501
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Leggett HC, Cornwallis CK, West SA. Mechanisms of pathogenesis, infective dose and virulence in human parasites. PLoS Pathog 2012; 8:e1002512. [PMID: 22359500 PMCID: PMC3280976 DOI: 10.1371/journal.ppat.1002512] [Citation(s) in RCA: 87] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2011] [Accepted: 12/15/2011] [Indexed: 01/10/2023] Open
Abstract
The number of pathogens that are required to infect a host, termed infective dose, varies dramatically across pathogen species. It has recently been predicted that infective dose will depend upon the mode of action of the molecules that pathogens use to facilitate their infection. Specifically, pathogens which use locally acting molecules will require a lower infective dose than pathogens that use distantly acting molecules. Furthermore, it has also been predicted that pathogens with distantly acting immune modulators may be more virulent because they have a large number of cells in the inoculums, which will cause more harm to host cells. We formally test these predictions for the first time using data on 43 different human pathogens from a range of taxonomic groups with diverse life-histories. We found that pathogens using local action do have lower infective doses, but are not less virulent than those using distant action. Instead, we found that virulence was negatively correlated with infective dose, and higher in pathogens infecting wounded skin, compared with those ingested or inhaled. More generally, our results show that broad-scale comparative analyses can explain variation in parasite traits such as infective dose and virulence, whilst highlighting the importance of mechanistic details.
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Affiliation(s)
- Helen C Leggett
- Department of Zoology, Oxford University, Oxford, United Kingdom.
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502
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Wargo AR, Kell AM, Scott RJ, Thorgaard GH, Kurath G. Analysis of host genetic diversity and viral entry as sources of between-host variation in viral load. Virus Res 2012; 165:71-80. [PMID: 22310066 DOI: 10.1016/j.virusres.2012.01.010] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2011] [Revised: 01/21/2012] [Accepted: 01/22/2012] [Indexed: 12/16/2022]
Abstract
Little is known about the factors that drive the high levels of between-host variation in pathogen burden that are frequently observed in viral infections. Here, two factors thought to impact viral load variability, host genetic diversity and stochastic processes linked with viral entry into the host, were examined. This work was conducted with the aquatic vertebrate virus, Infectious hematopoietic necrosis virus (IHNV), in its natural host, rainbow trout. It was found that in controlled in vivo infections of IHNV, a suggestive trend of reduced between-fish viral load variation was observed in a clonal population of isogenic trout compared to a genetically diverse population of out-bred trout. However, this trend was not statistically significant for any of the four viral genotypes examined, and high levels of fish-to-fish variation persisted even in the isogenic trout population. A decrease in fish-to-fish viral load variation was also observed in virus injection challenges that bypassed the host entry step, compared to fish exposed to the virus through the natural water-borne immersion route of infection. This trend was significant for three of the four virus genotypes examined and suggests host entry may play a role in viral load variability. However, high levels of viral load variation also remained in the injection challenges. Together, these results indicate that although host genetic diversity and viral entry may play some role in between-fish viral load variation, they are not major factors. Other biological and non-biological parameters that may influence viral load variation are discussed.
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Affiliation(s)
- Andrew R Wargo
- U.S. Geological Survey, Western Fisheries Research Center, 6505 NE 65th Street, Seattle, WA 98115-5016, USA.
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503
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Lannou C. Variation and selection of quantitative traits in plant pathogens. ANNUAL REVIEW OF PHYTOPATHOLOGY 2012; 50:319-38. [PMID: 22702351 DOI: 10.1146/annurev-phyto-081211-173031] [Citation(s) in RCA: 79] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
The first section presents the quantitative traits of pathogenicity that are most commonly measured by plant pathologists, how the expression of those traits is influenced by environmental factors, and why the traits must be taken into account for understanding pathogen evolution in agricultural systems. Particular attention is given to the shared genetic control of these traits by the host and the pathogen. Next, the review discusses how quantitative traits account for epidemic development and how they can be related to pathogen fitness. The main constraints that influence the evolution of quantitative traits in pathogen populations are detailed. Finally, possible directions for research on the management of pathogen virulence (as defined by evolutionists) and host quantitative resistance are presented. The review evaluates how the theoretical corpus developed by epidemiologists and evolutionists may apply to plant pathogens in the context of agriculture. The review also analyzes theoretical papers and compares the modeling hypotheses to the biological characteristics of plant pathogens.
