Genetics of Biomphalaria glabrata and its effect on the outcome of Schistosoma mansoni infection

Parasite transmission in size-structured populations

Host heterogeneity can affect parasite transmission, but determining underlying traits and incorporating them into transmission models remains challenging. Body size is easily measured and affects numerous ecological interactions, including transmission. In the snail-schistosome system, larger snails have a higher exposure to parasites but lower susceptibility to infection per parasite. We quantified the effect of size-based heterogeneity on population-level transmission by conducting transmission trials in differently size-structured snail populations and competing size-dependent transmission models. Populations with greater proportions of large snails had lower prevalence, and small snails were shielded from infection by co-occurring large conspecifics. Furthermore, a fully dependent transmission model that incorporated body size in both exposure and susceptibility outperformed other candidate models considered. Incorporating traits such as body size, which are affected by and directly affect host ecology, into transmission models could yield insights into natural dynamics and disease mitigation in many systems.

Population genetic structure of Indoplanorbis exustus (Gastropoda: Planorbidae) in Thailand and its infection with trematode cercariae

Indoplanorbis exustus is a freshwater gastropod belonging to the family Planorbidae. This snail is widely distributed across the tropics and plays an important role as the intermediate host for trematodes. However, relatively little is understood regarding the genetic relationship between I. exustus and trematodes. The goals of this study were to investigate the current transmission status of trematode cercariae in I. exustus in Thailand and to examine the genetic diversity, genetic structure, and demographic history of I. exustus. We collected 575 I. exustus from 21 provinces across six regions of Thailand and investigated cercarial infections by using the shedding method. I. exustus from two provinces were infected with cercarial trematodes, and two types of cercarial stages were molecularly identified as furcocercous cercaria and xiphidiocercariae. Phylogenetic tree analysis based on 28S rDNA and ITS2 sequences demonstrated that furcocercous cercaria and xiphidiocercariae were closely clustered with a clade of Euclinostomum sp. and Xiphidiocercariae sp., respectively. Phylogenetic and network analyses of I. exustus haplotypes based on the COI, 16S rDNA, and ITS1 genes demonstrated four main clades. Only snails in clade A were distributed in all regions of Thailand and harbored trematode cercariae. The level of genetic diversity of I. exustus was relatively high, but most populations were not genetically different, thus suggesting the appearance of gene flow within the I. exustus populations. Overall, the haplotype network was star-shaped, thus suggesting the recent demographic expansion of populations. This result was also supported by the unimodal mode of the mismatch distribution graph and the large negative values of the neutrality tests. Therefore, the I. exustus snail was likely another freshwater snail of the invasive species in Thailand. This information will aid in monitoring the spread of the parasitic trematodes carried by I. exustus from different populations.

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.

PTC2 region genotypes counteract Biomphalaria glabrata population differences between M-line and BS90 in resistance to infection by Schistosoma mansoni

Background

Biomphalaria glabrata is a snail intermediate host for Schistosoma mansoni, a trematode responsible for human schistosomiasis. BS90 is one of the most well studied strains of B. glabrata owing to its high resistance to infection by most strains of S. mansoni. An F2 mapping study from 1999 identified two RAPD markers that associated with what appeared to be single-locus, dominant resistance by the BS90 population relative to the susceptible M-line population. One marker cannot be mapped, but the other, OPM-04, maps to within 5 Mb of PTC2, a region we recently showed has a very large effect on resistance within another snail population challenged by the same strain of parasite (PR1). Here we tested the hypothesis that the PTC2 region contains the causal gene/s that explain the iconic resistance of BS90 snails.

Methods

We used marker-assisted backcrossing to drive the BS90 version of the PTC2 region (+/-~1 Mb on either side) into an M-line (susceptible strain) genetic background, and the M-line version into a BS90 genetic background. We challenged the offspring with PR1-strain schistosomes and tested for effects of allelic variation in the PTC2 region in a common genetic background.

Results

Relative to M-line haplotypes, the BS90 haplotype actually confers enhanced susceptibility. So we reject our original hypothesis. One possible explanation for our result was that the causal gene linked to OPM-04 is near, but not in the PTC2 block that we introgressed into each line. So we used an F2 cross to independently test the effects of the PTC2 and OPM-04 regions in a randomized genetic background. We confirmed that the BS90 haplotype confers increased susceptibility, and we see a similar, although non-significant effect at OPM-04. We discuss possible reasons why our results differed so dramatically from those of the 1999 study. We also present Pacbio assemblies of the PTC2 and flanking region in BS90 and M-line, compare with previously published PTC2 haplotypes, and discuss candidate genes that might be behind the enhanced susceptibility of the BS90 haplotype.

Compatibility between snails and schistosomes: insights from new genetic resources, comparative genomics, and genetic mapping

The freshwater snail Biomphalaria glabrata is an important intermediate host of the parasite Schistosoma mansoni that causes human intestinal schistosomiasis. To better understand vector snail biology and help advance innovative snail control strategies, we have developed a new snail model consisting of two homozygous B. glabrata lines (iM line and iBS90) with sharply contrasting schistosome-resistance phenotypes. We produced and compared high-quality genome sequences for iM line and iBS90 which were assembled from 255 (N50 = 22.7 Mb) and 346 (N50 = 19.4 Mb) scaffolds, respectively. Using F2 offspring bred from the two lines and the newly generated iM line genome, we constructed 18 linkage groups (representing the 18 haploid chromosomes) covering 96% of the genome and identified three new QTLs (quantitative trait loci), two involved in snail resistance/susceptibility and one relating to body pigmentation. This study provides excellent genomic resources for unveiling complex vector snail biology, reveals genomic difference between resistant and susceptible lines, and offers novel insights into genetic mechanism of the compatibility between snail and schistosome.

Molecular characterization of thioester-containing proteins in Biomphalaria glabrata and their differential gene expression upon Schistosoma mansoni exposure

Schistosomiasis is a disease caused by trematode parasites of the genus Schistosoma that affects approximately 200 million people worldwide. Schistosomiasis has been a persistent problem in endemic areas as there is no vaccine available, currently used anti-helmintic medications do not prevent reinfection, and most concerning, drug resistance has been documented in laboratory and field isolates. Thus, alternative approaches to curtail this human disease are warranted. Understanding the immunobiology of the obligate intermediate host of these parasites, which include the freshwater snail Biomphalaria glabrata, may facilitate the development of novel methods to stop or reduce transmission to humans. Molecules from the thioester-containing protein (TEP) superfamily have been shown to be involved in immunological functions in many animals including corals and humans. In this study we identified, characterized, and compared TEP transcripts and their expression upon S. mansoni exposure in resistant and susceptible strains of B. glabrata snails. Results showed the expression of 11 unique TEPs in B. glabrata snails. These transcripts present high sequence identity at the nucleotide and putative amino acid levels between susceptible and resistant strains. Further analysis revealed differences in several TEPs’ constitutive expression levels between resistant and susceptible snail strains, with C3-1, C3-3, and CD109 having higher constitutive expression levels in the resistant (BS90) strain, whereas C3-2 and TEP-1 showed higher constitutive expression levels in the susceptible (NMRI) strain. Furthermore, TEP-specific response to S. mansoni miracidia exposure reiterated their differential expression, with resistant snails upregulating the expression of both TEP-4 and TEP-3 at 2 h and 48 h post-exposure, respectively. Further understanding the diverse TEP genes and their functions in invertebrate animal vectors will not only expand our knowledge in regard to this ancient family of immune proteins, but also offer the opportunity to identify novel molecular targets that could aid in the efforts to develop control methods to reduce schistosomiasis transmission.

