Double peaks reveal rare diplomonad sex

Subspecific Nomenclature of Giardia duodenalis in the Light of a Compared Population Genomics of Pathogens

Genetic and genomic data have long recognized that the species Giardia duodenalis is subdivided into at least eight genetic clusters that have been named "assemblages" by specialists in the field. Some of these assemblages have been given the status of species, with Linnean binames. In the framework of the predominant clonal evolution model (PCE), we have shown that, from an evolutionary point of view, G. duodenalis assemblages are equatable to "near-clades", that is to say: clades whose discreteness is somewhat clouded by occasional genetic exchange, but remain discrete and stable in space and time. The implications of this evolutionary status for the species described within G. duodenalis are discussed in light of the most recent genetic and genomic studies. The pattern of this species' subspecific genetic variability and genetic clustering appears to be very similar to the ones of various parasitic, fungal and bacteria species. This underlines the relevance of a compared population genomics of pathogenic species allowed by the broad framework of the PCE model.

Metabolic Reconstruction Elucidates the Lifestyle of the Last Diplomonadida Common Ancestor

Diplomonads are a group of microbial eukaryotes found in oxygen-poor environments. There are both parasitic (e.g., Giardia intestinalis) and free-living (e.g., Trepomonas) members in the group.

The identification of ancestral traits is essential to understanding the evolution of any group. In the case of parasitic groups, this helps us understand the adaptation to this lifestyle and a particular host. Most diplomonads are parasites, but there are free-living members of the group nested among the host-associated diplomonads. Furthermore, most of the close relatives within Fornicata are free-living organisms. This leaves the lifestyle of the ancestor unclear. Here, we present metabolic maps of four different diplomonad species. We identified 853 metabolic reactions and 147 pathways present in at least one of the analyzed diplomonads. Our study suggests that diplomonads represent a metabolically diverse group in which differences correlate with different environments (e.g., the detoxification of arsenic). Using a parsimonious analysis, we also provide a description of the putative metabolism of the last Diplomonadida common ancestor. Our results show that the acquisition and loss of reactions have shaped metabolism since this common ancestor. There is a net loss of reaction in all branches leading to parasitic diplomonads, suggesting an ongoing reduction in the metabolic capacity. Important traits present in host-associated diplomonads (e.g., virulence factors and the synthesis of UDP-N-acetyl-d-galactosamine) are shared with free-living relatives. The last Diplomonadida common ancestor most likely already had acquired important enzymes for the salvage of nucleotides and had a reduced capacity to synthesize nucleotides, lipids, and amino acids de novo, suggesting that it was an obligate host-associated organism.

IMPORTANCE Diplomonads are a group of microbial eukaryotes found in oxygen-poor environments. There are both parasitic (e.g., Giardia intestinalis) and free-living (e.g., Trepomonas) members in the group. Diplomonads are well known for their anaerobic metabolism, which has been studied for many years. Here, we reconstructed whole metabolic networks of four extant diplomonad species as well as their ancestors, using a bioinformatics approach. We show that the metabolism within the group is under constant change throughout evolutionary time, in response to the environments that the different lineages explore. Both gene losses and gains are responsible for the adaptation processes. Interestingly, it appears that the last Diplomonadida common ancestor had a metabolism that is more similar to extant parasitic than free-living diplomonads. This suggests that the host-associated lifestyle of parasitic diplomonads, such as the human parasite G. intestinalis, is an old evolutionary adaptation.

When is a "cryptic" species not a cryptic species: A consideration from the Holarctic micro-landsnail genus Euconulus (Gastropoda: Stylommatophora)

