Parthenogenesis and asexual multiplication among parasitic platyhelminths

Effect of Hydroxyurea on Morphology, Proliferation, and Protein Expression on Taenia crassiceps WFU Strain

Flatworms are known for their remarkable regenerative ability, one which depends on totipotent cells known as germinative cells in cestodes. Depletion of germinative cells with hydroxyurea (HU) affects the regeneration of the parasite. Here, we studied the reduction and recovery of germinative cells in T. crassiceps cysticerci after HU treatment (25 mM and 40 mM of HU for 6 days) through in vitro assays. Viability and morphological changes were evaluated. The recovery of cysticerci's mobility and morphology was evaluated at 3 and 6 days, after 6 days of treatment. The number of proliferative cells was evaluated using EdU. Our results show morphological changes in the size, shape, and number of evaginated cysticerci at the 40 mM dose. The mobility of cysticerci was lower after 6 days of HU treatment at both concentrations. On days 3 and 6 of recovery after 25 mM of HU treatment, a partial recovery of the proliferative cells was observed. Proteomic and Gene Ontology analyses identified modifications in protein groups related to DNA binding, DNA damage, glycolytic enzymes, cytoskeleton, skeletal muscle, and RNA binding.

Transcriptomic analysis of subarachnoid cysts of Taenia solium reveals mechanisms for uncontrolled proliferation and adaptations to the microenvironment

Subarachnoid neurocysticercosis (SANCC) is caused by an abnormally transformed form of the metacestode or larval form of the tapeworm Taenia solium. In contrast to vesicular parenchymal and ventricular located cysts that contain a viable scolex and are anlage of the adult tapeworm, the subarachnoid cyst proliferates to form aberrant membranous cystic masses within the subarachnoid spaces that cause mass effects and acute and chronic arachnoiditis. How subarachnoid cyst proliferates and interacts with the human host is poorly understood, but parasite stem cells (germinative cells) likely participate. RNA-seq analysis of the subarachnoid cyst bladder wall compared to the bladder wall and scolex of the vesicular cyst revealed that the subarachnoid form exhibits activation of signaling pathways that promote proliferation and increased lipid metabolism. These adaptions allow growth in a nutrient-limited cerebral spinal fluid. In addition, we identified therapeutic drug targets that would inhibit growth of the parasite, potentially increase effectiveness of treatment, and shorten its duration.

Form and Function in the Digenea, with an Emphasis on Host-Parasite and Parasite-Bacteria Interactions

This review covers the general aspects of the anatomy and physiology of the major body systems in digenetic trematodes, with an emphasis on new knowledge of the area acquired since the publication of the second edition of this book in 2019. In addition to reporting on key recent advances in the morphology and physiology of tegumentary, sensory, neuromuscular, digestive, excretory, and reproductive systems, and their roles in host-parasite interactions, this edition includes a section discussing the known and putative roles of bacteria in digenean biology and physiology. Furthermore, a brief discussion of current trends in the development of novel treatment and control strategies based on a better understanding of the trematode body systems and associated bacteria is provided.

CRISPR interference for sequence-specific regulation of fibroblast growth factor receptor A in Schistosoma mansoni

Employing the flatworm parasite Schistosoma mansoni as a model, we report the first application of CRISPR interference (CRISPRi) in parasitic helminths for loss-of-function studies targeting the SmfgfrA gene which encodes the stem cell marker, fibroblast growth factor receptor A (FGFRA). SmFGFRA is essential for maintaining schistosome stem cells and critical in the schistosome-host interplay. The SmfgfrA gene was targeted in S. mansoni adult worms, eggs and schistosomula using a catalytically dead Cas9 (dCas9) fused to a transcriptional repressor KRAB. We showed that SmfgfrA repression resulted in considerable phenotypic differences in the modulated parasites compared with controls, including reduced levels of SmfgfrA transcription and decreased protein expression of SmFGFRA, a decline in EdU (thymidine analog 5-ethynyl-2'-deoxyuridine, which specifically stains schistosome stem cells) signal, and an increase in cell apoptosis. Notably, reduced SmfgfrA transcription was evident in miracidia hatched from SmfgfrA-repressed eggs, and resulted in a significant change in miracidial behavior, indicative of a durable repression effect caused by CRISPRi. Intravenous injection of mice with SmfgfrA-repressed eggs resulted in granulomas that were markedly reduced in size and a decline in the level of serum IgE, emphasizing the importance of SmFGFRA in regulating the host immune response induced during schistosome infection. Our findings show the feasibility of applying CRISPRi for effective, targeted transcriptional repression in schistosomes, and provide the basis for employing CRISPRi to selectively perturb gene expression in parasitic helminths on a genome-wide scale.

Unique genetic structure of the human tapeworm Dibothriocephalus latus from the Alpine lakes region - a successful adaptation?