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'.

Clusters of polymorphic transmembrane genes control resistance to schistosomes in snail vectors

Schistosomiasis is a debilitating parasitic disease infecting hundreds of millions of people. Schistosomes use aquatic snails as intermediate hosts. A promising avenue for disease control involves leveraging innate host mechanisms to reduce snail vectorial capacity. In a genome-wide association study of Biomphalaria glabrata snails, we identify genomic region PTC2 which exhibits the largest known correlation with susceptibility to parasite infection (>15 fold effect). Using new genome assemblies with substantially higher contiguity than the Biomphalaria reference genome, we show that PTC2 haplotypes are exceptionally divergent in structure and sequence. This variation includes multi-kilobase indels containing entire genes, and orthologs for which most amino acid residues are polymorphic. RNA-Seq annotation reveals that most of these genes encode single-pass transmembrane proteins, as seen in another resistance region in the same species. Such groups of hyperdiverse snail proteins may mediate host-parasite interaction at the cell surface, offering promising targets for blocking the transmission of schistosomiasis.

Prevalence of Infection of Biomphalaria glabrata by Schistosoma mansoni and the risk of urban Schistosomiasis mansoni in Salvador, Bahia, Brazil

INTRODUCTION

Biomphalaria glabrata is considered to be responsible for the incidence of schistosomiasis in Brazil. Therefore, surveillance of areas where schistosomiasis is prevalent is fundamental for public health planning. This study was aimed to evaluate B. glabrata populations in water bodies of the city of Salvador, determine their distribution, estimate the prevalence of Schistosoma mansoni infections, characterize shed cercariae, and identify transmission foci.

METHODS

Malacological surveys were carried out in 17 water collections from Salvador. Snail species were identified based on shell and mantle characteristics. Snails were evaluated for S. mansoni infection by exposure to light and via real time polymerase chain reaction (qPCR) using S. mansoni-18S rRNA subunit specific primers.

RESULTS

1,403 B. glabrata were collected. Classical cercarial shedding indicated that 5 snails (0.4%) were positive for S. mansoni. A higher prevalence of infections was found in Horta de Saramandaia (5.5%) and Lagoa do IAT (1.9%). Non-Schistosoma larvae, such as Xiphidiocercaria, Strigeidae, Spirorchiidae and Clinostomidae, were observed in 3.2% of the snails. S. mansoni DNA was detected in 6.2% snails via qPCR.

CONCLUSIONS

B. glabrata is widely distributed in Salvador, as indicated by 7 water collections associated with a risk of schistosomiasis transmission. To our knowledge, this is the first study to identify B. glabrata eliminating cercariae of Clinostomidae, Strigeidae, and Spirorchiidae in Salvador. We propose that qPCR may be employed in combination with classical cercarial shedding. Estimating S. mansoni prevalence in snails by only considering the results of light exposure method classical into account may underestimate the problem.

Treading the Path towards Genetic Control of Snail Resistance to Schistosome Infection

Schistosomiasis remains the most important tropical snail-borne trematodiasis that threatens many millions of human lives. In achieving schistosomiasis elimination targets, sustainable control of the snail vectors represents a logical approach. Nonetheless, the ineffectiveness of the present snail control interventions emphasizes the need to develop new complementary strategies to ensure more effective control outcomes. Accordingly, the use of genetic techniques aimed at driving resistance traits into natural vector populations has been put forward as a promising tool for integrated snail control. Leveraging the Biomphalaria-Schistosoma model system, studies unraveling the complexities of the vector biology and those exploring the molecular basis of snail resistance to schistosome infection have been expanding in various breadths, generating many significant discoveries, and raising the hope for future breakthroughs. This review provides a compendium of relevant findings, and without neglecting the current existing gaps and potential future challenges, discusses how a transgenic snail approach may be adapted and harnessed to control human schistosomiasis.

Complete mitochondrial and rDNA complex sequences of important vector species of Biomphalaria, obligatory hosts of the human-infecting blood fluke, Schistosoma mansoni

Using high throughput Illumina sequencing technology, we determined complete sequences for the mitochondrial genome (mitogenome) and nuclear ribosomal DNA (rDNA) complex for three African freshwater snail taxa within the genus Biomphalaria, B. pfeifferi, B. sudanica and B. choanomphala, and for two laboratory strains of B. glabrata originating from the Neotropics. Biomphalaria snails are obligate vectors of the blood fluke Schistosoma mansoni, a major etiologic agent of human intestinal schistosomiasis. Our data show that mitogenomes from African and Neotropical Biomphalaria are highly conserved. With respect to rDNA, the two internal transcribed spacers (ITS1 and 2) were found to be highly variable whereas the three ribosomal RNA genes (28S, 5.8S and 18S rRNA) exhibited no or very limited variation. Our analyses reveal that the two taxa inhabiting Lake Victoria, B. sudanica and B. choanomphala, are very similar to one another relative to the similarity either shows to B. pfeifferi or B. glabrata. This new sequence information may prove useful for developing new markers for snail identification, environmental detection/monitoring purposes or for tracking epidemiology and snail dependencies of S. mansoni in endemic areas. It also provides new information pertinent to still unresolved questions in Biomphalaria systematics and nomenclature.

The influence of related and unrelated co-infections on parasite dynamics and virulence

Many parasitic infections increase the morbidity and mortality of host populations. Interactions between co-infecting parasites can influence virulence, the damage done to a host. Previous studies investigating the impacts of parasite co-infection on hosts have been limited by their inability to control parasite dosage, use consistent virulence metrics, or verify co-infection status. This study used molecular tools, known infection dosage, and multiple assessments over time to test whether parasite relatedness can predict virulence in co-infections, as well as whether competitive interactions between different parasite strains within a host are predictable over time. In addition, we examined the impacts of other parasite traits, such as infectivity, as alternative predictors of virulence and competition outcomes. Hosts with single-strain (related) parasite infections were found to have lower virulence in terms of host and parasite reproduction, supporting kin selection predictions. However, these infections also resulted in higher host mortality. We argue that mortality should not be used as a measurement of virulence in parasite systems that castrate hosts. Hosts were more susceptible to mixed strain (unrelated) parasite infections, indicating that co-infections may make resistance more costly to hosts. Co-infections were dynamic, with changes in parasite dominance over the course of the infection. The more infective parasite strain appeared to suppress the less infective strain, ultimately increasing host longevity. Our findings suggest that unrelated, or more diverse, parasite infections are associated with higher virulence, but that studies must consider their methodology and possible alternative explanations beyond kin selection to understand virulence outcomes.

Integrated multi-omic analyses in Biomphalaria-Schistosoma dialogue reveal the immunobiological significance of FREP-SmPoMuc interaction

The fresh water snail Biomphalaria glabrata is one of the vectors of the trematode pathogen Schistosoma mansoni, which is one of the agents responsible of human schistosomiasis. In this host-parasite interaction, co-evolutionary dynamic results into an infectivity mosaic known as compatibility polymorphism. Integrative approaches including large scale molecular approaches have been conducted in recent years to improve our understanding of the mechanisms underlying compatibility. This review presents the combination of integrated Multi-Omic approaches leading to the discovery of two repertoires of polymorphic and/or diversified interacting molecules: the parasite antigens S. mansoni polymorphic mucins (SmPoMucs) and the B. glabrata immune receptors fibrinogen-related proteins (FREPs). We argue that their interactions may be major components for defining the compatible/incompatible status of a specific snail/schistosome combination.

