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Experimental evolution in a Paramecium endosymbiont system

Thomas Berendonk - Dresden
Oliver Kaltz - Montpellier
Sunday, March 16, 2014 to Thursday, April 17, 2014

Environmental variation is expected to have strong influence on host-parasite interactions. Adaptation to a particular environment can involve the evolution of novel features, as well as the loss of functions. In parasites with a mixed mode of transmission, environmental conditions can influence host birth rate or host density, and thereby the efficacy of the two transmission modes. If selection acts on the predominant transmission mode, parasites with an initially mixed mode of transmission may evolve to become specialists of either vertical or horizontal transmission. We performed a long-term experiment using microcosm populations of Paramecium caudatum and its bacterial parasite Holospora undulata. The paramecia were kept in three different growth treatments; a high growth treatment replacing half of the medium every second day, a low growth treatment replacing 20% of the medium every days and an alternating treatment switching between high and low growth treatment every eight days. After approximately 800 host generations we conducted infection assays. We tested infectivity of the parasites and host resistance from the different treatment. The high growth parasites had lost the ability to infect hosts horizontally. Further, we found the strongest host resistance for paramecia of the high growth treatment. We believe that the loss of horizontal transmission is the consequence of several effects: no available hosts for infection within the high growing host population (Lipsitch et al. 96, Ebert 2013) due to epidemiological dynamics in the high growth treatment, missing co-infection success and increasing host resistance. Therefore, the loss of function may be the ultimate consequence of a trade-off between the two transmission modes, but not a result from the accumulation of deleterious mutations.

During my STSM period, I had the chance to write up the manuscript together with Oliver Kaltz. The performed studying period of one month was sufficient for manuscript preparation. The results are summarized and discussed in a manuscript, which is currently awaiting comments from our co-author Thomas Berendonk. During the experiment we have not studied the underlying genetic and mechanistic basis of host resistance and parasite’s loss of horizontal transmission. We discussed the idea of investigating the resistance of paramecia in a follow-up study.

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