Mating behaviour in Schistosomes: Are paired worms always faithful?

Parent-of-Origin-Dependent Gene Expression in Male and Female Schistosome Parasites

Schistosomes are the causative agents of schistosomiasis, a neglected tropical disease affecting over 230 million people worldwide. Additionally to their major impact on human health, they are also models of choice in evolutionary biology. These parasitic flatworms are unique among the common hermaphroditic trematodes as they have separate sexes. This so-called “evolutionary scandal” displays a female heterogametic genetic sex-determination system (ZZ males and ZW females), as well as a pronounced adult sexual dimorphism. These phenotypic differences are determined by a shared set of genes in both sexes, potentially leading to intralocus sexual conflicts. To resolve these conflicts in sexually selected traits, molecular mechanisms such as sex-biased gene expression could occur, but parent-of-origin gene expression also provides an alternative. In this work we investigated the latter mechanism, that is, genes expressed preferentially from either the maternal or the paternal allele, in Schistosoma mansoni species. To this end, transcriptomes from male and female hybrid adults obtained by strain crosses were sequenced. Strain-specific single nucleotide polymorphism (SNP) markers allowed us to discriminate the parental origin, while reciprocal crosses helped to differentiate parental expression from strain-specific expression. We identified genes containing SNPs expressed in a parent-of-origin manner consistent with paternal and maternal imprints. Although the majority of the SNPs was identified in mitochondrial and Z-specific loci, the remaining SNPs found in male and female transcriptomes were situated in genes that have the potential to explain sexual differences in schistosome parasites. Furthermore, we identified and validated four new Z-specific scaffolds.

Barcoding hybrids: heterogeneous distribution of Schistosoma haematobium × Schistosoma bovis hybrids across the Senegal River Basin

Hybridization events between Schistosoma species (Digenea, Platyhelminthes) are reported with increasing frequency, largely due to improved access to molecular tools. Nevertheless, little is known about the distribution and frequency of hybrid schistosomes in nature. Screening for hybrids on a large scale is complicated by the need for nuclear and mitochondrial sequence information, precluding a 'simple' barcoding approach. Here we aimed to determine and understand the spatiotemporal distribution of Schistosoma haematobium × Schistosoma bovis hybrids in the Senegal River Basin. From ten villages, distributed over the four main water basins, we genotyped a total of 1236 schistosome larvae collected from human urine samples using a partial mitochondrial cox1 fragment; a subset of 268 parasites was also genotyped using ITS rDNA. Hybrid schistosomes were unevenly distributed, with substantially higher numbers in villages bordering Lac de Guiers than in villages from the Lampsar River and the Middle Valley of the Senegal River. The frequency of hybrids per village was not linked with the prevalence of urinary schistosomiasis in that village. However, we did find a significant positive association between the frequency of hybrids per village and the prevalence of Schistosoma mansoni. We discuss the potential consequences of adopting a barcoding approach when studying hybrids in nature.

Getting a GRiPP on everyday schistosomiasis: experience from Zimbabwe

Schistosomiasis, commonly known as bilharzia, is a parasitic disease prevalent in Africa, Asia and South America. The majority of the cases occur in Sub-Saharan Africa where schistosomiasis is a major public health problem impacting on child health and development as well as adult health when infections become chronic. Control of schistosomiasis is by treatment of infected people with the antihelminthic drug praziquantel. Current schistosome control programmes advocated by the World Health Assembly in 2001 are aimed at regular school-based integrated deworming strategies in order to reduce development of severe morbidity, promote school health and to improve cognitive potential of children. Several countries in Africa have now embarked on national scale deworming programmes treating millions of children exposed to schistosomiasis in endemic areas without prior diagnosis of infection through mass drug administration programmes. Implementing such control programmes requires a concerted effort between scientists, policy makers, health practitioners and several other stake holders and of course a receptive community. This paper considers the contributions to global schistosome control efforts made by research conducted in Zimbabwe and the historical context and developments leading to the national schistosomiasis control programme in Zimbabwe giving an example of Getting Research into Policy and Practice.

