Spring 2009 Meeting: Abstracts

Papers

Rising from obscurity: parasites of marine protists

Kristina Hamilton

Department of Biological Sciences, School of the Environment and Society, Singleton Park, Swansea, SA2 8PP

The role of eukaryotic parasites in foodwebs, carbon cycling and in the organisation of marine ecosystems has long been underestimated. Whereas pathogens causing human and live-stock diseases have received attention from pathologists, epidemiologists, and ecologists, parasites of protists, biology's stepchildren themselves, have been neglected. In this introduction to parasites of marine protist, I aim to summarise what we have learned about these organisms since they were first described in the late 19th century. In particular, I will discuss the diversity of the known (morpho)species and explore their molecular phylogeny. I will take a look at their habitats and hosts, and introduce the dynamics of their epidemics in regards to host availability, presence of competitors and grazers, and abiotic environmental factors.

Protozoal parasites and human diarrhoeal disease

Paul R Hunter

School of Medicine, Health Policy and Practice, University of East Anglia, Norwich, UK

Diarrhoeal disease is responsible for some 1.8 million deaths each year mostly in children under 5 years old. The microbial pathogens implicated in the cause of diarrhoeal disease include viruses, bacteria and protozoa. Probably the two commonest protozoal pathogens are Cryptosporidium and Giardia, though in developing countries Entamoeba is also an important pathogen. Although microsporidium were considered to be another group of protozoal pathogens causing diarrhoea in immune compromised patients, predominantly idue to HIV infection, though recently these agent have been considered to be fungi. This presentation will described the clinical presentations of the three predominant protozoal pathogens and describe their epidemiology. Recent developments in our understanding of these organisms and their epidemiology come from molecular studies. These studies have demonstrated different species/sub-species with very different host ranges and transmission pathways.

Cops-1: a species determinant for anthroponotic Cryptosporidium species?

Maha Bouzid¹, Rachel Chalmers², Paul Hunter¹, Kevin Tyler¹.

¹: Biomedical research centre, School of Medicine, Health Policy and Practice, University of East Anglia, Norwich NR4 7TJ, England, UK.

²: UK Cryptosporidium Reference Unit, NPHS Microbiology Swansea, Singleton Hospital, Swansea SA2 8QA, UK.

Cryptosporidium is an apicomplexan parasite causing diarrheal illness in a wide range of hosts. Two species C. parvum and C. hominis cause the majority of human cases both as sporadic and outbreak related cases. Despite the ongoing efforts, little is still known about the pathogenesis and the virulence of this important pathogen. The genomes of these two anthroponotic species are sequenced and showed only 3-5% sequence divergence. Using comparative genomic tools, we identified 100 putative specific genes for each species, the majority corresponding to hypothetical proteins. These could be genetic determinant for host specificity. Six genes for each species were tested by PCR in a collection of ten clinical isolates (three C. hominis and seven C. parvum). These samples were isolated from symptomatic human cases of Cryptosporidium and were previously genotyped at the Cryptosporidium Reference Unit using the standard methods (PCR-RFLP, microsatellite and Real time PCR). In addition, three reference strains C. parvum IOWA, C. parvum Moredun and C. hominis TU502 were also tested. Our results showed that the majority of the genes tested were present in both C. hominis and C. parvum. Nevertheless, sequence analysis revealed species specific single nucleotide polymorphisms (SNPs), which would allow better understanding of molecular epidemiology and population structure of Cryptosporidium. Interestingly, only one gene Cops-1 was C. parvum specific. We are currently investigating the biological function and the potential of this gene as a species determinant.

Receptors for transferrin and lactoferrin in protozoan parasites

Dietmar Steverding

BioMedical Research Centre, School of Medicine, Health Policy and Practice. University of East Anglia, Norwich NR4 7TJ

Iron is an essential bio-element for almost all organisms including protozoan parasites. Within the mammalian host, iron is sequestered to withhold it from pathogens. The iron-binding proteins transferrin and lactoferrin reduce the availability of iron for pathogenic micro-organisms in body fluids and mucosal membranes. There is evidence that protozoan parasites express specific receptors for the uptake of transferrin and/or lactoferrin to utilise them as source of iron. The best characterised example is the transferrin receptor of bloodstream forms of Trypanosoma brucei. In Trypanosoma cruzi, Leishmania sp., Trichomonas vaginalis, Entamoeba histolytica, Plasmodium falciparum and Toxoplasma gondii, receptors for the uptake of transferrin and/or lactoferrin are not well described. Identification of the involved genes and functional analysis of the encoded polypeptides are required to confirm that the proteins described so far are indeed receptors for the uptake of transferrin and/or lactoferrin in these protozoan parasites. Here, the available information on transferrin and lactoferrin recptors of protozoan parasites is summarised with particular emphasis on structure, biochemical properties and function of the well established transferrin receptor of T. brucei.