Comparative immunogenomics of molluscs

Comparative immunology, studying both vertebrates and invertebrates, provided the earliest descriptions of phagocytosis as a general immune mechanism. However, the large scale of animal diversity challenges all-inclusive investigations and the field of immunology has developed by mostly emphasizing study of a few vertebrate species. In addressing the lack of comprehensive understanding of animal immunity, especially that of invertebrates, comparative immunology helps toward management of invertebrates that are food sources, agricultural pests, pathogens, or transmit diseases, and helps interpret the evolution of animal immunity. Initial studies showed that the Mollusca (second largest animal phylum), and invertebrates in general, possess innate defenses but lack the lymphocytic immune system that characterizes vertebrate immunology. Recognizing the reality of both common and taxon-specific immune features, and applying up-to-date cell and molecular research capabilities, in-depth studies of a select number of bivalve and gastropod species continue to reveal novel aspects of molluscan immunity. The genomics era heralded a new stage of comparative immunology; large-scale efforts yielded an initial set of full molluscan genome sequences that is available for analyses of full complements of immune genes and regulatory sequences. Next-generation sequencing (NGS), due to lower cost and effort required, allows individual researchers to generate large sequence datasets for growing numbers of molluscs. RNAseq provides expression profiles that enable discovery of immune genes and genome sequences reveal distribution and diversity of immune factors across molluscan phylogeny. Although computational de novo sequence assembly will benefit from continued development and automated annotation may require some experimental validation, NGS is a powerful tool for comparative immunology, especially increasing coverage of the extensive molluscan diversity. To date, immunogenomics revealed new levels of complexity of molluscan defense by indicating sequence heterogeneity in individual snails and bivalves, and members of expanded immune gene families are expressed differentially to generate pathogen-specific defense responses.

DYNAMIC AND GENETIC CONSEQUENCES OF VARIATION IN HORIZONTAL TRANSMISSION FOR A MICROPARASITIC INFECTION

Transmission to a new host is a critical step in the life cycle of a parasite. Variation in the characteristics of the transmission process, for example, due to host demography, is assumed to select for different variants of the parasite. We have experimentally tested how variation in the time to transmission (early or late after infection) and exposure to adverse conditions outside the host (immediate or delayed contact with new host) interact to determine the success of the infection in the next host, using the trypanosome Crithidia bombi infecting its bumblebee host, Bombus terrestris. These two experimentally manageable steps mimic the processes of within- and among-host selection for the parasite. We found that early transmission led to higher infection success in the next host as did immediate contact with the new host. However, there was no interaction between the two parameters as would be expected if early-transmitted variants, resulting from rapid multiplication within the host, would be less adapted to the conditions encountered during the between-host transfer or infection of the next host. Furthermore, typing the genetic variability of the parasites with microsatellites showed that the four different transmission routes of our experiment selected for different degrees of allelic diversity of the infecting parasite populations. The results support the idea that variation in the transmission process selects for different genotypic variants of the parasite. At the same time, the relationship of allelic diversity with infection intensity suggested that the coinfection model of May and Nowak (1995) may be appropriate, where each parasite is able to infect and multiply independent of others within the same host.

A multistrain approach to studying the mechanisms underlying compatibility in the interaction between Biomphalaria glabrata and Schistosoma mansoni

In recent decades, numerous studies have sought to better understand the mechanisms underlying the compatibility between Biomphalaria glabrata and Schistosoma mansoni. The developments of comparative transcriptomics, comparative genomics, interactomics and more targeted approaches have enabled researchers to identify a series of candidate genes. However, no molecular comparative work has yet been performed on multiple populations displaying different levels of compatibility. Here, we seek to fill this gap in the literature. We focused on B. glabrata FREPs and S. mansoni SmPoMucs, which were previously demonstrated to be involved in snail/schistosome compatibility. We studied the expression and polymorphisms of these factors in combinations of snail and schistosome isolates that display different levels of compatibility. We found that the polymorphism and expression levels of FREPs and SmPoMucs could be linked to the compatibility level of S. mansoni. These data and our complementary results obtained by RNA-seq of samples from various snail strains indicate that the mechanism of compatibility is much more complex than previously thought, and that it is likely to be highly variable within and between populations. This complexity must be taken into account if we hope to identify the molecular pathways that are most likely to be good targets for strategies aimed at blocking transmission of the parasite through the snail intermediate host.

Schistosomiasis is the second most widespread human tropical parasitic disease after malaria. It is caused by flatworms of the genus Schistosoma, and poses a considerable threat for human health in numerous Asian, African and South American countries. The World Health Organization has set the goal of eradicating schistosomiasis by 2025. However, no vaccine is available, and we currently have only one drug (praziquantel) that can effectively and efficiently treat the disease. As treatment by mass drug administration would enhance the risk of drug resistance in schistosome parasites, complementary strategies to fight this parasitic disease are urgently needed. Freshwater snails of the Biomphalaria genus act as intermediate hosts in the transmission of the schistosome species. Thus, learning more about the mechanisms of the interaction between these snails and the schistosomes could critically facilitate the identification of potential new candidate molecules that may be targeted to prevent schistosome transmission in the field.

The Compatibility Between Biomphalaria glabrata Snails and Schistosoma mansoni: An Increasingly Complex Puzzle

This review reexamines the results obtained in recent decades regarding the compatibility polymorphism between the snail, Biomphalaria glabrata, and the pathogen, Schistosoma mansoni, which is one of the agents responsible for human schistosomiasis. Some results point to the snail's resistance as explaining the incompatibility, while others support a "matching hypothesis" between the snail's immune receptors and the schistosome's antigens. We propose here that the two hypotheses are not exclusive, and that the compatible/incompatible status of a particular host/parasite couple probably reflects the balance of multiple molecular determinants that support one hypothesis or the other. Because these genes are involved in a coevolutionary arms race, we also propose that the underlying mechanisms can vary. Finally, some recent results show that environmental factors could influence compatibility. Together, these results make the compatibility between B. glabrata and S. mansoni an increasingly complex puzzle. We need to develop more integrative approaches in order to find targets that could potentially be manipulated to control the transmission of schistosomiasis.

Characterization of South American Snails of the Genus Biomphalaria (Basommatophora: Planorbidae) and Schistosoma mansoni (Platyhelminthes: Trematoda) in Molluscs by PCR-RFLP

The identification of snails of the genus Biomphalaria can be done using morphological characteristics which depends on the size of the snails and skill and knowledge of researcher. These methods sometimes are not adequate for identification of species. The PCR-RFLP, using the ITS region of the rDNA, has been used to identify Brazilian species of the genus Biomphalaria. Nevertheless, there is a lack of information about snails from other Latin American countries. In addition, some snails may be infected by Schistosoma mansoni and when submitted to PCR-RFLP they show molecular profiles different from those previously standardized for the other mollusc species. In this work the molecular profiles of 15 species and the subspecies were established by PCR-RFLP of ITS-rDNA with the enzyme DdeI. Moreover, the molecular profiles of host species, B. glabrata, B. straminea, B. tenagophila, and B. prona, infected by S. mansoni were also established. The molluscs were dissected to permit morphological identification. These results contribute to a correct identification of snails of the genus Biomphalaria and detection of these snails infected by S. mansoni.