Dibothriocephalus latus is the most frequent causative agent of fish-borne zoonosis (diphyllobothriosis) in Europe, where it is currently circulating mainly in the Alpine lakes region (ALR) and Russia. Three mitochondrial genes (cox1, cob and nad3) and 6 microsatellite loci were analysed to determine how is the recently detected triploidy/parthenogenesis in tapeworms from ALR displayed at the DNA level. A geographically distant population from the Krasnoyarsk Reservoir in Russia (RU-KR) was analysed as a comparative population. One or 2 alleles of each microsatellite locus was detected in plerocercoids from RU-KR, corresponding to the microsatellite pattern of a diploid organism. In contrast, 1–3 alleles were observed in tapeworms from ALR, in accordance with their triploidy. The high diversity of mitochondrial haplotypes in D. latus from RU-KR implied an original and relatively stable population, but the identical structure of mitochondrial genes of tapeworms from ALR was probably a consequence of a bottleneck typical of introduced populations. These results indicated that the diploid/sexually reproducing population from RU-KR was ancestral, located within the centre of the distribution of the species, and the triploid/parthenogenetically reproducing subalpine population was at the margin of the distribution. The current study revealed the allelic structure of the microsatellite loci in the triploid tapeworm for the first time.

Germinal development in embryonic rediae of the hemiuroid digenean Bunocotyle progenetica: an ultrastructural study

Embryonic development of reproductive organs in rediae of the digenean Bunocotyle progenetica was studied using transmission electron microscopy. The germinal primordium becomes morphologically distinct in early embryos as a weakly separated cell mass with a forming cavity. It consists of undifferentiated, differentiating, and supporting cells. As embryos develop, the supporting cells form a wall around the enlarging cavity. Other cells of the germinal primordium are incorporated into the wall as solitary cells or as small cell aggregations. Those situated posteriorly give rise to an incipient germinal mass functioning during postembryonic development. Undifferentiated and differentiating cells in the middle and the anterior part of the primordium ensure a considerable growth of the cavity wall, which incorporates solitary germinal cells. In advanced embryonic rediae, these cells mature, cleave, and give rise to germinal balls, which enter the forming brood cavity. In the most mature embryonic rediae, all these early cercarial embryos reside in a brood cavity, which is lined by that time with a syncytium continuous with the supporting tissue of the incipient germinal mass. Based on our results and the literature data, we suggest that the morphogenesis of the reproductive apparatus of the daughter parthenitae in hemiuroid digeneans may be characterized by (1) emergence of an incipient brood cavity within the germinal primordium, (2) formation of the cavity lining from the cells of the germinal primordium, (3) fragmentation and uneven distribution of the germinal material of the germinal primordium around the cavity, and (4) an anticipatory development of some of this germinal material.

Subarachnoid neurocysticercosis: emerging concepts and treatment

PURPOSE OF REVIEW

Subarachnoid neurocysticercosis (SUBNCC) is caused by a morphologically unique proliferative form of Taenia solium involving the subarachnoid spaces. Prolonged therapy based upon the pathophysiology of SUBNCC and long-term follow-up have shed light on the course of disease and led to highly improved outcomes.

RECENT FINDINGS

SUBNCC has a prolonged incubation period of between 10 and 25 years characterized by cyst proliferation and growth and invasion of contiguous spaces leading to mass effect (Stage 1). With induction of the host-immune responses, cysts degenerate leading to a predominately inflammatory arachnoiditis (Stage 2) causing hydrocephalus, infarcts, and other inflammatory based neurological manifestations. Inactive disease (Stage 3) may occur naturally but mostly is a result of successful treatment, which generally requires prolonged intensive anthelminthic and antiinflammatory treatments. Cerebral spinal fluid cestode antigen or cestode DNA falling to nondetectable levels predicts effective treatment. Prolonged treatment with extended follow-up has resulted in moderate disability and no mortality. Repeated short intensive 8-14-day courses of treatment are also used, but long-term outcomes and safety using this strategy are not reported.

SUMMARY

SUBNCC gives rise to a chronic arachnoiditis. Its unique ability to proliferate and induce inflammatory responses requires long-term anthelmintic and antiinflammatory medications.

Single-cell deconstruction of stem-cell-driven schistosome development

Schistosomes cause one of the most devastating neglected tropical diseases, schistosomiasis. Their transmission is accomplished through a complex life cycle with two obligate hosts and requires multiple radically different body plans specialized for infecting and reproducing in each host. Recent single-cell transcriptomic studies on several schistosome body plans provide a comprehensive map of their cell types, which include stem cells and their differentiated progeny along an intricate developmental hierarchy. This progress not only extends our understanding of the basic biology of the schistosome life cycle but can also inform new therapeutic and preventive strategies against the disease, as blocking the development of specific cell types through genetic manipulations has shown promise in inhibiting parasite survival, growth, and reproduction.

Innovations and Advances in Schistosome Stem Cell Research

Schistosomes infect about 250 million people globally causing the devastating and persistent disease of schistosomiasis. These blood flukes have a complicated life cycle involving alternating infection of freshwater snail intermediate and definitive mammalian hosts. To survive and flourish in these diverse environments, schistosomes transition through a number of distinct life-cycle stages as a result of which they change their body plan in order to quickly adapt to each new environment. Current research suggests that stem cells, present in adults and larvae, are key in aiding schistosomes to facilitate these changes. Given the recent advances in our understanding of schistosome stem cell biology, we review the key roles that two major classes of cells play in the different life cycle stages during intramolluscan and intramammalian development; these include the germinal cells of sporocysts involved in asexual reproduction in molluscan hosts and the neoblasts of adult worms involved in sexual reproduction in human and other mammalian hosts. These studies shed considerable new light in revealing the stem cell heterogeneity driving the propagation of the schistosome life cycle. We also consider the possibility and value of establishing stem cell lines in schistosomes to advance schistosomiasis research. The availability of such self-renewable resources will provide new platforms to study stem cell behavior and regulation, and to address fundamental aspects of schistosome biology, reproductive development and survival. In turn, such studies will create new avenues to unravel individual gene function and to optimize genome-editing processes in blood flukes, which may lead to the design of novel intervention strategies for schistosomiasis.