Proliferative kidney disease as an emerging disease: the importance of life cycle complexity and environmental change

Beth Okamura¹, Hanna Hartikainen¹, Sylvie Tops²

¹Department of Zoology, Natural History Museum, Cromwell Road, London SW7 5BD

²School of Biological Sciences, University of Reading, Whiteknights, Reading RG6 6AJ

Tetracapsuloides bryosalmonae is a myxozoan parasite that is the causative agent of proliferative kidney disease (PKD), a disease of all salmonids that can have devastating effects on both farmed and wild salmonid populations. The life cycle of T. bryosalmonae includes freshwater bryozoans as primary hosts with spores released from bryozoans being infectious to fish. We have characterised low virulence and vertical transmission in bryozoan hosts, traits that should promote the persistence of infection in local populations as well as the dissemination of the disease to new localities. The strategies employed by the parasite to exploit its colonial bryozoan hosts and the responses of both bryozoans and T. bryosalmonae to environmental variation (eutrophication and rising temperatures) are likely to contribute to the emerging status of PKD. A recent severe PKD outbreak in Norway suggests that infected bryozoans are widespread and that the disease is induced with changing environmental conditions. We anticipate that many emerging diseases represent such endemic diseases that manifest themselves in new ways.

A spotlight on genetic exchange in trypanosomes

Wendy Gibson¹, Lori Peacock^1,2, Vanessa Ferris^1,2 andMick Bailey²

1. Biological Sciences, University of Bristol, Bristol BS8 1UG, UK

2. Clinical Veterinary Science, University of Bristol, Langford, BS40 7DU, UK

Trypanosoma brucei undergoes genetic exchange when two different strains are co- transmitted through the insect vector, the tsetse fly. We recently developed a highly efficient system to visualise hybrid production directly in the tsetse fly by the use of green or red fluorescent markers, allowing flies with a mixed infection of the parental strains to be easily identified while successful mating is indicated by the production of yellow hybrids. In our initial cross of red and green fluorescent trypanosomes, yellow hybrids first appeared 13 days after infection and were seen only in the fly's salivary glands, implicating the salivary glands as the site of genetic exchange and the epimastigote as the lifecycle stage involved. In depth investigation has not yet revealed the nature of the intermediate stages, possibly because they are rare or transient. Inheritance of microsatellite alleles among the progeny was consistent with Mendelian expectations, but a large proportion of hybrid progeny were polyploid rather than diploid. It is not known what factors control compatibility in trypanosome crosses or whether mating types exist. All twelve F1 and back crosses carried out were successful, and there is thus no simple system of mating types in T. brucei. Hybrids were also demonstrated in single strain transmissions, confirming that intraclonal mating is possible, but without the requirement for out crossing reported previously. This means that it is no longer possible to assume that strains necessarily remain as genetic clones following fly transmission. Taken together, our results suggest that genetic exchange is a normal part of the transmission cycle of T. brucei rather than a rare event.

GIBSON, W., PEACOCK, L., FERRIS, V., WILLIAMS, K. and BAILEY, M. (2008). The use of yellow fluorescent hybrids to indicate mating in Trypanosoma brucei. Parasites and Vectors, 1, 4.

Are determinants of cell entry and virulence synonymous for American trypanosomes?

Claire Butler¹, Guy Wheeler¹, Edmundo Grisard^1,2, Kevin Tyler¹.

1: Biomedical research centre, School of Medicine, Health Policy and Practice, University of East Anglia, Norwich NR4 7TJ, England, UK.

2:Departamento de Microbiologia e Parasitologia, Universidade Federal de Santa Catarina, 88040-900, Florianópolis, Santa Catarina, Brazil.

