Cellular and humoral immune response between snail hosts and their parasites

In invertebrates, the innate immune system protects against a wide range of microbiological infections. Several immunological processes are involved in the interactive immune response between snails and their parasites, including phagocytosis, nitric oxide synthesis, phenol oxidase activity, lysozymes, and lectin formation. The immunological responses connected to the interaction between snails and parasites are discussed in detail in the current research. Understanding the nature of these interactive reactions will enable scientists to explore approaches to eliminate and cure parasitic infections.

Transcriptome profiling based on larvae at different time points after hatching provides a core set of gene resource for understanding the immune response mechanisms of the egg-protecting behavior against Vibrio anguillarum infection in Amphioctopus fangsiao

Mollusks have recently received increasing attention because of their unique immune systems. Mollusks such as Amphioctopus fangsiao are economically important cephalopods, and the effects of their egg-protecting behavior on the larval immune response are unclear. Meanwhile, little research has been done on the resistance response of cephalopod larvae infected with pathogenic bacteria such as Vibrio anguillarum. In this study, V. anguillarum was used to infect the primary hatching A. fangsiao larvae under different egg-protecting behaviors for 24 h, and a total of 7156 differentially expressed genes (DEGs) were identified at four time points after hatching based on transcriptome analysis. GO and KEGG enrichment analyses showed that multiple immune-related GO terms and KEGG signaling pathways were enriched. Protein-protein interaction networks (PPI networks) were used to search functional relationships between immune-related DEGs. Finally, 20 hub genes related to multiple gene functions or involved in multiple signaling pathways were identified, and their accuracy was verified using quantitative RT-PCR. PPI networks were first used to study the effects A. fangsiao larvae after infection with V. anguillarum under different egg-protecting behaviors. The results provide significant genetic resources for exploring invertebrate larval immune processes. The data lays a foundation for further study the immune response mechanisms for invertebrates after infection.

Identification, characterization, and antimicrobial activity of a novel big defensin discovered in a commercial bivalve mollusc, Tegillarca granosa

Molluscs, the second largest animal phylum on earth, primarily rely on cellular and humoral immune responses to fight against pathogen infection. Although antimicrobial peptides (AMPs) such as big defensin play crucial roles in the humoral immune response, it remains largely unknown in the ecological and economic important blood clam (Tegillarca granosa). In this study, a novel big defensin gene (TgBD) was identified in T. granosa through transcripts and whole genome searching. Bioinformatic analyses were conducted to explore the molecular characteristics of TgBD, and comparisons of TgBD with those reported in other molluscs were performed by multiple alignments and phylogenetic analysis. In addition, the expression patterns of TgBD in various tissues and upon bacterial challenge were investigated while the antimicrobial activity of synthetic N-terminal domain of TgBD was confirmed in vitro by radial diffusion experiment. Results obtained showed TgBD had an open reading frame (ORF) of 369 bp, encoding a prepropeptide containing a signal peptide and a propeptide. Similar to big defensins reported in other species, TgBD consists of a hydrophobic N-terminal domain containing β1-α1-α2-β2 folds and a cysteine-rich cationic C-terminal domain with three disulfide bonds between C1-C5, C2-C4, and C3-C6. Phylogenetic analysis showed that TgBD shared 76.80% similarity to its close relative ark shell (Scapharca broughtoni). In addition, TgBD expression was observed in all tissues investigated under normal conditions and was significantly induced by injection of Vibrio parahaemolyticus. Furthermore, synthetic N-terminal peptide of TgBD exhibited strong antimicrobial activity against Gram-positive bacteria tested. Our results indicated that TgBD is a constitutive and inducible acute phase AMP, which provides a universal and prompt protection for T. granosa.

An Overview of Transcriptional Responses of Schistosome-Susceptible (M line) or -Resistant (BS-90) Biomphalaria glabrata Exposed or Not to Schistosoma mansoni Infection

Background

We seek to provide a comprehensive overview of transcriptomics responses of immune-related features of the gastropod Biomphalaria glabrata (Bg) following exposure to Schistosoma mansoni (Sm), a trematode causing human schistosomiasis. Responses of schistosome-susceptible (M line, or SUS) and -resistant (BS-90, or RES) Bg strains were characterized following exposure to Sm for 0.5, 2, 8 or 40 days post-exposure (dpe).

Methods

RNA-Seq and differential expression analysis were undertaken on 56 snails from 14 groups. We considered 7 response categories: 1) constitutive resistance factors; 2) constitutive susceptibility factors; 3) generalized stress responses; 4) induced resistance factors; 5) resistance factors suppressed in SUS snails; 6) suppressed/manipulated factors in SUS snails; and 7) tolerance responses in SUS snails. We also undertook a gene co-expression network analysis. Results from prior studies identifying schistosome resistance/susceptibility factors were examined relative to our findings.

Results

A total of 792 million paired-end reads representing 91.2% of the estimated 31,985 genes in the Bg genome were detected and results for the 7 categories compiled and highlighted. For both RES and SUS snails, a single most supported network of genes with highly correlated expression was found.

