Contributed Papers
Tetracapsula bryosalmonae (PKX organism) in bryozoans, exhibits both myxosporean and actinosporean characters and represents an early branch in myxozoan evolution.Canning, E.U., Department of Biology, Imperial College, London, Curry, A. PHLS, Withington Hospital, Manchester M20 2LR, Feist, S.W. & Longshaw, M., MAFF, CEFAS Laboratory, Weymouth DT4 8UB and Okamura, B., School of Animal & Microbial Sciences, University of Reading, RG6 6AJ.
Tetracapsula bryosalmonae (PKX organism) causes proliferative kidney disease (PKD) in salmonid fish. Several species of bryozoans act as alternate hosts (possibly the primary hosts). Development in bryozoans involves formation of two types of cell within a sac-like compartment that is free-floating in the bryozoan coelomic cavity. Each of the larger cells (sporoplasmogenic) becomes enclosed by stellate cells which differentiate as four capsulogenic and four valve cells. The sporoplasmogenic cell undergoes meiosis and divides into two sporoplasms, each with a secondary cell. The bryozoan phase of T. bryosalmonae resembles the myxosporean phase of the better known myxozoan genera in lacking external projections on the spores and not having triradial symmetry. It resembles the actinosporean phase in undergoing chromosome reduction and lacking valve cell coverage of the points of exit of polar filaments. Unusual features of the genus Tetracapsula are the sac-like compartment enclosing developing stages, sporoplasmosomes of a unique structure, polar capsule formation without prior external tube manifestation and no deposition of strengthening elements by the valve cells. 18S rDNA sequence analysis shows that Tetracapsula emerged as an early branch in myxozoan evolution for which a new class is required, and indicates that bryozoans may have been the ancestral hosts of myxozoans.
Merozoite assembly in Plasmodium falciparum
Lawrence Bannister, John Hopkins, Ruth Fowler and Graham Mitchell, Medical and Dental Schools of Guy's, King's and St. Thomas' Hospitals, London; and Sanjeev Krishna, St. George's Hospital Medical School, London.
During schizogony in Plasmodium falciparum about 16 nuclei are formed, with a corresponding number of merozoite buds projecting from the parasite's perimeter. Within each bud, rhoptries, micronemes, dense granules and ribosomes are synthesised and put into position, with the nucleus, mitochondrion and plastid, which have all multiplied earlier in the schizont. During this period the merozoite pellicle membranes and the filamentous surface coat are completed. Some types of organelle trafficking may involve a narrow band of sub-surface microtubules anchored at the apical end of the merozoite. Dynein and kinesin may be involved in these translocations.
The formation of rhoptries and related structures appears to require a nucleus-related Golgi cisterna which both receives vesicles from the nuclear envelope, and buds off vesicles which fuse to generate the apical secretory organelles and subsurface membranes of the pellicle.
The study was supported by the Wellcome Trust (grant numbers 037082 and 048244 ) and the Special Trustees of St. Thomas' and of Guy's Hospital. SK is a Wellcome Trust Senior Research Fellow in Clinical Science.
The development of an automated method for determining protozoan grazing rates.
Jackie Parry and Karen Heaton, Division of Biological Sciences, IENS, Lancaster University, Lancaster, LA1 4YQ.
This paper describes the progress made in the development of an automated method for the determination of protozoan grazing rates, which employs a GFP-expressing bacterium as the tracer. Upon ingestion by the protozoan, acidification in the food vacuole leads to loss of the fluorescence within the prey cells, so a decrease in fluorescence of the culture occurs over time. This decrease is recorded by an automated multi-task plate reader (Victor 1420) and thus significantly reduces the time taken to generate data from grazing experiments. The procedure has been tested on laboratory cultures and natural samples, and selected results will be presented to illustrate the potential usefulness of the method.
Dinoflagellate blooms in two S. Worcestershire ponds.
John D. Dodge Biology Dept., Royal Holloway, University of London & 'The Old Farmhouse', Ashton-under-Hill, Worcs., WR11 6SW;
e-mail: Jdodge3458@aol.com
In February 1998 the water (7.5°C) in a small ornamental pond in this Worcestershire village was found to be coloured brown. Microscopic examination revealed the presence of a small dinoflagellate, identified as Gymnodinium inversum Nygaard, in large numbers of around 4000/ml. Over succeeding weeks the progress of the bloom, and the formation of motile planozygotes, followed by knobbly cysts, was monitored. By mid-March the dinoflagellate had completely disappeared from the water column. In 1999 G. inversum first appeared in early January (7.5°C) and reached populations of up to 9000 cells/ml in February and March. The bloom remained until late April ( 17°C) when it quickly collapsed coincident with a large increase in the numbers of ciliate and rotifer predators. Very few cysts were formed. In November 1999 (8.5°C) the Gymnodinium appeared again and reached almost 8000/ml in mid-January 2000. By the end of February the bloom had declined to 2000/ml with no sign of cyst formation.
In August 1999 samples acquired from a large spring-fed irrigation pond (The Moat) in the village were found to contain large numbers of around 5000 cells/ml of Gymnodinium pseudopalustre Schiller (= G. excavatum Nygaard) which coloured the water brown. This bloom soon collapsed, possibly due to a large water influx after heavy rains, but a small population was presentuntil the end of October. Water temperature varied from 17 to 20°C during the bloom. This species forms distinctive spiny cysts but none were observed.
These appear to be the first records in the British Isles of these two contrasting bloom-forming dinoflagellates, one having a winter optimum and the other, summer.
Some of the other protists present in significant numbers will be included in this presentation.
Sex in trypanosomes: give 'em the green light.
W. Gibson, L.E.H. Bingle, J.L. Eastlake and M.Bailey School of Biological Sciences, University of Bristol.
Sexual reproduction has been experimentally demonstrated in one trypanosome species, Trypanosoma brucei, but has never been observed directly. The process takes place in the tsetse fly vector, probably within the salivary glands. Genetic markers appear to be inherited in a Mendelian fashion, but no haploid intermediate stage has been identified and it is not known whether T. brucei can undergo meiosis. To pinpoint the exact location and lifecycle stage involved, we are exploiting the fluorescent reporter gene, GFP, under control of the bacterial Tet repressor. One parental clone has been transformed with both GFP and repressor constructs, so that it does not express GFP. After meiosis, segregation of the reporter and repressor genes should give rise to some hybrid progeny which fluoresce. Such fluorescent trypanosomes have now been observed in the salivary glands of tsetse flies carrying both parental lines and are currently being cloned for further genotypic analysis.