Genetic diversity of Schistosoma mansoni within and among individual hosts (Rattus rattus): infrapopulation differentiation at microspatial scale

Molecular approaches reveal weak sibship aggregation and a high dispersal propensity in a non-native fish parasite

Inferring parameters related to the aggregation pattern of parasites and to their dispersal propensity are important for predicting their ecological consequences and evolutionary potential. Nonetheless, it is notoriously difficult to infer these parameters from wildlife parasites given the difficulty in tracking these organisms. Molecular-based inferences constitute a promising approach that has yet rarely been applied in the wild. Here, we combined several population genetic analyses including sibship reconstruction to document the genetic structure, patterns of sibship aggregation, and the dispersal dynamics of a non-native parasite of fish, the freshwater copepod ectoparasite Tracheliastes polycolpus. We collected parasites according to a hierarchical sampling design, with the sampling of all parasites from all host individuals captured in eight sites spread along an upstream-downstream river gradient. Individual multilocus genotypes were obtained from 14 microsatellite markers, and used to assign parasites to full-sib families and to investigate the genetic structure of T. polycolpus among both hosts and sampling sites. The distribution of full-sibs obtained among the sampling sites was used to estimate individual dispersal distances within families. Our results showed that T. polycolpus sibs tend to be aggregated within sites but not within host individuals. We detected important upstream-to-downstream dispersal events of T. polycolpus between sites (modal distance: 25.4 km; 95% CI [22.9, 27.7]), becoming scarcer as the geographic distance from their family core location increases. Such a dispersal pattern likely contributes to the strong isolation-by-distance observed at the river scale. We also detected some downstream-to-upstream dispersal events (modal distance: 2.6 km; 95% CI [2.2-23.3]) that likely result from movements of infected hosts. Within each site, the dispersal of free-living infective larvae among hosts likely contributes to increasing genetic diversity on hosts, possibly fostering the evolutionary potential of T. polycolpus.

Reading between the vines: Hosts as islands for extreme holoparasitic plants

PREMISE OF THE STUDY

Partitioning of population genetic variation in plants may be affected by numerous factors including life history and dispersal characteristics. In parasitic plants, interactions with host populations may be an additional factor influencing partitioning. To test for hierarchical population genetic patterns related to obligate endoparasitism, we studied three species of Rafflesiaceae, which grow as extremely reduced endophytes infecting Tetrastigma vines in Southeast Asia.

METHODS

Microsatellite markers were developed and multilocus genotypes were determined for Rafflesia cantleyi, Rafflesia tuan-mudae, and Sapria himalayana and each of their Tetrastigma hosts. Relatedness among parasite individuals was estimated, and AMOVAs were used to determine levels of population genetic subdivision.

KEY RESULTS

Microsatellite genotypes for 340 paired parasite and host samples revealed that host vines were infected by numerous Rafflesiaceae individuals that may spread for up to 14 m within stem tissues. Surprisingly, Rafflesiaceae parasites within a given host are significantly more closely related to each other than individuals of the same species in other host individuals. The pattern of hierarchical population genetic subdivision we detected across species is likely due to limited seed dispersal with reinfection of natal host vines.

CONCLUSIONS

These findings demonstrate common population genetic patterns between animal and plant parasites, potentially indicating advantages of close relatives infecting hosts. This study also has important conservation implications for Rafflesiaceae since our data suggest that destruction of a single infected host vine could result in large genetic losses.

Signatures of mito-nuclear discordance in Schistosoma turkestanicum indicate a complex evolutionary history of emergence in Europe

High levels of molecular diversity were identified in mitochondrial cytochrome c oxidase (cox1) gene sequences of Schistosoma turkestanicum from Hungary. These cox1 sequences were all specific to Hungary which contrasted with the low levels of diversity seen in the nuclear internal transcribed spacer region (ITS) sequences, the majority of which were shared between China and Iran isolates. Measures of within and between host molecular variation within S. turkestanicum showed there to be substantial differences in molecular diversity, with cox1 being significantly more diverse than the ITS. Measures of haplotype frequencies revealed that each host contained its own subpopulation of genetically unique parasites with significant levels of differentiation. Pairwise mismatch analysis of cox1 sequences indicated S. turkestanicum populations to have a bimodal pairwise difference distribution and to be stable unlike the ITS sequences, which appeared to have undergone a recent population expansion event. Positive selection was also detected in the cox1 sequences, and biochemical modelling of the resulting protein illustrated significant mutational events causing an alteration to the isoelectric point of the cox1 protein, potentially altering metabolism. The evolutionary signature from the cox1 indicates local adaptation and long establishment of S. turkestanicum in Hungary with continual introgression of nuclear genes from Asian isolates. These processes have led to the occurrence of mito-nuclear discordance in a schistosome population.