Conclusions

1) Several constitutive differences in gene expression between SUS and RES snails were noted, the majority over-represented in RES; 2) There was little indication of a generalized stress response shared by SUS and RES snails at 0.5 or 2 dpe; 3) RES snails mounted a strong, multi-faceted response by 0.5 dpe that carried over to 2 dpe; 4) The most notable SUS responses were at 40 dpe, in snails shedding cercariae, when numerous features were either strongly down-regulated indicative of physiological distress or parasite manipulation, or up-regulated, suggestive of tolerance or survival-promoting effects; 5) Of 55 genes previously identified in genome wide mapping studies, 29 (52.7%) were responsive to Sm, as were many familiar resistance-associated genes (41.0%) identified by other means; 6) Both network analysis and remarkably specific patterns of expression of lectins and G protein-coupled receptors in categories 4, 6 and 7 were indicative of orchestrated responses of different suites of genes in SUS or RES snails following exposure to Sm.

The Role of Anti-Viral Effector Molecules in Mollusc Hemolymph

Molluscs are major contributors to the international and Australian aquaculture industries, however, their immune systems remain poorly understood due to limited access to draft genomes and evidence of divergences from model organisms. As invertebrates, molluscs lack adaptive immune systems or 'memory', and rely solely on innate immunity for antimicrobial defence. Hemolymph, the circulatory fluid of invertebrates, contains hemocytes which secrete effector molecules with immune regulatory functions. Interactions between mollusc effector molecules and bacterial and fungal pathogens have been well documented, however, there is limited knowledge of their roles against viruses, which cause high mortality and significant production losses in these species. Of the major effector molecules, only the direct acting protein dicer-2 and the antimicrobial peptides (AMPs) hemocyanin and myticin-C have shown antiviral activity. A better understanding of these effector molecules may allow for the manipulation of mollusc proteomes to enhance antiviral and overall antimicrobial defence to prevent future outbreaks and minimize economic outbreaks. Moreover, effector molecule research may yield the description and production of novel antimicrobial treatments for a broad host range of animal species.

Toll like receptors and their evolution in the lymnaeid freshwater snail species Radix auricularia and Lymnaea stagnalis, key intermediate hosts for zoonotic trematodes

One of the major evolutionarily conserved pathways in innate immunity of invertebrates is the toll-like receptor (TLR) pathway. However, little is known of the TLR protein family in gastropod molluscs despite their role in the transmission of human diseases, especially the common lymnaeid freshwater snail species Radix auricularia and Lymnaea stagnalis, key intermediate hosts of zoonotic trematodes. Using comparative genomics and gene prediction approaches utilising the freshwater snail Biomphalaria glabrata genome as a reference ten putative TLR proteins were identified in both R. auricularia and L. stagnalis. Phylogenetic analyses revealed that unlike other molluscs the lymnaeid species also possessed class 1 TLRs, previously thought to be unique to B. glabrata. Gene duplication events were also seen across the TLR classes in the lymnaeids with several of the genes appearing to exist as potential tandem elements in R. auricularia. Each predicted TLR was shown to possess the typical the leucine-rich repeat extracellular and TIR intracellular domains and both single cysteine clusters and multiple cysteine clusters TLRs were identified in both lymnaeid species. Principle component analyses of 3D models of the predicted TLRs showed that class 1 and 5 proteins did not cluster based on similarity of structure, suggested to be potential adaptation to a range of pathogens. This study provides the first detailed account of TLRs in lymnaeids and affords a platform for further research into the role of these proteins into susceptibility and compatibility of these snails with trematodes and their role in transmission.

Transcriptome profiling based on protein-protein interaction networks provides a set of core genes for understanding the immune response mechanisms of the egg-protecting behavior in Octopus ocellatus

Protection via of the immune system is indispensable to the life of organisms. Within an immune network, problems with a given link will affect the normal life activities of the organism. Octopus ocellatus is cephalopod widely distributed throughout the world's oceans. Because of its unique nervous system and locomotive organs, research on this species has gradually increased in recent years. Many immune response mechanisms associated with behaviors of O. ocellatus are still unclear. Moreover, as a factor affecting the normal growth of O. ocellatus, egg protection has rarely been considered in previous behavioral studies. In this study, we analyzed the transcriptome profile of gene expression in O. ocellatus larvae, and identified 5936 differentially expressed genes (DEGs). GO and KEGG enrichment analyses were used to search for immune-related DEGs. Protein-protein interaction networks were constructed to examine the interactions between immune-related genes. Fifteen hub genes involved in multiple KEGG signaling pathways or with multiple protein-protein interaction relationships were obtained and verified by quantitative RT-PCR. We first studied the effects of egg protection on the immunity of O. ocellatus larvae by means of protein-protein interaction networks, and the results provide valuable genetic resources for understanding the immunity of invertebrate larvae. The data serve as a foundation for further research on the egg-protecting behavior of invertebrates.