Naive use of molecular data may lead to ambiguous conclusions, especially within the context of "cryptic" species. Here, we integrated molecular and morphometric data to evaluate phylogenetic relationships in the widespread terrestrial micro-snail genus, Euconulus. We analyzed mitochondrial (16S + COII) and nuclear (ITS1 + ITS2) sequence across 94 populations from Europe, Asia and North America within the nominate species E. alderi, E. fulvus and E. polygyratus, and used the southeastern USA E. chersinus, E. dentatus, and E. trochulus as comparative outgroups. Phylogeny was reconstructed using four different reconstruction methods to identify robust, well-supported topological features. We then performed discriminant analysis on shell measurements between these genetically-identified species-level clades. These analyses provided evidence for a biologically valid North American "cryptic" species within E. alderi. However, while highly supported polyphyletic structure was also observed within E. fulvus, disagreement in placement of individuals between mtDNA and nDNA clades, lack of morphological differences, and presence of potential hybrids imply that these lineages do not rise to the threshold as biologically valid cryptic species, and rather appear to simply represent a complex of geographically structured populations within a single species. These results caution that entering into a cryptic species hypothesis should not be undertaken lightly, and should be optimally supported along multiple lines of evidence. Generally, post-hoc analyses of macro-scale features should be conducted to attempt identification of previously ignored diagnostic traits. If such traits cannot be found, i.e. in the case of potentially "fully cryptic" species, additional criteria should be met to propound a cryptic species hypothesis, including the agreement in tree topology among both mtDNA and nDNA, and little (or no) evidence of hybridization based on a critical analysis of sequence chromatograms. Even when the above conditions are satisfied, it only implies that the cryptic species hypothesis is plausible, but should optimally be subjected to further careful examination.

Swarming and Aggregation in the Parasitic Diplomonad Flagellate Spironucleus vortens

Pathogenicity, evolutionary history, and unusual cell organization of diplomonads are well known, particularly for Giardia and Spironucleus; however, behavior of these aerotolerant anaerobes is largely unknown. Addressing this deficit, we studied behavior of the piscine diplomonad Spironucleus vortens (ATCC 50386) in in vitro culture. Spironucleus vortens trophozoites from Angelfish, Pterophyllum scalare, were maintained axenically in modified liver digest, yeast extract, and iron (LYI) medium, at 22 °C in the dark, and subcultured weekly. Cultures were monitored every 1-2 d, by removing an aliquot, and loading cells into a hemocytometer chamber, or onto a regular microscope slide. We observed three distinct swimming behaviors: (i) spontaneous formation of swarms, reaching 200 μm in diameter, persisting for up to several min in situ, (ii) directional movement of the swarm, via collective motility, and (iii) independent swimming of trophozoites to form a band (aggregation), presumably at the location of optimal environmental conditions. These behaviors have not previously been reported in Spironucleus. The observation that flagellate motility can change, from individual self-propulsion to complex collective swarming motility, prompts us to advocate S. vortens as a new model for study of group behavioral dynamics, complementing emerging studies of collective swimming in flagellated bacteria.

The first multilocus genotype analysis of Giardia intestinalis in humans in the Czech Republic

To date, genotyping data on giardiasis have not been available in the Czech Republic. In this study, we characterized 47 human isolates of Giardia intestinalis from symptomatic as well as asymptomatic giardiasis cases. Genomic DNA from trophozoites was tested by PCR-sequence analysis at three loci (β-giardin, glutamate dehydrogenase and triose phosphate isomerase). Sequence analysis showed assemblages A and B in 41 (87.2%) and six (12.8%) isolates, respectively. Two of the 41 assemblage A samples were genotyped as sub-assemblage AI, and 39 were genotyped as sub-assemblage AII. Four previously identified multilocus genotypes (MLGs: AI-1, AII-1, AII-4 and AII-9) and six likely novel variations of MLGs were found. In agreement with previous studies, sequences from assemblage B isolates were characterized by a large genetic variability and by the presence of heterogeneous positions, which prevent the definition of MLGs. This study also investigated whether there was a relationship between the assemblage and clinical data (including drug resistance). However, due to the large number of genotypes and the relatively small number of samples, no significant associations with the clinical data were found.

Genetic variability and transcontinental sharing of Giardia duodenalis infrapopulations determined by glutamate dehydrogenase gene