Modelling the effects of mass drug administration on the molecular epidemiology of schistosomes

As national governments scale up mass drug administration (MDA) programs aimed to combat neglected tropical diseases (NTDs), novel selection pressures on these parasites increase. To understand how parasite populations are affected by MDA and how to maximize the success of control programmes, it is imperative for epidemiological, molecular and mathematical modelling approaches to be combined. Modelling of parasite population genetic and genomic structure, particularly of the NTDs, has been limited through the availability of only a few molecular markers to date. The landscape of infectious disease research is being dramatically reshaped by next-generation sequencing technologies and our understanding of how repeated selective pressures are shaping parasite populations is radically altering. Genomics can provide high-resolution data on parasite population structure, and identify how loci may contribute to key phenotypes such as virulence and/or drug resistance. We discuss the incorporation of genetic and genomic data, focussing on the recently sequenced Schistosoma spp., into novel mathematical transmission models to inform our understanding of the impact of MDA and other control methods. We summarize what is known to date, the models that exist and how population genetics has given us an understanding of the effects of MDA on the parasites. We consider how genetic and genomic data have the potential to shape future research, highlighting key areas where data are lacking, and how future molecular epidemiology knowledge can aid understanding of transmission dynamics and the effects of MDA, ultimately informing public health policy makers of the best interventions for NTDs.

The sex lives of parasites: investigating the mating system and mechanisms of sexual selection of the human pathogen Schistosoma mansoni

The mating systems of internal parasites are inherently difficult to investigate although they have important implications for the evolutionary biology of the species, disease epidemiology, and are important considerations for control measures. Using parentage analyses, three topics concerning the mating biology of Schistosoma mansoni were investigated: the number of mates per adult male and female, variance in reproductive success among individuals, and the potential role for sexual selection on male body size and also mate choice for genetically dissimilar individuals. Results indicated that schistosomes were mostly monogamous, and evidence of only one mate change occurred over a period of 5-6 weeks. One male was polygynous and contained two females in its gynecophoral canal although offspring were only detected for one of the females. Even though they were primarily monogamous and the sex ratio near even, reproductive success was highly variable, indicating a potential role for sexual selection. Male body size was positively related to reproductive success, consistent with sexual selection via male-male competition and female choice for large males. However, relatedness of pairs was not associated with their reproductive success. Finally, genetically identical individuals differed significantly in their reproductive output and identical males in their body size, indicating important partner and environmental effects on these traits.

Mating interactions between Schistosoma haematobium and S. mansoni

Schistosoma haematobium and S. mansoni are two medically important schistosomes, commonly occurring sympatrically in Africa and so potentially able to infect the same human host. Experiments were designed to study the mating behaviour of these two species in mixed infections in hamsters. Analysis of the data obtained showed that both heterospecific and homospecific pairs readily form. No significant difference was seen between the two species in their ability in forming pairs, however, S. mansoni showed a greater homospecific mate preference. Analysis of the data using the Mantel-Haenszel test suggests that mating competition does occur between S. haematobium and S. mansoni, the former being the more dominant species. Both species appeared to be able to change mate, with S. haematobium showing a greater ability in taking S. mansoni females away from S. mansoni males when introduced into a pre-established S. mansoni infection highlighting the competitiveness of S. haematobium. The significance of the results is discussed in relation to the epidemiological consequences occurring in Senegal, and other areas where both species are sympatric.

INCESTUOUS MATE PREFERENCE BY A SIMULTANEOUS HERMAPHRODITE WITH STRONG INBREEDING DEPRESSION

Inbreeding depression and its consequences for mate choice have been extensively studied in free-living animals. However, very little is known about its significance for parasites, although it is well recognized that the mating systems of parasites can have important implications for their epidemiology and evolution. In this article, we show that the cestode Schistocephalus solidus shows incestuous mate preference despite evidence for very strong inbreeding depression. When given the simultaneous choice between mating with a sibling and an unrelated partner, on average, the cestode preferred its sibling. To explain this surprising result, we present three hypotheses that suggest different benefits to fitness of incestuous mating, which could, alone or in concert, outweigh the cost of inbreeding depression.