Evolution of Cellular Immunity Effector Cells; Perspective on Cytotoxic and Phagocytic Cellular Lineages

The immune system has evolved to protect organisms from infections caused by bacteria, viruses, and parasitic pathogens. In addition, it provides regenerative capacities, tissue maintenance, and self/non-self recognition of foreign tissues. Phagocytosis and cytotoxicity are two prominent cellular immune activities positioned at the base of immune effector function in mammals. Although these immune mechanisms have diversified into a wide heterogeneous repertoire of effector cells, it appears that they share some common cellular and molecular features in all animals, but also some interesting convergent mechanisms. In this review, we will explore the current knowledge about the evolution of phagocytic and cytotoxic immune lineages against pathogens, in the clearance of damaged cells, for regeneration, for histocompatibility recognition, and in killing virally infected cells. To this end, we give different immune examples of multicellular organism models, ranging from the roots of bilateral organisms to chordate invertebrates, comparing to vertebrates' lineages. In this review, we compare cellular lineage homologies at the cellular and molecular levels. We aim to highlight and discuss the diverse function plasticity within the evolved immune effector cells, and even suggest the costs and benefits that it may imply for organisms with the meaning of greater defense against pathogens but less ability to regenerate damaged tissues and organs.

Examining adaptive evolution of immune activity: opportunities provided by gastropods in the age of 'omics'

Parasites threaten all free-living organisms, including molluscs. Understanding the evolution of immune defence traits in natural host populations is crucial for predicting their long-term performance under continuous infection risk. Adaptive trait evolution requires that traits are subject to selection (i.e. contribute to organismal fitness) and that they are heritable. Despite broad interest in the evolutionary ecology of immune activity in animals, the understanding of selection on and evolutionary potential of immune defence traits is far from comprehensive. For instance, empirical observations are only rarely in line with theoretical predictions of immune activity being subject to stabilizing selection. This discrepancy may be because ecoimmunological studies can typically cover only a fraction of the complexity of an animal immune system. Similarly, molecular immunology/immunogenetics studies provide a mechanistic understanding of immunity, but neglect variation that arises from natural genetic differences among individuals and from environmental conditions. Here, we review the current literature on natural selection on and evolutionary potential of immune traits in animals, signal how merging ecological immunology and genomics will strengthen evolutionary ecological research on immunity, and indicate research opportunities for molluscan gastropods for which well-established ecological understanding and/or 'immune-omics' resources are already available. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

Morphology and activities of cell populations of haemocytes in Oncomelania hupensis following Schistosoma japonicum infection

Oncomelania hupensis is the only obligatory intermediate host of Schistosoma japonicum, the pathogen of zoonosis schistosomiasis. Haemocytes play a critical role in the cellular immune defence of O. hupensis against S. japonicum challenge. Here, the morphology and classification of haemocytes of O. hupensis were investigated by Giemsa staining and light microscopy, combining with the scanning and transmission electron microscopy and flow cytometry. Granulocytes and hyalinocytes were confirmed as two main types of haemocytes, account for ~ 10% and ~ 90% of all haemocytes, with size varying in 4.3-10.9 μm and 0.4-30.8 μm, respectively. Subpopulations can be identified further by granule feature, shape, size, and surface and inner structure of cells. The heterogeneity in morphology implied varied developmental process and function of haemocyte subpopulations. After the S. japonicum challenge, haemocytes of O. hupensis respond to S. japonicum invasion immediately. The dynamic change of haemocyte subpopulations indicates that the small hyalinocyte could differentiate into a larger one or granulocyte after S. japonicum challenge, and the granulocytes and larger hyalinocytes play leading roles in early defence reaction, but in different ways. Phagocytosis and apoptosis of haemocytes in O. hupensis were proved to be related to immune defence against S. japonicum, with the combined effect of granulocytes and larger hyalinocytes. However, the main pathway of each subpopulation to take effect in different periods need further investigation.

Transcriptome profiling of Lymnaea stagnalis (Gastropoda) for ecoimmunological research

BACKGROUND

Host immune function can contribute to numerous ecological/evolutionary processes. Ecoimmunological studies, however, typically use one/few phenotypic immune assays and thus do not consider the complexity of the immune system. Therefore, "omics" resources that allow quantifying immune activity across multiple pathways are needed for ecoimmunological models. We applied short-read based RNAseq (Illumina NextSeq 500, PE-81) to characterise transcriptome profiles of Lymnaea stagnalis (Gastropoda), a multipurpose model snail species. We used a genetically diverse snail stock and exposed individuals to immune elicitors (injury, bacterial/trematode pathogens) and changes in environmental conditions that can alter immune activity (temperature, food availability).

RESULTS

Immune defence factors identified in the de novo assembly covered elements broadly described in other gastropods. For instance, pathogen-recognition receptors (PRR) and lectins activate Toll-like receptor (TLR) pathway and cytokines that regulate cellular and humoral defences. Surprisingly, only modest diversity of antimicrobial peptides and fibrinogen related proteins were detected when compared with other taxa. Additionally, multiple defence factors that may contribute to the phenotypic immune assays used to quantify antibacterial activity and phenoloxidase (PO)/melanisation-type reaction in this species were found. Experimental treatments revealed factors from non-self recognition (lectins) and signalling (TLR pathway, cytokines) to effectors (e.g., antibacterial proteins, PO enzymes) whose transcription depended on immune stimuli and environmental conditions, as well as components of snail physiology/metabolism that may drive these effects. Interestingly, the transcription of many factors (e.g., PRR, lectins, cytokines, PO enzymes, antibacterial proteins) showed high among-individual variation.