Gastropod-Borne Helminths: A Look at the Snail-Parasite Interplay

More than 300 million people suffer from a range of diseases caused by gastropod-borne helminths, predominantly flatworms and roundworms, whose life cycles are characterized by a diversified ecology and epidemiology. Despite the plethora of data on these parasites, very little is known of the fundamental biology of their gastropod intermediate hosts, or of the interactions occurring at the snail-helminth interface. In this article, we focus on schistosomes and metastrongylids of human and animal significance, and review current knowledge of snail-parasite interplay. Future efforts aimed at elucidating key elements of the biology and ecology of the snail intermediate hosts, together with an improved understanding of snail-parasite interactions, will aid to identify, plan, and develop new strategies for disease control focused on gastropod intermediate hosts.

Genome-Wide Scan and Test of Candidate Genes in the Snail Biomphalaria glabrata Reveal New Locus Influencing Resistance to Schistosoma mansoni

BACKGROUND

New strategies to combat the global scourge of schistosomiasis may be revealed by increased understanding of the mechanisms by which the obligate snail host can resist the schistosome parasite. However, few molecular markers linked to resistance have been identified and characterized in snails.

METHODOLOGY/PRINCIPAL FINDINGS

Here we test six independent genetic loci for their influence on resistance to Schistosoma mansoni strain PR1 in the 13-16-R1 strain of the snail Biomphalaria glabrata. We first identify a genomic region, RADres, showing the highest differentiation between susceptible and resistant inbred lines among 1611 informative restriction-site associated DNA (RAD) markers, and show that it significantly influences resistance in an independent set of 439 outbred snails. The additive effect of each RADres resistance allele is 2-fold, similar to that of the previously identified resistance gene sod1. The data fit a model in which both loci contribute independently and additively to resistance, such that the odds of infection in homozygotes for the resistance alleles at both loci (13% infected) is 16-fold lower than the odds of infection in snails without any resistance alleles (70% infected). Genome-wide linkage disequilibrium is high, with both sod1 and RADres residing on haplotype blocks >2 Mb, and with other markers in each block also showing significant effects on resistance; thus the causal genes within these blocks remain to be demonstrated. Other candidate loci had no effect on resistance, including the Guadeloupe Resistance Complex and three genes (aif, infPhox, and prx1) with immunological roles and expression patterns tied to resistance, which must therefore be trans-regulated.

CONCLUSIONS/SIGNIFICANCE

The loci RADres and sod1 both have strong effects on resistance to S. mansoni. Future approaches to control schistosomiasis may benefit from further efforts to characterize and harness this natural genetic variation.

Advances in gastropod immunity from the study of the interaction between the snail Biomphalaria glabrata and its parasites: A review of research progress over the last decade

This review summarizes the research progress made over the past decade in the field of gastropod immunity resulting from investigations of the interaction between the snail Biomphalaria glabrata and its trematode parasites. A combination of integrated approaches, including cellular, genetic and comparative molecular and proteomic approaches have revealed novel molecular components involved in mediating Biomphalaria immune responses that provide insights into the nature of host-parasite compatibility and the mechanisms involved in parasite recognition and killing. The current overview emphasizes that the interaction between B. glabrata and its trematode parasites involves a complex molecular crosstalk between numerous antigens, immune receptors, effectors and anti-effector systems that are highly diverse structurally and extremely variable in expression between and within host and parasite populations. Ultimately, integration of these molecular signals will determine the outcome of a specific interaction between a B. glabrata individual and its interacting trematodes. Understanding these complex molecular interactions and identifying key factors that may be targeted to impairment of schistosome development in the snail host is crucial to generating new alternative schistosomiasis control strategies.

Impact of the age of Biomphalaria alexandrina snails on Schistosoma mansoni transmission: modulation of the genetic outcome and the internal defence system of the snail

Of the approximately 34 identified Biomphalaria species,Biomphalaria alexandrina represents the intermediate host of Schistosoma mansoni in Egypt. Using parasitological and SOD1 enzyme assay, this study aimed to elucidate the impact of the age of B. alexandrina snails on their genetic variability and internal defence against S. mansoni infection. Susceptible and resistant snails were reared individually for self-reproduction; four subgroups of their progeny were used in experiment. The young susceptible subgroup showed the highest infection rate, the shortest pre-patent period, the highest total cercarial production, the highest mortality rate and the lowest SOD1 activity. Among the young and adult susceptible subgroups, 8% and 26% were found to be resistant, indicating the inheritance of resistance alleles from parents. The adult resistant subgroup, however, contained only resistant snails and showed the highest enzyme activity. The complex interaction between snail age, genetic background and internal defence resulted in great variability in compatibility patterns, with the highest significant difference between young susceptible and adult resistant snails. The results demonstrate that resistance alleles function to a greater degree in adults, with higher SOD1 activity and provide potential implications for Biomphalaria control. The identification of the most susceptible snail age enables determination of the best timing for applying molluscicides. Moreover, adult resistant snails could be beneficial in biological snail control.

Hyperdiverse gene cluster in snail host conveys resistance to human schistosome parasites

Schistosomiasis, a neglected global pandemic, may be curtailed by blocking transmission of the parasite via its intermediate hosts, aquatic snails. Elucidating the genetic basis of snail-schistosome interaction is a key to this strategy. Here we map a natural parasite-resistance polymorphism from a Caribbean population of the snail Biomphalaria glabrata. In independent experimental evolution lines, RAD genotyping shows that the same genomic region responds to selection for resistance to the parasite Schistosoma mansoni. A dominant allele in this region conveys an 8-fold decrease in the odds of infection. Fine-mapping and RNA-Seq characterization reveal a <1Mb region, the Guadeloupe Resistance Complex (GRC), with 15 coding genes. Seven genes are single-pass transmembrane proteins with putative immunological roles, most of which show strikingly high nonsynonymous divergence (5-10%) among alleles. High linkage disequilibrium among three intermediate-frequency (>25%) haplotypes across the GRC, a significantly non-neutral pattern, suggests that balancing selection maintains diversity at the GRC. Thus, the GRC resembles immune gene complexes seen in other taxa and is likely involved in parasite recognition. The GRC is a potential target for controlling transmission of schistosomiasis, including via genetic manipulation of snails.

Schistosomes are water-borne blood-flukes that are transmitted by snail vectors. They infect over 200 million people in more than 70 countries and cause severe and chronic disability. Snails naturally vary in resistance to this parasite even within species, so bolstering snail resistance in the wild would block transmission. We artificially selected snails for resistance and observed a rapid evolutionary response, with the greatest change occurring in the same genomic region in two independent trials. We subsequently confirmed that the selected haplotype conveys resistance to infection by schistosomes. The extraordinarily high sequence divergence among haplotypes in this region appears to be elevated due to ongoing natural selection, likely via host-parasite co-evolution. We observed the highest variation in genes encoding putative parasite recognition proteins, suggesting that these control the resistance phenotype in a manner reminiscent of immune gene complexes in other taxa. Thus, this gene cluster presents a potential new target to interfere with parasite transmission at the vector stage.