Inbreeding in stochastic subdivided mating systems: the genetic consequences of host spatial structure, aggregated transmission dynamics and life history characteristics in parasite populations

Inbreeding in parasite populations can have important epidemiological and evolutionary implications. However, theoretical models have predominantly focussed on the evolution of parasite populations under strong selection or in epidemic situations, and our understanding of neutral gene dynamics in parasite populations at equilibrium has been limited to verbal arguments or conceptual models. This study focusses on how host-parasite population dynamics affects observed levels of inbreeding in a random sample of parasites from an infinite population of hosts by bridging traditional genetic and parasitological processes utilizing a backward-forward branching Markov process embedded within a flexible statistical framework, the logarithmic-poisson mixture model. My results indicate that levels of inbreeding in parasites are impacted by demographic and/or transmission dynamics (subdivided mating, aggregated transmission dynamics and host spatial structure), and that this inbreeding is poorly estimated by 'equilibrium' levels of inbreeding calculated assuming regular systems of mating. Specifically, the model reveals that at low levels of inbreeding (F ≤ 0.1), equilibrium levels of inbreeding are lower than those observed, while at high levels of inbreeding the opposite pattern occurs. The model also indicates that inbreeding could have important epidemiological implications (e.g., the spread of recessive drug resistance genes) by directly impacting the observed frequency of rare homozygotes in parasite populations. My results indicate that frequencies of rare homozygotes are affected by aggregated transmission dynamics and host spatial structure, and also that an increase in the frequency of rare homozygotes can be caused by a decrease in effective population size solely due to the presence of a subdivided breeding system.

The population genetic structure of Biomphalaria choanomphala in Lake Victoria, East Africa: implications for schistosomiasis transmission

Background

The freshwater snail Biomphalaria acts as the intermediate host of Schistosoma mansoni, a globally important human parasite. Understanding the population structure of intermediate host species can elucidate transmission dynamics and assist in developing appropriate control methods.

Methods

We examined levels of population genetic structure and diversity in 29 populations of Biomphalaria choanomphala collected around the shoreline of Lake Victoria in Uganda, Kenya and Tanzania, where S. mansoni is hyper-endemic. Molecular markers were utilized to estimate the degree to which snail populations are genetically differentiated from one another.

Results

High levels of snail genetic diversity were found coupled with evidence of geographically-determined population structure but low levels of local inbreeding. The data are consistent with an effect of schistosome infection on population structure of intermediate host snails, but other factors, such as habitat and historical demographic changes, could also be important determinants of the degree of population genetic structure in Biomphalaria choanomphala.

Conclusions

The low stratification of populations and high genetic diversity indicates potentially less local compatibility with intermediate snail populations than previously theorized, and highlights the importance of coordinated parasite control strategies across the region.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0524-4) contains supplementary material, which is available to authorized users.

Inbreeding within human Schistosoma mansoni: do host-specific factors shape the genetic composition of parasite populations?

The size, structure and distribution of host populations are key determinants of the genetic composition of parasite populations. Despite the evolutionary and epidemiological merits, there has been little consideration of how host heterogeneities affect the evolutionary trajectories of parasite populations. We assessed the genetic composition of natural populations of the parasite Schistosoma mansoni in northern Senegal. A total of 1346 parasites were collected from 14 snail and 57 human hosts within three villages and individually genotyped using nine microsatellite markers. Human host demographic parameters (age, gender and village of residence) and co-infection with Schistosoma haematobium were documented, and S. mansoni infection intensities were quantified. F-statistics and clustering analyses revealed a random distribution (panmixia) of parasite genetic variation among villages and hosts, confirming the concept of human hosts as 'genetic mixing bowls' for schistosomes. Host gender and village of residence did not show any association with parasite genetics. Host age, however, was significantly correlated with parasite inbreeding and heterozygosity, with children being more infected by related parasites than adults. The patterns may be explained by (1) genotype-dependent 'concomitant immunity' that leads to selective recruitment of genetically unrelated worms with host age, and/or (2) the 'genetic mixing bowl' hypothesis, where older hosts have been exposed to a wider variety of parasite strains than children. The present study suggests that host-specific factors may shape the genetic composition of schistosome populations, revealing important insights into host-parasite interactions within a natural system.

