The protistan parasite Perkinsus marinus is resistant to selected reactive oxygen species

Perkinsus marinus in bioreactor: growth and a cost-reduced growth medium

Perkinsus marinus (Perkinsea) is an osmotrophic facultative intracellular marine protozoan responsible for "Dermo" disease in the eastern oyster, Crassostrea virginica. In 1993 in vitro culture of P. marinus was developed in the absence of host cells. Compared to most intracellular protozoan parasites, the availability of P. marinus to grow in the absence of host cells has provided the basis to explore its use as a heterologous expression system. As the genetic toolbox is becoming available, there is also the need for larger-scale cultivation and lower-cost media formulations. Here, we took an industrial approach to scaled-up growth from a small culture flask to bioreactors, which required developing new cultivation parameters, including aeration, mixing, pH, temperature control, and media formulation. Our approach also enabled more real-time data collection on growth. The bioreactor cultivation method showed similar or accelerated growth rates of P. marinus compared to culture in T-flasks. Redox measurements indicated sufficient oxygen availability throughout the cultivation. Replacing fetal bovine serum with chicken serum showed no differences in the growth rate and a 60% reduction in the medium cost. This study opens the door to furthering P. marinus as a valid heterologous expression system by showing the ability to grow in bioreactors.

ONE-SENTENCE SUMMARY

Perkinsus marinus, a microbial parasite of oysters that could be useful for developing vaccines for humans, has been shown to grow well in laboratory equipment that can be expanded to commercial scale using a less expensive growth formula than usual laboratory practice.

Perkinsus marinus suppresses in vitro eastern oyster apoptosis via IAP-dependent and caspase-independent pathways involving TNFR, NF-kB, and oxidative pathway crosstalk

The protozoan parasite Perkinsus marinus causes Dermo disease in eastern oysters, Crassostrea virginica, and can suppress apoptosis of infected hemocytes using incompletely understood mechanisms. This study challenged hemocytes in vitro with P. marinus for 1 h in the presence or absence of caspase inhibitor Z-VAD-FMK or Inhibitor of Apoptosis protein (IAP) inhibitor GDC-0152. Hemocytes exposure to P. marinus significantly reduced granulocyte apoptosis, and pre-incubation with Z-VAD-FMK did not affect P. marinus-induced apoptosis suppression. Hemocyte pre-incubation with GDC-0152 prior to P. marinus challenge further reduced apoptosis of granulocytes with engulfed parasite, but not mitochondrial permeabilization. This suggests P. marinus-induced apoptosis suppression may be caspase-independent, affect an IAP-involved pathway, and occur downstream of mitochondrial permeabilization. P. marinus challenge stimulated hemocyte differential expression of oxidation-reduction, TNFR, and NF-kB pathways. WGCNA analysis of P. marinus expression in response to hemocyte exposure revealed correlated protease, kinase, and hydrolase expression that could contribute to P. marinus-induced apoptosis suppression.

CRISPR/Cas9 Ribonucleoprotein-Based Genome Editing Methodology in the Marine Protozoan Parasite Perkinsus marinus

Perkinsus marinus (Perkinsozoa), a close relative of apicomplexans, is an osmotrophic facultative intracellular marine protozoan parasite responsible for "Dermo" disease in oysters and clams. Although there is no clinical evidence of this parasite infecting humans, HLA-DR40 transgenic mice studies strongly suggest the parasite as a natural adjuvant in oral vaccines. P. marinus is being developed as a heterologous gene expression platform for pathogens of medical and veterinary relevance and a novel platform for delivering vaccines. We previously reported the transient expression of two rodent malaria genes Plasmodium berghei HAP2 and MSP8. In this study, we optimized the original electroporation-based protocol to establish a stable heterologous expression method. Using 20 μg of pPmMOE[MOE1]:GFP and 25.0 × 106 P. marinus cells resulted in 98% GFP-positive cells. Furthermore, using the optimized protocol, we report for the first time the successful knock-in of GFP at the C-terminus of the PmMOE1 using ribonucleoprotein (RNP)-based CRISPR/Cas9 gene editing methodology. The GFP was expressed 18 h post-transfection, and expression was observed for 8 months post-transfection, making it a robust and stable knock-in system.

Biochemical Characterization of Oyster and Clam Galectins: Selective Recognition of Carbohydrate Ligands on Host Hemocytes and Perkinsus Parasites

Both vertebrates and invertebrates display active innate immune mechanisms for defense against microbial infection, including diversified repertoires of soluble and cell-associated lectins that can effect recognition and binding to potential pathogens, and trigger downstream effector pathways that clear them from the host internal milieu. Galectins are widely distributed and highly conserved lectins that have key regulatory effects on both innate and adaptive immune responses. In addition, galectins can bind to exogenous (“non-self”) carbohydrates on the surface of bacteria, enveloped viruses, parasites, and fungi, and function as recognition receptors and effector factors in innate immunity. Like most invertebrates, eastern oysters (Crassostrea virginica) and softshell clams (Mya arenaria) can effectively respond to most immune challenges through soluble and hemocyte-associated lectins. The protozoan parasite Perkinsus marinus, however, can infect eastern oysters and cause “Dermo” disease, which is highly detrimental to both natural and farmed oyster populations. The sympatric Perkinsus chesapeaki, initially isolated from infected M. arenaria clams, can also be present in oysters, and there is little evidence of pathogenicity in either clams or oysters. In this review, we discuss selected observations from our studies on the mechanisms of Perkinsus recognition that are mediated by galectin-carbohydrate interactions. We identified in the oyster two galectins that we designated CvGal1 and CvGal2, which strongly recognize P. marinus trophozoites. In the clam we also identified galectin sequences, and focused on one (that we named MaGal1) that also recognizes Perkinsus species. Here we describe the biochemical characterization of CvGal1, CvGal2, and MaGal1 with focus on the detailed study of the carbohydrate specificity, and the glycosylated moieties on the surfaces of the oyster hemocytes and the two Perkinsus species (P. marinus and P. chesapeaki). Our goal is to gain further understanding of the biochemical basis for the interactions that lead to recognition and opsonization of the Perkinsus trophozoites by the bivalve hemocytes. These basic studies on the biology of host-parasite interactions may contribute to the development of novel intervention strategies for parasitic diseases of biomedical interest.