Roughly a third of those which have contracted the protozoan parasite Trypanosoma cruzi will go on to manifest the symptoms of Chagas disease. Although there is little evidence for spontaneous cure of what is generally held to be a life-long infection there is a profound heterogeneity in the pathology arising, which stems in part from the genotype of the parasite, and in part from the genotype and immune status of the infected host. A close cousin of T. cruzi, Trypanosoma rangeli is also infectious, causing life-long infections in humans, but it is completely apathogenic. Our investigations of T. cruzi surface determinants have demonstrated the critical importance of trans-sialidase and trans-sialidase like molecules in host cell entry and our use of their heterologous expression in T. rangeli has led to the elucidation of hitherto cryptic life-cycle stages with important translational implications.

Phylogeography of Tetracapsuloides bryosalmonae (Myxozoa), a parasite of salmonid fish and freshwater Bryozoa

Hanna Hartikainen and Beth Okamura

Department of Zoology, Natural History Museum, Cromwell Road, London SW7 5BD

Outbreaks of Proliferative Kidney Disease (PKD) have increased in frequency and severity over the last decade, afflicting both farmed and wild salmonid fish, and causing significant economic losses in Europe and North America. The causative agent of PKD, the myxozoan Tetracapsuloides bryosalmonae, uses freshwater bryozoans as definitive hosts. Recent field surveys in Europe have substantially extended the number of bryozoan populations known to harbour T. bryosalmonae, and the availability of material from a range of new locations has provided the basis for the phylogeographic study reported here. Analysis of ITS-1 sequences revealed generally low genetic divergence between T. bryosalmonae populations, whether individuals were from bryozoan or fish hosts, and little geographical clustering of variants. However, the 136 sequences were arrayed in 3 clusters that differ in their genetic variability and geographical provenance. The largest cluster, a poorly resolved polychotomy, contained sequences of low variability from locations covering much of the known European range of the parasite, including the UK. A smaller cluster, also poorly resolved internally, included mainly English sequences. Conversely, the third cluster contained sequences of continental European origin and unlike the other two clusters, its members displayed relatively high genetic divergence. We found no sequences characteristic of members of the North American clade of T. bryosalmonae.

* Presenting author

Deep phylogeny of protozoa and the multiple origins of parasitism (Presidential address)

Thomas Cavalier-Smith

Department of Zoology, South Parks Road, Oxford, OX1 3PS, UK

I shall primarily discuss the deep phylogeny of protozoa and other protists with special reference to the results of recent multi-gene sequence trees, the evolution of protozoan body plans, and the still uncertain position of the root of the eukaryote evolutionary tree. Eukaryotes have been divided into two major groups, the unikonts and bikonts. The unikonts primarily comprise the Amoebozoa, Choanozoa, animals and Fungi, whereas the bikonts comprise the Excavata, Plantae, chromalveolates and Rhizaria. Recent multigene trees suggest that Rhizaria and centrohelid heliozoa may be derived ultimately from chromalveolates by independent losses of the red algal chloroplast that was acquired symbiogenetically by the ancestral chromalveolate. I shall give special attention to evidence for the monophyly and internal phylogeny of Amoebozoa and Rhizaria (especially Cercozoa and its remarkable diversity), to the paraphyly of Choanozoa, to Heliozoa, and to the possible polyphyly, relationships, and special evolutionary significance of Apusozoa (Apusomonadida, Planomonadida, Micronuclearia). I shall point out where the major groups of protozoan parasites fit on the evolutionary tree and briefly discuss multiple origins of parasitism within Cercozoa.

pSNPs: A New Tool for Comparative Genomics of Eukaryotic Microorganisms?

Ian N Roberts¹, Michael J T O'Kelly², Alexander van Oudenaarden², Stephen A James¹ and Robert P Davey¹

¹National Collection of Yeast Cultures, Institute of Food Research, Norwich Research Park, Colney, Norwich, NR4 7UA, UK, ²Department of Physics, Massachusetts Institute of Technology, Cambridge MA 02139, USA

Current methods of genome sequencing do not allow assembly of ribosomal DNA (rDNA) repeats. It is generally assumed by genome projects that these tandem repeats will in any event have been homogenised by evolutionary forces such as sister chromatid exchange and gene conversion. We have investigated rDNA sequence variation in whole genome shotgun sequences from a variety of different strains of the yeast Saccharomyces cerevisiae and have found unexpectedly high levels of rDNA sequence variation within individual genomes. We suggest the term pSNPs (partial Single Nucleotide Polymorphisms) to describe this variation. We propose that pSNPs may provide a valuable new measure of genomic interrelatedness and stability.

