Résumé
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 ciliate Paramecium
caudatum to pairs of parasites from the Holospora species complex (H.
undulata, H. caryophila and H. obtusa) 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 2 parasites. There was a clear pattern of competitive
superiority, with H. caryophila persisting in all doubly inoculated
populations (with either H. undulata or H. obtusa), and H. undulata
tending to exclude H. obtusa. This mirrored infection success in single
infections, with H. caryophila having a higher infection prevalence in
single inoculations, followed by H. undulata then H. 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 parasites always wins, may be of interest for public health
management, especially if the winning parasite is avirulent, as is
seemingly the case here.