Lacking catalase, a protistan parasite draws on its photosynthetic ancestry to complete an antioxidant repertoire with ascorbate peroxidase

Background

Antioxidative enzymes contribute to a parasite’s ability to counteract the host’s intracellular killing mechanisms. The facultative intracellular oyster parasite, Perkinsus marinus, a sister taxon to dinoflagellates and apicomplexans, is responsible for mortalities of oysters along the Atlantic coast of North America. Parasite trophozoites enter molluscan hemocytes by subverting the phagocytic response while inhibiting the typical respiratory burst. Because P. marinus lacks catalase, the mechanism(s) by which the parasite evade the toxic effects of hydrogen peroxide had remained unclear. We previously found that P. marinus displays an ascorbate-dependent peroxidase (APX) activity typical of photosynthetic eukaryotes. Like other alveolates, the evolutionary history of P. marinus includes multiple endosymbiotic events. The discovery of APX in P. marinus raised the questions: From which ancestral lineage is this APX derived, and what role does it play in the parasite’s life history?

Results

Purification of P. marinus cytosolic APX activity identified a 32 kDa protein. Amplification of parasite cDNA with oligonucleotides corresponding to peptides of the purified protein revealed two putative APX-encoding genes, designated PmAPX1 and PmAPX2. The predicted proteins are 93% identical, and PmAPX2 carries a 30 amino acid N-terminal extension relative to PmAPX1. The P. marinus APX proteins are similar to predicted APX proteins of dinoflagellates, and they more closely resemble chloroplastic than cytosolic APX enzymes of plants. Immunofluorescence for PmAPX1 and PmAPX2 shows that PmAPX1 is cytoplasmic, while PmAPX2 is localized to the periphery of the central vacuole. Three-dimensional modeling of the predicted proteins shows pronounced differences in surface charge of PmAPX1 and PmAPX2 in the vicinity of the aperture that provides access to the heme and active site.

Conclusions

PmAPX1 and PmAPX2 phylogenetic analysis suggests that they are derived from a plant ancestor. Plant ancestry is further supported by the presence of ascorbate synthesis genes in the P. marinus genome that are similar to those in plants. The localizations and 3D structures of the two APX isoforms suggest that APX fulfills multiple functions in P. marinus within two compartments. The possible role of APX in free-living and parasitic stages of the life history of P. marinus is discussed.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1465-5) contains supplementary material, which is available to authorized users.

Oyster disease in a changing environment: Decrypting the link between pathogen, microbiome and environment

Shifting environmental conditions are known to be important triggers of oyster diseases. The mechanism(s) behind these synergistic effects (interplay between host, environment and pathogen/s) are often not clear, although there is evidence that shifts in environmental conditions can affect oyster immunity, and pathogen growth and virulence. However, the impact of shifting environmental parameters on the oyster microbiome and how this affects oyster health and susceptibility to infectious pathogens remains understudied. In this review, we summarise the major diseases afflicting oysters with a focus on the role of environmental factors that can catalyse or amplify disease outbreaks. We also consider the potential role of the oyster microbiome in buffering or augmenting oyster disease outbreaks and suggest that a deeper understanding of the oyster microbiome, its links to the environment and its effect on oyster health and disease susceptibility, is required to develop new frameworks for the prevention and management of oyster diseases.

Regulation of apoptosis-related genes during interactions between oyster hemocytes and the alveolate parasite Perkinsus marinus

The alveolate Perkinsus marinus is the most devastating parasite of the eastern oyster Crassostrea virginica. The parasite is readily phagocytosed by oyster hemocytes, but instead of intracellular killing and digestion, P. marinus can survive phagocytosis and divide in host cells. This intracellular parasitism is accompanied by a regulation of host cell apoptosis. This study was designed to gain a better understanding of the molecular mechanisms of apoptosis regulation in oyster hemocytes following exposure to P. marinus. Regulation of apoptosis-related genes in C. virginica, and apoptosis-regulatory genes in P. marinus, were investigated via qPCR to assess the possible pathways involved during these interactions. In vitro experiments were also carried out to evaluate the effect of chemical inhibitors of P. marinus antioxidant processes on hemocyte apoptosis. Results indicate the involvement of the mitochondrial pathway (Bcl-2, anamorsin) of apoptosis in C. virginica exposed to P. marinus. In parallel, the antioxidants peroxiredoxin and superoxide dismutase were regulated in P. marinus exposed to C. virginica hemocytes suggesting that apoptosis regulation in infected oysters may be mediated by anti-oxidative processes. Chemical inhibition of P. marinus superoxide dismutase resulted in a marked increase of reactive oxygen species production and apoptosis in infected hemocytes. The implication of oxygen-dependent apoptosis during P. marinus infection and disease development in C. virginica is discussed.

Molecular and cellular characterization of apoptosis in flat oyster a key mechanisms at the heart of host-parasite interactions

Bonamia ostreae has been associated with the decline of flat oyster Ostrea edulis populations in some European countries. This obligatory intracellular parasite persists and multiplies into hemocytes. Previous in vitro experiments showed that apoptosis is activated in hemocytes between 1 h and 4 h of contact with the parasite. The flat oyster uses the apoptosis pathway to defend against B. ostreae. However, the parasite might be also able to modulate this response in order to survive in its host. In order to investigate this hypothesis the apoptotic response of the host was evaluated using flow cytometry, transmission electron microscopy and by measuring the response of genes involved in the apoptotic pathway after 4 h. In parallel, the parasite response was investigated by measuring the expression of B. ostreae genes involved in different biological functions including cell cycle and cell death. Obtained results allow describing molecular apoptotic pathways in O. edulis and confirm that apoptosis is early activated in hemocytes after a contact with B. ostreae. Interestingly, at cellular and molecular levels this process appeared downregulated after 44 h of contact. Concurrently, parasite gene expression appeared reduced suggesting that the parasite could inhibit its own metabolism to escape the immune response.

An Agar-Based Method for Plating Marine Protozoan Parasites of the Genus Perkinsus

The genus Perkinsus includes protozoan parasites of mollusks responsible for losses in the aquaculture industry and hampering the recovery of natural shellfish beds worldwide, and they are a key taxon for understanding intracellular parasitism adaptations. The ability to propagate the parasite in liquid media, in the absence of the host, has been crucial for improving understanding of its biology; however, alternative techniques to grow the parasite are needed to explore other basic aspects of the Perkinsus spp. biology. We optimized a DME: Ham's F12-5% FBS- containing solid agar medium for plating Perkinsus marinus. This solid medium supported trophozoite propagation both by binary fission and schizogony. Colonies were visible to the naked eye 17 days after plating. We tested the suitability of this method for several applications, including the following: 1) Subcloning P. marinus isolates: single discrete P. marinus colonies were obtained from DME: Ham's F12-5% FBS- 0.75% agar plates, which could be further propagated in liquid medium; 2) Subcloning engineered Perkinsus mediterraneus MOE[MOE]: GFP by streaking cultures on plates; 3) Chemical susceptibility: Infusing the DME: Ham's F12-5% FBS- 0.75% agar plates with triclosan resulted in inhibition of the parasite propagation in a dose-dependent manner. Altogether, our plating method has the potential for becoming a key tool for investigating diverse aspects of Perkinsus spp. biology, developing new molecular tools, and for biotechnological applications.