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504
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505
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Osnas EE, Dobson AP. EVOLUTION OF VIRULENCE IN HETEROGENEOUS HOST COMMUNITIES UNDER MULTIPLE TRADE-OFFS. Evolution 2011; 66:391-401. [DOI: 10.1111/j.1558-5646.2011.01461.x] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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506
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Alizon S, Michalakis Y. THE TRANSMISSION-VIRULENCE TRADE-OFF AND SUPERINFECTION: COMMENTS TO SMITH. Evolution 2011; 65:3633-8; discussion 3639-41. [DOI: 10.1111/j.1558-5646.2011.01432.x] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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507
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Laine AL. Context-dependent effects of induced resistance under co-infection in a plant-pathogen interaction. Evol Appl 2011; 4:696-707. [PMID: 25568016 PMCID: PMC3352536 DOI: 10.1111/j.1752-4571.2011.00194.x] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2011] [Accepted: 05/12/2011] [Indexed: 12/31/2022] Open
Abstract
The ability of a parasite strain to establish and grow on its host may be drastically altered by simultaneous infection by other parasite strains, and dynamics under multiple infection have been suggested to be a major force driving pathogen evolution. Here, I studied whether hosts' induced defenses mediate dynamics of multiple infection of the fungal pathogen, Podosphaera plantaginis, infecting Plantago lanceolata. A laboratory study of sequential infections, where interaction between pathogen strains was prevented, showed that ability to establish remained unaffected, but prior infection elevates the host's resistance to the degree that subsequent infection development is significantly reduced. However, when inoculated plants and their healthy controls were planted back into their natural populations, hosts with prior infection became more heavily infected by the subsequent infections than the initially healthy plants. Hence, a controlled short-term laboratory study is a poor predictor of the host's ability to mediate multiple infection during the course of natural epidemics. These results have applied implications for priming where the plants' defenses are elicited to provide protection against further attack, highlighting the importance of testing priming under natural conditions for relevant time scales.
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Affiliation(s)
- Anna-Liisa Laine
- Metapopulation Research Group, Department of Biosciences, University of Helsinki Finland ; Department of Biology, University of Turku Turku, Finland
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508
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Mideo N, Nelson WA, Reece SE, Bell AS, Read AF, Day T. Bridging scales in the evolution of infectious disease life histories: application. Evolution 2011; 65:3298-310. [PMID: 22023593 DOI: 10.1111/j.1558-5646.2011.01382.x] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Within- and between-host disease processes occur on the same timescales, therefore changes in the within-host dynamics of parasites, resources, and immunity can interact with changes in the epidemiological dynamics to affect evolutionary outcomes. Consequently, studies of the evolution of disease life histories, that is, infection-age-specific patterns of transmission and virulence, have been constrained by the need for a mechanistic understanding of within-host disease dynamics. In a companion paper (Day et al. 2011), we develop a novel approach that quantifies the relevant within-host aspects of disease through genetic covariance functions. Here, we demonstrate how to apply this theory to data. Using two previously published datasets from rodent malaria infections, we show how to translate experimental measures into disease life-history traits, and how to quantify the covariance in these traits. Our results show how patterns of covariance can interact with epidemiological dynamics to affect evolutionary predictions for disease life history. We also find that the selective constraints on disease life-history evolution can vary qualitatively, and that "simple" virulence-transmission trade-offs that are often the subject of experimental investigation can be obscured by trade-offs within one trait alone. Finally, we highlight the type and quality of data required for future applications.
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Affiliation(s)
- Nicole Mideo
- Centre for Immunity, Infection, and Evolution, School of Biological Sciences, University of Edinburgh, Edinburgh, UK.
