Effects of shortened host life span on the evolution of parasite life history and virulence in a microbial host-parasite system

Thibault Nidelet,Oliver Kaltz,Jacob C Koella

doi:10.1186/1471-2148-9-65

Abstract

BackgroundEcological 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.ResultsAfter 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.ConclusionThe 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.

Highlights

Ecological factors play an important role in the evolution of parasite exploitation strategies
Understanding the factors that shape the evolution of parasite life history and virulence is a major issue in evolutionary biology [1,2,3,4], with important implications in applied and medical contexts [5,6]
A general prediction concerning environmental factors is that increased extrinsic host mortality selects for higher virulence [[11,14,16,16], but see [17]]

Summary

Introduction

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, be more virulent. To compensate for the loss of future transmission, parasites should evolve to grow more rapidly within the host, start transmission earlier and, be more virulent This prediction can be restated in terms of classical lifehistory theory: if future reproduction is compromised, we expect selection for earlier age at maturity and increased investment in early reproduction [18]. It is important to understand how parasite life-history, such as latency time or fecundity, responds to selection imposed by such a treatment [20,21]

Methods

Results

Discussion

Conclusion