Structural, functional, and evolutionary aspects of galectins in aquatic mollusks: From a sweet tooth to the Trojan horse

Galectins constitute a conserved and widely distributed lectin family characterized by their binding affinity for β-galactosides and a unique binding site sequence motif in the carbohydrate recognition domain (CRD). In spite of their structural conservation, galectins display a remarkable functional diversity, by participating in developmental processes, cell adhesion and motility, regulation of immune homeostasis, and recognition of glycans on the surface of viruses, bacteria and protozoan parasites. In contrast with mammals, and other vertebrate and invertebrate taxa, the identification and characterization of bona fide galectins in aquatic mollusks has been relatively recent. Most of the studies have focused on the identification and domain organization of galectin-like transcripts or proteins in diverse tissues and cell types, including hemocytes, and their expression upon environmental or infectious challenge. Lectins from the eastern oyster Crassostrea virginica, however, have been characterized in their molecular, structural and functional aspects and some notable features have become apparent in the galectin repertoire of aquatic mollusks. These including less diversified galectin repertoires and different domain organizations relative to those observed in vertebrates, carbohydrate specificity for blood group oligosaccharides, and up regulation of galectin expression by infectious challenge, a feature that supports their proposed role(s) in innate immune responses. Although galectins from some aquatic mollusks have been shown to recognize microbial pathogens and parasites and promote their phagocytosis, they can also selectively bind to phytoplankton components, suggesting that they also participate in uptake and intracellular digestion of microalgae. In addition, the experimental evidence suggests that the protozoan parasite Perkinsus marinus has co-evolved with the oyster host to be selectively recognized by the oyster hemocyte galectins over algal food or bacterial pathogens, thereby subverting the oyster's innate immune/feeding recognition mechanisms to gain entry into the host cells.

Galectin CvGal2 from the Eastern Oyster (Crassostrea virginica) Displays Unique Specificity for ABH Blood Group Oligosaccharides and Differentially Recognizes Sympatric Perkinsus Species

Galectins are highly conserved lectins that are key to multiple biological functions, including pathogen recognition and regulation of immune responses. We previously reported that CvGal1, a galectin expressed in phagocytic cells (hemocytes) of the eastern oyster (Crassostrea virginica), is hijacked by the parasite Perkinsus marinus to enter the host, where it causes systemic infection and death. Screening of an oyster hemocyte cDNA library revealed a novel galectin, which we designated CvGal2, with four tandemly arrayed carbohydrate recognition domains (CRDs). Phylogentic analysis of the CvGal2 CRDs suggests close relationships with homologous CRDs from CvGal1. Glycan array analysis, however, revealed that, unlike CvGal1 which preferentially binds to the blood group A tetrasaccharide, CvGal2 recognizes both blood group A and B tetrasaccharides and related structures, suggesting that CvGal2 has broader binding specificity. Furthermore, SPR analysis demonstrated significant differences in the binding kinetics of CvGal1 and CvGal2, and structural modeling revealed substantial differences in their interactions with the oligosaccharide ligands. CvGal2 is homogeneously distributed in the hemocyte cytoplasm, is released to the extracellular space, and binds to the hemocyte surface. CvGal2 binds to P. marinus trophozoites in a dose-dependent and β-galactoside-specific manner. Strikingly, negligible binding of CvGal2 was observed for Perkinsus chesapeaki, a sympatric parasite species mostly prevalent in the clams Mya arenaria and Macoma balthica. The differential recognition of Perkinsus species by the oyster galectins is consistent with their relative prevalence in oyster and clam species and supports their role in facilitating parasite entry and infectivity in a host-preferential manner.

Immune responses to infectious diseases in bivalves

Many species of bivalve mollusks (phylum Mollusca, class Bivalvia) are important in fisheries and aquaculture, whilst others are critical to ecosystem structure and function. These crucial roles mean that considerable attention has been paid to the immune responses of bivalves such as oysters, clams and mussels against infectious diseases that can threaten the viability of entire populations. As with many invertebrates, bivalves have a comprehensive repertoire of immune cells, genes and proteins. Hemocytes represent the backbone of the bivalve immune system. However, it is clear that mucosal tissues at the interface with the environment also play a critical role in host defense. Bivalve immune cells express a range of pattern recognition receptors and are highly responsive to the recognition of microbe-associated molecular patterns. Their responses to infection include chemotaxis, phagolysosomal activity, encapsulation, complex intracellular signaling and transcriptional activity, apoptosis, and the induction of anti-viral states. Bivalves also express a range of inducible extracellular recognition and effector proteins, such as lectins, peptidoglycan-recognition proteins, thioester bearing proteins, lipopolysaccharide and β1,3-glucan-binding proteins, fibrinogen-related proteins (FREPs) and antimicrobial proteins. The identification of FREPs and other highly diversified gene families in bivalves leaves open the possibility that some of their responses to infection may involve a high degree of pathogen specificity and immune priming. The current review article provides a comprehensive, but not exhaustive, description of these factors and how they are regulated by infectious agents. It concludes that one of the remaining challenges is to use new "omics" technologies to understand how this diverse array of factors is integrated and controlled during infection.