Single-cell analysis of Schistosoma mansoni identifies a conserved genetic program controlling germline stem cell fate

Schistosomes are parasitic flatworms causing one of the most prevalent infectious diseases from which millions of people are currently suffering. These parasites have high fecundity and their eggs are both the transmissible agents and the cause of the infection-associated pathology. Given its biomedical significance, the schistosome germline has been a research focus for more than a century. Nonetheless, molecular mechanisms that regulate its development are only now being understood. In particular, it is unknown what balances the fate of germline stem cells (GSCs) in producing daughter stem cells through mitotic divisions versus gametes through meiosis. Here, we perform single-cell RNA sequencing on juvenile schistosomes and capture GSCs during de novo gonadal development. We identify a genetic program that controls the proliferation and differentiation of GSCs. This program centers around onecut, a homeobox transcription factor, and boule, an mRNA binding protein. Their expressions are mutually dependent in the schistosome male germline, and knocking down either of them causes over-proliferation of GSCs and blocks germ cell differentiation. We further show that this germline-specific regulatory program is conserved in the planarian, schistosome's free-living evolutionary cousin, but the function of onecut has changed during evolution to support GSC maintenance.

The in vivo transcriptome of Schistosoma mansoni in the prominent vector species Biomphalaria pfeifferi with supporting observations from Biomphalaria glabrata

BACKGROUND

The full scope of the genes expressed by schistosomes during intramolluscan development has yet to be characterized. Understanding the gene products deployed by larval schistosomes in their snail hosts will provide insights into their establishment, maintenance, asexual reproduction, ability to castrate their hosts, and their prolific production of human-infective cercariae. Using the Illumina platform, the intramolluscan transcriptome of Schistosoma mansoni was investigated in field-derived specimens of the prominent vector species Biomphalaria pfeifferi at 1 and 3 days post infection (d) and from snails shedding cercariae. These S. mansoni samples were derived from the same snails used in our complementary B. pfeifferi transcriptomic study. We supplemented this view with microarray analyses of S. mansoni from B. glabrata at 2d, 4d, 8d, 16d, and 32d to highlight robust features of S. mansoni transcription, even when a different technique and vector species was used.

PRINCIPAL FINDINGS

Transcripts representing at least 7,740 (66%) of known S. mansoni genes were expressed during intramolluscan development, with the greatest number expressed in snails shedding cercariae. Many transcripts were constitutively expressed throughout development featuring membrane transporters, and metabolic enzymes involved in protein and nucleic acid synthesis and cell division. Several proteases and protease inhibitors were expressed at all stages, including some proteases usually associated with cercariae. Transcripts associated with G-protein coupled receptors, germ cell perpetuation, and stress responses and defense were well represented. We noted transcripts homologous to planarian anti-bacterial factors, several neural development or neuropeptide transcripts including neuropeptide Y, and receptors that may be associated with schistosome germinal cell maintenance that could also impact host reproduction. In at least one snail the presence of larvae of another digenean species (an amphistome) was associated with repressed S. mansoni transcriptional activity.

CONCLUSIONS/SIGNIFICANCE

This in vivo study, emphasizing field-derived snails and schistosomes, but supplemented with observations from a lab model, provides a distinct view from previous studies of development of cultured intramolluscan stages from lab-maintained organisms. We found many highly represented transcripts with suspected or unknown functions, with connection to intramolluscan development yet to be elucidated.

Stem cell heterogeneity drives the parasitic life cycle of Schistosoma mansoni

Schistosomes are parasitic flatworms infecting hundreds of millions of people. These parasites alternate between asexual reproduction in molluscan hosts and sexual reproduction in mammalian hosts; short-lived, water-borne stages infect each host. Thriving in such disparate environments requires remarkable developmental plasticity, manifested by five body plans deployed throughout the parasite’s life cycle. Stem cells in Schistosoma mansoni provide a potential source for such plasticity; however, the relationship between stem cells from different life-cycle stages remains unclear, as does the origin of the germline, required for sexual reproduction. Here, we show that subsets of larvally derived stem cells are likely sources of adult stem cells and the germline. We also identify a novel gene that serves as the earliest marker for the schistosome germline, which emerges inside the mammalian host and is ultimately responsible for disease pathology. This work reveals the stem cell heterogeneity driving the propagation of the schistosome life cycle.

Parasitic flatworms called schistosomes infect around 250 million people, causing the disease schistosomiasis. Schistosomes live complex lives, spending part of their life cycle inside snails and part of it inside mammals; short-lived, water-borne stages infect each of these hosts. To thrive in such different environments, schistosomes go through several life-cycle stages. At each stage the flatworms transition to a new body plan adapted to its new environment. Understanding how these transitions occur could help researchers devise new strategies for eliminating these parasites.

Previous research suggested that stem cells help schistosomes transition to new body plans. Stem cells have the ability to transform into many different cell types, and have been found in schistosome larvae and adults. However, the relationship between the larval and adult stem cells was not clear.