Microevolutionary data of Giardia duodenalis sub-assemblages is a prerequisite for determining the invasion zoonotic patterns of the parasite. To infer transmission patterns that could not be differentiated by the phenotypic features, a population genetic investigation is crucial for the elucidation of the genetic structure of G. duodenalis among the continents. Forty G. duodenalis positive fecal samples were collected from different foci of Northwest Iran. The specimens were subjected to Trichrome staining and sucrose gradient flotation. DNA samples were extracted, amplified, and sequenced by targeting glutamate dehydrogenase (gdh) gene. The global gdh sequences of sub-assemblages AII and BIV retrieved from NCBI GenBank were analyzed to estimate diversity indices, neutrality indices, and gene migration tests. Sequencing analyses indicated various levels of genetic variability of sub-assemblages AII and BIV among the five continents. Sub-assemblage BIV had greater genetic variability (haplotype diversity: 0.975; nucleotide diversity: 0.04246) than sub-assemblage AII. The statistical Fst value demonstrated that the genetic structure of sub-assemblages AII and BIV are moderately differentiated between European-American populations (Fst: 0.05352-0.15182), whereas a significant differentiation was not seen among other geographical population pairs. We conclude that a high gene flow of G. duodenalis sub-assemblages AII and BIV is unequivocally sharing among the continents. The current findings strengthen our knowledge to assess the evolutionary patterns of G. duodenalis in endemic foci of the world and it will become the basis of public health policy to control human giardiasis.

Is Predominant Clonal Evolution a Common Evolutionary Adaptation to Parasitism in Pathogenic Parasitic Protozoa, Fungi, Bacteria, and Viruses?

We propose that predominant clonal evolution (PCE) in microbial pathogens be defined as restrained recombination on an evolutionary scale, with genetic exchange scarce enough to not break the prevalent pattern of clonal population structure. The main features of PCE are (1) strong linkage disequilibrium, (2) the widespread occurrence of stable genetic clusters blurred by occasional bouts of genetic exchange ('near-clades'), (3) the existence of a "clonality threshold", beyond which recombination is efficiently countered by PCE, and near-clades irreversibly diverge. We hypothesize that the PCE features are not mainly due to natural selection but also chiefly originate from in-built genetic properties of pathogens. We show that the PCE model obtains even in microbes that have been considered as 'highly recombining', such as Neisseria meningitidis, and that some clonality features are observed even in Plasmodium, which has been long described as panmictic. Lastly, we provide evidence that PCE features are also observed in viruses, taking into account their extremely fast genetic turnover. The PCE model provides a convenient population genetic framework for any kind of micropathogen. It makes it possible to describe convenient units of analysis (clones and near-clades) for all applied studies. Due to PCE features, these units of analysis are stable in space and time, and clearly delimited. The PCE model opens up the possibility of revisiting the problem of species definition in these organisms. We hypothesize that PCE constitutes a major evolutionary strategy for protozoa, fungi, bacteria, and viruses to adapt to parasitism.

Prevalence and Genetic Diversity of Giardia duodenalis and Cryptosporidium spp. among School Children in a Rural Area of the Amhara Region, North-West Ethiopia

Backgroud

Giardia duodenalis and Cryptosporidium spp. are enteric protozoan causing gastrointestinal illness in humans and animals. Giardiasis and cryptosporidiosis are not formally considered as neglected tropical diseases, but belong to the group of poverty-related infectious diseases that impair the development and socio-economic potential of infected individuals in developing countries.

Methods

We report here the prevalence and genetic diversity of G. duodenalis and Cryptosporidium spp. in children attending rural primary schools in the Bahir Dar district of the Amhara Region, Ethiopia. Stool samples were collected from 393 children and analysed by molecular methods. G. duodenalis was detected by real-time PCR, and the assemblages and sub-assemblages were determined by multilocus sequence-based genotyping of the glutamate dehydrogenase and β-giardin genes of the parasite. Detection and identification of Cryptosporidium species was carried out by sequencing of a partial fragment of the small-subunit ribosomal RNA gene.

Principal Findings

The PCR-based prevalences of G. duodenalis and Cryptosporidium spp. were 55.0% (216/393) and 4.6% (18/393), respectively. A total of 78 G. duodenalis isolates were successfully characterized, revealing the presence of sub-assemblages AII (10.3%), BIII (28.2%), and BIV (32.0%). Discordant typing results AII/AIII and BIII/BIV were identified in 7.7% and 15.4% of the isolates, respectively. An additional five (6.4%) isolates were assigned to assemblage B. No mixed infections of assemblages A+B were found. Extensive genetic variation at the nucleotide level was observed within assemblage B (but no within assemblage A), resulting in the identification of a large number of sub-types. Cryptosporidium diversity was demonstrated by the occurrence of C. hominis, C. parvum, and C. viatorum in the population under study.