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509
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Vale PF, Wilson AJ, Best A, Boots M, Little TJ. Epidemiological, evolutionary, and coevolutionary implications of context-dependent parasitism. Am Nat 2011; 177:510-21. [PMID: 21460572 DOI: 10.1086/659002] [Citation(s) in RCA: 83] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022]
Abstract
Abstract Victims of infection are expected to suffer increasingly as parasite population growth increases. Yet, under some conditions, faster-growing parasites do not appear to cause more damage, and infections can be quite tolerable. We studied these conditions by assessing how the relationship between parasite population growth and host health is sensitive to environmental variation. In experimental infections of the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa, we show how easily an interaction can shift from a severe interaction, that is, when host fitness declines substantially with each unit of parasite growth, to a tolerable relationship by changing only simple environmental variables: temperature and food availability. We explored the evolutionary and epidemiological implications of such a shift by modeling pathogen evolution and disease spread under different levels of infection severity and found that environmental shifts that promote tolerance ultimately result in populations harboring more parasitized individuals. We also find that the opportunity for selection, as indicated by the variance around traits, varied considerably with the environmental treatment. Thus, our results suggest two mechanisms that could underlie coevolutionary hotspots and coldspots: spatial variation in tolerance and spatial variation in the opportunity for selection.
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Affiliation(s)
- Pedro F Vale
- Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Ashworth Labs, West Mains Road, Edinburgh, Scotland, United Kingdom.
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510
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Day T, Alizon S, Mideo N. BRIDGING SCALES IN THE EVOLUTION OF INFECTIOUS DISEASE LIFE HISTORIES: THEORY. Evolution 2011; 65:3448-61. [DOI: 10.1111/j.1558-5646.2011.01394.x] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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511
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Peñaranda MMD, Wargo AR, Kurath G. In vivo fitness correlates with host-specific virulence of Infectious hematopoietic necrosis virus (IHNV) in sockeye salmon and rainbow trout. Virology 2011; 417:312-9. [PMID: 21745673 DOI: 10.1016/j.virol.2011.06.014] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2011] [Revised: 06/02/2011] [Accepted: 06/06/2011] [Indexed: 11/26/2022]
Abstract
The relationship between virulence and overall within-host fitness of the fish rhabdovirus Infectious hematopoietic necrosis virus (IHNV) was empirically investigated in vivo for two virus isolates belonging to different IHNV genogroups that exhibit opposing host-specific virulence. U group isolates are more virulent in sockeye salmon and M group isolates are more virulent in rainbow trout. In both single and mixed infections in the two fish hosts, the more virulent IHNV type exhibited higher prevalence and higher viral load than the less virulent type. Thus, a positive correlation was observed between higher in vivo fitness and higher host-specific virulence in sockeye salmon and rainbow trout. Comparisons of mean viral loads in single and mixed infections revealed no evidence for limitation due to competition effects between U and M viruses in either rainbow trout or sockeye salmon co-infections.
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Affiliation(s)
- Ma Michelle D Peñaranda
- Graduate Program in Pathobiology, Department of Global Health, University of Washington, Seattle, WA 98195, USA.
