Evolution

Published by: The Society for the Study of Evolution



Evolution 60(7):1358-1371. 2006
doi: http://dx.doi.org/10.1554/05-611.1

WITHIN-HOST COMPETITION IN GENETICALLY DIVERSE MALARIA INFECTIONS: PARASITE VIRULENCE AND COMPETITIVE SUCCESS

Andrew S. Bella, Jacobus C. de Roode2, Derek Sima, and Andrew F. Readab

aInstitutes of Evolution, Immunology and Infection Research, School of Biological Sciences, Kings Buildings, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, United Kingdom

b

2Present address: Institute of Ecology, Ecology Building, University of Georgia, Athens, Georgia 30602-2202

Abstract

Humans and animals often become coinfected with pathogen strains that differ in virulence. The ensuing interaction between these strains can, in theory, be a major determinant of the direction of selection on virulence genes in pathogen populations. Many mathematical analyses of this assume that virulent pathogen lineages have a competitive advantage within coinfected hosts and thus predict that pathogens will evolve to become more virulent where genetically diverse infections are common. Although the implications of these studies are relevant to both fundamental biology and medical science, direct empirical tests for relationships between virulence and competitive ability are lacking. Here we use newly developed strain-specific real-time quantitative polymerase chain reaction protocols to determine the pairwise competitiveness of genetically divergent Plasmodium chabaudi clones that represent a wide range of innate virulences in their rodent host. We found that even against their background of widely varying genotypic and antigenic properties, virulent clones had a competitive advantage in the acute phase of mixed infections. The more virulent a clone was relative to its competitor, the less it suffered from competition. This result confirms our earlier work with parasite lines derived from a single clonal lineage by serial passage and supports the virulence-competitive ability assumption of many theoretical models. To the extent that our rodent model captures the essence of the natural history of malaria parasites, public health interventions which reduce the incidence of mixed malaria infections should have beneficial consequences by reducing the selection for high virulence.

J. Koella

Received: December 5, 2005; Accepted: April 16, 2006



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Fig. 1.  Clone differences in virulence and parasite densities in single-clone infections. Parasite density in the acute phase of infection is the summed total of all parasites present during this period. Plotted points are the means of the number of mice surviving through to the end of the infection period (day 35). Error bars are ±1 SEM. Open circles represent data from experiment 1 and closed circles data from experiment 2. Error bars are missing for AJ in experiment 1 because only a single mouse survived the entire acute phase of infection in this treatment group. Vertical error bars for AT in experiment 2 are smaller than the symbol; horizontal error bars for CB in experiment 1 are based on just two animals

Fig. 2.  Densities of individual parasite clones through time (mean ± 1 SEM), showing the four clones in single-clone infections (left columns), and then comparing their performance alone and in competition with each of the other clones in experiment 1 (a, above) and experiment 2 (b, next page). Means are based on the number of mice alive at that point (maximum n = 5). The days postinfection of mouse deaths are indicated as open circles (individual infections) or closed circles (in competition)

Fig. 2.  Continued.

Fig. 3.  Parasite density of the four clones individually and in pairwise competition during the acute (a, above) and chronic (b, next page) phases of infection in each experiment. For the acute phase of infection (a), experiment 1 panels (E1: panels A, C, E, G, I) show peak parasite densities, whereas experiment 2 panels (E2: B, D, F, H, J) show total parasite densities. Bars represent mean (±1 SEM) of the number of mice surviving up to the end of each phase (maximum n = 5). A single asterisk indicates a single mouse in a treatment group and two asterisks a treatment group in which all mice died

Fig. 3.  Continued

Fig. 4.  Relationship between the relative virulence of clones and the extent of competitive suppression (A, B) and competitiveness (C, D) in competition. Data are pooled across both experiments during the acute (A, C) and chronic (B, D) phases of infections. Relative virulence is the anemia induced by the two competing clones when on their own, expressed as the anemia induced by least virulent competitor as a fraction of that induced by the more virulent. Thus, a value of 1.0 means the competing clones induce equal levels of anemia; a value of 0.5 that the less virulent clone induces half the anemia of the more virulent. Competitive suppression is the proportional reduction in clone density due to the presence of a competitor (1, competitive exclusion; 0, no suppression; <0, facilitation); competitiveness is clone frequency in mixed infections (see text). Dotted lines are least squares regression lines for plotted data. In (A) open circles indicate datapoints from experiment 1 and closed circles datapoints from experiment 2. Solid lines in panels (C) and (D) indicate the null expectation if there was no competition present between the two coinfecting clones (see Materials and Methods). There are a total of 11, not 12, pairwise competitions because there were no surviving mice in the CB + AJ coinfection group in experiment 1

Fig. 5.  Densities of individual parasite clones through time in mixed intections (mean ± 1 SEM). Panels (A–F) experiment 1 (E1), panels (G–L) experiment 2 (E2). Means are based on the number of mice alive at that point (maximum n = 5). Traces terminate at the time point of the last mouse death if all mice in a treatment group died

table

Table 1. Real-time quantitative polymerase chain reaction assays: primer and probe sequences and details of assay specificities.

table

Table 1. Extended

table

Table 2. Parasite densities during the acute and chronic phases of infection. Competition during the acute phase in experiment 1 is based on peak parasite densities rather than total parasite densities (see trait definitions in Materials and Methods). “Competition” is the effect of the presence of a competitor, irrespective of the identity of the clone (i.e., a comparison of the focal clone's performance in mixed infections relative to that achieved alone). “Competitor” is whether the performance of the focal clone in mixed infections varied depending on the identity of the competitor (i.e., a comparison of the performance of the focal clone against each of the three other clones). Significant results indicate suppression of the focal clone, with the exception of an asterisk, which indicates increased numbers (facilitation) of the focal clone in at least some mixed infections. A dash indicates that the clone was absent from the experiment; NP, not possible, where too few mice survived to the chronic phase to enable statistical testing

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