Physiological and pathological changes in the eastern oyster Crassostrea virginica infested with the trematode Bucephalus sp. and exposed to the toxic dinoflagellate Alexandrium fundyense

Effects of experimental exposure to Alexandrium fundyense, a Paralytic Shellfish Toxin (PST) producer known to affect bivalve physiological condition, upon eastern oysters, Crassostrea virginica with a variable natural infestation of the digenetic trematode Bucephalus sp. were determined. After a three-week exposure to cultured A. fundyense or to a control algal treatment with a non-toxic dinoflagellate, adult oysters were assessed for a suite of variables: histopathological condition, hematological variables (total and differential hemocyte counts, morphology), hemocyte functions (Reactive Oxygen Species (ROS) production and mitochondrial membrane potential), and expression in gills of genes involved in immune responses and cellular protection (MnSOD, CAT, GPX, MT-IV, galectin CvGal) or suspected to be (Dominin, Segon). By comparing individual oysters infested heavily with Bucephalus sp. and uninfested individuals, we found altered gonad and digestive gland tissue and an inflammatory response (increased hemocyte concentration in circulating hemolymph and hemocyte infiltrations in tissues) associated with trematode infestation. Exposure to A. fundyense led to a higher weighted prevalence of infection by the protozoan parasite Perkinsus marinus, responsible for Dermo disease. Additionally, exposure to A. fundyense in trematode-infested oysters was associated with the highest prevalence of P. marinus infection. These observations suggest that the development of P. marinus infection was advanced by A. fundyense exposure, and that, in trematode-infested oysters, P. marinus risk of infection was higher when exposed to A. fundyense. These effects were associated with suppression of the inflammatory response to trematode infestation by A. fundyense exposure. Additionally, the combination of trematode infestation and A. fundyense exposure caused degeneration of adductor muscle fibers, suggesting alteration of valve movements and catch state, which could increase susceptibility to predation. Altogether, these results suggest that exposure of trematode-infested oysters to A. fundyense can lead to overall physiological weakness that decrease oyster defense mechanisms.

Identification of MMV Malaria Box inhibitors of Perkinsus marinus using an ATP-based bioluminescence assay

"Dermo" disease caused by the protozoan parasite Perkinsus marinus (Perkinsozoa) is one of the main obstacles to the restoration of oyster populations in the USA. Perkinsus spp. are also a concern worldwide because there are limited approaches to intervention against the disease. Based on the phylogenetic affinity between the Perkinsozoa and Apicomplexa, we exposed Perkinsus trophozoites to the Medicines for Malaria Venture Malaria Box, an open access compound library comprised of 200 drug-like and 200 probe-like compounds that are highly active against the erythrocyte stage of Plasmodium falciparum. Using a final concentration of 20 µM, we found that 4 days after exposure 46% of the compounds were active against P. marinus trophozoites. Six compounds with IC50 in the µM range were used to compare the degree of susceptibility in vitro of eight P. marinus strains from the USA and five Perkinsus species from around the world. The three compounds, MMV666021, MMV665807 and MMV666102, displayed a uniform effect across Perkinsus strains and species. Both Perkinsus marinus isolates and Perkinsus spp. presented different patterns of response to the panel of compounds tested, supporting the concept of strain/species variability. Here, we expanded the range of compounds available for inhibiting Perkinsus proliferation in vitro and characterized Perkinsus phenotypes based on their resistance to six compounds. We also discuss the implications of these findings in the context of oyster management. The Perkinsus system offers the potential for investigating the mechanism of action of the compounds of interest.

Excreted Leishmania peruviana and Leishmania amazonensis iron-superoxide dismutase purification: specific antibody detection in Colombian patients with cutaneous leishmaniasis

Leishmania sp. survival in the vertebrate host depends on the host macrophage immune response as well as on the parasite's defense against free radicals. Iron-superoxide dismutase (Fe-SOD) is a key antioxidant enzyme that contributes to radical superoxide dismutation, preventing the disease from surging and propagating itself. Leishmania sp. has various Fe-SOD isoforms, one of which (Fe-SODe) is excreted into the medium and, being highly immunogenic, can be considered a very good molecular marker. In this work, we purified the Fe-SOD enzymes excreted by L. peruviana and L. amazonensis and studied them as antigens in serodiagnosis. We used ELISA and Western blot techniques to test 51 human cutaneous leishmaniasis sera from Colombia. All 51 patients presented with dermal injuries caused by unknown Leishmania species. The results observed with the purified proteins were compared with those obtained when total soluble lysate and unpurified Fe-SODe were used as the antigen fraction. Thus, we conclude that the purified enzymes are more sensitive and specific than their unpurified counterparts and that there is no cross-reactivity between them.

Humanized HLA-DR4 mice fed with the protozoan pathogen of oysters Perkinsus marinus (Dermo) do not develop noticeable pathology but elicit systemic immunity

Perkinsus marinus (Phylum Perkinsozoa) is a marine protozoan parasite responsible for “Dermo” disease in oysters, which has caused extensive damage to the shellfish industry and estuarine environment. The infection prevalence has been estimated in some areas to be as high as 100%, often causing death of infected oysters within 1–2 years post-infection. Human consumption of the parasites via infected oysters is thus likely to occur, but to our knowledge the effect of oral consumption of P. marinus has not been investigated in humans or other mammals. To address the question we used humanized mice expressing HLA-DR4 molecules and lacking expression of mouse MHC-class II molecules (DR4.EA⁰) in such a way that CD4 T cell responses are solely restricted by the human HLA-DR4 molecule. The DR4.EA⁰ mice did not develop diarrhea or any detectable pathology in the gastrointestinal tract or lungs following single or repeated feedings with live P. marinus parasites. Furthermore, lymphocyte populations in the gut associated lymphoid tissue and spleen were unaltered in the parasite-fed mice ruling out local or systemic inflammation. Notably, naïve DR4.EA⁰ mice had antibodies (IgM and IgG) reacting against P. marinus parasites whereas parasite specific T cell responses were undetectable. Feeding with P. marinus boosted the antibody responses and stimulated specific cellular (IFNγ) immunity to the oyster parasite. Our data indicate the ability of P. marinus parasites to induce systemic immunity in DR4.EA⁰ mice without causing noticeable pathology, and support rationale grounds for using genetically engineered P. marinus as a new oral vaccine platform to induce systemic immunity against infectious agents.

