Effects of shortened host life span on the evolution of parasite life history and virulence in a microbial host-parasite system
1 UPMC University Paris 06, Laboratoire de Parasitologie Evolutive – UMR 7103, 7 quai St-Bernard, 75252 Paris, France
2 UMR de Génétique Végétale, University Paris Sud/INRA/CNRS/INAPG, Ferme du Moulon, 91190 Gif-sur-Yvette, France
3 Division of Biology, Imperial College London, Silwood Park Campus, Ascot SL5 7PY, UK
4 Institut des Sciences de l'Evolution, UMR 5554, Université de Montpellier 2, CC 065, Place Eugène Bataillon, 34095 Montpellier, France
BMC Evolutionary Biology 2009, 9:65 doi:10.1186/1471-2148-9-65Published: 25 March 2009
Ecological factors play an important role in the evolution of parasite exploitation strategies. A common prediction is that, as shorter host life span reduces future opportunities of transmission, parasites compensate with an evolutionary shift towards earlier transmission. They may grow more rapidly within the host, have a shorter latency time and, consequently, be more virulent. Thus, increased extrinsic (i.e., not caused by the parasite) host mortality leads to the evolution of more virulent parasites. To test these predictions, we performed a serial transfer experiment, using the protozoan Paramecium caudatum and its bacterial parasite Holospora undulata. We simulated variation in host life span by killing hosts after 11 (early killing) or 14 (late killing) days post inoculation; after killing, parasite transmission stages were collected and used for a new infection cycle.
After 13 cycles (≈ 300 generations), parasites from the early-killing treatment were less infectious, but had shorter latency time and higher virulence than those from the late-killing treatment. Overall, shorter latency time was associated with higher parasite loads and thus presumably with more rapid within-host replication.
The analysis of the means of the two treatments is thus consistent with theory, and suggests that evolution is constrained by trade-offs between virulence, transmission and within-host growth. In contrast, we found little evidence for such trade-offs across parasite selection lines within treatments; thus, to some extent, these traits may evolve independently. This study illustrates how environmental variation (experienced by the host) can lead to the evolution of distinct parasite strategies.