Conclusions

Our data suggest an epidemiological scenario with an elevated transmission intensity of a wide range of G. duodenalis genetic variants. Importantly, the elevated degree of genetic diversity observed within assemblage B is consistent with the occurrence of intra-assemblage recombination in G. duodenalis.

Molecular Genotyping of Giardia duodenalis Isolates from Symptomatic Individuals Attending Two Major Public Hospitals in Madrid, Spain

BACKGROUND

The flagellate protozoan Giardia duodenalis is an enteric parasite causing human giardiasis, a major gastrointestinal disease of global distribution affecting both developing and industrialised countries. In Spain, sporadic cases of giardiasis have been regularly identified, particularly in pediatric and immigrant populations. However, there is limited information on the genetic variability of circulating G. duodenalis isolates in the country.

METHODS

In this longitudinal molecular epidemiological study we report the diversity and frequency of the G. duodenalis assemblages and sub-assemblages identified in 199 stool samples collected from 184 individual with symptoms compatible with giardiasis presenting to two major public hospitals in Madrid for the period December 2013-January 2015. G. duodenalis cysts were initially detected by conventional microscopy and/or immunochomatography on stool samples. Confirmation of the infection was performed by direct immunofluorescence and real-time PCR methods. G. duodenalis assemblages and sub-assemblages were determined by multi-locus genotyping of the glutamate dehydrogenase (GDH) and β-giardin (BG) genes of the parasite. Sociodemographic and clinical features of patients infected with G. duodenalis were also analysed.

PRINCIPAL FINDINGS

Of 188 confirmed positive samples from 178 giardiasis cases a total of 124 G. duodenalis isolates were successfully typed at the GDH and/or the BG loci, revealing the presence of sub-assemblages BIV (62.1%), AII (15.3%), BIII (4.0%), AI (0.8%), and AIII (0.8%). Additionally, 6.5% of the isolates were only characterised at the assemblage level, being all of them assigned to assemblage B. Discordant genotype results AII/AIII or BIII/BIV were also observed in 10.5% of DNA isolates. A large number of multi-locus genotypes were identified in G. duodenalis assemblage B, but not assemblage A, isolates at both the GDH and BG loci, confirming the high degree of genetic variability observed in other molecular surveys. BIV was the most prevalent genetic variant of G. duodenalis found in individuals with symptomatic giardiasis in the population under study.

CONCLUSIONS

Human giardiasis is an ongoing public health problem in Spain affecting primarily young children under four years of age but also individuals of all age groups. Our typing and sub-typing results demonstrate that assemblage B is the most prevalent G. duodenalis assemblage circulating in patients with clinical giardiasis in Central Spain. Our analyses also revealed a large genetic variability in assemblage B (but not assemblage A) isolates of the parasite, corroborating the information obtained in similar studies in other geographical regions. We believe that molecular data presented here provide epidemiological evidence at the population level in support of the existence of genetic exchange within assemblages of G. duodenalis.

Comparative genomic analyses of freshly isolated Giardia intestinalis assemblage A isolates

Background

The diarrhea-causing protozoan Giardia intestinalis makes up a species complex of eight different assemblages (A-H), where assemblage A and B infect humans. Comparative whole-genome analyses of three of these assemblages have shown that there is significant divergence at the inter-assemblage level, however little is currently known regarding variation at the intra-assemblage level. We have performed whole genome sequencing of two sub-assemblage AII isolates, recently axenized from symptomatic human patients, to study the biological and genetic diversity within assemblage A isolates.

Results

Several biological differences between the new and earlier characterized assemblage A isolates were identified, including a difference in growth medium preference. The two AII isolates were of different sub-assemblage types (AII-1 [AS175] and AII-2 [AS98]) and showed size differences in the smallest chromosomes. The amount of genetic diversity was characterized in relation to the genome of the Giardia reference isolate WB, an assemblage AI isolate. Our analyses indicate that the divergence between AI and AII is approximately 1 %, represented by ~100,000 single nucleotide polymorphisms (SNP) distributed over the chromosomes with enrichment in variable genomic regions containing surface antigens. The level of allelic sequence heterozygosity (ASH) in the two AII isolates was found to be 0.25–0.35 %, which is 25–30 fold higher than in the WB isolate and 10 fold higher than the assemblage AII isolate DH (0.037 %). 35 protein-encoding genes, not found in the WB genome, were identified in the two AII genomes. The large gene families of variant-specific surface proteins (VSPs) and high cysteine membrane proteins (HCMPs) showed isolate-specific divergences of the gene repertoires. Certain genes, often in small gene families with 2 to 8 members, localize to the variable regions of the genomes and show high sequence diversity between the assemblage A isolates. One of the families, Bactericidal/Permeability Increasing-like protein (BPIL), with eight members was characterized further and the proteins were shown to localize to the ER in trophozoites.