Protozoan parasites of bivalve molluscs: literature follows culture

Bivalve molluscs are key components of the estuarine environments as contributors to the trophic chain, and as filter -feeders, for maintaining ecosystem integrity. Further, clams, oysters, and scallops are commercially exploited around the world both as traditional local shellfisheries, and as intensive or semi-intensive farming systems. During the past decades, populations of those species deemed of environmental or commercial interest have been subject to close monitoring given the realization that these can suffer significant decline, sometimes irreversible, due to overharvesting, environmental pollution, or disease. Protozoans of the genera Perkinsus, Haplosporidium, Marteilia, and Bonamia are currently recognized as major threats for natural and farmed bivalve populations. Since their identification, however, the variable publication rates of research studies addressing these parasitic diseases do not always appear to reflect their highly significant environmental and economic impact. Here we analyzed the peer- reviewed literature since the initial description of these parasites with the goal of identifying potential milestone discoveries or achievements that may have driven the intensity of the research in subsequent years, and significantly increased publication rates. Our analysis revealed that after initial description of the parasite as the etiological agent of a given disease, there is a time lag before a maximal number of yearly publications are reached. This has already taken place for most of them and has been followed by a decrease in publication rates over the last decade (20- to 30- year lifetime in the literature). Autocorrelation analyses, however, suggested that advances in parasite purification and culture methodologies positively drive publication rates, most likely because they usually lead to novel molecular tools and resources, promoting mechanistic studies. Understanding these trends should help researchers in prioritizing research efforts for these and other protozoan parasites, together with their development as model systems for further basic and translational research in parasitic diseases.

Immunological responses of the mangrove oysters Crassostrea gasar naturally infected by Perkinsus sp. in the Mamanguape Estuary, Paraíba state (Northeastern, Brazil)

Perkinsus genus includes protozoan parasites of marine mollusks, especially bivalves. In the last four years, this parasite has been detected in mangrove oysters Crassostrea rhizophorae and Crassostrea gasar from the Northeastern region of Brazil. Hemocytes are the key cells of the oyster immune system, being responsible for a variety of cellular and humoral reactions, such as phagocytosis, encapsulation and the release of several effector molecules that control the invasion and proliferation of microorganisms. In Brazil, there is little information on perkinsosis and none on the immune responses of native oysters' species against Perkinsus spp. The objective of this study was to determine the effects of natural infection by Perkinsus sp. on the immunological parameters of mangrove oysters C. gasar cultured in the Mamanguape River Estuary (Paraíba, Brazil). Adults oysters (N = 40/month) were sampled in December 2011, March, May, August and October 2012. Gills were removed and used to determine the presence and intensity of the Perkinsus sp. infection, according to a scale of four levels (1-4), using the Ray's fluid thioglycollate medium assay. Immunological parameters were measured in hemolymph samples by flow cytometry, including: total hemocyte count (THC), differential hemocyte count (DHC), cell mortality, phagocytic capacity, and production of Reactive Oxygen Species (ROS). The plasma was used to determine the hemagglutination activity. The results showed the occurrence of Perkinsus sp. with the highest mean prevalence (93.3%) seen so far in oyster populations in Brazil. Despite that, no oyster mortality was associated. In contrast, we observed an increase in hemocyte mortality and a suppression of two of the main defense mechanisms, phagocytosis and ROS production in infected oysters. The increase in the percentage of blast-like cells on the hemolymph, and the increase in THC in oysters heavily infected (at the maximum intensity, 4) suggest an induction of hemocytes proliferation. The immunological parameters varied over the studied months, which may be attributed to the dynamics of infection by Perkinsus sp. The results of the present study demonstrate that Perkinsus sp. has a deleterious effect on C. gasar immune system, mainly in high intensities, which likely renders oysters more susceptible to other pathogens and diseases.

Early host-pathogen interactions in a marine bivalve: Crassostrea virginica pallial mucus modulates Perkinsus marinus growth and virulence

Perkinsus marinus is an important protistan parasite of the eastern oyster Crassostrea virginica. Recent findings showed that oyster pallial organs (mantle, gills) are a major portal of entry for the parasite. Therefore, mucus covering these organs represents the first host effectors encountered by P. marinus. This study consisted of several experiments designed to investigate the effect of oyster pallial mucus on the growth, protease production and infectivity of P. marinus. In each experiment, P. marinus performance in cultures supplemented with pallial mucus (mantle, gill, or both) was compared to that of parasite cells grown in unsupplemented media or in cultures supplemented with oyster plasma or digestive extracts. P. marinus grown in media supplemented with C. virginica mantle mucus showed a significantly higher growth rate than cultures enriched with the other supplemental extracts, while cultures grown in gill mucus promoted higher protease production. Conversely, P. marinus grown in cultures supplemented with pallial mucus of the non-compatible host Crassostrea gigas (Pacific oyster) were dramatically inhibited. Challenge experiments showed a significant increase in P. marinus virulence in cultures supplemented with C. virginica pallial mucus as compared to unsupplemented cultures or to those supplemented with digestive extract or plasma. These results suggest that C. virginica mucus plays a significant role in the pathogenesis of P. marinus by enhancing the proliferation and the infectivity of this devastating parasite. The contrasting results obtained with both oyster species indicate that P. marinus host specificity may begin in the mucus.

A Review of Current State of Knowledge Concerning Perkinsus marinus Effects on Crassostrea virginica (Gmelin) (the Eastern Oyster)

The eastern oyster, Crassostrea virginica (Gmelin), is both an important component of our estuaries and an important farmed food animal along the east and south coasts of the United States. Its populations have been significantly diminished in the wild due to decades of overfishing beginning in the 1890s. Unfortunately, in 1950, a new disease in eastern oysters caused by the protistan agent, Perkinsus marinus, was identified. The disease, resulting from infection with this protozoan, leads to high mortality of both wild and cultured eastern oysters. Current restoration efforts are hampered by the disease, as is the aquaculture of this economically important food. The parasite infects hemocytes and causes hemolytic anemia and general degeneration of the tissues, leading to death. Ongoing research efforts are attempting to develop oysters resistant to the disease. Transport regulations exist in may states. Infection with P. marinus is listed as a reportable disease by the World Health Organization.

Early host-pathogen interactions in marine bivalves: evidence that the alveolate parasite Perkinsus marinus infects through the oyster mantle during rejection of pseudofeces

Parasites have developed myriad strategies to reach and infect their specific hosts. One of the most common mechanisms for non-vector transmitted parasites to reach the internal host environment is by ingestion during feeding. In this study, we investigated the mechanisms of oyster host colonization by the alveolate Perkinsus marinus and focused on how oysters process infective waterborne P. marinus cells during feeding in order to determine the portal(s) of entry of this parasite to its host. We also compared the infectivity of freely-suspended cells of P. marinus with that of cells incorporated into marine aggregates to link changes in particle processing by the feeding organs with infection success and route. Finally, we evaluated the effect of oyster secretions (mucus) covering the feeding organs on P. marinus physiology because these host factors are involved in the processing of waterborne particles. The ensemble of results shows a unique mechanism for infection by which the parasite is mostly acquired during the feeding process, but not via ingestion. Rather, infection commonly occurs during the rejection of material as pseudofeces before reaching the mouth. The pseudofeces discharge area, a specialized area of the mantle where unwanted particles are accumulated for rejection as pseudofeces, showed significantly higher parasite loads than other host tissues including other parts of the mantle. Aggregated P. marinus cells caused significantly higher disease prevalence and infection intensities when compared to freely-suspended parasite cells. Mucus covering the mantle caused a quick and significant increase in parasite replication rates suggesting rapid impact on P. marinus physiology. A new model for P. marinus acquisition in oysters is proposed.