Digenean-gastropod host associations inform on aspects of specific immunity in snails

Gastropod immunology is informed importantly by the study of the frequent encounters snails endure with digeneans (digenetic trematodes). One of the hallmarks of gastropod-digenean associations is their specificity: any particular digenean parasite species is transmitted by a limited subset of snail taxa. We discuss the nature of this specificity, including its immunological basis. We then review studies of the model gastropod Biomphalaria glabrata indicating that the baseline responses of snails to digeneans can be elevated in a specific manner. Studies incorporating molecular and functional approaches are then highlighted, and are further suggestive of the capacity for specific gastropod immune responses. These studies have led to the compatibility polymorphism hypothesis: the interactions between diversified fibrinogen-related proteins (FREPs) and diverse carbohydrate-decorated polymorphic parasite antigens determine recognition and trigger specific immunity. Complex glycan structures are also likely to play a role in the host specificity typifying snail-digenean interactions. We conclude by noting the dynamic and consequential interactions between snails and digeneans can be considered as drivers of diversification of digenean parasites and in the development and maintenance of specific immunity in gastropods.

Fibrinogen-Related Proteins (FREPs) in Mollusks

Anti-parasite responses of the snail Biomphalaria glabrata involve antigen-reactive plasma lectins termed fibrinogen-related proteins (FREPs) comprising a C-terminal fibrinogen (FBG) domain and one or two upstream immunoglobulin domains. FREPs are highly polymorphic; they derive from several gene families with multiple loci and alleles that are diversified by exon loss, alternative splicing, and random somatic mutation (gene conversion and point mutations). Individual B. glabrata snails have dynamically distinct FREP sequence repertoires. The immune relevance of B. glabrata FREPs is indicated by FREP binding to polymorphic antigens of (snail-specific) digenean parasites and altered resistance of B. glabrata to digeneans following RNAi knockdown of FREPs. The compatibility polymorphism hypothesis proposes that FREP mutation increases the range of germline-encoded immune recognition in B. glabrata to counter antigenically-varied parasites. Somatic mutation may result from sequence exchange among tandemly arranged FREP genes in the genome, and analysis of sequence variants also suggests involvement of cytidine deaminase-like activity or epigenetic regulation. Without current indications of selection or retention of effective sequence variants toward immunological memory, FREP diversification is thought to afford B. glabrata immunity that is anticipatory but not adaptive. More remains to be learned about this system; other mollusks elaborate diversified lectins consisting of single FBG domains, and bona fide FREPs were reported from additional gastropod species, but these may not be diversified. Future comparative immunological studies and gene discovery driven by next-generation sequencing will further clarify taxonomic distribution of FREP diversification and the underlying mutator mechanisms as a component of immune function in mollusks.

Schistosomes and snails: a molecular encounter

Biomphalaria glabrata snails play an integral role in the transmission of Schistosoma mansoni, the causative agent for human schistosomiasis in the Western hemisphere. For the past two decades, tremendous advances have been made in research aimed at elucidating the molecular basis of the snail/parasite interaction. The growing concern that there is no vaccine to prevent schistosomiasis and only one effective drug in existence provides the impetus to develop new control strategies based on eliminating schistosomes at the snail-stage of the life cycle. To elucidate why a given snail is not always compatible to each and every schistosome it encounters, B. glabrata that are either resistant or susceptible to a given strain of S. mansoni have been employed to track molecular mechanisms governing the snail/schistosome relationship. With such snails, genetic markers for resistance and susceptibility were identified. Additionally, differential gene expression studies have led to the identification of genes that underlie these phenotypes. Lately, the role of schistosomes in mediating non-random relocation of gene loci has been identified for the first time, making B. glabrata a model organism where chromatin regulation by changes in nuclear architecture, known as spatial epigenetics, orchestrated by a major human parasite can now be investigated. This review will highlight the progress that has been made in using molecular approaches to describe snail/schistosome compatibility issues. Uncovering the signaling networks triggered by schistosomes that provide the impulse to turn genes on and off in the snail host, thereby controlling the outcome of infection, could also yield new insights into anti-parasite mechanism(s) that operate in the human host as well.

Multi-parasite host susceptibility and multi-host parasite infectivity: a new approach of the Biomphalaria glabrata/Schistosoma mansoni compatibility polymorphism

In this study, we analyze the degree of susceptibility/un-susceptibility of five strains of Biomphalaria glabrata from different geographical origins successively challenged with a panel of 4 Schistosoma mansoni strains. A total of 20 homopatric and heteropatric host-parasite combinations were tested with exposure doses of 1, 10, 20, 30 and 50 miracidia per individual host. By doing this, we characterized each B. glabrata strain by its "multi-parasite susceptibility phenotype" that reflects better the efficiency of their defense mechanism against not only one, but a diversity of schistosome stocks. In the same time, all the S. mansoni strains used were characterized, by their "multi-host infectivity phenotype" that reflects the level of infectivity they display when confronted to diverse snail populations. Based on these results it is possible to select different homogenous stocks of snails with different spectrum of susceptibility/un-susceptibility for several parasite strains. This will be a useful tool for future functional studies conducted to understand the genetics and molecular basis of the compatibility polymorphism in this host/parasite model.

Resistance of Biomphalaria glabrata 13-16-R1 snails to Schistosoma mansoni PR1 is a function of haemocyte abundance and constitutive levels of specific transcripts in haemocytes

Continuing transmission of human intestinal schistosomiasis depends on the parasite's access to susceptible snail intermediate hosts (often Biomphalaria glabrata). Transmission fails when parasite larvae enter resistant individuals in wild snail populations. The genetic basis for differences in snail susceptibility/resistance is being intensively investigated as a means to devise novel control strategies based on resistance genes. Reactive oxygen species produced by the snail's defence cells (haemocytes) are effectors of resistance. We hypothesised that genes relevant to production and consumption of reactive oxygen species would be expressed differentially in the haemocytes of snail hosts with different susceptibility/resistance phenotypes. By restricting the genetic diversity of snails, we sought to facilitate identification of resistance genes. By inbreeding, we procured from a 13-16-R1 snail population with both susceptible and resistant individuals 52 lines of B. glabrata (expected homozygosity ~87.5%), and determined the phenotype of each in regard to susceptibility/resistance to Schistosoma mansoni. The inbred lines were found to have line-specific differences in numbers of spreading haemocytes; these were enumerated in both juvenile and adult snails. Lines with high cell numbers were invariably resistant to S. mansoni, whereas lines with lower cell numbers could be resistant or susceptible. Transcript levels in haemocytes were quantified for 18 potentially defence-related genes. Among snails with low cell numbers, the different susceptibility/resistance phenotypes correlated with differences in transcript levels for two redox-relevant genes: an inferred phagocyte oxidase component and a peroxiredoxin. Allograft inflammatory factor (potentially a regulator of leucocyte activation) was expressed at higher levels in resistant snails regardless of spread cell number. Having abundant spreading haemocytes is inferred to enable a snail to kill parasite sporocysts. In contrast, snails with fewer spreading haemocytes seem to achieve resistance only if specific genes are expressed constitutively at levels that are high for the species.

Genetic variation between Biomphalaria alexandrina snails susceptible and resistant to Schistosoma mansoni infection

Much effort has been made to control schistosomiasis infection in Egypt. However, enduring effects from such strategies have not yet been achieved. In this study, we sought to determine the genetic variability related to the interaction between Biomphalaria alexandrina snails and Schistosoma mansoni. Using RAPD-PCR with eight (10 mers) random primers, we were able to determine the polymorphic markers that differed between snails susceptible and resistant to Schistosoma mansoni infection using five primers out of the eight. Our results suggest that the RAPD-PCR technique is an efficient means by which to compare genomes and to detect genetic variations between schistosomiasis intermediate hosts. The RAPD technique with the above-noted primers can identify genomic markers that are specifically related to the Biomphalaria alexandrina/Schistosoma mansoni relationship in the absence of specific nucleotide sequence information. This approach could be used in epidemiologic surveys to investigate genetic diversity among Biomphalaria alexandrina snails. The ability to determine resistant markers in Biomphalaria alexandrina snails could potentially lead to further studies that use refractory snails as agents to control the spread of schistosomiasis.

