X-cell parasites: an emerging threat to marine fish

X-cells - parasites in fish Parasites and their effects on ecosystems have received little attention, but now new DNA sequence data reveals a new and unknown diversity. The last 10 years of research have revealed a large and enigmatic biodiversity among eukaryotic microorganisms. We do not know how they look like, what kind of ecological adaptations they have and how widespread they are. But what is quite certain is that large quantities of these species are parasites or live in close interaction with other organisms. They represent a hidden diversity with potentially enormous impact on ecosystems and economically important aquaculture. In cod, tumors in pseudobranchia can be full of cells from other species. Some of these are known as X-cells because they have a characteristic X-shape. In cod (Gadus morhua L.) X-cell infection causes reduce growth, and the occurrence of natural populations of fishes may be significant. Despite major impact on fish health, there is little that is known about X-cell diversity and ecology, and it is uncertain what kind of phylogenetic placement these species have. This project therefore aims to investigate: ? Diversity and distribution of X-cells in nature ? The phylogenetic position of the X cells. ? Lifecycle and density of cells using FISH methods ? Genes important for infection biology of parasites through comparative genomics studies. The project has managed to uncover a larger diversity of X-cell and close relatives in natural habitats and in various body parts of cod. In addition, the project reveals that X-cell actually consists of many different species, and that all these have a phylogenetic origin within Perkinsea, which already is known as parasites in oysters and amphibians. In addition, we surveyed most of the genetic material in two different tumors of X-cells and developed new informatics tools simplifying identification of parasites, which in turn will be beneficial for applied research projects and relevant institutions. The team behind the project has consisted of scientists from Norway and abroad, including: research group of Dag Klaveness (original project) and Kamran Shalchian-Tabrizi (new project, both University of Oslo), David Bass (Natural History Museum, London), Mark Freeman (University of Malaya, Malaysia), Håkon Hansen (NVI). In addition, Egil Karlsbakk (IMR, Bergen) played a central role. Furthermore, a postdoc, a researcher, one PhD and three master students played central roles. The work has generated several manuscripts and three master dissertations. This summer, a PhD project will be ready for submission.

X-cells are alveolate protists related to the economically important shellfish parasite Perkinsus, amphibian parasites, and more distantly to the human malaria pathogen Plasmodium. X-cells parasitise fish and are known from five teleost Orders, including cod and salmonids, and are likely to represent elevated risks in aquaculture conditions. Molecular studies suggest that X-cells are much more diverse, and are likely to affect many more fish groups, than currently known. Our project will use molecular bio logy techniques and bioinformatics to construct a far more realistic picture of X-cell diversity than current available. We will use these results to determine which environmental factors are the best predictors of X-cell presence and infection, the bioge ography of X-cell lineages, and to improve the phylogeny of the perkinsids so that their evolutionary position with respect to other parasites is clarified. We will also screen fish samples from a range of species and sampling sites so that new X-cell par asites can be firmly associated with their host species. These results will provide a new baseline for making aquacultural decisions with respect to fish disease. We will also use comparative transcriptomics to understand the mechanisms by whi ch X-cells infect cod, and describe the parasitic interaction in terms of gene expression levels in both X-cell and cod. The recent publication of the cod genome and importance of cod aquaculture in Norway make this a research priority. Our results will h ave broader implications as related work is currently underway for other alveolate parasites, so subsequent comparative genomics analyses will show how parasitism evolved in alveolates as a whole. The genome-wide sequence data from the transcriptomes will also enable us to include X-cells in phylogenomic (multi-gene) analyses, which can be highly resolving and show exactly where in the tree of life X-cells belong.