CONCLUSIONS

Our results indicate several uniform aspects of gastropod immunity, but also apparent differences between L. stagnalis and some previously examined taxa. Interestingly, in addition to immune defence factors that responded to immune elicitors and changes in environmental conditions, many factors showed high among-individual variation across experimental snails. We propose that such factors are highly important to be included in future ecoimmunological studies because they may be the key determinants of differences in parasite resistance among individuals both within and between natural snail populations.

Immunological Responses of Marine Bivalves to Contaminant Exposure: Contribution of the -Omics Approach

The increasing number of data studies on the biological impact of anthropogenic chemicals in the marine environment, together with the great development of invertebrate immunology, has identified marine bivalves as a key invertebrate group for studies on immunological responses to pollutant exposure. Available data on the effects of contaminants on bivalve immunity, evaluated with different functional and molecular endpoints, underline that individual functional parameters (cellular or humoral) and the expression of selected immune-related genes can distinctly react to different chemicals depending on the conditions of exposure. Therefore, the measurement of a suite of immune biomarkers in hemocytes and hemolymph is needed for the correct evaluation of the overall impact of contaminant exposure on the organism's immunocompetence. Recent advances in -omics technologies are revealing the complexity of the molecular players in the immune response of different bivalve species. Although different -omics represent extremely powerful tools in understanding the impact of pollutants on a key physiological function such as immune defense, the -omics approach has only been utilized in this area of investigation in the last few years. In this work, available information obtained from the application of -omics to evaluate the effects of pollutants on bivalve immunity is summarized. The data shows that the overall knowledge on this subject is still quite limited and that to understand the environmental relevance of any change in immune homeostasis induced by exposure to contaminants, a combination of both functional assays and cutting-edge technology (transcriptomics, proteomics, and metabolomics) is required. In addition, the utilization of metagenomics may explain how the complex interplay between the immune system of bivalves and its associated bacterial communities can be modulated by pollutants, and how this may in turn affect homeostatic processes of the host, host–pathogen interactions, and the increased susceptibility to disease. Integrating different approaches will contribute to knowledge on the mechanism responsible for immune dysfunction induced by pollutants in ecologically and economically relevant bivalve species and further explain their sensitivity to multiple stressors, thus resulting in health or disease.

RNA-seq: The early response of the snail Physella acuta to the digenetic trematode Echinostoma paraensei

To analyze the response of the snail Physella acuta to Echinostoma paraensei, a compatible digenetic trematode, Illumina RNA-seq data were collected from snails with early infection (5 snails at 2 days post-exposure [DPE]) and established infection (4 snails, 8 DPE), and 7 control (unexposed) snails. A reference transcriptome (325,563 transcripts, including 98% of eukaryotic universal single-copy orthologs; BUSCO) and a draft P. acuta genome (employing available genomic Illumina reads; 799,945 scaffolds, includes 88% BUSCO genes) were assembled to guide RNA-seq analyses. Parasite exposure of P. acuta led to 10,195 differentially expressed (DE) genes at 2 DPE and 8,876 DE genes at 8 DPE with only 18% of up-regulated and 22% of down-regulated sequences shared between these time points. Gene ontology (GO) analysis yielded functional annotation of only 1.2% of DE genes but did not indicate major changes in biological activities of P. acuta between 2 and 8 DPE. Increased insights were achieved by analysis of expression profiles of 460 immune-relevant DE transcripts, identified by BLAST and InterProScan. Physella acuta has expanded gene families that encode immune-relevant domains, including CD109/TEP, GTPase IMAP, Limulus agglutination factor (dermatopontin), FReD (≥82 sequences with fibrinogen-related domains), and transcripts that combine C-type lectin (C-LECT) and C1q domains, novel among metazoa. Notably, P. acuta expressed sequences from these immune gene families at all time points, but the assemblages of unique transcripts from particular immune gene families differed between 2 and 8 DPE. The shift in profiles of DE immune genes, from early exposure to parasite establishment, suggests that compatible P. acuta initially respond to infection but switch to express immune genes that likely are less effective against E. paraensei but counter other types of (opportunistic) pathogens and parasites. We propose that the latter expression profile is part of an extended phenotype of E. paraensei, imposed upon P. acuta through parasite manipulation of the host, following successful parasite establishment in the snail after 2 DPE.

Bulinus truncatus transcriptome – a resource to enable molecular studies of snail and schistosome biology

Despite advances in high-throughput sequencing and bioinformatics, molecular investigations of snail intermediate hosts that transmit parasitic trematodes are scant. Here, we report the first transcriptome for Bulinus truncatus – a key intermediate host of Schistosoma haematobium – a blood fluke that causes urogenital schistosomiasis in humans. We assembled this transcriptome from short- and long-read RNA-sequence data. From this transcriptome, we predicted 12,998 proteins, 58% of which had orthologs in Biomphalaria glabrata – an intermediate host of Schistosoma mansoni – a blood fluke that causes hepato-intestinal schistosomiasis. We predicted that select protein groups are involved in signal transduction, cell growth and death, the immune system, environmental adaptation and/or the excretory/secretory system, suggesting roles in immune responses, pathogen defence and/or parasite-host interactions. The transcriptome of Bu. truncatus provides a useful resource to underpin future molecular investigations of this and related snail species, and its interactions with pathogens including S. haematobium. The present resource should enable comparative investigations of other molluscan hosts of socioeconomically important parasites in the future.