The Antimicrobial Defense of the Pacific Oyster, Crassostrea gigas. How Diversity may Compensate for Scarcity in the Regulation of Resident/Pathogenic Microflora

Healthy oysters are inhabited by abundant microbial communities that vary with environmental conditions and coexist with immunocompetent cells in the circulatory system. In Crassostrea gigas oysters, the antimicrobial response, which is believed to control pathogens and commensals, relies on potent oxygen-dependent reactions and on antimicrobial peptides/proteins (AMPs) produced at low concentrations by epithelial cells and/or circulating hemocytes. In non-diseased oysters, hemocytes express basal levels of defensins (Cg-Defs) and proline-rich peptides (Cg-Prps). When the bacterial load dramatically increases in oyster tissues, both AMP families are driven to sites of infection by major hemocyte movements, together with bactericidal permeability/increasing proteins (Cg-BPIs) and given forms of big defensins (Cg-BigDef), whose expression in hemocytes is induced by infection. Co-localization of AMPs at sites of infection could be determinant in limiting invasion as synergies take place between peptide families, a phenomenon which is potentiated by the considerable diversity of AMP sequences. Besides, diversity occurs at the level of oyster AMP mechanisms of action, which range from membrane lysis for Cg-BPI to inhibition of metabolic pathways for Cg-Defs. The combination of such different mechanisms of action may account for the synergistic activities observed and compensate for the low peptide concentrations in C. gigas cells and tissues. To overcome the oyster antimicrobial response, oyster pathogens have developed subtle mechanisms of resistance and evasion. Thus, some Vibrio strains pathogenic for oysters are equipped with AMP-sensing systems that trigger resistance. More generally, the known oyster pathogenic vibrios have evolved strategies to evade intracellular killing through phagocytosis and the associated oxidative burst.

The natural resistance-associated macrophage protein from the protozoan parasite Perkinsus marinus mediates iron uptake

Microbial pathogens succeed in acquiring essential metals such as iron and manganese despite their limited availability because of the host's immune response. The eukaryotic natural resistance-associated macrophage proteins mediate uptake of divalent metals and, during infection, may compete directly for metal acquisition with the pathogens' transporters. In this study, we characterize the Nramp gene family of Perkinsus marinus, an intracellular parasite of the eastern oyster, and through yeast complementation, we demonstrate for the first time for a protozoan parasite that Nramp imports environmental Fe. Three PmNramp isogenes differ in their exon-intron structures and encode transcripts that display a trans splicing leader at the 5' end. The protein sequences share conserved properties predicted for the Nramp/Solute carrier 11 (Slc11) family, such as 12-transmembrane segment (TMS) topology (N- and C-termini cytoplasmic) and preferential conservation of four TMS predicted to form a pseudosymmetric proton/metal symport pathway. Yeast fet3fet4 mutant complementation assays showed iron transport activity for PmNramp1 and a fusion chimera of the PmNramp3 hydrophobic core and PmNramp1 N- and C-termini. PmNramp1 site-directed mutagenesis demonstrated that Slc11 invariant and predicted pseudosymmetric motifs (TMS1 Asp-Pro-Gly and TMS6 Met-Pro-His) are key for transport function. PmNramp1 TMS1 mutants D76E, G78A, and D76E/G78A prevented membrane protein expression, while TMS6 M250A, H252Y, and M250A/H252Y specifically abrogated Fe uptake; the TMS6 H252Y mutation also correlates with divergence from Nramp specificity for divalent metals.

Use of OmpU porins for attachment and invasion of Crassostrea gigas immune cells by the oyster pathogen Vibrio splendidus

OmpU porins are increasingly recognized as key determinants of pathogenic host Vibrio interactions. Although mechanisms remain incompletely understood, various species, including the human pathogen Vibrio cholera, require OmpU for host colonization and virulence. We have shown previously that OmpU is essential for virulence in the oyster pathogen Vibrio splendidus LGP32. Here, we showed that V. splendidus LGP32 invades the oyster immune cells, the hemocytes, through subversion of host-cell actin cytoskeleton. In this process, OmpU serves as an adhesin/invasin required for β-integrin recognition and host cell invasion. Furthermore, the major protein of oyster plasma, the extracellular superoxide dismutase Cg-EcSOD, is used as an opsonin mediating the OmpU-promoted phagocytosis through its RGD sequence. Finally, the endocytosed bacteria were found to survive intracellularly, evading the host defense by preventing acidic vacuole formation and limiting reactive oxygen species production. We conclude that (i) V. splendidus is a facultative intracellular pathogen that manipulates host defense mechanisms to enter and survive in host immune cells, and (ii) that OmpU is a major determinant of host cell invasion in Vibrio species, used by V. splendidus LGP32 to attach and invade oyster hemocytes through opsonisation by the oyster plasma Cg-EcSOD.

The Alveolate Perkinsus marinus: biological insights from EST gene discovery

Background

Perkinsus marinus, a protozoan parasite of the eastern oyster Crassostrea virginica, has devastated natural and farmed oyster populations along the Atlantic and Gulf coasts of the United States. It is classified as a member of the Perkinsozoa, a recently established phylum considered close to the ancestor of ciliates, dinoflagellates, and apicomplexans, and a key taxon for understanding unique adaptations (e.g. parasitism) within the Alveolata. Despite intense parasite pressure, no disease-resistant oysters have been identified and no effective therapies have been developed to date.

Results

To gain insight into the biological basis of the parasite's virulence and pathogenesis mechanisms, and to identify genes encoding potential targets for intervention, we generated >31,000 5' expressed sequence tags (ESTs) derived from four trophozoite libraries generated from two P. marinus strains. Trimming and clustering of the sequence tags yielded 7,863 unique sequences, some of which carry a spliced leader. Similarity searches revealed that 55% of these had hits in protein sequence databases, of which 1,729 had their best hit with proteins from the chromalveolates (E-value ≤ 1e-5). Some sequences are similar to those proven to be targets for effective intervention in other protozoan parasites, and include not only proteases, antioxidant enzymes, and heat shock proteins, but also those associated with relict plastids, such as acetyl-CoA carboxylase and methyl erythrithol phosphate pathway components, and those involved in glycan assembly, protein folding/secretion, and parasite-host interactions.