Wang et al. used transcriptional profiling, a technique that measures the genes currently in use in different cells, to study the stem cells in the schistosome species Schistosoma mansoni. This uncovered four types of stem cell, each of which uses a slightly different combination of genes. Examining the behaviour of these cells at different schistosome life-cycle stages revealed that certain larval stem cells produce adult stem cells. Other larval stem cells seem to be the source of the ‘germline’ cells that make gametes (egg and sperm) and allow the parasites to reproduce sexually.

Schistosomes only produce germline cells when they are inside mammals. Wang et al. found that as juvenile flatworms develop inside mouse blood vessels, a gene called eledh becomes active in some of their stem cells. Further investigation showed that this activity is the earliest indicator that germline cells are developing and is also required for proper development of the germline. This knowledge, along with future work to characterize the roles of the stem cell populations identified by Wang et al., could ultimately help researchers develop new ways to stop the spread of schistosomiasis.

Nanomedicine Approaches Against Parasitic Worm Infections

Nanomedicine approaches have the potential to transform the battle against parasitic worm (helminth) infections, a major global health scourge from which billions are currently suffering. It is anticipated that the intersection of two currently disparate fields, nanomedicine and helminth biology, will constitute a new frontier in science and technology. This progress report surveys current innovations in these research fields and discusses research opportunities. In particular, the focus is on: (1) major challenges that helminth infections impose on mankind; (2) key aspects of helminth biology that inform future research directions; (3) efforts to construct nanodelivery platforms to target drugs and genes to helminths hidden in their hosts; (4) attempts in applying nanotechnology to enable vaccination against helminth infections; (5) outlooks in utilizing nanoparticles to enhance immunomodulatory activities of worm-derived factors to cure allergy and autoimmune diseases. In each section, achievements are summarized, limitations are explored, and future directions are assessed.

Definitive Hosts of Versteria Tapeworms (Cestoda: Taeniidae) Causing Fatal Infection in North America

We previously reported fatal infection of a captive Bornean orangutan with metacestodes of a novel taeniid tapeworm, Versteria sp. New data implicate mustelids as definitive hosts of these tapeworms in North America. At least 2 parasite genetic lineages circulate in North America, representing separate introductions from Eurasia.

Oxytocin mediated behavior in invertebrates: An evolutionary perspective

The molecular and functional conservation of oxytocin-related neuropeptides in behavior is striking. In animals separated by at least 600 million years of evolution, from roundworms to humans, oxytocin homologs play critical roles in the modulation of reproductive behavior and other biological functions. Here, we review the roles of oxytocin in invertebrate behavior from an evolutionary perspective. We begin by tracing the evolution of oxytocin through the invertebrate animal lineages, and then describe common themes in invertebrate behaviors that are mediated by oxytocin-related peptides, including reproductive behavior, learning and memory, food arousal, and predator/prey relationships. Finally, we discuss interesting future directions that have recently become experimentally tractable. Studying oxytocin in invertebrates offers precise insights into the activity of neuropeptides on well-defined neural circuits; the principles that emerge may also be represented in the more complex vertebrate brain. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 128-142, 2017.

Biology and Systematics of Echinococcus

The biology of Echinococcus, the causative agent of echinococcosis (hydatid disease) is reviewed with emphasis on the developmental biology of the adult and metacestode stages of the parasite. Major advances include determining the origin, structure and functional activities of the laminated layer and its relationship with the germinal layer; and the isolation, in vitro establishment and characterization of the multipotential germinal cells. Future challenges are to identify the mechanisms that provide Echinococcus with its unique developmental plasticity and the nature of activities at the parasite-host interface, particularly in the definitive host. The revised taxonomy of Echinococcus is presented and the solid nomenclature it provides will be essential in understanding the epidemiology of echinococcosis.

Thermodynamics of egg production, development and hatching in trematodes

Temperature is a key factor influencing the rate of biological processes of ectothermic animals and is intrinsically linked to climate change. Trematode parasites may be potentially susceptible to temperature changes and, in order to develop a predictive framework of their response to climate change, large-scale analyses are needed. In particular, the biology of the egg of all species is at some time influenced by environmental conditions. The present study uses Arrhenius activation energy (E*), a common measure of temperature-mediated reaction rates, to analyse experimental data from the scientific literature on the effects of temperature on the production, development and hatching of trematode eggs. Egg production declines at high temperatures, with habitat-specific climatic factors determining the optimal thermal range. Egg development, as is typical of invertebrates, shows a simple response to temperature, with minimal differences between mid- (35-60°) and low-latitude (<35°) species. Egg hatching demonstrates variable thermodynamics with high E* values at low temperature ranges and thermostability at mid-temperatures, before declining at high temperature ranges, with wide thermostable zones being a common feature. Comparisons between development and hatching indicate that these two parameters demonstrate different thermodynamical responses. The significance of these results in furthering our understanding of trematode egg biology under natural conditions is discussed.

Fatal metacestode infection in Bornean orangutan caused by unknown Versteria species

A captive juvenile Bornean orangutan (Pongo pygmaeus) died from an unknown disseminated parasitic infection. Deep sequencing of DNA from infected tissues, followed by gene-specific PCR and sequencing, revealed a divergent species within the newly proposed genus Versteria (Cestoda: Taeniidae). Versteria may represent a previously unrecognized risk to primate health.