Conclusions

Giardia genomes are modular with highly conserved core regions mixed up by variable regions containing high levels of ASH, SNPs and variable surface antigens. There are significant genomic variations in assemblage A isolates, in terms of chromosome size, gene content, surface protein repertoire and gene polymorphisms and these differences mainly localize to the variable regions of the genomes. The large genetic differences within one assemblage of G. intestinalis strengthen the argument that the assemblages represent different Giardia species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1893-6) contains supplementary material, which is available to authorized users.

Population expansion and gene flow in Giardia duodenalis as revealed by triosephosphate isomerase gene

Background

Giardia duodenalis is a protozoan parasite that can cause significant diarrhoeal diseases. Knowledge of population genetics is a prerequisite for ascertaining the invasion patterns of this parasite. In order to infer evolutionary patterns that could not be uncovered based on the morphological features, a population genetic study with the incorporation of molecular marker was carried out to access the genetic structure of G. duodenalis isolated from the Malaysian population and the global populations.

Methods

A total of 154 samples positive for Giardia, collected from different Malaysian communities, were subjected to DNA amplification and sequencing targeting three genetic loci (tpi, gdh, and bg). The tpi sequences together with sequences from the global data obtained from the NCBI GenBank were used for genetic diversity analyses including identification of haplotypes, haplotype diversity, nucleotide diversity, Tajima’s D and Fu and Li’s D, gene flow and genetic differentiation tests.

Results

Analysis of the Malaysian and global data showed that assemblages A, B, and E (the most prevalent assemblages in humans and animals), have different levels of genetic diversity. Assemblage B had the highest level of both haplotype diversity and nucleotide diversity, followed by assemblage E. The analysis also revealed population expansion and high gene flow in all assemblages. No clear genetic structure was observed across five continents (i.e., the Americas, Europe, Asia, Australia and Africa). However, median joining network of assemblage B formed a cluster that was exclusively isolated from Asia while other haplotypes were well dispersed across the continents.

Conclusions

This study provides new insight into the genetic diversity of Giardia assemblages in different geographical regions. The significant result shown by gene flow and genetic differentiation analyses as well as test of neutrality among the populations should have brought a clearer picture to the dynamics and distribution of Giardia infection.

Sex is a ubiquitous, ancient, and inherent attribute of eukaryotic life

Sexual reproduction and clonality in eukaryotes are mostly seen as exclusive, the latter being rather exceptional. This view might be biased by focusing almost exclusively on metazoans. We analyze and discuss reproduction in the context of extant eukaryotic diversity, paying special attention to protists. We present results of phylogenetically extended searches for homologs of two proteins functioning in cell and nuclear fusion, respectively (HAP2 and GEX1), providing indirect evidence for these processes in several eukaryotic lineages where sex has not been observed yet. We argue that (i) the debate on the relative significance of sex and clonality in eukaryotes is confounded by not appropriately distinguishing multicellular and unicellular organisms; (ii) eukaryotic sex is extremely widespread and already present in the last eukaryotic common ancestor; and (iii) the general mode of existence of eukaryotes is best described by clonally propagating cell lines with episodic sex triggered by external or internal clues. However, important questions concern the relative longevity of true clonal species (i.e., species not able to return to sexual procreation anymore). Long-lived clonal species seem strikingly rare. We analyze their properties in the light of meiotic sex development from existing prokaryotic repair mechanisms. Based on these considerations, we speculate that eukaryotic sex likely developed as a cellular survival strategy, possibly in the context of internal reactive oxygen species stress generated by a (proto) mitochondrion. Thus, in the context of the symbiogenic model of eukaryotic origin, sex might directly result from the very evolutionary mode by which eukaryotic cells arose.