Reductions in genetic diversity of Schistosoma mansoni populations under chemotherapeutic pressure: the effect of sampling approach and parasite population definition

Detecting potential changes in genetic diversity in schistosome populations following chemotherapy with praziquantel (PZQ) is crucial if we are to fully understand the impact of such chemotherapy with respect to the potential emergence of resistance and/or other evolutionary outcomes of interventions. Doing so by implementing effective, and cost-efficient sampling protocols will help to optimise time and financial resources, particularly relevant to a disease such as schistosomiasis currently reliant on a single available drug. Here we explore the effect on measures of parasite genetic diversity of applying various field sampling approaches, both in terms of the number of (human) hosts sampled and the number of transmission stages (miracidia) sampled per host for a Schistosoma mansoni population in Tanzania pre- and post-treatment with PZQ. In addition, we explore population structuring within and between hosts by comparing the estimates of genetic diversity obtained assuming a 'component population' approach with those using an 'infrapopulation' approach. We found that increasing the number of hosts sampled, rather than the number of miracidia per host, gives more robust estimates of genetic diversity. We also found statistically significant population structuring (using Wright's F-statistics) and significant differences in the measures of genetic diversity depending on the parasite population definition. The relative advantages, disadvantages and, hence, subsequent reliability of these metrics for parasites with complex life-cycles are discussed, both for the specific epidemiological and ecological scenario under study here and for their future application to other areas and schistosome species.

More than meets the eye: detecting cryptic microgeographic population structure in a parasite with a complex life cycle

Nonrandom recruitment of parasites among hosts can lead to genetic differentiation among hosts and mating dynamics that promote inbreeding. It has been hypothesized that strictly aquatic parasites with intermediate hosts will behave as panmictic populations among hosts because ample opportunity exists for random mixing of unrelated individuals during transmission to the definitive host. A previous allozyme study on the marine trematode Lecithochirium fusiforme did not support this hypothesis; in that, there was genetic differentiation among, and significant heterozygote deficiencies within, definitive hosts. We revisit this system and use microsatellites to obtain multilocus genotypes. Our goal was to determine whether cryptic subgroups and/or the presence of clones could account for the apparent deviation from 'panmixia'. We find strong evidence for cryptic subdivision (three genetic clusters) that causes the Wahlund effect and differentiation among definitive hosts. After accounting for these cryptic groups, we see panmictic genetic structure among definitive hosts that is consistent with the 'high mixing in aquatic habitats' hypothesis. We see evidence for cotransmission of clones in all three clusters, but this level of clonal structure did not have a major impact in causing deviations from Hardy-Weinberg equilibrium, and only affected genetic differentiation among hosts in one cluster. A cursory examination of the data may have led to incorrect conclusions about nonrandom transmission. However, it is obvious in this system that there is more than meets the eye in relation to the actual make-up of parasite populations. In general, the methods we employ will be useful for elucidating hidden patterns in other organisms where cryptic structure may be common (e.g. those with limited morphology or complex life histories).

Transmission dynamics of two strains of Schistosoma mansoni utilizing novel intermediate and definitive hosts

The intimate host-parasite relationship mandates adaptation to the genetic and phenotypic variability of their counterparts. Here, inbred and outcrossed strains of Schistosoma mansoni were challenged with "local" and "novel" intermediate and definitive hosts to examine effects of genetic variability and novelty on infection success and dynamics. Genetically distinct lines of Biomphalaria glabrata intermediate hosts exposed to inbred and outcrossed S. mansoni larvae were assessed for differences in both snail and parasite life-history parameters. Cercariae from each parasite-snail treatment were used to infect "local" and "novel" Mus musculus definitive hosts to assess parasite infectivity and fitness. Outcrossed parasites significantly reduced snail growth, were more productive, and induced greater host mortality than inbred parasites. Mouse strain did not influence parasite infectivity or reproduction, but parasite and snail host genetic background did, affecting both sex-specific infectivity and parasite productivity. Overall, genetic background of S. mansoni and its intermediate snail host altered life history traits and transmission dynamics of the parasite throughout its life cycle.