Trematode reproduction in the molluscan host: an ultrastructural study of the germinal mass in the rediae of Himasthla elongata (Mehlis, 1831) (Digenea: Echinostomatidae)

The germinal mass in Himasthla elongata rediae was studied in detail using transmission electron microscopy. It was shown to be a specialized reproductive organ consisting of germinal cells at various maturation stages, supporting cells and stem cells. The germinal mass also contains early cercarial embryos emerging as a result of cleavage division of mature germinal cells. The stem cells that give rise to germinal cells have heterochromatin-rich nuclei with distinct nucleoli and scarce cytoplasm containing mainly free ribosomes and few mitochondria. The differentiating germinal cells undergo a growth, which is accompanied by an emergence of annulate lamellae and the nuage in their cytoplasm, a noticeable development of RER and Golgi apparatus and an increase in the number of mitochondria. The mitochondria form a large group at one of the cell poles. During differentiation, the nucleus and nucleolus of the germinal cell enlarge while the chromatin becomes gradually less condensed. The supporting tissue of the germinal mass is made up of cells connected by septate junctions. These supporting cells are distinctly different in cellular shape and nuclear ultrastructure. Their outgrowths form a tight meshwork housing stem cells, germinal cells and early cercarial embryos. The cytoplasm of the supporting cells in the mesh area is separated into fine parallel layers by labyrinthine narrow cavities communicating with the intercellular space. The supporting tissue contains differentiating and degenerating cells which indicates its renewal. The results of this ultrastructural study lend support to the hypothesis that the germinal cells of digeneans are germ line cells.

Functional genomic characterization of neoblast-like stem cells in larval Schistosoma mansoni

Schistosomes infect hundreds of millions of people in the developing world. Transmission of these parasites relies on a stem cell-driven, clonal expansion of larvae inside a molluscan intermediate host. How this novel asexual reproductive strategy relates to current models of stem cell maintenance and germline specification is unclear. Here, we demonstrate that this proliferative larval cell population (germinal cells) shares some molecular signatures with stem cells from diverse organisms, in particular neoblasts of planarians (free-living relatives of schistosomes). We identify two distinct germinal cell lineages that differ in their proliferation kinetics and expression of a nanos ortholog. We show that a vasa/PL10 homolog is required for proliferation and maintenance of both populations, whereas argonaute2 and a fibroblast growth factor receptor-encoding gene are required only for nanos-negative cells. Our results suggest that an ancient stem cell-based developmental program may have enabled the evolution of the complex life cycle of parasitic flatworms.

DOI:http://dx.doi.org/10.7554/eLife.00768.001

Schistosomiasis—a disease caused by parasitic flatworms known as schistosomes—affects more than 200 million people worldwide, mainly in tropical regions, and in public health importance is second only to malaria (according to the World Health Organization). Chronic infection leads to damage to internal organs, and the disease is responsible for roughly 250,000 deaths each year.

The schistosome parasite has a complex life cycle, and the worms are capable of infecting mammals during just one stage of this cycle. Infection occurs through contact with contaminated freshwater, with the infectious form of the parasite burrowing through skin. Once inside the body, the parasites mature into adults, before reproducing sexually and laying eggs that are excreted by their host back into the water supply.

However, to generate the form of the parasite that can infect mammals, schistosomes must first infect an intermediate host, namely a freshwater snail. When the larval form of the parasite—which cannot infect mammals—enters the snail, the larvae undergo an unusual type of asexual embryogenesis. This results in thousands of parasites that are capable of infecting mammals. Studies suggest that a population of cells known as germinal cells are responsible for this transformation and replication process, but little is known about these cells at the molecular level.

Here, Wang et al. report the gene expression profile of these cells in a species of schistosome, and use RNA-mediated silencing techniques to explore the functions of the genes. This analysis revealed that the germinal cells have a molecular signature similar to that of neoblasts—adult pluripotent stem cells found in free-living flatworms such as planarians. Neoblasts can develop into any cell type in the body, enabling planarians to repair or even replace damaged body parts.

The similarity between neoblasts and germinal cells led Wang et al. to suggest that schistosomes may have evolved their parasitic life cycle partly by adapting a program of development based on stem cells in non-parasitic worms.

DOI:http://dx.doi.org/10.7554/eLife.00768.002

Quantifying the biomass of parasites to understand their role in aquatic communities

By infecting multiple host species and acting as a food resource, parasites can affect food web topography and contribute to ecosystem energy transfer. Owing to the remarkable secondary production of some taxa, parasite biomass – although cryptic – can be comparable to other invertebrate and vertebrate groups. More resolved estimates of parasite biomass are therefore needed to understand parasite interactions, their consequences for host fitness, and potential influences on ecosystem energetics. We developed an approach to quantify the masses of helminth parasites and compared our results with those of biovolume-based approaches. Specifically, we massed larval and adult parasites representing 13 species and five life stages of trematodes and cestodes from snail and amphibian hosts. We used a replicated regression approach to quantify dry mass and compared these values with indirect biovolume estimates to test the validity of density assumptions. Our technique provided precise estimates (R² from 0.69 to 0.98) of biomass across a wide range of parasite morphotypes and sizes. Individual parasites ranged in mass from 0.368 ± 0.041 to 320 ± 98.1 μg. Among trematodes, adult parasites tended to be the largest followed by rediae, with nonclonal larval stages (metacercariae and cercariae) as the smallest. Among similar morphotypes, direct estimates of dry mass and the traditional biovolume technique provided generally comparable estimates (although important exceptions also emerged). Finally, we present generalized length-mass regression equations to calculate trematode mass from length measurements, and discuss the most efficient use of limited numbers of parasites. By providing a novel method of directly estimating parasite biomass while also helping to validate more traditional methods involving length-mass conversion, our findings aim to facilitate future investigations into the ecological significance of parasites, particularly with respect to ecosystem energetics. In addition, this novel technique can be applied to a wide range of difficult-to-mass organisms.