Diversification, dioecy and dimorphism in schistosomes

In addition to causing one of the great neglected diseases of the world, schistosomes have unusual biological features that further command attention, including their habit of living in the blood of their hosts and the presence of separate, dimorphic sexes. Phylogenetic studies including a more complete sampling of pivotal and rare schistosomes and their relatives, provide an improved framework for interpreting schistosome biology. From such studies, it is inferred that schistosomes are exclusively parasites of endotherms. It is argued that a commitment to life in the endothermic hepatic portal system favored a filiform body form for egg deposition and led to the emergence of dioecy. Schistosome sexual dimorphism and mating systems have subsequently been influenced by the duration of opportunities for transmission and by the nature of the vascular habitats in which they live. A comparative perspective provides valuable insights for interpreting the biology of schistosomes, including the species that cause disease in humans.

Test of pangamy by genetic analysis of Schistosoma mansoni pairs within its natural murine host in Guadeloupe

Mating system plays a determinant role in the maintenance and distribution of genetic variations. It can be assessed indirectly by analyzing the distribution of the genetic variability within populations or directly by considering how mating pairs are formed. In the present study, 71 pairs of adult Schistosoma mansoni worms sampled from naturally infected rats were genotyped to investigate how male and female schistosomes paired according to their genetic relatedness. Among all samples, pangamy, the random association between males and females, could not be rejected. Whereas the schistosome mating system has been intensively studied under experimental conditions, to the best of our knowledge, our study is the first to attempt to understand the way in which males and females pair in natural conditions.

Interactions between Schistosoma haematobium and Schistosoma mansoni in humans in north Cameroon

OBJECTIVES

To analyse the relationships between the frequency of ectopic localizations of Schistosoma haematobium and S. mansoni eggs.

METHODS

Studies were conducted in 11 villages in north Cameroon, around Bessoum, a village where an epidemic of bloody diarrhoea caused by S. mansoni occurred in 1997.

RESULTS

The results revealed infection prevalence rates of 70.5% for S. haematobium and 30.8% for S. mansoni. Interestingly, S. mansoni eggs were found in 14.5% of the urine samples and S. haematobium eggs in 3% of the stool samples. These ectopic eliminations of schistosome eggs resulted from sexual interactions between the two species of schistosomes, and from a spill-over of high infection loads. The clinical study showed that the morbidity was lower in individuals with mixed infections and high loads of S. haematobium than in those with S. mansoni infections only, suggesting a possible lowering effect of S. haematobium infection on S. mansoni morbidity.

DISCUSSION

The results obtained in human populations are discussed in relation to the known schistosome interspecific interactions in animal models.

Sexual Biology of Schistosomes

Schistosomes are unusual, together with some of the didymozoidae, in that they are dioecious instead of being hermaphrodite. This gonochorism is accompanied with morphological, ecological, behavioural and molecular differences between the male and the female parasites all through their life cycle. This review is an overview of the sexual biology of schistosomes and aims to provide the most recent information that may help to build future control strategies against these parasites. It proposes a new view of the life cycle of schistosomes, taking into account the sexual status of each developmental stage. It presents the relevant information available on the genetic and phenotypic sexual dimorphisms of these parasites; it proposes a comparison between the host-male parasite and the host-female parasite interactions in both the molluscan intermediate and the mammalian definitive hosts; it exposes the male-female parasite interactions that exist in both the mollusc and the mammalian hosts at the parasite individual and populational levels. This review highlights the domains of research that are still unexplored but that would be of great interest for a better knowledge of the sexual way of life of the parasites which are still responsible for one of the most important human parasitic diseases in the world.

Schistosome male-female interaction: induction of germ-cell differentiation

Male and female schistosomes are permanently paired while they live in the bloodstream of their vertebrate hosts. Female schistosomes produce eggs only when they are in intimate association with a male. Here, I combine classical cytological knowledge about the cellular processes in the female that are affected by the male with recent molecular results that are beginning to allow speculation about the signalling events involved.