The Transcriptome Sequence of Dientamoeba fragilis Offers New Biological Insights on its Metabolism, Kinome, Degradome and Potential Mechanisms of Pathogenicity

Dientamoeba fragilis is a human bowel parasite with a worldwide distribution. Dientamoeba was once described as a rare and harmless commensal though recent reports suggest it is common and potentially pathogenic. Molecular data on Dientamoeba is scarce which limits our understanding of this parasite. To address this, sequencing of the Dientamoeba transcriptome was performed. Messenger RNA was extracted from cultured Dientamoeba trophozoites originating from clinical stool specimens, and sequenced using Roche GS FLX and Illumina HiSeq technologies. In total 6,595 Dientamoeba transcripts were identified. These sequences were analysed using the BLAST2GO software suite and via BLAST comparisons to sequences available from TrichDB, GenBank, MEROPS and kinase.com. Several novel KEGG pathway maps were generated and gene ontology analysis was also performed. These results are thoroughly discussed guided by knowledge available for other related protozoa. Attention is paid to the novel biological insights afforded by this data including peptidases and kinases of Dientamoeba, as well as its metabolism, novel chemotherapeutics and possible mechanisms of pathogenicity. Currently, this work represents the largest contribution to our understanding of Dientamoeba molecular biology and also represents a major contribution to our understanding of the trichomonads generally, many of which are important pathogens of humans and animals.

Cryptosporidium,Giardia, Cryptococcus, Pneumocystis genetic variability: cryptic biological species or clonal near-clades?

An abundant literature dealing with the population genetics and taxonomy of Giardia duodenalis, Cryptosporidium spp., Pneumocystis spp., and Cryptococcus spp., pathogens of high medical and veterinary relevance, has been produced in recent years. We have analyzed these data in the light of new population genetic concepts dealing with predominant clonal evolution (PCE) recently proposed by us. In spite of the considerable phylogenetic diversity that exists among these pathogens, we have found striking similarities among them. The two main PCE features described by us, namely highly significant linkage disequilibrium and near-clading (stable phylogenetic clustering clouded by occasional recombination), are clearly observed in Cryptococcus and Giardia, and more limited indication of them is also present in Cryptosporidium and Pneumocystis. Moreover, in several cases, these features still obtain when the near-clades that subdivide the species are analyzed separately (“Russian doll pattern”). Lastly, several sets of data undermine the notion that certain microbes form clonal lineages simply owing to a lack of opportunity to outcross due to low transmission rates leading to lack of multiclonal infections (“starving sex hypothesis”). We propose that the divergent taxonomic and population genetic inferences advanced by various authors about these pathogens may not correspond to true evolutionary differences and could be, rather, the reflection of idiosyncratic practices among compartmentalized scientific communities. The PCE model provides an opportunity to revise the taxonomy and applied research dealing with these pathogens and others, such as viruses, bacteria, parasitic protozoa, and fungi.

Micropathogen species definition is extremely difficult, since concepts applied to higher organisms (the biological species concept) are inadequate. In particular, the pathogens here surveyed have given rise to long-lasting controversies about their species status and that of the genotypes that subdivide them. The population genetic approach based on the predominant clonal evolution (PCE) concept proposed by us could bring simple solutions to these controversies, since it permits the description of clearly defined evolutionary entities (clonal multilocus genotypes and near-clades [incompletely isolated clades]) that could be the basis for species description, if the concerned specialists find it justified for applied research. The PCE model also provides a convenient framework for applied studies (molecular epidemiology, vaccine and drug design, clinical research) dealing with these pathogens and others.

The genome of Spironucleus salmonicida highlights a fish pathogen adapted to fluctuating environments