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First transcriptome to represent Bulinus truncatus – a snail intermediate host of Schistosoma haematobium.

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Select protein groups of Bu. truncatus are inferred to associate with innate immune responses against pathogens.

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Transcriptome provides a resource for future studies of parasite-host interactions and snail-host resistance to pathogens.

Evolution and Potential Function in Molluscs of Neuropeptide and Receptor Homologues of the Insect Allatostatins

The allatostatins (ASTs), AST-A, AST-B and AST-C, have mainly been investigated in insects. They are a large group of small pleotropic alloregulatory neuropeptides that are unrelated in sequence and activate receptors of the rhodopsin G-protein coupled receptor family (GPCRs). The characteristics and functions of the homologue systems in the molluscs (Buccalin, MIP and AST-C-like), the second most diverse group of protostomes after the arthropods, and of high interest for evolutionary studies due to their less rearranged genomes remains to be explored. In the present study their evolution is deciphered in molluscs and putative functions assigned in bivalves through meta-analysis of transcriptomes and experiments. Homologues of the three arthropod AST-type peptide precursors were identified in molluscs and produce a larger number of mature peptides than in insects. The number of putative receptors were also distinct across mollusc species due to lineage and species-specific duplications. Our evolutionary analysis of the receptors identified for the first time in a mollusc, the cephalopod, GALR-like genes, which challenges the accepted paradigm that AST-AR/buccalin-Rs are the orthologues of vertebrate GALRs in protostomes. Tissue transcriptomes revealed the peptides, and their putative receptors have a widespread distribution in bivalves and in the bivalve Mytilus galloprovincialis, elements of the three peptide-receptor systems are highly abundant in the mantle an innate immune barrier tissue. Exposure of M. galloprovincialis to lipopolysaccharide or a marine pathogenic bacterium, Vibrio harveyi, provoked significant modifications in the expression of genes of the peptide precursor and receptors of the AST-C-like system in the mantle suggesting involvement in the immune response. Overall, our study reveals that homologues of the arthropod AST-systems in molluscs are potentially more complex due to the greater number of putative mature peptides and receptor genes. In bivalves they have a broad and varying tissue distribution and abundance, and the elements of the AST-C-like family may have a putative function in the immune response.

Physiology and immunity of the invasive giant African snail, Achatina (Lissachatina) fulica, intermediate host of Angiostrongylus cantonensis

As one of the most successful invasive land snail species, Achatina (Lissachatina) fulica Bowdich, 1822 has achieved wide global distribution, particularly in (sub)tropical regions, with further dispersal likely due to climate change. This species of giant African snails (up to 17 cm shell length) is a pest that has extensive negative impact on agriculture and can serve as vector for several parasites, including Angiostrongylus cantonensis, a nematode parasite that causes (human) eosinophilic meningitis, an emergent disease. Investigation showed that A. cantonensis infection negatively impacts the metabolism of A. fulica by depleting polysaccharide stores of the intermediate host, compromising the energy balance of the snail. A review of the literature indicates that A. fulica possesses potent innate type immune defenses to counter infection, including phagocytic hemocytes capable of deploying reactive oxygen species and lectins for non-self recognition, a serine protease-dependent coagulation response (not observed in other taxa of gastropods), as well as antimicrobial proteins including achacin, an antimicrobial protein. A recent chromosome level genome assembly will facilitate progressively detailed characterization of these immune features of A. fulica. We strongly encourage further immunological studies of A. fulica, ranging from organismal level to molecular biology to gain better understanding of the A. fulica internal defense response to nematode pathogens like A. cantonensis and the contribution of immune function to the invasiveness of (snail) species. Characterization of immunity of A. fulica, representing the understudied Stylommatophora (panpulmonate landsnails) will also broaden the comparative immunology of Gastropoda.

Snail defence responses to parasite infection: The Lymnaea stagnalis-Trichobilharzia szidati model

Lymnaea stagnalis is a common freshwater gastropod. Importantly, the snail serves as the intermediate host for more than one hundred species of digenetic trematodes, including the avian schistosome Trichobilharzia szidati, a causative agent of cercarial dermatitis in humans. Infection of L. stagnalis by T. szidati initiates a dynamic confrontation between the host and the parasite that culminates in immunocompatibility ensuring survival and development of larvae. Unfortunately, the molecular mechanisms determining this immunocompatibility remain poorly characterised. By employing a variety of immune elicitors, including chemical compounds, PAMPs and bacteria, research in the last two decades has elucidated some of the molecular processes that regulate the snail internal defence response such as haemocyte signalling pathways. These discoveries provide a framework for future studies of molecular interactions between T. szidati and L. stagnalis to help elucidate factors and mechanisms enabling transmission of schistosome parasites. Moreover, support from recently available next generation sequence data and CRISPR-enabled functional genomics should further enable L. stagnalis as an important model for comparative immunology and contribute to a more comprehensive understanding of immune functions in gastropod molluscs.