CA88, a nuclear repetitive DNA sequence identified in Schistosoma mansoni, aids in the genotyping of nine Schistosoma species of medical and veterinary importance

CA88 is the first long nuclear repetitive DNA sequence identified in the blood fluke, Schistosoma mansoni. The assembled S. mansoni sequence, which contains the CA88 repeat, has 8,887 nucleotides and at least three repeat units of approximately 360 bp. In addition, CA88 also possesses an internal CA microsatellite, identified as SmBr18. Both PCR and BLAST analysis have been used to analyse and confirm the CA88 sequence in other S. mansoni sequences in the public database. PCR-acquired nuclear repetitive DNA sequence profiles from nine Schistosoma species were used to classify this organism into four genotypes. Included among the nine species analysed were five sequences of both African and Asian lineages that are known to infect humans. Within these genotypes, three of them refer to recognised species groups. A panel of four microsatellite loci, including SmBr18 and three previously published loci, has been used to characterise the nine Schistosoma species. Each species has been identified and classified based on its CA88 DNA fingerprint profile. Furthermore, microsatellite sequences and intra-specific variation have also been observed within the nine Schistosoma species sequences. Taken together, these results support the use of these markers in studying the population dynamics of Schistosoma isolates from endemic areas and also provide new methods for investigating the relationships between different populations of parasites. In addition, these data also indicate that Schistosoma magrebowiei is not a sister taxon to Schistosoma mattheei, prompting a new designation to a basal clade.

Transmission of Schistosoma japonicum in marshland and hilly regions of China: parasite population genetic and sibship structure

The transmission dynamics of Schistosoma japonicum remain poorly understood, as over forty species of mammals are suspected of serving as reservoir hosts. However, knowledge of the population genetic structure and of the full-sibship structuring of parasites at two larval stages will be useful in defining and tracking the transmission pattern between intermediate and definitive hosts. S. japonicum larvae were therefore collected in three marshland and three hilly villages in Anhui Province of China across three time points: April and September-October 2006, and April 2007, and then genotyped with six microsatellite markers. Results from the population genetic and sibling relationship analyses of the parasites across two larval stages demonstrated that, within the marshland, parasites from cattle showed higher genetic diversity than from other species; whereas within the hilly region, parasites from dogs and humans displayed higher genetic diversity than those from rodents. Both the extent of gene flow and the estimated proportion of full-sib relationships of parasites between two larval stages indicated that the cercariae identified within intermediate hosts in the marshlands mostly came from cattle, whereas in the hilly areas, they were varied between villages, coming primarily from rodents, dogs or humans. Such results suggest a different transmission process within the hilly region from within the marshlands. Moreover, this is the first time that the sibling relationship analysis was applied to the transmission dynamics for S. japonicum.

Schistosoma japonicum involves two obligatory host stages, with asexual reproduction within a molluscan host and sexual reproduction within a mammalian host. Having over 40 species of mammals suspected of being potential reservoirs complicates the transmission patterns. Understanding the complex transmission patterns is further hampered by the ethical and logistical difficulty in sampling adult worms from mammalian hosts. However, the two free-swimming larval stages, cercariae (released from a mollusc and then infective to a mammal) and miracidia (hatched from eggs passed in a mammal's faeces, and then infective to a mollusc), are available, and elucidating the genetic composition of parasites at theses two stages could provide information of infection processes. Here we sampled cercariae during April 2006, miracidia during September-October 2006, and cercariae during April 2007 in three marshland and three hilly villages in Anhui Province of China, and, using microsatellite markers, analyzed the population genetic structure and, for the first time, the familial relationships of parasites at different stages. We found contrasting population structures of parasites, and host species-associated diversities and transmission patterns of parasites between and within two regions. Moreover, we demonstrate that the successful application of sibship analyses to infection process provides an alternative approach to the dissection of transmission dynamics.