Testing local-scale panmixia provides insights into the cryptic ecology, evolution, and epidemiology of metazoan animal parasites

When every individual has an equal chance of mating with other individuals, the population is classified as panmictic. Amongst metazoan parasites of animals, local-scale panmixia can be disrupted due to not only non-random mating, but also non-random transmission among individual hosts of a single host population or non-random transmission among sympatric host species. Population genetics theory and analyses can be used to test the null hypothesis of panmixia and thus, allow one to draw inferences about parasite population dynamics that are difficult to observe directly. We provide an outline that addresses 3 tiered questions when testing parasite panmixia on local scales: is there greater than 1 parasite population/species, is there genetic subdivision amongst infrapopulations within a host population, and is there asexual reproduction or a non-random mating system? In this review, we highlight the evolutionary significance of non-panmixia on local scales and the genetic patterns that have been used to identify the different factors that may cause or explain deviations from panmixia on a local scale. We also discuss how tests of local-scale panmixia can provide a means to infer parasite population dynamics and epidemiology of medically relevant parasites.

An infectious topic in reticulate evolution: introgression and hybridization in animal parasites

Little attention has been given to the role that introgression and hybridization have played in the evolution of parasites. Most studies are host-centric and ask if the hybrid of a free-living species is more or less susceptible to parasite infection. Here we focus on what is known about how introgression and hybridization have influenced the evolution of protozoan and helminth parasites of animals. There are reports of genome or gene introgression from distantly related taxa into apicomplexans and filarial nematodes. Most common are genetic based reports of potential hybridization among congeneric taxa, but in several cases, more work is needed to definitively conclude current hybridization. In the medically important Trypanosoma it is clear that some clonal lineages are the product of past hybridization events. Similarly, strong evidence exists for current hybridization in human helminths such as Schistosoma and Ascaris. There remain topics that warrant further examination such as the potential hybrid origin of polyploid platyhelminths. Furthermore, little work has investigated the phenotype or fitness, and even less the epidemiological significance of hybrid parasites.

Morphological study of anomalous "laminated" brood capsules in cystic echinococcosis in humans and sheep

"Anomalous", as yet unreported forms of brood capsules (BC) in cystic echinococcosis larvae from humans and sheep are described, thought to be the likely effect of degenerative involution of the parasite. Morphological examination showed an inner laminated layer (LL) in the brood capsules in intimate contact with the internal wall surface, suggesting the term "laminated brood capsules" (LBC). Analysis conducted on LBC from cysts of different typology invariably revealed them to contain just dead and highly degenerated protoscoleces (PSC). Structural and ultrastructural aspects of these peculiar forms are described and possible processes of formation discussed. Comparison of germinal layer (GL), BC wall and PSC organization recognize a "tegumental" and a "non-tegumental" pole, coinciding in GL and PSC and inverted in BC. A quite reversible PSC-or-LL switch governed by complex mechanisms is proposed, of which PSC viability/death is assumed to be an essential part, that under certain conditions makes in vivo LL production inwards possible in BC.

An ultrastructural study of reproduction in the sporocysts of Prosorhynchoides gracilescens (Digenea: Bucephalidae)

The germinal development in Prosorhynchoides gracilescens sporocysts was studied using electron microscopy. The germinal cells proliferated and developed within multiple floating germinal masses located in the terminal portions of sporocyst branches. The germinal masses were composed of supporting and germinal cells. Supporting cells possessed numerous flattened extensions that spread around and between developing germinal cells to form three-dimensional mesh network, which maintained the integrity of the germinal mass. Morphological evidences of close interactions between supporting and germinal cells were numerous gap junctions between the two cell types and phagocytosis of small fragments of germinal cells cytoplasm by supporting cells. The germinal cells displayed structural differences that seemed to reflect their sequential developmental changes. These changes included (1) cell growth and increase of organelles number, (2) dispersion of nuclear chromatin and increase of nucleolus size, (3) polarization of the cell, (4) appearance of specific structures such as nuage and laminated inclusions. The germinal cells left the germinal masses to finish their differentiation in the body cavity and then cleaved to give rise to cercarial embryos. Ultrastructural features of the germinal elements of P. gracilescens sporocysts are discussed in the light of existing controversy concerning the nature of the germinal sacs reproduction.

