Abstracts (first author)

Talk 

Fish farms select for increased phenotypic plasticity in growth and virulence in a fish pathogen

Author(s): Pulkkinen K, Ketola T, Laakso J, Mappes J, Sundberg L

Summary:

Opportunistic pathogens generally face two vastly different environments - within the host and outside host. One mechanism allowing for adaptation to alternating environments is switching between phenotypes (phenotypic plasticity). The opportunistic fish pathogen Flavobacterium columnare can be found from natural waters and from fish farms and it exhibits two reversible colony morphologies; a non-virulent “rough” and a virulent “rhizoid” morphology. As compared to natural waters, fish farms can be considered as extreme environments in terms of available host resources, but also in terms of stress caused by chemical and antibiotic treatments. Fish farms could thus be expected to impose higher selection pressures for coping between the within and outside host environment, and to select for increased phenotypic plasticity. To test these ideas we measured growth parameters of rhizoid and rough colony morphotypes of F. columnare isolates both from natural waters and from disease outbreaks at fish farms in different resource concentrations and temperatures, and tested their virulence with a zebrafish challenge model. We found that the non-virulent “rough” morphotypes had a higher growth rate and lower virulence than the “rhizoid” morphotypes, but only if the isolate was originating from the fish farms. This suggests that phenotypic plasticity between two morphotypes of opportunistic pathogen and their characteristic traits is clearly selected for in fish farms rather than in the natural environment.



Abstracts (coauthor)

Summary:

Even though parasitism is the most common lifestyle on earth, the impact of the lifestyle on parasite genomes is still poorly known. Adaptation to a specialized niche has been shown to cause reduction of genome size in intracellular parasites (bacteria and eukaryotes) when compared to free-living organisms. However, this genomic reduction has been suggested to be connected to intracellular lifestyle only. We explored the effect of parasitism on genome size in four metazoan taxa: flatworms, nematodes, annelida and arthropods. We found that the genome size was significantly smaller in parasitic taxa when compared to closest free-living taxa in all the studied metazoans. Our results advocate that despite of the high variation in the taxonomic position, evolutionary history and diverse life cycles, parasitism as a lifestyle promotes genomic reduction. The selection for genomic reduction associated with parasitic lifestyle must be strong and adaptive. We suggest that small genome size benefits parasites by decreasing the costs of genome replication, thus increasing their growth and likelihood of successful transmission, leading to enhanced virulence. Because the number of sequenced metazoan parasite genomes is low, it is difficult to estimate if reduction in genome size is due to gene loss or by compaction, both of which are observed in protozoan parasites. In many protozoan parasites genome is reduced by loss of even complete core metabolic pathways, leaving the parasites parasitizing directly on the function of host genes. If similar findings are made with metazoan parasites, we suggest that parasitism on host genes, functions and metabolism could be considered as a core component of the definition of virulence (harm caused by the pathogen leading to reduction of host fitness).

Summary:

Flavobacterium columnare is a gram-negative bacterial pathogen that causes columnaris disease in freshwater aquaculture. Columnaris outbreaks occur at fish farms during summer months and may cause mortality up to 100 %. Virulence of environmental isolates of F. columnare has been found to be lower than those isolated during disease outbreaks at fish farms. In order to understand factors selecting for the higher virulence at fish farms, we studied if the bacterial dose, exposure time (transient or continuous), or nutrients have an effect on the virulence of F. columnare. Three F. columnare strains were used in two separate experiments: a non-virulent strain B398 isolated from the lake and two virulent strains from disease outbreaks (B185 and B67). In the first experiment zebrafish (Danio rerio) and rainbow trout (Oncorhynchus mykiss) fingerlings were individually infected with bath immersion (transient challenge) with 9 different doses of bacterial strains B185, B398, and a mixture of these strains. In the second experiment the bacteria (strains B185, B398 and B67) were added in three doses directly into aquaria (continuous challenge) where zebrafish and rainbow trout were maintained. Longevity of fish was monitored for five days in both experiments, and the infection verified by bacterial culture from gills.
We found bacterial dose to have a positive effect on mortality of both fish species. Increase in nutrients had a significantly positive effect on columnaris infection and fish mortality. The non-virulent strain was able to infect the fish when introduced in continuous exposure, but not in transient challenge. Our results suggest that the continuous exposure to bacteria at fish farms combined with a high nutrient level can promote virulence also in environmental non-virulent bacteria. In addition, the zebrafish can be used as a functional model host to study F. columnare virulence and infection dynamics in the laboratory.

Contacts

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Address

XIV Congress of the European Society for Evolutionary Biology

Organization Team
Department of Animal Biology (DBA)
Faculty of Sciences of the University of Lisbon
P-1749-016 Lisbon
Portugal

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Computational Biology & Population Genomics Group 
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