A natural focus of the blood fluke Orientobilharzia turkestanica (Skrjabin, 1913) (Trematoda: Schistosomatidae) in red deer (Cervus elaphus) in Hungary

The large American liver fluke, Fascioloides magna - introduced to Europe before 1875 - was the first non-indigenous trematode to be detected in Hungarian cervids in 1995. As the most precious deer population became infected in the Gemenc game reserve a study has been launched to assess the extent and the effect of F. magna infections and to examine the options for the treatment of infected deer populations. Livers of red deer shot during regular hunting were submitted for the survey and were investigated by necropsy for the presence of F. magna and other liver dwelling trematodes. Recovered parasites were counted, sorted and stored in 70% ethanol. Unexpectedly, blood flukes were found in liver blood vessels at necropsy. They were identified as Orientobilharzia turkestanica (Skrjabin, 1913) by their morphological features as well as by molecular methods. This parasite is widespread in the middle belt of Asia from Korea to the eastern part of Turkey and infects mainly bovids. It was never found in red deer until now and this is the first report of its occurrence in Hungary and Europe. A subsequent search for the local intermediate host of O. turkestanica in the Gemenc area identified only the pulmonate snail Radix auricularia (Linnaeus, 1758) [syn: Lymnaea auricularia] as a vector capable of producing cercariae of this trematode. Cercariae of O. turkestanica were examined morphologically and were confirmed to be the larvae of this trematode by PCR. O. turkestanica appears to be fairly common in red deer of the Gemenc area. No clinical or pathological effect of the infection could be identified in this study. A form of cercarial dermatitis locally called "water mange" was formerly attributed to the larvae of avian shistosomes. It regularly occurred among fishermen fishing in shallow ponds remaining after floods along the Danube in the Gemenc area. This isolated habitat of O. turkestanica presents an exceptional opportunity to study epidemical situations of typical mammalian schistosomosis under temperate climatic conditions.

Wild and domesticated animals as reservoirs of Schistosomiasis mansoni in Brazil

Natural infection with Schistosoma mansoni in wild vertebrates and domesticated animals in Brazil is described in this review from an epidemiological viewpoint. Some species of wild rodents are small-sized animals, with a short expectation of life, a limited territory, and present high infection rates under natural conditions. A successful maintenance of the parasite's life cycle under artificial conditions can be achieved with Biomphalaria glabrata. On the other hand, despite showing low natural infection rates, cattle are very susceptible to infection under experimental conditions (using calves of Holstein lineage, cross-bred with the Gir lineage). Due to their large size (just one calf may harbor a number of worms higher than a whole colony of aquatic rodents) and their longevity, cattle are a potential reservoir for the maintenance and dissemination of the disease. There is thus a need of new studies to gain a better understanding about the actual role of these animals in the epidemiology of S. mansoni.

Effects of laboratory culture on compatibility between snails and schistosomes

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

Genetic diversity and population structuring of Schistosoma mansoni in a Brazilian village

The digenean trematode Schistosoma mansoni is responsible for chronic schistosomiasis worldwide, and in Brazil alone an estimated 35 million people are at risk. To evaluate epidemiological patterns among human definitive hosts, we assessed genetic diversity and population subdivision of S. mansoni infrapopulations in human hosts from the highly endemic village of Virgem das Graças in the state of Minas Gerais, Brazil. We believe this is the largest such survey to date. Genetic diversity of parasites, measured over eight polymorphic microsatellite loci, was relatively high and standard measures of inbreeding indicated that the population was panmictic. Furthermore, there was no significant isolation-by-distance of parasite infrapopulations, and measures of population subdivision indicated significant but low to moderate levels of population differentiation. We conclude that patients within this village sample from a broad range of schistosome genetic diversity and effectively act as "genetic mixing bowls" for the parasites. These results contrast with those previously observed in the Brazilian village of Melqui ades and thus provide the opportunity for comparisons of environmental and epidemiological differences that are likely to influence host-parasite coevolution and parasite transmission.