Compatibility polymorphism in snail/schistosome interactions: From field to theory to molecular mechanisms

Coevolutionary dynamics in host-parasite interactions potentially lead to an arms race that results in compatibility polymorphism. The mechanisms underlying compatibility have remained largely unknown in the interactions between the snail Biomphalaria glabrata and Schistosoma mansoni, one of the agents of human schistosomiasis. This review presents a combination of data obtained from field and laboratory studies arguing in favor of a matching phenotype model to explain compatibility polymorphism. Investigations focused on the molecular determinants of compatibility have revealed two repertoires of polymorphic and/or diversified molecules that have been shown to interact: the parasite antigens S. mansoni polymorphic mucins and the B. glabrata fibrinogen-related proteins immune receptors. We hypothesize their interactions define the compatible/incompatible status of a specific snail/schistosome combination. This line of thought suggests concrete approaches amenable to testing in field-oriented studies attempting to control schistosomiasis by disrupting schistosome-snail compatibility.

Reversing the resistance phenotype of the Biomphalaria glabrata snail host Schistosoma mansoni infection by temperature modulation

Biomphalaria glabrata snails that display either resistant or susceptible phenotypes to the parasitic trematode, Schistosoma mansoni provide an invaluable resource towards elucidating the molecular basis of the snail-host/schistosome relationship. Previously, we showed that induction of stress genes either after heat-shock or parasite infection was a major feature distinguishing juvenile susceptible snails from their resistant counterparts. In order to examine this apparent association between heat stress and snail susceptibility, we investigated the effect of temperature modulation in the resistant snail stock, BS-90. Here, we show that, incubated for up to 4 hrs at 32°C prior to infection, these resistant snails became susceptible to infection, i.e. shedding cercariae at 5 weeks post exposure (PE) while unstressed resistant snails, as expected, remained resistant. This suggests that susceptibility to infection by this resistant snail phenotype is temperature-sensitive (ts). Additionally, resistant snails treated with the Hsp 90 specific inhibitor, geldanamycin (GA) after heat stress, were no longer susceptible to infection, retaining their resistant phenotype. Consistently, susceptible snail phenotypes treated with 100 mM GA before parasite exposure also remained uninfected. These results provide direct evidence for the induction of stress genes (heat shock proteins; Hsp 70, Hsp 90 and the reverse transcriptase [RT] domain of the nimbus non-LTR retrotransposon) in B. glabrata susceptibility to S. mansoni infection and characterize the resistant BS-90 snails as a temperature-sensitive phenotype. This study of reversing snail susceptibility phenotypes to S. mansoni provides an opportunity to directly track molecular pathway(s) that underlie the B. glabrata snail's ability to either sustain or destroy the S. mansoni parasite.

Biomphalaria glabrata snails that are either resistant or susceptible to the parasite, Schistosoma mansoni, have been an invaluable resource in studies aimed at understanding the molecular basis of the snail/schistosome interaction. Schistosomes cause the chronic debilitating disease schistosomiasis. Thus, it is hoped that dissecting pathways that underlie the snail/schistosome relationship might translate into alternative control strategies that will include blocking transmission of the parasite at the snail-stage of its development. Induction of stress genes is a feature distinguishing early exposed juvenile susceptible NMRI snails from resistant BS-90 snail stocks. To further analyze this apparent involvement of stress induction and snail susceptibility, here we applied heat stress to the resistant BS-90 snail, enhancing induction of stress genes (Hsp 70, Hsp 90 and RT) prior to infection. Results showed these resistant snails became susceptible, indicating resistance as being a temperature sensitive phenotype in these snails. Stressed resistant snails treated with the Hsp 90 specific inhibitor, geldanamycin, prior to exposure, were, however, shown to maintain their refractory phenotype. Interestingly, inhibitor treated susceptible snails also became non-susceptible. Collectively, these data point to stress induction as an important early step in the ability of S. mansoni to infect juvenile B. glabrata snails.

In vitro encystment of Himasthla elongata cercariae (Digenea, Echinostomatidae) in the haemolymph of blue mussels Mytilus edulis as a tool for assessing cercarial infectivity and molluscan susceptibility

Infectivity of Himasthla elongata cercariae to mussels, their second intermediate hosts, and resistance by these hosts to infection were assessed on the basis of the cercariae's ability to encyst in mussel haemolymph in vitro. A series of experimental in vivo infections of mussels with batches of cercariae, each batch released from a different single infected mollusc and referred to as a clone (due to their shared genotype), demonstrated that the results of the in vitro tests corresponded to the actual indices of infectivity/susceptibility of the parasites and their hosts. Most cercarial clones had high infectivity, with a few clones having very high or, at the other extreme, very low infectivity. A similar pattern was revealed in mussel resistance to cercarial infection. Most of the molluscs tested were moderately susceptible to cercarial infection, but at each extreme a small fraction (less than 10%) displayed very high or very low susceptibility. It was shown that there were no totally compatible or totally incompatible 'cercaria clone/mussel' combinations. Results obtained are compared with the data on intra-population variability using the characters parasite infectivity/host compatibility for trematode/mollusc-first intermediate host associations. Results are made relevant to actual infection levels in mussel settlements at the White Sea.

Differences in the number of hemocytes in the snail host Biomphalaria tenagophila, resistant and susceptible to Schistosoma mansoni infection

The relationships between schistosomiasis and its intermediate host, mollusks of the genus Biomphalaria, have been a concern for decades. It is known that the vector mollusk shows different susceptibility against parasite infection, whose occurrence depends on the interaction between the forms of trematode larvae and the host defense cells. These cells are called amebocytes or hemocytes and are responsible for the recognition of foreign bodies and for phagocytosis and cytotoxic reactions. The defense cells mediate the modulation of the resistant and susceptible phenotypes of the mollusk. Two main types of hemocytes are found in the Biomphalaria hemolymph: the granulocytes and the hyalinocytes. We studied the variation in the number (kinetics) of hemocytes for 24 h after exposing the parasite to genetically selected and non-selected strains of Biomphalaria tenagophila, susceptible or not to infection by Schistosoma mansoni. The differences were analyzed referred to the variations in the number of hemocytes in mollusks susceptible or not to infection by S. mansoni. The hemolymph of the selected and non-selected snails was collected, and hemocytes were counted using a Neubauer chamber at six designated periods: 0 h (control, non-exposed individuals), 2 h, 6 h, 12 h, 18 h and, 24 h after parasite exposure. Samples of hemolymph of five selected mollusks and five non-selected mollusks were separately used at each counting time. There was a significant variation in the number of hemocytes between the strains, which indicates that defense cells have different behaviors in resistant and susceptible mollusks.