De novo assembly, characterization of tissue-specific transcriptomes and identification of immune related genes from the scallop Argopecten purpuratus

The scallop Argopecten purpuratus is one of the most economically important cultured mollusks on the coasts from Chile and Peru but its production has declined, in part, due to the emergence of mass mortality events of unknown origin. Driven by this scenario, increasing progress has been made in recent years in the comprehension of immune response mechanisms in this species. However, it is still not entirely understood how different mucosal interfaces participate and cooperate with the immune competent cells, the hemocytes, in the immune defense. Thus, in this work we aimed to characterize the transcriptome of three tissues with immune relevance from A. purpuratus by next-generation sequencing and de novo transcriptome assembly. For this, 18 cDNA libraries were constructed from digestive gland, gills and hemocytes tissues of scallops from different immune conditions and sequenced by the Illumina HiSeq4000 platform. A total of 967.964.884 raw reads were obtained and 967.432.652 clean reads were generated. The clean reads were de novo assembled into 46.601 high quality contigs and 32.299 (69.31%) contigs were subsequently annotated. In addition, three de novo specific assemblies were performed from clean reads obtained from each tissue cDNA libraries for their comparison. Gene ontology (GO) and KEGG analyses revealed that annotated sequences from digestive gland, gills and hemocytes could be classified into both general and specific subcategory terms and known biological pathways, respectively, according to the tissue nature. Finally, several immune related candidate genes were identified, and the differential expression of tissue-specific genes was established, suggesting they could display specific roles in the host defense. The data presented in this study provide the first insight into the tissue specific transcriptome profiles of A. purpuratus, which should be considered for further research on the interplay between the hemocytes and mucosal immune responses.

From the raw bar to the bench: Bivalves as models for human health

Bivalves, from raw oysters to steamed clams, are popular choices among seafood lovers and once limited to the coastal areas. The rapid growth of the aquaculture industry and improvement in the preservation and transport of seafood have enabled them to be readily available anywhere in the world. Over the years, oysters, mussels, scallops, and clams have been the focus of research for improving the production, managing resources, and investigating basic biological and ecological questions. During this decade, an impressive amount of information using high-throughput genomic, transcriptomic and proteomic technologies has been produced in various classes of the Mollusca group, and it is anticipated that basic and applied research will significantly benefit from this resource. One aspect that is also taking momentum is the use of bivalves as a model system for human health. In this review, we highlight some of the aspects of the biology of bivalves that have direct implications in human health including the shell formation, stem cells and cell differentiation, the ability to fight opportunistic and specific pathogens in the absence of adaptive immunity, as source of alternative drugs, mucosal immunity and, microbiome turnover, toxicology, and cancer research. There is still a long way to go; however, the next time you order a dozen oysters at your favorite raw bar, think about a tasty model organism that will not only please your palate but also help unlock multiple aspects of molluscan biology and improve human health.

Changes in hemocytes of Biomphalaria glabrata infected with Echinostoma paraensei and exposed to glyphosate-based herbicide

The immune system of snails is highly sensitive to pollutants, which can suppress its immune response. We investigated the effects of exposure to the glyphosate-based herbicide Roundup® Original on the snail Biomphalaria glabrata infected by the platyhelminth Echinostoma paraensei by evaluating changes in the snail's internal defense system. Four cohorts were studied: control group, infected snails, snails treated with Roundup®, and snails infected and treated with Roundup®. The hemocyte viability was assessed, morphological differentiation of cells was observed and flow cytometry was performed to determine the morphology, viability and the lectin expression profiles. The frequencies of dead hemocytes were lower in the infected group and higher in both pesticide treated groups. Three cell types were identified: blast-like cells, hyalinocytes and granulocytes. The highest number of all types of hemocytes, as well as the highest number of dead cells, were observed in the infected, pesticide-treated group. The association between infection and herbicide exposure greatly increased the frequency of dead hemocytes, suggesting that this condition impairs the internal defense system of B. glabrata making the snails more vulnerable to parasitic infections.

Evolutionary and functional analysis of Cyclina sinensis c-Jun AP-1 gene in response to LPS stimulation

The transcription factor activator protein-1 (AP-1) plays an essential and critical role in the regulation of numerous downstream genes involved in various physiological and chemical responses. In this study, we identified a full-length cDNA of the c-Jun AP-1 gene (termed Csc-Jun) from the transcriptome library in Cyclina sinensis. The cDNA contains an 825-bp open reading frame that encodes a 274-amino acid protein sequence, including a characteristic Jun transcription factor domain and a highly conserved basic leucine zipper (bZIP) signature that shares 90% identity to that of Ruditapes philippinarum. Furthermore, a phylogenetic analysis using MrBayes and PhyML software (with Bayesian and maximum likelihood approaches, respectively) revealed that the c-Jun AP-1 family genes might be involved in adapting to various environments in different invertebrates. We implemented the PAML software with the maximum likelihood method to further select and verify the positive selection sites (PSSs) in the Mollusca c-Jun AP-1 genes, and we detected four PSSs located in the Jun transcription factor domain. In addition, a spatial expression analysis showed that the Csc-Jun cDNA transcript was ubiquitously expressed in all of the tested tissues and was strongly expressed in the hepatopancreas and weakly expressed in the tissues of the hemocytes, gill filaments, mantle and adductor muscle. Quantitative real-time PCR showed that the expression profiles of Csc-Jun were significantly upregulated at different times in all of the tested tissues when challenged with lipopolysaccharide (LPS). Furthermore, knockdown of Csc-Jun by RNA interference resulted in a higher mortality of C. sinensis following LPS exposure. Finally, we explored the function of the TLR13-MyD88 signaling pathway in the innate immunity of C. sinensis by RNA interference and immune challenges. The results revealed that the mRNA expression levels of Csc-Jun were all decreased (P < 0.01) in normal and stimulated C. sinensis hemocytes. These data collectively indicated that the c-Jun AP-1 gene might play vital roles in innate immunity and provide new evidence for the evolutionary patterns of innate immune genes in Mollusca.