Acknowledgements: We thank Richard Durbin and David Carter (Sanger Institute) and Ed Louis and Gianni Liti (University of Nottingham) for scientific advice and the BBSRC for financial support.

Upside Down/Inside Out: How a major evolutionary leap could result from a minor regulatory switch - and have no intermediates

Barry Leadbeater

School of Biosciences, University of Birmingham, UK

One of the challenges in molecular and evolutionary biology is to explain the link between 'major' evolutionary leaps and the underlying genetic changes. For a long time it was thought that substantial changes in morphology would be accompanied by equally dramatic upheavals at the genetic level. However, several examples have come to light whereby relatively minor changes in regulation, rather than the modification or creation of genes, have driven major evolutionary innovations.

The loricate choanoflagellates, with their unique basket-like loricae, present an interesting example at the morphological level of how a relatively minor regulatory change could explain a major evolutionary innovation. The uniqueness of the lorica demonstrates the unequivocal monophyly of this choanoflagellate group and yet there are two major clades that differ in the order of events leading to lorica assembly. The result is that one clade (nudiform) contains only five extant species, limited in both morphology and ecological diversity, whereas the other (tectiform) comprises more that 150 species that have successfully colonised many microniches within the marine environment. Could the difference be due to a regulatory change which not only produced viable offspring but conferred an advantage over its antecedents?

New insights into the production of DMS and DMSP by marine protists

Gill Malin and Amandine Caruana

Laboratory for Global Marine and Atmospheric Chemistry, School of Environmental Sciences, University of East Anglia

Dimethyl sulphide [(CH₃)₂S; DMS] is a volatile sulphur compound that makes the connection between algae and clouds. DMS plays a key role in the global sulphur cycle because it is the main vehicle for the transfer of sulphur between the relatively sulphur-rich oceans and terrestrial ecosystems where sulphur is in much shorter supply. A small portion of the DMS that is found in marine aquatic systems is emitted to the air where it oxidises rapidly to form acidic sulphate aerosol particles. As well as influencing atmospheric chemistry, the aerosol particles may also cool the Earth's climate directly or indirectly by acting as cloud-condensation nuclei. DMS derives from a zwitterionic precursor compound known as dimethylsulphoniopropionate (DMSP; [CH₃)₂S⁺CH₂ CH₂COO^-] which is found in a range of seaweeds and protists. A range of evidence suggests that DMSP can potentially be used as a compatible solute, a cryoprotectant, an antioxidant, an overflow metabolite under conditions of unbalanced growth or grazing deterrent. Hence, it appears to be a multifunctional compound. When cells die due to autolysis, grazing or viral lysis some of the DMSP released is released as DMS due to the action of algal or bacterial enzymes. This talk will begin with an overview of the biogeochemical roles of DMS, the production of DMSP by protists, the physiology and microbial ecology of DMS production. We will then turn to our group's research on a range of different DMSP-producing protists groups. Emphasis will be given to recent research on Dinoflagellates - which are considered to be one of the major DMSP-producing phytoplankton groups. This enigmatic taxon displays a range of different plastid groups, about 50% of them are heterotrophic and it seems likely that even more are mixotrophic. Taking our laboratory data along with published data for this group highlights the high variability in DMSP content within dinoflagellates. We have also looked at DMSP production in one heterotrophic species, Crypthecodinium cohnii in detail and found that intracellular DMSP concentration increases substantially in carbon-depleted cultures. These new data give fresh insight into DMSP production in this protist group and underline some important gaps in our current knowledge.

Widespread replacement of an "essential" gene within Opisthokonta

Martin Carr,

Biology Department, University of York

eEF1A and EFL are two closely related families of elongation factors within the GTPase superfamily. eEF1A is a core translation protein with homologues present in both archaea and bacteria; EFL is a more recently discovered family and its presence in genomes appears to coincide with the absence of eEF1A. EFL has a discontiguous distribution within eukaryotes, indicating either lateral transfer or, perhaps less likely, ancient paralogy followed by lineage assortment. Within Opisthokonta EFL has been discovered in three major groups - Fungi, Choanoflagellida and Mesomycetozoa - however each of these lineages also possesses species that encode eEF1A rather than EFL.

