Résumé
Evolutionary costs of parasite resistance arise if genes conferring
resistance reduce fitness in the absence of parasites. Thus,
parasite-mediated selection may lead to increased resistance and a
correlated decrease in fitness, whereas relaxed parasite-mediated
selection may lead to reverse evolution of increased fitness and a
correlated decrease in resistance. We tested this idea in experimental
populations of the protozoan Paramecium caudatum and the parasitic
bacterium Holospora undulata. After 8 years, resistance to infection and
asexual reproduction were compared among paramecia from (i) infected
populations, (ii) uninfected naive populations and (iii) previously
infected, parasite-free (recovered) populations. Paramecia from infected
populations were more resistant (+12%), but had lower reproduction (-15%)
than naive paramecia, indicating an evolutionary trade-off between
resistance and fitness. Recovered populations showed similar reproduction
to naive populations; however, resistance of recently (<3 years)
recovered populations was similar to paramecia from infected populations,
while longer (>3 years) recovered populations were as susceptible
as naive populations. This suggests a weak, convex trade-off between
resistance and fitness, allowing recovery of fitness, without complete
loss of resistance, favouring the maintenance of a generalist strategy of
intermediate fitness and resistance. Our results indicate that
(co)evolution with parasites can leave a genetic signature in disease-free
populations.