Clonal diversity of the marine trematode Maritrema novaezealandensis within intermediate hosts: the molecular ecology of parasite life cycles

We quantified the clonal diversity of the New Zealand marine trematode Maritrema novaezealandensis (n = 1250) within Zeacumantus subcarinatus snail (n = 25) and Macrophthalmus hirtipes crab (n = 25) intermediate hosts using four to six microsatellite loci, and investigated the potential biological and physical factors responsible for the observed genetic patterns. Individual snails harboured one to five trematode genotypes and 48% of snails were infected by multiple parasite genotypes. Overall, the number of parasite genotypes did not increase with snail size, but was highest in intermediate-sized snails. Significantly larger numbers of parasite genotypes were detected in crabs (relative to snails; P < 0.001), with 16-25 genotypes recovered from individual crabs. Although crabs are typically infected by small numbers of cercariae sourced from many snails, they are occasionally infected by large numbers of cercariae sourced from single snails. The latter cases explain the significant genetic differentiation of trematode populations detected among their crab hosts (F(ST) = 0.009, P < 0.001). Our results suggest that the timing of infection and/or intraspecific competition among parasite clones within snails determine(s) the diversity of parasite clones that snails harbour. The presence of a large number of infected snails and tidal mixing of cercariae prior to infection results in crabs potentially harbouring hundreds of parasite genotypes despite the crabs' territorial behaviour.

Evidence for multiple mitochondrial lineages of Fasciola hepatica (liver fluke) within infrapopulations from cattle and sheep

The economic, veterinary, and medical impact of the parasite Fasciola hepatica, liver fluke, is difficult to alleviate due to increasing incidences of resistance to the principal anthelmintic drugs. These have occurred in widely separated regions. The rate of response to selection imposed by such drugs will be dependent on the genetic variation present in the F. hepatica gene pool, but this is at present unknown. We have assessed the genetic diversity of mitochondrial haplotypes found in the infrapopulation of flukes recovered from a calf of known provenance and from six other cattle and sheep hosts located in Ireland and four from elsewhere. Our results revealed that at least ten different mitochondrial composite PCR-restriction fragment length polymorphism haplotypes had been acquired by a single animal in 1 year, and there was comparable diversity in six other definitive hosts carrying field-acquired infections. The extent of divergence between these fluke lineages suggests that they predate the last ice age and, thus, cannot have developed in Northern Europe. A consequence of this high level of diversity is that there will be frequent selection for anthelmintic resistance and rapid responses to climatic changes.

Development and application of an ethically and epidemiologically advantageous assay for the multi-locus microsatellite analysis of Schistosoma mansoni

Non-availability of adult worms from living hosts remains a key problem in population genetic studies of schistosomes. Indirect sampling involving passage through laboratory animals presents significant ethical and practical drawbacks, and may result in sampling biases such as bottlenecking processes and/or host-induced selection pressures. The novel techniques reported here for sampling, storage and multi-locus microsatellite analysis of larval Schistosoma mansoni, allowing genotyping of up to 7 microsatellite loci from a single larva, circumvent these problems. The utility of these assays and the potential problems of laboratory passage, were evaluated using 7 S. mansoni population isolates collected from school-children in the Hoima district of Uganda, by comparing the associated field-collected miracidia with adult worms and miracidia obtained from a single generation in laboratory mice. Analyses of laboratory-passaged material erroneously indicated the presence of geographical structuring in the population, emphasizing the dangers of indirect sampling for population genetic studies. Bottlenecking and/or other sampling effects were demonstrated by reduced variability of adult worms compared to their parent field-collected larval samples. Patterns of heterozygote deficiency were apparent in the field-collected samples, which were not evident in laboratory-derived samples, potentially indicative of heterozygote advantage in establishment within laboratory hosts. Genetic distance between life-cycle stages in the majority of isolates revealed that adult worms and laboratory-passaged miracidia clustered together whilst segregating from field miracidia, thereby further highlighting the utility of this assay.

Genetic filtering and optimal sampling of Schistosoma mansoni populations

Allelic variation in 6 microsatellite markers was compared between frozen Schistosoma mansoni eggs and laboratory-passaged worms originating from the same 5 fecal samples obtained from Brazilian residents. Based on allelic richness values, the number of alleles detected per locus did not differ between egg and worm DNA templates. However, our ability to score loci differed between these DNA templates, with worms providing more scored loci per individual than eggs. Differences also existed between the worms and eggs in the identity of the specific alleles that were detected. Additionally, we observed a reduction in homozygous genotypes among laboratory-passaged worms relative to the eggs. Allelic diversity curves were calculated by genotyping all worms from a representative host sample to determine the relationship between the number of alleles detected at a locus and the number of worms genotyped. Curves for the 5 residents' worm infrapopulations for each of the loci were very similar. The equation y=19.55 x ln(x) + 9.992 explained the association between sampling effort (x) and number of alleles detected (y) with an R(2) of 0.775. In conclusion, egg DNA templates and allelic diversity curves can benefit efforts to discern the sociological, ecological and evolutionary forces impacting the genetic diversity and disease epidemiology of human schistosomes.