Recent studies on the reproductive biology of the schistosomes and their relevance to speciation in the Digenea

The members of the family Schistosomatidae, dioecious Digenea, are discussed with regard to their distribution, intermediate and definitive host-parasite relationships. The biological species concept is considered together with the difficulties of its application to Schistosoma spp. and the Digenea. The correlation between pairing of adult schistosomes, physical and sexual development and the maintenance of reproductive potential is emphasised. Development of the female reproductive system does not depend upon species-specific pairing. In some combinations, e.g., Schistosoma haematobium/Schistosoma intercalatum and Schistosoma bovis/Schistosoma curassoni, a specific mate choice system apparently does not exist, whereas it does in other combinations, e.g., Schistosoma mansoni/Schistosoma intercalatum. In mixed infections change of mate may occur and when the opportunity arises heterospecific pairs of worms will change partners to conspecific pairs. Interspecific pairing in adult schistosomes will lead to either hybridisation or parthenogenesis. Yet the majority of schistosomes that inhabit the same definitive host maintain their genetic identity: specific mate recognition, site selection within the host and heterologous immunity have been suggested as isolating mechanisms. Experimental intraspecific crosses have enabled evaluation of the degree to which some populations separated and became reproductively isolated through pre-mating isolating mechanisms, indicative of incipient speciation, e.g., the Lower Guinea and Zaire strains of S. intercalatum. The occurrence and significance of parthenogenesis in schistosomes and other species of Digenea are discussed. The consequences of interspecific mating interactions in schistosomes with regard to parasite epidemiology, interspecific competition and genetic heterogeneity are debated. Geographical isolation and host specificity represent important pre-zygotic isolating mechanisms. It is suggested that site selection within the host and heterologous immunity may both reduce interspecific genetic interchange when digenean parasites utilise the same definitive host.

The early stages of Echinococcus granulosus development

The paper points out the essential aspects of host/parasite relationship during the development of oncospheres (ONC) of Echinococcus granulosus into an early cyst. Secretions of the penetration glands in the hatched ONC causes lysis of host tissue during penetration of the activated ONC. It also protects the parasite against the host's immune response while developing the laminated layer. The microvilli, compressed under the plasma membrane in the hatched ONC, are uplifted in the activated larvae; they increase in number and size and are substituted by short and truncated microtriches in metacestodes 3 days old. At that time appears the first lamination of the laminated layer that surrounds the metacestodes as an electron-dense matrix composed of fine microfibrillated material and remnants of sloughed microvilli. The second lamination appears by day 6-8 and is more electron-dense than the first one. The laminated layer is of parasite origin, formed of a series that emanates from the germinal membrane. It is suggested that Vg secretory vesicles, that are elaborated in the perikaryon of the germinal zone and continuously carried to the syncytium via the microtubular-cytoskeleton, are responsible for the laminated layer formation and are involved in initial evasion of the immune response of the host. The cyclical production of laminations could be necessary to create layers that can ultimately be sloughed off as the cyst grows and serve to divert the host cellular response to the parasite.

Hepatic tissue culture model for study of host-parasite interactions in alveolar echinococcosis

An in vitro model for growth and differentiation of the metacestode tissue of the tapeworm Echinococcus multilocularis is described. This model simulates the organotropism of the parasite toward the liver of the intermediate host. In the presence of collagen-embedded primary hepatocytes from rats and humans, which can be kept in culture for 2 to 3 months, the parasitic vesicles grew by exogenous budding and multiplied about 12-fold within 3 weeks. In contrast, without the hepatocytes, the metacestodes rapidly degenerated. Development of protoscolices was seen only in the presence of rat hepatocytes but not in coculture of the metacestodes with hepatocytes of human origin, thus reflecting the in vivo situation during infection of rodents and in alveolar echinococcosis in humans. The experiments indicated that growth of the metacestodes and development of protoscolices depended on soluble low-molecular-weight factors released by the hepatocytes. The in vitro-grown metacestodes did not differ morphologically from the larvae found in infected intermediate hosts, and their infectivity was completely maintained. This report describes the first in vitro model of alveolar echinococcosis and will be the basis for future studies on host-parasite interactions of this important zoonosis.

Parthenogenesis in Schistosomatidae

Among which species, in what situations and how, does parthenogenesis occur in the biology of reproduction of schistosomes? Here, José Jourdane, Daniéle Imbert-Establet and Louis Albert Tchuem Tchuenté review the literature on parthenogenesis in schistosomes, and debate the evolutionary aspects of this type of reproduction.

Interspecific stimulation of parthenogenesis in Schistosoma intercalatum and S. mansoni

Experimental studies of mating behaviour of Schistosoma intercalatum and Schistosoma mansoni in mixed infections in mice showed that in simultaneous infections, without the possibility of choice of mate, heterologous pairing occurs readily. The paired females reach sexual maturity, are inseminated and lay parthenogenetic eggs. Miracidia originating from the S. mansoni male x S. intercalatum female are non infective to either Biomphalaria glabrata or Bulinus forskalii, whereas those from the reverse cross show a very low infectivity to only B. glabrata. The resulting haploid male cercariae also show a very low infectivity to mice (1.1%) and consequently only a very small number of adult worms develop. It appears from this study, on chromosomal and electrophoretic evidence, that generative (haploid) parthenogenesis occurs in S. mansoni females paired with S. intercalatum males.

Parthenogenesis in the genus Schistosoma: electrophoretic evidence for this reproduction system in S. japonicum and S. mansoni

Mixed infections with Schistosoma japonicum and S. mansoni were carried out in mice. S. japonicum females paired with S. mansoni males developed normally and produced numerous viable eggs; very little sperm was found in the female genital tract. The eggs yielded many miracidia infective to Oncomelania hupensis, the host of S. japonicum. Cercariae arising from miracidia developed into male worms with an electrophoretic pattern of malate dehydrogenase (MDH) corresponding only to the maternal species S. japonicum. S. mansoni females paired with S. japonicum produced few viable eggs; sperm was found in the female genital tract. Miracidia hatched from some of these eggs were infective to Biomphalaria glabrata, the host of S. mansoni. Cercariae arising from miracidia developed into female worms with an electrophoretic pattern of MDH typical of the maternal species S. mansoni. It was concluded that S. japonicum females paired with S. japonicum males reproduce parthenogenetically. Parthenogenesis in schistosomes is discussed.