Conclusions

Our transcriptome analysis of P. marinus, the first for any member of the Perkinsozoa, contributes new insight into its biology and taxonomic position. It provides a very informative, albeit preliminary, glimpse into the expression of genes encoding functionally relevant proteins as potential targets for chemotherapy, and evidence for the presence of a relict plastid. Further, although P. marinus sequences display significant similarity to those from both apicomplexans and dinoflagellates, the presence of trans-spliced transcripts confirms the previously established affinities with the latter. The EST analysis reported herein, together with the recently completed sequence of the P. marinus genome and the development of transfection methodology, should result in improved intervention strategies against dermo disease.

Development of an in vitro assay to examine intracellular survival of Perkinsus marinus trophozoites upon phagocytosis by oyster (Crassostrea virginica and Crassostrea ariakensis) hemocytes

Perkinsus marinus is a facultative intracellular parasite that causes "Dermo" disease in the eastern oyster Crassostrea virginica. Although hemocytes from healthy oysters rapidly phagocytize P. marinus trophozoites, they fail to efficiently kill them. Instead, trophozoites survive and proliferate, eventually overwhelming the host. Because Chesapeake Bay oyster populations have been reduced to unprecedented levels, the introduction of the Suminoe oyster, Crassostrea ariakensis (synonymous C. rivularis), has recently been proposed. Although this species is refractory to developing Dermo disease, it can be infected by Perkinsus spp. and, thus, the mechanistic basis of its disease resistance remains intriguing. To examine whether the resistance to develop Dermo is due to a high capacity of C ariakensis hemocytes to kill internalized P. marinus, we developed an in vitro assay to compare intracellular survival and proliferation of P. marinus in C. virginica and C ariakensis hemocytes. Our results revealed that P. marinus cultured trophozoites have a similar capacity for in vitro survival within hemocytes from both oyster species, suggesting that the resistance of C. ariakensis to develop Dermo disease is most likely due to reduced parasite pathogenicity for the latter oyster species, rather than to infectivity. Together with the currently available P. marinus genome, EST sequences, and the transfection methodology we recently developed, this assay should significantly contribute to a rigorous identification of the P. marinus genes responsible for its intrahemocytic survival.

Serine protease inhibitor cvSI-1 potential role in the eastern oyster host defense against the protozoan parasite Perkinsus marinus

The serine protease inhibitor cvSI-1, purified from plasma of eastern oysters, inhibited the proliferation of the protozoan parasite Perkinsus marinus in vitro. In situ hybridization located cvSI-1 gene expression in basophil cells of the digestive tubules and cvSI-1 expression measured by real-time quantitative reverse transcriptase polymerase chain reaction was several hundred folds greater in digestive glands than in other organs examined or circulating hemocytes. cvSI-1 gene expression was also significantly greater in winter than in summer. Finally, cvSI-1 gene expression and plasma protease inhibitory activity in oysters selected for increased resistance to P. marinus were significantly greater than in unselected oysters. These findings support the hypothesis that cvSI-1 plays a role in eastern oyster host defense against P. marinus possibly through inhibition of parasite proliferation.

Infection with the protozoan parasite Bonamia ostreae modifies in vitro haemocyte activities of flat oyster Ostrea edulis

Bonamia ostreae is an intracellular protozoan parasite, infecting haemocytes of the European flat oyster Ostrea edulis. Oyster defence mechanisms mainly rely on haemocytes. In the present study in vitro interactions between parasites and flat oyster haemocytes were investigated using flow cytometry and light microscopy. Haemocyte parameters including: non specific esterase activity, reactive oxygen species (ROS) production and phagocytosis were monitored using flow cytometry after 2 h cell incubation with live and dead B. ostreae. Two ratios of parasites per haemocyte were tested (5:1 and 10:1), haemocytes alone were used as controls and the experiment was carried out three times. Flow cytometry revealed a decrease of non specific esterase activities and ROS production by haemocytes after incubation with live parasites, while there was little difference in phagocytosis activity when compared with controls. Similarly, dead parasites induced a decrease in haemocyte activities but to a lesser extent compared to live parasites. These results suggest that B. ostreae actively contributes to the modification of haemocyte activities in order to ensure its own intracellular survival.

Peroxisome proliferation in Foraminifera inhabiting the chemocline: an adaptation to reactive oxygen species exposure?

Certain foraminiferal species are abundant within the chemocline of marine sediments. Ultrastructurally, most of these species possess numerous peroxisomes complexed with the endoplasmic reticulum (ER); mitochondria are often interspersed among these complexes. In the Santa Barbara Basin, pore-water bathing Foraminifera and co-occurring sulfur-oxidizing microbial mats had micromolar levels of hydrogen peroxide (H(2)O(2)), a reactive oxygen species that can be detrimental to biological membranes. Experimental results indicate that adenosine triphosphate concentrations are significantly higher in Foraminifera incubated in 16 microM H(2)O(2) than in specimens incubated in the absence of H(2)O(2). New ultrastructural and experimental observations, together with published results, lead us to propose that foraminiferans can utilize oxygen derived from the breakdown of environmentally and metabolically produced H(2)O(2). Such a capability could explain foraminiferal adaptation to certain chemically inhospitable environments; it would also force us to reassess the role of protists in biogeochemistry, especially with respect to hydrogen and iron. The ecology of these protists also appears to be tightly linked to the sulfur cycle. Finally, given that some Foraminifera bearing peroxisome-ER complexes belong to evolutionarily basal groups, an early acquisition of the capability to use environmental H(2)O(2) could have facilitated diversification of foraminiferans during the Neoproterozoic.