Speciation in parasites: a population genetics approach

Parasite speciation and host-parasite coevolution should be studied at both macroevolutionary and microevolutionary levels. Studies on a macroevolutionary scale provide an essential framework for understanding the origins of parasite lineages and the patterns of diversification. However, because coevolutionary interactions can be highly divergent across time and space, it is important to quantify and compare the phylogeographic variation in both the host and the parasite throughout their geographical range. Furthermore, to evaluate demographic parameters that are relevant to population genetics structure, such as effective population size and parasite transmission, parasite populations must be studied using neutral genetic markers. Previous emphasis on larger-scale studies means that the connection between microevolutionary and macroevolutionary events is poorly explored. In this article, we focus on the spatial fragmentation of parasites and the population genetics processes behind their diversification in an effort to bridge the micro- and macro-scales.

Molecular ecology of parasites: elucidating ecological and microevolutionary processes

We review studies that have used molecular markers to address ecological and microevolutionary processes in parasites. Our goal is to highlight areas of research that may be of particular interest in relation to the parasitic lifestyle, and to draw attention to areas that require additional study. Topics include species identification, phylogeography, host specificity and speciation, population genetic structure, modes of reproduction and transmission patterns, and searching for loci under selection.

F-statistics under alternation of sexual and asexual reproduction: a model and data from schistosomes (platyhelminth parasites)

Accurate inferences on population genetics data require a sound underlying theoretical null model. Nearly nothing is known about the gene dynamics of organisms with complex life cycles precluding any biological interpretation of population genetics parameters. In this article, we used an infinite island model to derive the expectations of those parameters for the life cycle of a dioecious organism obligatorily alternating sexual and asexual reproductions as it is the case for schistosomes (plathyhelminth parasites). This model allowed us to investigate the effects of the degree of mixing among individuals coming from different subpopulations at each new generation (represented in the model by the migration rates before and after clonal reproductions) and the variance in the reproductive success of individuals during the clonal phase. We also consider the effects of different migration rates and degrees of clonal reproductive skew between male and female individuals. Results show that the variance in the reproductive success of clones is very important in shaping the distribution of the genetic variability both within and among subpopulations. Thus, higher variance in the reproductive success of clones generates heterozygous excesses within subpopulations and also increases genetic differentiation between them. Migration occurring before and after asexual reproduction has different effects on the patterns of F(IS) and F(ST). When males and females display different degrees of reproductive skew or migration rates, we observe differences in their respective population genetic structure. While results of the model apply to any organism alternating sexual and clonal reproductions (e.g. all parasitic trematodes, many plants, and all aphididae), we finally confront some of these theoretical expectations to empirical data from Schistosoma mansoni infecting Rattus rattus in Guadeloupe.

Molecular ecology of Schistosoma mansoni transmission inferred from the genetic composition of larval and adult infrapopulations within intermediate and definitive hosts

We investigated the genotypic composition of the digenetic parasite Schistosoma mansoni for its adult stages within the definitive host (the wild rat, Rattus rattus) and for the larval stages within the intermediate host (the snail, Biomphalaria glabrata) both collected at the same transmission site. Our analyses are based upon the recognition and distribution of 200 different multilocus genotypes generated by RAPD markers. While intramolluscan larval infrapopulations are characterized by a low infection rate (0.6 % on average) and low intra-host genetic diversity (1.1 genotype on average per infected snail), adult infrapopulations within rats showed a high infection rate (94%) and a substantial intra-host genetic diversity (34 genotypes on average) linked to high intensities (160 worms per host on average). A single definitive host bearing 105 different genotypes harboured 52 % of the total genetic diversity detected within the whole parasite population. Analysis of the genetic data allowed the identification of various ecological, behavioural and immunological factors which are likely to enhance transmission of multiple parasite genotypes towards the vertebrate hosts. From the distribution of repeated identical multilocus genotypes within the parasite population and among the hosts, we have inferred different parameters of the cercarial transmission efficiency as well as patterns and processes by which vertebrate hosts acquire infection in the field.