Spironucleus salmonicida causes systemic infections in salmonid fish. It belongs to the group diplomonads, binucleated heterotrophic flagellates adapted to micro-aerobic environments. Recently we identified energy-producing hydrogenosomes in S. salmonicida. Here we present a genome analysis of the fish parasite with a focus on the comparison to the more studied diplomonad Giardia intestinalis. We annotated 8067 protein coding genes in the ∼12.9 Mbp S. salmonicida genome. Unlike G. intestinalis, promoter-like motifs were found upstream of genes which are correlated with gene expression, suggesting a more elaborate transcriptional regulation. S. salmonicida can utilise more carbohydrates as energy sources, has an extended amino acid and sulfur metabolism, and more enzymes involved in scavenging of reactive oxygen species compared to G. intestinalis. Both genomes have large families of cysteine-rich membrane proteins. A cluster analysis indicated large divergence of these families in the two diplomonads. Nevertheless, one of S. salmonicida cysteine-rich proteins was localised to the plasma membrane similar to G. intestinalis variant-surface proteins. We identified S. salmonicida homologs to cyst wall proteins and showed that one of these is functional when expressed in Giardia. This suggests that the fish parasite is transmitted as a cyst between hosts. The extended metabolic repertoire and more extensive gene regulation compared to G. intestinalis suggest that the fish parasite is more adapted to cope with environmental fluctuations. Our genome analyses indicate that S. salmonicida is a well-adapted pathogen that can colonize different sites in the host.

Studies of model organisms are very powerful. However, to appreciate the enormous diversity of genetic and cell biological processes we need to extend the number of available model organisms. For example, there are very few model organisms for diverse microbial eukaryotes, a group of organisms which indeed represents the vast majority of the eukaryotic diversity. To this end, we have developed a system to do genetic modification on the Atlantic salmon pathogen Spironucleus salmonicida. Using this system we could show that the organism is capable of producing hydrogen within specialised compartments. Here we present the genome sequence of S. salmonicida together with a thorough annotation. We compare the results with the closest available model organism, the human intestinal parasite Giardia intestinalis. The fish parasite has a more elaborate system for regulation of gene expression, as well as a larger metabolic capacity. This indicates that S. salmonicida is a well-adapted pathogen that can deal with fluctuating environments, an important trait to be able to establish systemic infections in the host. The development of S. salmonicida into a model system will benefit the studies of fish infections, as well as cell biological processes.

Reproductive clonality of pathogens: a perspective on pathogenic viruses, bacteria, fungi, and parasitic protozoa

We propose that clonal evolution in micropathogens be defined as restrained recombination on an evolutionary scale, with genetic exchange scarce enough to not break the prevalent pattern of clonal population structure, a definition already widely used for all kinds of pathogens, although not clearly formulated by many scientists and rejected by others. The two main manifestations of clonal evolution are strong linkage disequilibrium (LD) and widespread genetic clustering ("near-clading"). We hypothesize that this pattern is not mainly due to natural selection, but originates chiefly from in-built genetic properties of pathogens, which could be ancestral and could function as alternative allelic systems to recombination genes ("clonality/sexuality machinery") to escape recombinational load. The clonal framework of species of pathogens should be ascertained before any analysis of biomedical phenotypes (phylogenetic character mapping). In our opinion, this model provides a conceptual framework for the population genetics of any micropathogen.

Nuclear inheritance and genetic exchange without meiosis in the binucleate parasite Giardia intestinalis

The protozoan parasite Giardia intestinalis (also known as Giardia lamblia) is a major waterborne pathogen. During its life cycle, Giardia alternates between the actively growing trophozoite, which has two diploid nuclei with low levels of allelic heterozygosity, and the infectious cyst, which has four nuclei and a tough outer wall. Although the formation of the cyst wall has been studied extensively, we still lack basic knowledge about many fundamental aspects of the cyst, including the sources of the four nuclei and their distribution during the transformation from cyst into trophozoite. In this study, we tracked the identities of the nuclei in the trophozoite and cyst using integrated nuclear markers and immunofluorescence staining. We demonstrate that the cyst is formed from a single trophozoite by a mitotic division without cytokinesis and not by the fusion of two trophozoites. During excystation, the cell completes cytokinesis to form two daughter trophozoites. The non-identical nuclear pairs derived from the parent trophozoite remain associated in the cyst and are distributed to daughter cells during excystation as pairs. Thus, nuclear sorting (such that each daughter cell receives a pair of identical nuclei) does not appear to be a mechanism by which Giardia reduces heterozygosity between its nuclei. Rather, we show that the cyst nuclei exchange chromosomal genetic material, perhaps as a way to reduce heterozygosity in the absence of meiosis and sex, which have not been described in Giardia. These results shed light on fundamental aspects of the Giardia life cycle and have implications for our understanding of the population genetics and cell biology of this binucleate parasite.