Screening of 12 choanoflagellate species determined that 10 species encode EFL, with eEF1A appearing to be restricted to a single freshwater sublineage. Within Mesomycetozoa EFL appears to be restricted to a single sublineage, with all other species encoding eEF1A. EFL is widespread within the "lower" fungi, yet appears to be absent from Dikarya. The distribution of EFL within choanoflagellates can be explained by a single lateral transfer event into an ancestral stem-group species followed by lineage assortment. A secondary lateral transfer event appears to have occurred from the choanoflagellates to Mesomycetozoa. In contrast, EFL distribution in Fungi requires at least two lateral transfer events and multiple losses of both EFL and eEF1A.

Parallel tag environmental DNA sequencing opens new windows for protistan ecology and diversity research

Thorsten Stoeck, David Bass, Markus Nebel, Richard Christen, Meredith D. M. Jones, Hans-Werner Breiner, Thomas A. Richards

Sequencing of small subunit ribosomal RNA gene clone libraries has shown that lineage diversity of natural eukaryotic microbial assemblages is far greater than suggested by their morphological diversity. However, this method is labour intensive, expensive, and methodologically biased. Novel massively parallel tag sequencing technology (454 sequencing) offers an unprecedented intensity and scale of sampling, which has already initiated a radical revision of our understanding of prokaryote community structure. Using 454 sequencing we show that ten litres of anoxic Norwegian fjord water can harbour in the order of 10,000 genetically distinct microbial eukaryote lineages. The majority of these comprise an extensive, hyperdiverse 'rare biosphere'. 454 sequencing detects many more lineages across a much wider range of taxonomic groups than clone library data from the same site. We demonstrate that 454 sequencing is a very powerful new tool for eukaryotic microbial diversity analyses, and outline current/forthcoming projects that will develop this approach further.

Identification of the Shikimate pathway in Acanthamoeba species

Fiona L. Henriquez, Sara J. Campbell, Craig W. Roberts

Our previous work has demonstrated that the shikimate pathway was present in the last universal common ancestor (LUCA) of eukaryotes and that it has potential as an antimicrobial agent target (Roberts, 1998 Nature 393, 801; Campbell, 2004. Intern J Parasitol 34, 5; Richards 2006; Euk Cell 5, 1517). The shikimate pathway is a 7 enzymatic pathway that synthesizes chorismate, which in turn serves as a precursor to folate, ubiquinone and aromatic amino acids. Crucially it is absent from mammals. Herein we characterise the shikimate pathway in Acanthamoeba and determine its potential as an antimicrobial target in the opportunistic infections that Acanthamoeba may cause.

Characterisation of Imidazole glycerol phostphate dehydratase in Acanthamoeba castellanii

Sara J. Campbell, Craig W. Roberts, Fiona L. Henriquez

Acanthamoeba species can cause severe disease in the eye (Acanthamoeba keratitis) and in the central nervous system (Granulomatous Amoebic Encephalitis). Current treatments for AK are limited and not completely effective Herein, we examine the potential of imidazole glycerol phosphate dehydratase, which belongs to the histidine biosynthesis pathway as a potential anti-acanthamoebic target.

Temperature relations of testate amoebae

Humphrey G Smith,

Coventry University, James Starley Building, Coventry CV1 5FB

A knowledge of the influence of temperature, in interaction with other environmental variables, is important in seeking to explain the distribution and abundance of testate populations. There is evidence that testates can show plasticity of size and form. Testates in laboratory culture are frequently larger than those observed in the wild. Wanner (1999) determined reversible effects of temperature on the test sizes of Trinema spp, Cyclopyxis spp and Euglypha spp in experimental cultures. This paper contrasts Wanner's results with the variability observed in natural populations of Phryganella acropodia, Assulina muscorum and Corythion dubium from the Antarctic zone. Results suggest that tests of rapidly growing populations can be smaller than those which survive when temperatures below 5⁰C cause high mortality. Small euglyphid species, which are abundant and ubiquitous, show facultative psychrophily and maintain trophic activity between 0⁰ and 5⁰ C.

Wanner M (1999) Acta Protozoologica 38 15-29.