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512
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Ping J, Keleta L, Forbes NE, Dankar S, Stecho W, Tyler S, Zhou Y, Babiuk L, Weingartl H, Halpin RA, Boyne A, Bera J, Hostetler J, Fedorova NB, Proudfoot K, Katzel DA, Stockwell TB, Ghedin E, Spiro DJ, Brown EG. Genomic and protein structural maps of adaptive evolution of human influenza A virus to increased virulence in the mouse. PLoS One 2011; 6:e21740. [PMID: 21738783 PMCID: PMC3128085 DOI: 10.1371/journal.pone.0021740] [Citation(s) in RCA: 68] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/25/2010] [Accepted: 06/10/2011] [Indexed: 12/11/2022] Open
Abstract
Adaptive evolution is characterized by positive and parallel, or repeated selection of mutations. Mouse adaptation of influenza A virus (IAV) produces virulent mutants that demonstrate positive and parallel evolution of mutations in the hemagglutinin (HA) receptor and non-structural protein 1 (NS1) interferon antagonist genes. We now present a genomic analysis of all 11 genes of 39 mouse adapted IAV variants from 10 replicate adaptation experiments. Mutations were mapped on the primary and structural maps of each protein and specific mutations were validated with respect to virulence, replication, and RNA polymerase activity. Mouse adapted (MA) variants obtained after 12 or 20–21 serial infections acquired on average 5.8 and 7.9 nonsynonymous mutations per genome of 11 genes, respectively. Among a total of 115 nonsynonymous mutations, 51 demonstrated properties of natural selection including 27 parallel mutations. The greatest degree of parallel evolution occurred in the HA receptor and ribonucleocapsid components, polymerase subunits (PB1, PB2, PA) and NP. Mutations occurred in host nuclear trafficking factor binding sites as well as sites of virus-virus protein subunit interaction for NP, NS1, HA and NA proteins. Adaptive regions included cap binding and endonuclease domains in the PB2 and PA polymerase subunits. Four mutations in NS1 resulted in loss of binding to the host cleavage and polyadenylation specificity factor (CPSF30) suggesting that a reduction in inhibition of host gene expression was being selected. The most prevalent mutations in PB2 and NP were shown to increase virulence but differed in their ability to enhance replication and demonstrated epistatic effects. Several positively selected RNA polymerase mutations demonstrated increased virulence associated with >300% enhanced polymerase activity. Adaptive mutations that control host range and virulence were identified by their repeated selection to comprise a defined model for studying IAV evolution to increased virulence in the mouse.
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Affiliation(s)
- Jihui Ping
- Department of Biochemistry, Microbiology and Immunology, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
- Emerging Pathogens Research Centre, University of Ottawa, Ottawa, Ontario, Canada
- Canadian Institutes of Health Research (CIHR) Canadian Influenza Pathogenesis Team, University of Ottawa, Ottawa, Ontario, Canada
| | - Liya Keleta
- Department of Biochemistry, Microbiology and Immunology, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
- Emerging Pathogens Research Centre, University of Ottawa, Ottawa, Ontario, Canada
- Canadian Institutes of Health Research (CIHR) Canadian Influenza Pathogenesis Team, University of Ottawa, Ottawa, Ontario, Canada
| | - Nicole E. Forbes
- Department of Biochemistry, Microbiology and Immunology, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
- Emerging Pathogens Research Centre, University of Ottawa, Ottawa, Ontario, Canada
- Canadian Institutes of Health Research (CIHR) Canadian Influenza Pathogenesis Team, University of Ottawa, Ottawa, Ontario, Canada
| | - Samar Dankar
- Department of Biochemistry, Microbiology and Immunology, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
- Emerging Pathogens Research Centre, University of Ottawa, Ottawa, Ontario, Canada
- Canadian Institutes of Health Research (CIHR) Canadian Influenza Pathogenesis Team, University of Ottawa, Ottawa, Ontario, Canada
| | - William Stecho
- Department of Biochemistry, Microbiology and Immunology, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
- Emerging Pathogens Research Centre, University of Ottawa, Ottawa, Ontario, Canada
| | - Shaun Tyler
- National Microbiology Laboratory, Canadian Science Centre for Human and Animal Health, Public Health Agency of Canada, Winnipeg, Manitoba, Canada
| | - Yan Zhou
- Vaccine and Infectious Disease Organization, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
- Canadian Institutes of Health Research (CIHR) Canadian Influenza Pathogenesis Team, University of Ottawa, Ottawa, Ontario, Canada
| | - Lorne Babiuk
- Vaccine and Infectious Disease Organization, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
- Canadian Institutes of Health Research (CIHR) Canadian Influenza Pathogenesis Team, University of Ottawa, Ottawa, Ontario, Canada
| | - Hana Weingartl
- National Centre for Foreign Animal Disease, Canadian Food Inspection Agency, Winnipeg, Manitoba, Canada