Plastid-derived genes in the nonphotosynthetic alveolate Oxyrrhis marina

Reconstructing the history of plastid acquisition and loss in the alveolate protists is a difficult problem because our knowledge of the distribution of plastids in extant lineages is incomplete due to the possible presence of cryptic, nonphotosynthetic plastids in several lineages. The discovery of the apicoplast in apicomplexan parasites has drawn attention to this problem and, more specifically, to the question of whether many other nonphotosynthetic lineages also contain cryptic plastids or are derived from plastid-containing ancestors. Oxyrrhis marina is one such organism: It is a heterotrophic, early-branching member of the dinoflagellate lineage for which there is no evidence of a plastid. To investigate the possibility that O. marina is derived from a photosynthetic ancestor, we have generated and analyzed a large-scale EST database and searched for evidence of plastid-derived genes. Here, we describe 8 genes whose phylogeny shows them to be derived from plastid-targeted homologues. These genes encode proteins from several pathways known to be localized in the plastids of other algae, including synthesis of tetrapyrroles, isoprenoids, and amino acids, as well as carbon metabolism and oxygen detoxification. The 5' end of 5 cDNAs were also characterized using cap-dependent or spliced leader-mediated reverse transcriptase-polymerase chain reaction, revealing that at least 4 of these genes have retained leaders that are similar in nature to the plastid-targeting signals of other secondary plastids, suggesting that these proteins may be targeted to a cryptic organelle. At least 2 genes do not encode such leaders, and their products may presently function in the cytosol. Altogether, the presence of plastid-derived genes in O. marina shows that its ancestors contained a plastid, and the pathways represented by the genes and presence of targeting signals on at least some of the genes further suggests that a relict organelle may still exist to fulfill plastid metabolic functions.

A Galectin of Unique Domain Organization from Hemocytes of the Eastern Oyster (Crassostrea virginica) Is a Receptor for the Protistan Parasite Perkinsus marinus

Invertebrates display effective innate immune responses for defense against microbial infection. However, the protozoan parasite Perkinsus marinus causes Dermo disease in the eastern oyster Crassostrea virginica and is responsible for catastrophic damage to shellfisheries and the estuarine environment in North America. The infection mechanisms remain unclear, but it is likely that, while filter feeding, the healthy oysters ingest P. marinus trophozoites released to the water column by the infected neighboring individuals. Inside oyster hemocytes, trophozoites resist oxidative killing, proliferate, and spread throughout the host. However, the mechanism(s) for parasite entry into the hemocyte are unknown. In this study, we show that oyster hemocytes recognize P. marinus via a novel galectin (C. virginica galectin (CvGal)) of unique structure. The biological roles of galectins have only been partly elucidated, mostly encompassing embryogenesis and indirect roles in innate and adaptive immunity mediated by the binding to endogenous ligands. CvGal recognized a variety of potential microbial pathogens and unicellular algae, and preferentially, Perkinsus spp. trophozoites. Attachment and spreading of hemocytes to foreign surfaces induced localization of CvGal to the cell periphery, its secretion and binding to the plasma membrane. Exposure of hemocytes to Perkinsus spp. trophozoites enhanced this process further, and their phagocytosis could be partially inhibited by pretreatment of the hemocytes with anti-CvGal Abs. The evidence presented indicates that CvGal facilitates recognition of selected microbes and algae, thereby promoting phagocytosis of both potential infectious challenges and phytoplankton components, and that P. marinus subverts the host's immune/feeding recognition mechanism to passively gain entry into the hemocytes.

Role of nitric oxide in the defenses of Crassostrea virginica to experimental infection with the protozoan parasite Perkinsus marinus

We investigated the role of nitric oxide (NO) in the responses of the Eastern oyster, Crassostrea virginica, to the protozoan parasite Perkinsus marinus, causative agent of Dermo disease. P. marinus induced a slight but significant increase in NO production by oyster hemocytes in vitro, comparable to the increase induced by the immune stimulants phorbol myristrate acetate (PMA) and lipopolysaccharide (LPS). P. marinus also activated the NO response in oysters in vivo, as shown by induction of a protein reacting with a universal NO synthase (NOS) antibody in hemocytes and the presence of high levels of nitrite in plasma. Treatment of experimentally infected oysters with the NOS inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME) resulted in a transient decrease in NO levels in oyster plasma and a significant increase in the number of parasites at early time points after infection. The NO donor, S-nitroso-N-acetyl-penicillamine (SNAP) caused a significant inhibition in the proliferation of P. marinus cultured cells after 24 h of incubation. These results indicate that NO has a role in decreasing parasite loads at early time points after infection.

Structures of PmSOD1 and PmSOD2, two superoxide dismutases from the protozoan parasite Perkinsus marinus

Perkinsus marinus, a facultative intracellular parasite of the eastern oyster Crassostrea virginica, is responsible for mass mortalities of oyster populations. P. marinus trophozoites survive and proliferate within oyster hemocytes, invading most tissues and fluids, thus causing a systemic infection that eventually kills the host. The phagocytosis of P. marinus trophozoites lacks a respiratory burst, suggesting that the parasite has mechanisms that actively abrogate the host's oxidative defense responses. One mechanism and the first line of defense against oxidative damage is the dismutation of superoxide radical to molecular oxygen and hydrogen peroxide by superoxide dismutases (SODs). P. marinus possesses two iron-cofactored SODs, PmSOD1 and PmSOD2. Here, the crystallization and X-ray structures of both PmSOD1 and PmSOD2 are presented.

GENE ORGANIZATION AND EXPRESSION OF THE DIVALENT CATION TRANSPORTER NRAMP IN THE PROTISTAN PARASITE PERKINSUS MARINUS

Trophozoites of the protistan parasite Perkinsus marinus reside and proliferate inside phagosomelike structures of hemocytes from the host, the eastern oyster Crassostrea virginica. In a murine model, it has been proposed that the outcome of intracellular parasite-host interactions is determined, at least in part, by the activity of the host's divalent cation transporter natural resistance-associated macrophage protein 1 (Nramp1). Although nucleotide sequences from members of the Nramp family in protozoan parasites have recently become available in public databases, little is known about their molecular, structural, and functional aspects that may relate to the parasite's survival of intracellular killing by the host. The complementary DNA (cDNA) sequence of the Nramp from P. marinus (PmNramp) was obtained by polymerase chain reaction amplification with degenerated primers, followed by rapid amplification of cDNA ends. The 2,082-bp cDNA sequence encoded a predicted protein of 558 amino acids. PmNramp is a single-copy gene composed of 7 exons and 6 short introns (44-61 bp) with the canonical splicing signal (GT/AG). A phylogenetic analysis indicates that P. marinus and apicomplexan Nramp genes derive from a common "archetype" Nramp ancestor. However, the apicomplexan Nramps are highly divergent from the P. marinus sequence and the rest of the archetype Nramp group. Preliminary studies suggest that expression of PmNramp in in vitro-cultured P. marinus trophozoites is modulated by metals and by exogenous oxidative stress.