New models for new perspectives in the biology of senescence

This essay presents examples of life history patterns that are not often discussed. Life spans range nearly a million-fold between different species of higher animals and plants and must be species characteristics under considerable genetic control. A comparative approach to senescence reveals a vast variety in temporal organization, both among species as well as between and within populations that may vary over as large ranges of scale and qualitative characteristics as do morphological and biochemical variations. Species comparisons across many levels of biological organization involving the life histories of many species besides the usual few mammals, insects, and nematodes importantly expand the view of mechanisms that limit life spans. The presumption that age-correlated changes are all adverse to some degree and that most components of an organism should decay as the life span is approached is reevaluated.

Echinococcus: biology and strain variation

Biology and strain variation in the causative agent of hydatid disease is reviewed with emphasis on developmental and genetic aspects. In vitro cultivation experiments have made a significant contribution to current knowledge of the developmental plasticity of Echinococcus. However, the mechanisms which regulate and determine developmental strategies in the parasite, as well as the characteristics, source and cytodifferentiation of germinal cells, are not understood. The nature, significance and origin of strain variation in Echinococcus are examined. Before we can fully appreciate the phenotypic consequences of genetic differentiation between populations, we need to know something about the genetic and environmental components of variation in traits such as development rate, host preference, host specificity, virulence and drug resistance. There is an urgent need for research on the developmental pathways by which genetic differences within and between strains of E. granulosus are translated to phenotypic differences in these traits.

Genetic exchange and evolutionary relationships in protozoan and helminth parasites

The study of genetic exchange systems and the use of genetic analysis has been relatively limited in parasites leading to considerable gaps in our basic knowledge. This lack of knowledge makes it difficult to draw firm conclusions as to how these systems evolved. An additional problem is also raised by the difficulties in defining evolutionary distances particularly with the unicellular protozoa, using classical ultrastructural and cytological criteria. While these difficulties have by no means been overcome, the use of rapid sequencing techniques applied to the ribosomal genes has allowed measurement of evolutionary distances, and considerable advances in our understanding of the genetic exchange systems in a few parasitic protozoa have recently been made. The conclusions from these recent sets of analyses are reviewed and then examined together in order to discuss the evolution of genetic exchange systems in parasitic protozoa. The evolutionary distances defined by ribosome sequence analysis show that parasites are an extremely divergent group, with distances which, in some cases, are orders of magnitude greater than the distances between mammals and fish; furthermore these studies suggest that the parasitic protozoa or their free-living ancestors are extremely ancient. These findings support the view that parasitism has occurred independently many times and that the parasitic life-style has been adopted by evolutionarily distinct groups. The recent observation of a non-obligatory genetic system in the diploid but evolutionary ancient kinetoplastid Trypanosoma brucei suggests that diploidy and meiosis are extremely old. The observation, in parasitic protozoa and helminths, that selfing or non-obligatory mating is a common feature suggests that these processes may be strategies to overcome the cost of meiosis. In this context, the question of what selective forces maintain genetic exchange is discussed.

Schistosoma mansoni: vitelline gland development in females from single sex infections

Vitelline gland development and incomplete parthenogenesis in 65-week-old female Schistosoma mansoni from single sex infections is described. Vitelline gland development was restricted to the extreme posterior end of each worm and the developed vitelline lobules showed the normal sequence of development and maturation. The ovary, however, was always in an immature state. Egg production occurred although the eggs were non-viable. The results suggest that, whilst male stimulation is not necessary to initiate vitelline gland development and egg production, it appears to be essential for the complete maturation of the ovary. The possible importance of ovarian maturation to the production of viable eggs is discussed.

Replicating helminth parasites of man

Most helminth parasites of man are unable to replicate within the human host. Thus, the worm burden of an infected person (on which the pathology largely depends - see Box 1) is a function of the number of infective forms to which the person is exposed. But for some species of helminths, the ability to replicate in man has a marked effect on the course and duration of infection, and for the pathogenesis of disease. In this review, David Grove discusses the mechanisms by which such replication may occur, and considers how this ability affects our approach to therapy and control.

Suppression of Taenia crassiceps during concurrent infections with Mesocestoides corti in mice

Parasite burden (by volume) was measured in female mice of three strains given single or concurrent infections of Mesocestoides corti and Taenia crassiceps. Significant suppression of T. crassiceps volume was observed over a range of M. corti inocula and occurred irrespective of whether M. corti was introduced before or after infection with T. crassiceps. Suppression of T. crassiceps volume was greatest with larger inocula of M. corti and with increased duration of M. corti infection. No significant difference was found in the intraperitoneal volume of M. corti from mice given single or simultaneous concurrent infections of M. corti and T. crassiceps. When M. corti were inoculated into mice with an established infection of T. crassiceps a significantly smaller volume of M. corti was retrieved at post-mortem from the peritoneal cavities of these mice, than from mice given just M. corti.