- Canadian Institutes of Health Research (CIHR) Canadian Influenza Pathogenesis Team, University of Ottawa, Ottawa, Ontario, Canada
| | - Rebecca A. Halpin
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
| | - Alex Boyne
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
| | - Jayati Bera
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
| | - Jessicah Hostetler
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
| | - Nadia B. Fedorova
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
| | - Katie Proudfoot
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
| | - Dan A. Katzel
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
| | - Tim B. Stockwell
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
| | - Elodie Ghedin
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
- Center for Vaccine Research, Department of Computational and Systems Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, United States of America
| | - David J. Spiro
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
- Viral Genomics Group, J. Craig Venter Institute, Rockville, Maryland, United States of America
| | - Earl G. Brown
- Department of Biochemistry, Microbiology and Immunology, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada
- Emerging Pathogens Research Centre, University of Ottawa, Ottawa, Ontario, Canada
- Canadian Institutes of Health Research (CIHR) Canadian Influenza Pathogenesis Team, University of Ottawa, Ottawa, Ontario, Canada
- Canadian Institutes of Health Research (CIHR) Canadian Influenza Pathogenesis Team, University of Ottawa, Ottawa, Ontario, Canada
- * E-mail:
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513
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Alizon S, Lion S. Within-host parasite cooperation and the evolution of virulence. Proc Biol Sci 2011; 278:3738-47. [PMID: 21561974 DOI: 10.1098/rspb.2011.0471] [Citation(s) in RCA: 47] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Infections by multiple genotypes are common in nature and are known to select for higher levels of virulence for some parasites. When parasites produce public goods (PGs) within the host, such co-infections have been predicted to select for lower levels of virulence. However, this prediction is based on simplifying assumptions regarding epidemiological feedbacks on the multiplicity of infections (MOI). Here, we analyse the case of parasites producing a PG (for example, siderophore-producing bacteria) using a nested model that ties together within-host and epidemiological processes. We find that the prediction that co-infection should select for less virulent strains for PG-producing parasites is only valid if both parasite transmission and virulence are linear functions of parasite density. If there is a trade-off relationship such that virulence increases more rapidly than transmission, or if virulence also depends on the total amount of PGs produced, then more complex relationships between virulence and the MOI are predicted. Our results reveal that explicitly taking into account the distribution of parasite strains among hosts could help better understand the selective pressures faced by parasites at the population level.
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Affiliation(s)
- Samuel Alizon
- Laboratoire MIVEGEC (UMR CNRS 5290, IRD 224, UM1, UM2) 911 avenue Agropolis, B.P. 64501, 34394 Montpellier Cedex 5, France.
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514
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Evolution of virulence driven by predator–prey interaction: Possible consequences for population dynamics. J Theor Biol 2011; 276:181-91. [DOI: 10.1016/j.jtbi.2011.02.007] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2010] [Revised: 02/05/2011] [Accepted: 02/08/2011] [Indexed: 11/21/2022]
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515
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Long GH, Boots M. How can immunopathology shape the evolution of parasite virulence? Trends Parasitol 2011; 27:300-5. [PMID: 21531628 DOI: 10.1016/j.pt.2011.03.012] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2010] [Revised: 03/31/2011] [Accepted: 03/31/2011] [Indexed: 01/05/2023]
Abstract
Immunopathology (immune-mediated pathology) is a ubiquitous cause of disease during infection, but how will parasite exploitation strategies evolve in its presence? Immunopathology can act to increase parasite fitness if it increases transmission rate, but can equally act to decrease parasite fitness if it increases host mortality. The focus here is on understanding how immunopathology, mediated through different immune mechanisms, can influence parasite fitness and how experimental manipulations of the immune system can be carried out to examine this. A better understanding of how parasite fitness scales with, or responds to, immunopathology is crucial to understanding the nature of selection acting on parasite virulence traits and will allow more informed predictions to be made regarding the trajectory of parasite virulence evolution.