The Diversity and Ecology of Planomonas Zooflagellates (Apusozoa)

Edvard Glücksman and Thomas Cavalier-Smith, Department of Zoology, University of Oxford, United Kingdom

Planomonas is an important genus of gliding biciliate zooflagellate protozoa in the recently established protozoan phylum Apusozoa. Previously the most widespread members of this diverse genus were all misidentified as a single morphospecies, Ancyromonas sigmoides, one of the most frequently reported zooflagellates in marine sediment surveys. Despite this, little is known about their diversity and ecology.

Using differential interference contrast light microscopy and high definition video recording as well as 18S rRNA and ITS gene sequencing, we examined Planomonas diversity in marine environmental samples obtained at several sites in Europe, the Middle East, Oceania, North America, and Africa. Our early results confirm that Planomonas are widespread in marine and freshwater environments and show that they vary genotypically and phenotypically across environmental locations. We have evidence for several more lineages than previously recognized.

Actin and its role in Gliding Motility of Apicomplexan Gregarines

King C.A., Preston T.M., Baum J.,Sleep J.

In the early 1980s there was evidence that actin was absent in the apicomplexan Gregarina. Subsequently actin was detected although not at the high levels found in crawling cells (approx. 200µM) in which overt motility is low (cultured fibroblasts 1 µm per min or less. In the Apicomplexa the level of actin is low (10-20 µM) mostly in the G-actin form but gliding rates are high (Gregarina polymorpha 1-10µm per sec; Porospora gigantea 60µm per sec).

Since latex beads can be translocated over the cell surface in the opposite direction to that seen in gliding,we have used this as a model for study of motility over the pellicle of Gregarina trophozoites (200µm+ long). Their size enables the use of laser-trap technology. 2 µm beads were translocated erratically from the anterior to posterior (0 to 8 µm per sec) in Gregarina. This would fit with a model in which the pellicle surface motility apparatus can be assembled /disassembled over short time intervals. Molecular biological investigations of Apicomplexa detected the presence of Formins (Baum) but not Arp2/3 material (found in crawling cells). Thus the actin-binding protein Formin(s) might act as a nucleator for actin thereby generating an active actin-myosin complex within the pellicle.

The laser-trap can hold a 2µm bead on the surface of a Gregarina and if the actin-myosin motors in the pellicle generate a force of at least 50pN then the bead could escape (Sleep). This would involve the recruitment of at least 10 myosins per instant and thus at least 400 myosins per second. If we assume a step distance for actin-myosin to be 10nm then the distance travelled per sec would be 4000nm/4µm about the value of translocation observed. Whereas in Prorospora gigantean the value for bead translocation is more than 10 times this value.

Phylogenetic relationships among peritrich ciliates: new perspectives based on small subunit rRNA gene sequence data

Alan Warren¹^*, Kuidong Xu², Yingchun Gong³, Weibo Song⁴

¹Department of Zoology, Natural History Museum, Cromwell Road, London SW7 5BD, UK

²Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

³Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan 430072, China

⁴Laboratory of Protozoology, Ocean University of China, Qingdao 266003, China

The ciliate subclass Peritrichia has long been recognized as a monophyletic assemblage comprising two orders, namely Sessilida (>750 spp.) and Mobilida (>250 spp.). In order to re-evaluate the phylogenetic relationships among the peritrichs, SS rRNA gene sequences were determined for 16 species representing seven genera, i.e. five sessilids (Epicarchesium, Pseudovorticella, Vorticella, Zoothamnium and Zoothamnopsis) and two mobilids (Trichodina and Urceolaria). Phylogenetic analyses were carried out on these, and previously published, data and the main findings were as follows. (1) Taxa with stalks that contract independently and in a spiral fashion, and their stalkless relatives, form a well-supported clade representing a refined family Vorticellidae, separate from those whose stalks have a continuous myoneme and contract in a zig-zag fashion (Zoothmniidae). (2) Epicarchesium and Pseudovorticella, both of which have reticulate silverline systems, are more closely related to each other than to other vorticellids, suggesting that the silverline system may be phylogenetically more informative than coloniality within the Vorticellidae. (3) The mobilids never clustered with the sessilids, but instead formed a clade that is sister to the peniculines at a basal position within the class Oligohymenophorea. By contrast, the sessilids formed a clade that is sister to the hymenostomes at a terminal position within the Oligohymenophorea. Preliminary data based on α-tubulin gene sequences also support the separation of the sessilids and mobilids. If these findings are verified then it is possible that the similarity of the oral apparatus, and the reduced somatic ciliation, in the mobilid and sessilid peritrichs are a result of convergent evolution driven by their similar life-styles and feeding strategies.