Differential lncRNA expression profiles reveal the potential roles of lncRNAs in antiviral immune response of Crassostrea gigas

Long noncoding RNAs (lncRNAs) may play widespread roles in various biological processes. However, systematic profiles of lncRNAs in the biological responses of Pacific Oyster (Crassostrea gigas) to pathogen infection have not yet been demonstrated. Here, we have conducted an exhaustive comparative transcriptome analysis using a bioinformatics approach to exam the functions of lncRNAs response to Ostreid herpesvirus 1μVar (OsHV-1μVar) challenge. In total, 101 differentially expressed lncRNAs (DE-lncRNA) during OsHV-1μVar infections were identified. Compared with differentially expressed mRNAs (DE-mRNA), DE-lncRNAs are shorter in terms of overall length but longer in terms of exon length. These lncRNAs shared similar characteristics with previously reported invertebrate lncRNAs, such as relatively low GC content, low exon number and low sequence conservation, but low expression level were not observed. 20 DE-lncRNAs are typically co-expressed with their neighboring genes annotated as GO terms (GO: 0044237), indicating that these lncRNAs are involved in binding and cellular process functions in cis mode. The weighted gene co-expression network (WGCNA) analysis resulted in 15 modules. The highlighted blue module was specifically demonstrated a co-expression relationship between 14 DE-lncRNAs and 17 immune-related DE-mRNAs (IR-DE-mRNA). Three hub lncRNAs within this module were co-expressed with one hub IR-DE-mRNA involved in fibrinogen-related protein. It was speculated that lncRNAs is extensively involved in oyster antiviral innate immune system. The present study will facilitate subsequently experimental studies to unravel the function of lncRNAs in marine invertebrate response to pathogen infection.

Two macrophage migration inhibitory factors (MIFs) from the clam Ruditapes philippinarum: Molecular characterization, localization and enzymatic activities

Macrophage migration inhibitory factor (MIF) is an evolutionarily ancient cytokine-like factor and plays a critical role in both innate and adaptive immunity. In the present study, two MIFs (designed as RpMIF-1 and RpMIF-2, respectively) were identified and characterized from the clam Ruditapes philippinarum by rapid amplification of cDNA ends (RACE) approaches. The full-length cDNA of RpMIF-1 and RpMFI-2 consisted of 531 and 722 nucleotides, encoding a polypeptide of 113 and 114 amino acid residues, respectively. Multiple alignments and phylogenetic analysis revealed that both RpMIF-1 and RpMIF-2 belonged to the MIF family. The conserved catalytic-site Pro² for tautomerase activity was identified in the deduced amino acid sequences of RpMIFs. Both RpMIF-1 and RpMIF-2 transcripts were constitutively expressed in examined tissues of R. philippinarum with dominant expression in hepatopancreas, gills and hemocytes. Immunolocalization analysis showed that RpMIF-1 and RpMIF-2 proteins were expressed in examined tissues with the exception of adductor muscle and foot. After Vibrio anguillarum and Micrococcus luteus challenge, the mRNA expression of RpMIFs was significantly modulated in hemocytes, gills and hepatopancreas. Recombinant RpMIF-1 and RpMIF-2 proteins possessed significant tautomerase activity and oxidoreductase activity, indicating that these two proteins was perhaps involved in inflammatory responses. In summary, our results suggested that RpMIF-1 and RpMIF-2 played an important role in the innate immunity of R. philippinarum.

Comparative immunological study of the snail Physella acuta (Hygrophila, Pulmonata) reveals shared and unique aspects of gastropod immunobiology

The freshwater snail Physella acuta was selected to expand the perspective of comparative snail immunology. Analysis of Physella acuta, belonging to the Physidae, taxonomic sister family to Planorbidae, affords family-level comparison of immune features characterized from Biomphalaria glabrata, the model snail often used to interpret general gastropod immunity. To capture constitutive and induced immune sequences, transcriptomes of an individual Physella acuta snail, 12 h post injection with bacteria (Gram -/+) and one sham-exposed snail were recorded with 454 pyrosequencing. Assembly yielded a combined reference transcriptome containing 24,288 transcripts. Additionally, genomic Illumina reads were obtained (∼15-fold coverage). Recovery of transcripts for two macin-like antimicrobial peptides (AMPs), 12 aplysianins, four LBP/BPIs and three physalysins indicated that Physella acuta shares a similar organization of antimicrobial defenses with Biomphalaria glabrata, contrasting a modest AMP arsenal with a diverse set of antimicrobial proteins. The lack of predicted transmembrane domains in all seven Physella acuta PGRP transcripts supports the notion that gastropods do not employ cell-bound PGRP receptors, different from ecdysozoan invertebrates yet similar to mammals (vertebrate deuterostomes). The well-documented sequence diversification by Biomphalaria glabrata FREPs (immune lectins comprising immunoglobulin superfamily domains and fibrinogen domains), resulting from somatic mutations of a large FREP gene family is hypothesized to be unique to Planorbidae; Physella acuta revealed just two bonafide FREP genes and these were not diversified. Furthermore, the flatworm parasite Echinostoma paraensei, confirmed here to infect both snail species, did not evoke from Physella acuta the abundant expression of FREP proteins at 2, 4 and 8 days post exposure that was previously observed from Biomphalaria glabrata. The Physella acuta reference transcriptome also revealed 24 unique transcripts encoding proteins consisting of a single fibrinogen-related domain (FReDs), with a short N-terminal sequence encoding either a signal peptide, transmembrane domain or no predicted features. The Physella acuta FReDs are candidate immune genes based on implication of similar sequences in immunity of bivalve molluscs. Overall, comparative analysis of snails of sister families elucidated the potential for taxon-specific immune features and investigation of strategically selected species will provide a more comprehensive view of gastropod immunity.