Schistosoma japonicum: interactions of successive infections in pigs and mice using polymerase chain reaction-based identification of individual worms

The study reported here investigated acquired resistance of mice and pigs to challenge-infections with Schistosoma japonicum. Two morphologically indistinguishable isolates of the parasite (from the Anhui and Zhejiang provinces of China), which could be typed by polymerase chain reaction-linked restriction fragment length polymorphism analysis (PCR-RFLP), were used for the infections. In two parallel infection studies, 60 female outbred NMRI mice and 29 Danish Landrace/Yorkshire/Duroc crossbred pigs were used. Two of the groups received a primary infection with either the Anhui or the Zhejiang isolate, respectively. The remaining groups received a primary infection with the Zhejiang isolate and challenge-infections with the Anhui isolate at either week 2, 3, 4 or 6 post primary infection. The results of the study indicated that both mice and pigs are partially resistant to challenge-infection from week 4 post primary infection. Resistance appeared to decrease in pigs 6 weeks after primary infection, while it remained effective in mice. These results suggest that the mechanism responsible for acquired resistance in mice and pigs may not be the same and support the theory that worm burdens in pigs receiving repeated infection are in a balance between acquisition and loss of worms.

Elucidation of Schistosoma japonicum population dynamics in pigs using PCR-based identification of individuals representing distinct cohorts

The study reported here investigated the interactions of successive infections and acquired resistance of pigs to challenge infections of Schistosoma japonicum. Two morphologically indistinguishable geographical isolates from China (from Anhui and Zhejiang provinces) were used for the infections. The worms of the two isolates were distinguishable by PCR-linked restriction fragment length polymorphism analysis of the nicotinamide adenine dinucleotide phosphate dehydrogenase I gene of the mitochondrial genome. Thirty-two pigs divided into seven groups were used in the experiment. Two groups received a single infection by either the Anhui or the Zhejiang isolate. In Challenge Groups 1, 4, 6, 8 and 12, a primary infection of the Zhejiang isolate was followed by a challenge infection with the Anhui isolate at week 1, 4, 6, 8 or 12 after the primary infection. In this way it was possible to determine whether worms recovered by perfusion originated from the primary or the challenge infection. Only the challenge infection at week 1 resulted in a higher worm burden when compared with a single primary infection with the Zhejiang isolate. The results showed that challenge worms were able to establish, and that the proportion of worms originating from challenge infection increased at the later challenge infections, however without an increase in the total number of worms. In addition, mixed pairs of the two isolates were found in all challenge-infected groups. The results indicate that pigs are able to mount a partial resistance against re-infection with S. japonicum by 4 weeks after a primary infection, but that worms of the challenge infections eventually replace the primary infection. The finding of mixed pairs of the two isolates indicates that worms of S. japonicum are either polygamous or able to wait in solitude for up to 12 weeks for a partner.

Chemical communication and mate attraction in echinostomes

Mate attraction is widespread among animals and appears to facilitate mating and to prevent hybridisation between closely related species. In this study we investigated mate preference between two geographical isolates of Echinostoma caproni (Trematoda, Platyhelminth) and another species of the genus Echinostoma E. sp. Because previous experiments showed a partial reproductive isolation between echinostome isolates, we examined the possibility that such isolation resulted from differential mate attraction. We compared intra-isolate, inter-isolate and interspecific pairings using two in vitro experimental designs. In the first experiment we compared mate attraction of two individuals belonging to or not belonging to the same isolate, while in the second experiment we examined mate choice when individuals were in the presence of individuals from both the same isolate and from a different isolate or a different species. Distances between worms were measured over a period of 90 min. Results from both experiments suggested that mate attraction was similar for intra-isolate, inter-isolate or interspecific combinations. This lack of mate preference in vitro would therefore support an alternative hypothesis of a reproductive isolation through sperm selection.