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Affiliation(s)
- Gráinne H Long
- Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield, S10 2TN, UK.
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516
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Roche B, Drake JM, Rohani P. The curse of the Pharaoh revisited: evolutionary bi-stability in environmentally transmitted pathogens. Ecol Lett 2011; 14:569-75. [PMID: 21496194 DOI: 10.1111/j.1461-0248.2011.01619.x] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
It is increasingly evident that for a number of high-profile pathogens, transmission involves both direct and environmental pathways. Much of the distinguished evolutionary theory has, however, focused on each of transmission component separately. Herein, we use the framework of adaptive dynamics to study the evolutionary consequences of mixed transmission. We find that environmental transmission can select for increased virulence when direct transmission is low. Increasing the efficiency of direct transmission gives rise to an evolutionary bi-stability, with coexistence of different levels of virulence. We conclude that the overlooked contribution of environmental transmission may explain the curious appearance of high virulence in pathogens that are typically only moderately pathogenic, as observed for avian influenza viruses and cholera.
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Affiliation(s)
- Benjamin Roche
- Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI 48109, USA.
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517
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Wargo AR, Kurath G. In vivo fitness associated with high virulence in a vertebrate virus is a complex trait regulated by host entry, replication, and shedding. J Virol 2011; 85:3959-67. [PMID: 21307204 PMCID: PMC3126118 DOI: 10.1128/jvi.01891-10] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2010] [Accepted: 01/29/2011] [Indexed: 12/14/2022] Open
Abstract
The relationship between pathogen fitness and virulence is typically examined by quantifying only one or two pathogen fitness traits. More specifically, it is regularly assumed that within-host replication, as a precursor to transmission, is the driving force behind virulence. In reality, many traits contribute to pathogen fitness, and each trait could drive the evolution of virulence in different ways. Here, we independently quantified four viral infection cycle traits, namely, host entry, within-host replication, within-host coinfection fitness, and shedding, in vivo, in the vertebrate virus Infectious hematopoietic necrosis virus (IHNV). We examined how each of these stages of the viral infection cycle contributes to the fitness of IHNV genotypes that differ in virulence in rainbow trout. This enabled us to determine how infection cycle fitness traits are independently associated with virulence. We found that viral fitness was independently regulated by each of the traits examined, with the largest impact on fitness being provided by within-host replication. Furthermore, the more virulent of the two genotypes of IHNV we used had advantages in all of the traits quantified. Our results are thus congruent with the assumption that virulence and within-host replication are correlated but suggest that infection cycle fitness is complex and that replication is not the only trait associated with virulence.
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Affiliation(s)
- Andrew R Wargo
- Western Fisheries Research Center, 6505 NE 65th Street, Seattle, WA 98115-5016, USA.
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518
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Parker BJ, Barribeau SM, Laughton AM, de Roode JC, Gerardo NM. Non-immunological defense in an evolutionary framework. Trends Ecol Evol 2011; 26:242-8. [PMID: 21435735 DOI: 10.1016/j.tree.2011.02.005] [Citation(s) in RCA: 139] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2010] [Revised: 02/09/2011] [Accepted: 02/10/2011] [Indexed: 11/18/2022]
Abstract
After parasite infection, invertebrates activate immune system-based defenses such as encapsulation and the signaling pathways of the innate immune system. However, hosts are often able to defend against parasites without using these mechanisms. The non-immunological defenses, such as behaviors that prevent or combat infection, symbiont-mediated defense, and fecundity compensation, are often ignored but can be important in host-parasite dynamics. We review recent studies showing that heritable variation in these traits exists among individuals, and that they are costly to activate and maintain. We also discuss findings from genome annotation and expression studies to show how immune system-based and non-immunological defenses interact. Placing these studies into an evolutionary framework emphasizes their importance for future studies of host-parasite coevolution.
