Parasite competitive exclusion

In a project lead by Alison Duncan, we have published anarticle in OIKOS, looking at population-level outcomes of competition between three parasites ofParamecium caudatum.

Duncan, A., Dusi, E., Schrallhammer, M., Berendonk, T., & Kaltz, O. (2018). Population-level dynamics in experimental mixed infections: evidence for competitive exclusion among bacterial parasites of Paramecium caudatum.Oikos, 1–10. doi:10.1111/oik.05280

Summary.Parasites frequently share their host populations with other parasites. However, little is known about how different parasites respond to competition with diverse com­petitor species in the within-host and between-host environments. We explored the repeatability of competition by simultaneously exposing microcosm populations of the ciliateParamecium caudatumto pairs of parasites from theHolosporaspecies complex (H. undulata,H. caryophilaandH. obtusa). We measured how competition affected the persistence and prevalence of each compared to single infections, across three host genotypes. Three weeks post-inoculation we identified the presence of each parasite using fluorescence in situ hybridisation (FISH). Competitive exclusion (62/72) was more common than co-existence (10/72) in populations inoculated with two para­sites. There was a clear pattern of competitive superiority, withH.caryophilapersist­ing in all doubly inoculated populations (with eitherH. undulataorH. obtusa), andH. undulatatending to excludeH. obtusa. This mirrored infection success in single infections, withH. caryophilahaving a higher infection prevalence in single inocula­tions, followed byH. undulatathenH. obtusa. The probability of persistence in co-inoculations did not change across the different host genotypes, and prevalence was the same as in single infections. Our results are consistent with superinfection models, which assume the competitive exclusion of parasites upon contact within the same host. Furthermore, such non-random competitive epidemiological dynamics, where one parasite always wins, may be of interest for public health management, especially if the winning parasite is avirulent, as is seemingly the case here.