Sex-specific correlation between heterozygosity and clone size in the trematode Schistosoma mansoni

The mode of reproduction (sexual and/or asexual) and the mating system determine the patterns of gene transmission and genotype formation across generations. Schistosoma mansoni is a dioecious trematode that necessarily alternates sexual and asexual reproduction during its life cycle. In a previous study of the distribution of S. mansoni genetic variability within and between definitive host individuals, we noticed that deleting multilocus genotypes from each infrapopulation so as to keep only one copy of each multilocus genotype, seemed to have a substantial effect on FIS values. More precisely, female FIS increased when repeated genotypes were removed whereas no effect was observed on male FIS. This suggested that multilocus genotypes at high frequency tended to be more heterozygous. The aim of the present study is specifically to test and analyse this phenomenon. We demonstrate that the number of repetitions per clone correlates with individual heterozygosity. This effect is however, sex-specific: only female clone size correlates with heterozygosity. We discuss this phenomenon in relation to the heterozygosity-fitness relationship and the sex-specific response to inbreeding depression.

DEPLETION OF SCHISTOSOMA MANSONI LUNG-STAGE SCHISTOSOMULA CHOLESTEROL BY METHYL-β-CYCLODEXTRIN DRAMATICALLY INCREASES SPECIFIC ANTIBODY BINDING TO SURFACE MEMBRANE ANTIGENS

Schistosoma mansoni lung-stage larvae appear to not bind antibodies from radiation vaccine or infection sera in the membrane immunofluorescence test. However, treatment of ex vivo lung-stage schistosomula with methyl-beta-cyclodextrin, a hydrophobic oligosaccharide that specifically extracts cholesterol from plasma membranes, induced readily detectable binding of specific antibodies in a concentration- and time-dependent manner. Surface membrane antigen binding of specific antibodies was also conclusively demonstrated by quantitative absorption of anti-schistosome sera with intact ex vivo larvae. These data together suggest that confinement of lung-stage schistosomula surface membrane antigens in cholesterol-rich sites allows only monovalent antibody binding, which can be detected by absorption and not by direct serology.

A comparative parasitologic study on Biomphalaria glabrata snail and C3H/He mice infected with human and murine isolates of Schistosoma mansoni derived from Sumidouro, Rio de Janeiro, Brazil

Experiments were carried out to analyze the biological characteristics of two sympatric isolates of Schistosoma mansoni derived from humans and murines in a low endemic transmission area (Sumidouro county, state of Rio de Janeiro, Brazil). Sympatric reared-laboratory Biomphalaria glabrata and C3H/He mice were used as experimental hosts. Parameters assessed comprised: precercarial period, infectivity and mortality (snails), prepatent period, infectivity (percentage of cercariae maturation into adult worm) and intestinal egg count (mice). The murine isolate showed a shorter precercarial period and higher infectivity than human isolate (p<0.05). This biological heterogenicity did not correspond to the vertebrate data because any biological parameter presented significant difference (p>0.05). These data suggest that both isolates are local sub-populations, providing support for the hypotheses that in a same biotope mixed populations or sub-populations circulate among their main host (human beings) and/or rodent as an anfixenous infection.

Sex-specific genetic structure in Schistosoma mansoni: evolutionary and epidemiological implications

We studied the population genetic structure of 360 and 1247 adult Schistosoma mansoni using seven microsatellite and seven random amplified polymorphic DNA (RAPD) markers, respectively. Parasites were collected from their natural definitive host Rattus rattus in Guadeloupe (West Indies). We found a sex-specific genetic structure, a pattern never before reported in a parasitic organism. Male genotypes were more randomly distributed among rats than female genotypes. This interpretation was consistent with a lower differentiation between hosts for males relative to females, the higher genetic similarity between females in the same host and the observed local (i.e. within-individual-host) differences in allele frequencies between the two sexes. We discuss our results using ecological and immunological perspectives on host-parasite relationships. These results change our view on the epidemiology of schistosomiasis, a serious disease affecting humans in African and American intertropical zones.