Time series analysis of the transcriptional responses of Biomphalaria glabrata throughout the course of intramolluscan development of Schistosoma mansoni and Echinostoma paraensei

Successful colonization of a compatible snail host by a digenetic trematode miracidium initiates a complex, proliferative development program requiring weeks to reach culmination in the form of production of cercariae which, once started, may persist for the remainder of the life span of the infected snail. How are such proliferative and invasive parasites able to circumvent host defenses and establish chronic infections? Using a microarray designed to monitor the internal defense and stress-related responses of the freshwater snail Biomphalaria glabrata, we have undertaken a time course study to monitor snail responses following exposure to two different trematode species to which the snail is susceptible: the medically important Schistosoma mansoni, exemplifying sporocyst production in its larval development, or Echinostoma paraensei, representing an emphasis on rediae production in its larval development. We sampled eight time points (0.5, 1, 2, 4, 8, 16 and 32 days p.i.) that cover the period required for cercariae to be produced. Following exposure to S. mansoni, there was a preponderance of up-regulated over down-regulated array features through 2 days p.i. but by 4 days p.i. and thereafter, this pattern was strongly reversed. For E. paraensei, there was a preponderance of down-regulated array features over up-regulated features at even 0.5 days p.i., a pattern that persists throughout the course of infection except for 1 day p.i., when up-regulated array features slightly outnumbered down-regulated features. Examination of particular array features revealed several that were up-regulated by both parasites early in the course of infection and one, fibrinogen related protein 4 (FREP 4), that remained significantly elevated throughout the course of infection with either parasite, effectively serving as a marker of infection. Many defense-related transcripts were persistently down-regulated, including several fibrinogen-containing lectins and homologs of molecules best known from vertebrate phagocytic cells. Our results are consistent with earlier studies suggesting that both parasites are able to interfere with host defense responses, including a tendency for E. paraensei to do so more rapidly and strongly than S. mansoni. They further suggest mechanisms for how trematodes are able to establish the chronic infections necessary for their continued success.

Low genetic diversity in a snail intermediate host (Biomphalaria pfeifferi Krass, 1848) and schistosomiasis transmission in the Senegal River Basin

Population genetic perturbations of intermediate hosts, often a consequence of human pressure on environmental resources, can precipitate unexpectedly severe disease outbreaks. Such disturbances are set to become increasingly common following range changes concomitant with climate shifts, dwindling natural resources and major infrastructure changes such as hydroprojects. Construction of the Diama dam in the Senegal River Basin (SRB) reduced river salinity, enabling the freshwater snail intermediate host Biomphalaria pfeifferi to rapidly expand its distribution. A serious public health problem ensued, with an epidemic of intestinal schistosomiasis occurring in the previously schistosome-free Richard-Toll region within 2 years. The current study aimed to assess the population variability of B. pfeifferi in the SRB, and speculate upon its subsequent impact on host-parasite interactions following such engineered ecological change. Genetic variation at nine polymorphic microsatellite loci revealed little population differentiation in SRB snails compared with those from natural habitats in Zimbabwe, where Schistosoma mansoni transmission is much lower. 'Open' SRB habitats are associated with greater water contact, smaller population sizes and less genetic diversity, with sites downstream of Richard-Toll showing greater inter- and intrapopulation variation, concomitant with less frequent human contact. These observations may be explained by rapid expansion into pristine habitat selecting for high fecundity genotypes at the expense of schistosome resistance, presenting S. mansoni with genetically homogenous highly fecund susceptible populations around the focal point, promoting development of a highly compatible host-parasite relationship. Longitudinal study of such systems may prove important in predicting public health risks engendered by future environmental engineering projects.

Demographic responses to multigeneration cadmium exposure in two strains of the freshwater gastropod, Biomphalaria glabrata

A life table response experiment (LTRE) was used to quantify the population-level effects of continuous, multigeneration cadmium exposure on two strains of the freshwater gastropod, Biomphalaria glabrata: the parasite-resistant BS90 and parasite-susceptible NMRI strains. Snails were exposed to waterborne cadmium for three consecutive generations. Survival, growth, and reproduction were measured empirically and incorporated into a stage-based, deterministic population model. Cadmium significantly affected hatching success, time to maturity, and juvenile and adult survival in both strains. There were significant effects of generation on fecundity, hatching success, time to maturity and juvenile survival in NMRI, and time to maturity and adult survival in BS90. Cadmium significantly affected the population growth rate, lambda, in BS90. Cadmium, generation, and the cadmium x generation interaction had significant effects on lambda in NMRI. At the high cadmium exposure, lambda for NMRI showed a decrease from generation 1 to generation 2, followed by an increase from generation 2 to generation 3. The lambda value in high-cadmium BS90 steadily decreased over the three generations, while NMRI at this same concentration was similar to the controls. The results indicate that strain-specific differences in response to multigeneration cadmium exposure are evident in B. glabrata. Moreover, effects seen in the first generation are not necessarily indicative of effects in subsequent generations. Changes in lambda over the course of the three-generation exposure suggest that acclimation and/or adaptation to cadmium may have occurred, particularly in NMRI at the high cadmium exposure level.

Parasite-altered behaviour: impact of infection and starvation on mating inBiomphalaria glabrata

Very little is known about parasite-altered mating behaviour. This paper describes two experiments on the impact of infection and starvation on mating inBiomphalaria glabratasnails. In the first experiment, snails were infected with the parasitic trematodeSchistosoma mansoni. During both the early and the late stages of infection, snails were allowed to mate with individuals of the same infection status and uninfected controls. Their mating activities were filmed and later analysed. The mating frequencies of patently infected (shedding) snails were found to be lower than those of controls. This is thought to result from stress induced by the pathology. Successfully infected snails were found to mate more often as males than as females. This is seen as a compensation for the reduced fecundity caused by trematode infection. Successfully infected snails also exhibited partner choice. More matings were found between snails of the opposite infection status than among snails of the same infection status. This may be explained by the good genes hypothesis and can be seen as a specific response to schistosome infection. In the second experiment, starved snails were allowed to mate with starved and control snails. They mated less often than controls and more often as males than as females, controls showing the opposite pattern. This may be considered to be a general stress response. No partner choice has been found with starved snails. It is suggested that partner choice in infected snails occurs in response to specific rather than general stress.

Differences in cysteine protease activity in Schistosoma mansoni-resistant and -susceptible Biomphalaria glabrata and characterization of the hepatopancreas cathepsin B Full-length cDNA

Biomphalaria glabrata snails are known to display a wide range of susceptibility phenotypes to Schistosoma mansoni infection depending on the genetics of both the snail and the invading parasite. Evidence exists for a role of hydrolytic enzymes in the defense of molluscs against invading parasites. To elucidate the role of these enzymes in the outcome of infection in the snail, proteolysis was examined in parasite-resistant and -susceptible snails. Zymographs of extracts from the whole snail or hepatopancreas indicated higher proteolytic activity in resistant, compared with susceptible, snails. Lytic activity coincided with a high-molecular-weight smear (220 to 66 kDa) that was abrogated by the cysteine protease inhibitor trans-epoxysuccinyl-l-leucylamido-(4-guanidino)butane. Quantitative flourimetric assays showed 3.5-fold higher activity in resistant than in susceptible snails. From a hepatopancreas cDNA library, several cysteine protease encoding expressed sequence tags including the full-length cDNA for cathepsin B were identified. Sequence analysis revealed that this cathepsin B belonged to the C1A family of peptidases characterized by the presence of the catalytic cysteine-histidine dyad, the "occluding loop," signal sequence, and cleavage sites for the prepro and propeptides. Quantitative real-time reverse transcriptase-polymerase chain reaction showed higher up-regulation of cathepsin B transcript in resistant than in the susceptible snail after parasite exposure.