Supported by: the Knowledge Innovation Program of the Chinese Academy of Sciences (No. KZCX2-YW-417); National Natural Science Foundation of China (Nos. 30670280 and 40576072); the '100 Talents Project' of the Chinese Academy of Sciences; The Darwin Initiative.

Posters

Is Prodiscocephalus a hypotrich ciliate? A phylogenetic analysis based on morphogenetic and 18S rRNA gene sequence data (Ciliophora: Spirotrichea)

Chen Shao¹, Lifang Li¹, Alan Warren² and Weibo Song¹

¹Laboratory of Protozoology, Ocean University of China, Qingdao 266003, China

²Department of Zoology, Natural History Museum, Cromwell Road, London SW7 5BD

The Prodiscocephalus-like ciliates, or discocephalines, are traditionally considered to be euplotid hypotrichs but whose precise systematic position has long been uncertain, mainly because of the paucity of morphogenetic data, with only two discocephalines having been investigated, and a complete absence of molecular data. In the present study the cortical development of Prodiscocephalus borrori was observed during binary division. Five features were observed that are characteristic of stichotrichs: (1) the oral primordium in the opisthe occurs de novo on the cell surface; (2) the undulating membrane in the proter is derived from the parental structure; (3) there are >5 FTV-cirral anlagen; (4) the two marginal rows form intrakinetally; (5) the dorsal anlagen are formed in two groups. By contrast, only two features are typical of euplotid hypotrichs, i.e. (1) several caudal cirri are formed from the rightmost DK-anlagen; (2) the FVT-cirral anlagen are formed in the primary mode, indicating that the discocephalines are more closely related to the stichotrichs than to the euplotids. Based on a combination of morphological and morphogenetic data, a phylogenetic tree was constructed which also suggests that the discocephalines group within the stichotrichs and separate from the euplotids. In addition, the complete small subunit rRNA (SS rRNA) gene of P. borrori was sequenced and analyzed. In the resulting SS rRNA tree, P. borrori is sister to the Stichotrichia-Oligotrichia-Choreotrichia assemblage, albeit with low bootstrap support, and separate from the euplotids. These data suggest that the discocephalines should be considered as a distinct group, at the rank of order or even subclass, within the class Spirotrichea Bütschli, 1889.

Histidine biosynthesis in Acanthamoeba castellanii

Susan Manson, Elena Poulos, Sara J. Campbell, Craig W. Roberts, Fiona L. Henriquez

Compounds that prevent or cure Acanthamoeba castellanii infection are limited and not completely effective. There is a critical need for new antimicrobials against this pathogen. Herein we describe genetic evidence of all the ten enzymes belonging to the histidine biosynthesis pathway in Acanthamoeba. The complete characterisation of this pathway may provide potential new target for antimicrobial activity.

Mixotrophic protists and catastrophic darkness

Harriet Jones¹, Charles S. Cockell², Claire Goodson¹, Nicola Price¹, Annika Simpson¹, Benjamin Thomas¹

¹School of Biology, University of East Anglia, Norwich, NR4 7TJ, UK

²CEPSAR, Open University, Milton Keynes, MK7 6AA, UK

Catastrophically darkened photic zone conditions in water bodies are thought to be induced by a diversity of mechanisms including asteroid and comet impacts and large-scale volcanic eruptions. Previous modelling studies on asteroid and comet impacts have assumed that below an incident light level of 1% photosynthesis would cease. The ability of phototrophs to survive or even grow below this light level might dramatically improve their chances of surviving such crises. The data from this, and other, studies show how mixotrophs can survive and grow at very low light intensities. Both mixotrophs, and phototrophs which survived 6 month darkness, were found to recover quickly when returned to light conditions. A collapse of photosynthetic food chains following natural catastrophes that causes a sudden darkening is one of the favoured mechanisms for biotic crises following the K-T impact event leaving many ecosystems to depend on detritus. However, the effects on food webs and microbial interactions may not be as straightforward as a simple collapse. Mixotrophy might provide some significant resilience for microbial foodwebs during such crises.