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Innate immune system of bdelloid rotifers

Final Activity Report Summary - BDELLOID ROTIFERS (Innate immune system of bdelloid rotifers.)

Since the MC candidate K. Van Doninck has been granted a Lecturer position at FUNDP (Belgium), the current project was ended after 9 months.

Bdelloid rotifers are freshwater micro-invertebrates being the most successful group of multicellular organisms reproducing asexually. The success of bdelloids has been called "an evolutionary scandal" because all theories on the evolution of sex predict that bdelloids should be extinct. Their existence indicates that they must have evolved alternate means of achieving the advantages conferred by sexual reproduction. One of the advantages of sex is that recombination efficiently creates genetic variation which allows a species to avoid being driven to extinction by pathogens. Bdelloids cannot create genetic variability by recombination; yet live in environments where they are exposed to parasites, a paradox! Until now, no research has been conducted on this topic and it is timely to start a project studying the parasites of bdelloids and their response to infection. Only fungi of the genus Rotiferophthora have been described as pathogens of bdelloid rotifers (work by G. Barron). The main objectives of this project were threefold, the isolation and characterisation of microbes infecting bdelloids, challenging uninfected bdelloids with pathogens and examining the bdelloid innate immune response.

For the first objective, a comparative field study on parasitism was conducted. Bdelloids were sampled from different freshwater habitats; particular emphasis was placed on collecting bdelloids from sewage plants since C. Ricci did find few infected bdelloids in this type of habitat. The sampled bdelloids were examined under a light microscope to investigate evidence of infection based on several criteria typical for infected Daphnia and for fungus infected bdelloids. Unfortunately, for all the populations examined, few individuals could be identified as swollen, coloured milkish-white and slow moving, but none of those individuals did start an infection when exposed to 'normal' individuals.

For the second objective, we did expose two different bdelloid species, Adineta vaga and Philodina roseola, to the fungus Rotiferophthora minutispora, known to infect bdelloids (work by Glockling). Populations of A. vaga and P. roseola and single P. roseola individuals were exposed to the fungus grown on cornmeal agar (CMA) and 2% water agar (WA). Survival, mortality, fecundity and behavior of bdelloids were recorded. Our aim was twofold; establish whether the fungus does infect the tested bdelloid species and whether behavioural changes are observed in infected bdelloid individuals. Our results indicate that R. minutispora does not act as a pathogen of the two tested bdelloid species. Dead individuals with fungal infections were not apparent and the numbers of living individuals were overall similar in control and inoculated cultures even if the bdelloids did not avoid the mycelium inoculum and laid most of their eggs there. An explanation for the lack of infection is the production of only dictyochlamydospores (resting spores) and no infective conidia by the fungus R.minutispora. Nevertheless, two interesting observations were made during this study.

First, a difference in behaviour between P.roseola and A.vaga was observed. Most A.vaga individuals, whom typically slide over the agar surface, were wholly contracted and immobile while P.roseola individuals were actively swimming, and no resting spores were observed on the CMA when A.vaga was present while many spores were visible in the presence of P.roseola. Purely speculative, the A.vaga individuals might encounter and ingest more spores due to their different locomotive abilities. The wholly contracted bdelloid is also observed during desiccation and it might be a defence mechanism to survive stressful periods.

Second, differences were observed between exposed and control P. roseola for the experiments started with single individuals. Within some replicates the birth rate was lowered compared to others. We have yet no explanation for these observations. K. Van Doninck, with another Rotiferophthora, will repeat these promising experiments with single bdelloid individuals.