Identification of Toll-like receptor family members in Oncomelania hupensis and their role in defense against Schistosoma japonicum

The amphibious snail, Oncomelania hupensis, primarily distributed in the Far East, is the only intermediate host of Schistosoma japonicum, which causes the most virulent form of schistosomiasis. Obligatory parasitism of snails is the main vehicle for human and livestock infection and depends primarily on parasite infectivity, snail defense capacity and specificity, and parasite-snail compatibility. Therefore, the schistosome-snail interaction is biomedically significant, particularly the molecular mechanisms involved in the innate immune response against S. japonicum. Several immune effectors and signaling pathways have been successfully identified in mollusks, especially in Biomphalaria glabrata, the intermediate snail host of S. mansoni; however, limited information is available for O. hupensis. Here, we identified 16 Toll-like receptors (TLRs) in O. hupensis. These O. hupensis TLRs (OhTLRs) are highly expressed in haemocytes, the primary immune cell of mollusks. Most of the OhTLRs were more highly expressed in female gonads than in other tissues, which may suggest maternal immune transfer in O. hupensis. After S. japonicum challenge, the expression levels of all of the OhTLRs were significantly up-regulated at 6 h post-challenge; many of the OhTLR expression levels were inhibited at later time points in haemocytes, while they were inhibited and fluctuated to varying degrees in other tissues. Additionally, we further determined the tissue-specific expression and dynamic response against S. japonicum of one of the TLR signaling adaptors, myeloid differentiation factor 88 (MyD88), from O. hupensis. Three OhMyD88 genes were highly expressed in haemocytes, and were up-regulated in haemocytes and inhibited in the head-foot muscle at the early time-point after S. japonicum challenge; however, these had slower changes and longer durations compared to OhTLRs. These results provide evidence suggesting that immune effectors are involved in innate immune responses of O. hupensis against S. japonicum and may play a role in the activation of different haemocytes, and not limited for the early response to S. japonicum invasion. Further investigation into the varied expression of OhTLRs in other tissues after S. japonicum challenge will improve our understanding of TLR function in innate immunity of O. hupensis.

Specific Pathogen Recognition by Multiple Innate Immune Sensors in an Invertebrate

Detection of pathogens by all living organisms is the primary step needed to implement a coherent and efficient immune response. This implies a mediation by different soluble and/or membrane-anchored proteins related to innate immune receptors called PRRs (pattern-recognition receptors) to trigger immune signaling pathways. In most invertebrates, their roles have been inferred by analogy to those already characterized in vertebrate homologs. Despite the induction of their gene expression upon challenge and the presence of structural domains associated with the detection of pathogen-associated molecular patterns in their sequence, their exact role in the induction of immune response and their binding capacity still remain to be demonstrated. To this purpose, we developed a fast interactome approach, usable on any host–pathogen couple, to identify soluble proteins capable of directly or indirectly detecting the presence of pathogens. To investigate the molecular basis of immune recognition specificity, different pathogens (Gram-positive bacterium, Micrococcus luteus; Gram-negative, Escherichia coli; yeast, Saccharomyces cerevisiae; and metazoan parasites, Echinostoma caproni or Schistosoma mansoni) were exposed to hemocyte-free hemolymph from the gastropod Biomphalaria glabrata. Twenty-three different proteins bound to pathogens were identified and grouped into three different categories based on their primary function. Each pathogen was recognized by a specific but overlapping set of circulating proteins in mollusk’s hemolymph. While known PRRs such as C-type lectins were identified, other proteins not known to be primarily involved in pathogen recognition were found, including actin, tubulin, collagen, and hemoglobin. Confocal microscopy and specific fluorescent labeling revealed that extracellular actin present in snail hemolymph was able to bind to yeasts and induce their clotting, a preliminary step for their elimination by the snail immune system. Aerolysin-like proteins (named biomphalysins) were the only ones involved in the recognition of all the five pathogens tested, suggesting a sentinel role of these horizontally acquired toxins. These findings highlight the diversity and complexity of a highly specific innate immune sensing system. It paves the way for the use of such approach on a wide range of host–pathogen systems to provide new insights into the specificity and diversity of immune recognition by innate immune systems.