Increase of intestinal schistosomiasis after praziquantel treatment in a Schistosoma haematobium and Schistosoma mansoni mixed focus

The recent emergence of a mixed focus of Schistosoma haematobium-Schistosoma mansoni, in the lower delta of the Senegal river, requires adapted control programmes. A mass treatment with praziquantel was organised in April 1994 by local authorities in three villages where populations had been examined. A total of 2042 subjects participated. In Savoigne S. haematobium prevailed (53% for prevalence), in Diagambaly S. haematobium (64%) and S. mansoni (76%) were both abundant, and in Boundoum S. mansoni prevailed (53%). Therapeutic coverage (80%) was assessed on a representative sample. A cohort of 968 treated subjects were followed-up 40, 100, 200 and 300 days after treatment. Six weeks after treatment, the average of egg excretion decreased by 95% for S. haematobium, ranging from 23 to one egg(s)/10 ml at Savoigne and from 14 to one egg(s)/10 ml at Diagambal. Conversely, egg excretion only decreased by 75% for S. mansoni, from 23 to six eggs/g at Boundoum and from 69 to 16 eggs/g at Diagambal, showing evidence of the low susceptibility of S. mansoni local strain to praziquantel. Ten months after treatment, reinfections with S. haematobium remained weak at Savoigne (two eggs/10 ml) while those with S. mansoni were so high at Boundoum (24 eggs/g) that they compensated the reduction of load induced by the treatment. At Diagambal, where the two parasites were present before treatment, the disappearance of the urinary schistosomiasis after treatment concurred with a dramatic increase of intestinal schistosomiasis. S. manoni egg excretion was seven times higher than before treatment (478 eggs/g). These different effects of treatment are discussed according to the ecology of transmission in the three villages.

PCR-based identification of individuals of Schistosoma japonicum representing different subpopulations using a genetic marker in mitochondrial DNA

A mitochondrial NADH dehydrogenase I gene fragment (NDI) was sequenced for three laboratory maintained isolates of Schistosoma japonicum. Comparison of sequences representing the isolates (originally obtained from the Anhui and Zhejiang provinces of the People's Republic of China, and from the Philippines) revealed inter-isolate sequence variations of 0.2-0.6% and no intra-isolate variation was found. The sequences also indicated that while the amplification products of the Zhejiang and Philippine isolates contained a recognition site for the endonuclease RsaI, there was no such site in the Anhui isolate. This was tested by digesting amplification products from a number of individual worms with RsaI. Then an infection experiment was designed to test the value of this genetic marker for studies of the population biology of S. japonicum in the final host. For this, the two Chinese isolates were used. Three groups of mice (A-C) were exposed firstly to a primary infection and then challenge-infected at weeks 4 and 7 of the experiment. In group A the first infection was done with the Anhui isolate, and the two others with the Zhejiang isolate, thereby providing a specific, detectable cohort. In groups B and C the Anhui isolate was used for the second and third infection. All mice were perfused 5 weeks after the last challenge infection, and the NDI was subsequently amplified from DNA of the perfused worms and digested with RsaI. The digestion revealed that while infection groups A and B contained mixed populations of the Anhui and Zhejiang isolates, only Zhejiang worms were present in group C. We concluded that the absence/presence of the RsaI site in the NDI provides a useful marker for the delineation of cohorts of S. japonicum.

Sex determination and female reproductive development in the genus Schistosoma: a review

Parasites of the genus Schistosoma were among the first metazoans to develop separate sexes, which is chromosomally determined in the fertilized egg. Despite the occurrence of specific sex chromosomes, the females of most Schistosomatidae species do not complete their somatic development and reach no sexual maturity without the presence of males. Indeed, the most controversial and at the same time most fascinating aspect about the sexual development of Schistosoma females lies on discover the nature of the stimulus produced by males that triggers and controls this process. Although the nature of the stimulus (physical or chemical) is a source of controversy, there is agreement that mating is a necessary requirement for maturation to occur and for migration of the female to a definitive final site of residence in the vascular system of the vertebrate host. It has also been proposed that the stimulus is not species-specific and, in some cases, not even genus-specific. Despite a vast literature on the subject, the process or processes underlying the meeting of males and females in the circulatory system have not been determined and as yet no consensus exists about the nature of the stimulus that triggers and maintains female development. In the studies about their role, Schistosoma males have been considered, at times pejoratively, the brother, the muscles or even the liver of females. Indeed, it still remains to be determined whether the stimulus responsible for female maturation involves the transfer of hormones, nutrients, neuromediators, mere tactile stimulation or a combination of chemotactic and thigmotactic factors.