Effects of laboratory culture on compatibility between snails and schistosomes

The genetic control of compatibility between laboratory strains of schistosomes and their snail hosts has been studied intensively since the 1970s. These studies show (1) a bewildering array of genotype-by-genotype interactions - compatibility between one pair of strains rarely predicts compatibility with other strains, and (2) evidence for a variety of (sometimes conflicting) genetic mechanisms. Why do we observe such variable and conflicting results? One possibility is that it is partly an artifact of the use of laboratory strains that have been in culture for many years and are often inbred. Here we show that results of compatibility trials between snails and schistosomes - all derived from the same natural population - depend very much on whether one uses laboratory-cultured or field-collected individuals. Explanations include environmental effects of the lab on either host or parasite, and genetic changes in either host or parasite during laboratory culture. One intriguing possibility is that genetic bottlenecks during laboratory culture cause the random fixation of alleles at highly polymorphic loci that control the matched/mismatched status of hosts and parasites. We show that a simple model of phenotype matching could produce dose response curves that look very similar to empirical observations. Such a model would explain much of the genotype-by-genotype interaction in compatibility observed among strains.

Interactions between natural populations of human and rodent schistosomes in the Lake Victoria region of Kenya: a molecular epidemiological approach

BACKGROUND

Schistosoma mansoni exists in a complex environmental milieu that may select for significant evolutionary changes in this species. In Kenya, the sympatric distribution of S. mansoni with S. rodhaini potentially influences the epidemiology, ecology, and evolutionary biology of both species, because they infect the same species of snail and mammalian hosts and are capable of hybridization.

METHODOLOGY/PRINCIPAL FINDINGS

Over a 2-year period, using a molecular epidemiological approach, we examined spatial and temporal distributions, and the overlap of these schistosomes within snails, in natural settings in Kenya. Both species had spatially and temporally patchy distributions, although S. mansoni was eight times more common than S. rodhaini. Both species were overdispersed within snails, and most snails (85.2% for S. mansoni and 91.7% for S. rodhaini) only harbored one schistosome genotype. Over time, half of snails infected with multiple genotypes showed a replacement pattern in which an initially dominant genotype was less represented in later replicates. The other half showed a consistent pattern over time; however, the ratio of each genotype was skewed. Profiles of circadian emergence of cercariae revealed that S. rodhaini emerges throughout the 24-hour cycle, with peak emergence before sunrise and sometimes immediately after sunset, which differs from previous reports of a single nocturnal peak immediately after sunset. Peak emergence for S. mansoni cercariae occurred as light became most intense and overlapped temporally with S. rodhaini. Comparison of schistosome communities within snails against a null model indicated that the community was structured and that coinfections were more common than expected by chance. In mixed infections, cercarial emergence over 24 hours remained similar to single species infections, again with S. rodhaini and S. mansoni cercarial emergence profiles overlapping substantially.

CONCLUSIONS/SIGNIFICANCE

The data from this study indicate a lack of obvious spatial or temporal isolating mechanisms to prevent hybridization, raising the intriguing question of how the two species retain their separate identities.

Biomphalaria glabrata cytosolic copper/zinc superoxide dismutase (SOD1) gene: Association of SOD1 alleles with resistance/susceptibility to Schistosoma mansoni

Variation in susceptibility of the snail Biomphalaria glabrata to infection by the parasite Schistosoma mansoni is, at least in part, genetically determined. Functional studies have demonstrated that hemocyte-mediated killing of the parasite involves hydrogen peroxide, the enzymatic product of superoxide dismutase (SOD). The present study identifies alleles of the gene coding for cytosolic copper/zinc SOD (SOD1). The resistance/susceptibility phenotypes and SOD1 genotypes were determined for 354 snails of the predominantly resistant 13-16-R1 strain of B. glabrata. Resistance to the parasite was found to be significantly associated with one allele of the SOD1 gene. Conversely, a separate SOD1 allele was significantly associated with susceptibility.

Biomphalaria tenagophila: dominant character of the resistance to Schistosoma mansoni in descendants of crossbreedings between resistant (Taim, RS) and susceptible (Joinville, SC) strains

The aim of the present work was to study parasitological, molecular, and genetic aspects in descendants of crossbreedings between a totally resistant Biomphalaria tenagophila strain (Taim, RS) and another one highly susceptible (Joinville, SC) to Schistosoma mansoni. Descendants F1 and F2 were submitted to S. mansoni infection (LE strain). The susceptibility rates for individuals from Group F1 were 0 to 0.6%, and from Group F2 was 7.2%. The susceptible individuals from Group F2 discharged a lower number of cercariae, when compared with the susceptible parental group, and in 2 out of 9 positive snails the cercarial elimination was discontinued. In order to identify genetic markers associated with resistance the genotype of parental snails and their offspring F1 and F2 were analyzed by means of the randomly amplified polymorphic DNA method. Nevertheless, it was not possible to detect any marker associated to resistance, but the results showed that in the mentioned species the resistance character is determined by two dominant genes.

Genetics of anti-parasite resistance in invertebrates

This review summarizes and compares available data on genetic and molecular aspects of resistance in four well-described invertebrate host-parasite systems: snail-schistosome, mosquito-malaria, mosquito-filarial worm, and Drosophila-wasp associations. It underlies that the major components of the immune reaction, such as hemocyte proliferation and/or activation, and production of cytotoxic radicals are common to invertebrate hosts. Identifying genes responsible for naturally occurring resistance will then be helpful to understand the mechanisms of invertebrate immune defenses and to determine how virulence factors are used by parasites to overcome host resistance. Based on these four well-studied models, invertebrate resistance appears as generally determined by one major locus or a few loci, displaying at least partial dominance. Interestingly, specificity of resistance is highly variable and would involve processes other than simple recognition mechanisms. Finally, resistance was shown to be generally costly but is nevertheless observed at high frequencies in many natural populations, suggesting a high potential for host parasite coevolution.

The mitochondrial genome of Biomphalaria glabrata (Gastropoda: Basommatophora), intermediate host of Schistosoma mansoni

The complete mitochondrial (Mt) genome of the gastropod Biomphalaria glabrata, a major intermediate host for the human parasite Schistosoma mansoni, was sequenced. The circular genome, the first determined from a basommatophoran snail, is AT rich (74.6%) and the smallest Mt genome (13,670 nucleotides [nt]) characterized from mollusks to date. Sequences from 2 B. glabrata strains, M-line and 1742, differed by only 18 nt. Phylogenetic analysis of 16S and ND1 sequences confirmed the Brazilian ancestry of both B. glabrata strains. Gene predictions indicated 22 transfer RNA, 12S and 16S ribosomal RNA (rRNA), and 13 protein-encoding genes, as is typical for metazoans. Of the mollusk Mt genomes currently known, the gene order was most similar to that of stylommatophoran gastropods, concordant with the monophyly of pulmonate gastropods. Screening of GenBank (expressed sequence tags database [dbEST]) with the Mt sequence identified 108 entries from B. glabrata as Mt-derived sequences, including 12S and 16S rRNA sequences. Moreover, 11 sequences originating from the Mt genome of B. glabrata were identified among EST entries ascribed to intramolluskan stages of S. mansoni. The availability of this Mt sequence will facilitate further molecular investigations into the biology of Biomphalaria sp. and interactions between this intermediate host and intramolluskan stages of S. mansoni.