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Affiliation(s)
- Benjamin J Parker
- Department of Biology, Emory University, O. Wayne Rollins Research Center, 1510 E. Clifton Rd. N.E., Atlanta, GA 30322, USA
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519
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Brusini J, Robin C, Franc A. Parasitism and maintenance of diversity in a fungal vegetative incompatibility system: the role of selection by deleterious cytoplasmic elements. Ecol Lett 2011; 14:444-52. [PMID: 21382145 DOI: 10.1111/j.1461-0248.2011.01602.x] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
In fungi, horizontal transmission of deleterious cytoplasmic elements is reduced by the vegetative incompatibility system. This self/non-self recognition system may select for greater diversity of fungal incompatibility phenotypes in a frequency-dependent manner but the link between the diversity of fungal phenotypes and the virulence of cytoplasmic parasites has been poorly studied. We used an epidemiological model to show that even when transmission between incompatibility types is permitted, parasite pressure can lead to high levels of polymorphism for vegetative incompatibility systems. Moreover, high levels of polymorphism in host populations can select for less virulent cytoplasmic parasites. This feedback mechanism between parasite virulence and vegetative incompatibility system polymorphism of host populations may account for the general avirulence of most known mycoviruses. Furthermore, this mechanism provides a new perspective on the particular ecology and evolution of the host/parasite interactions acting between fungi and their cytoplasmic parasites.
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Affiliation(s)
- Jérémie Brusini
- INRA, UMR 1202 Biodiversity, Genes & Communities, 69 Route d'Arcachon, F-33610 Cestas, France.
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520
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de Roode JC, Rarick RM, Mongue AJ, Gerardo NM, Hunter MD. Aphids indirectly increase virulence and transmission potential of a monarch butterfly parasite by reducing defensive chemistry of a shared food plant. Ecol Lett 2011; 14:453-61. [PMID: 21375682 DOI: 10.1111/j.1461-0248.2011.01604.x] [Citation(s) in RCA: 47] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
Parasites and hosts live in communities consisting of many interacting species, but few studies have examined how communities affect parasite virulence and transmission. We studied a food web consisting of two species of milkweed, two milkweed herbivores (monarch butterfly and oleander aphid) and a monarch butterfly-specific parasite. We found that the presence of aphids increased the virulence and transmission potential of the monarch butterfly's parasite on one milkweed species. These increases were associated with aphid-induced decreases in the defensive chemicals of milkweed plants. Our experiment suggests that aphids can indirectly increase the virulence and transmission potential of monarch butterfly parasites, probably by altering the chemical composition of a shared food plant. These results indicate that species that are far removed from host-parasite interactions can alter such interactions through cascading indirect effects in the food web. As such, indirect effects within ecological communities may drive the dynamics and evolution of parasites.
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Affiliation(s)
- Jacobus C de Roode
- Department of Biology, Emory University, 1510 Clifton Road, Atlanta, GA 30322, USA.
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521
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Malaria and trypanosome transmission: different parasites, same rules? Trends Parasitol 2011; 27:197-203. [PMID: 21345732 PMCID: PMC3087881 DOI: 10.1016/j.pt.2011.01.004] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2010] [Revised: 01/19/2011] [Accepted: 01/20/2011] [Indexed: 11/04/2022]
Abstract
African trypanosomes produce different specialized stages for within-host replication and between-host transmission and therefore face a resource allocation trade-off between maintaining the current infection (survival) and investment into transmission (reproduction). Evolutionary theory predicts the resolution of this trade-off will significantly affect virulence and infectiousness. The application of life history theory to malaria parasites has provided novel insight into their strategies for survival and reproduction; how this framework can now be applied to trypanosomes is discussed. Specifically, predictions for how parasites trade-off investment in survival and transmission in response to variation in the within-host environment are outlined. An evolutionary approach has the power to explain why patterns of investment vary between strains and during infections, giving important insights into parasite biology.
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522
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Chapuis É, Pagès S, Emelianoff V, Givaudan A, Ferdy JB. Virulence and pathogen multiplication: a serial passage experiment in the hypervirulent bacterial insect-pathogen Xenorhabdus nematophila. PLoS One 2011; 6:e15872. [PMID: 21305003 PMCID: PMC3031541 DOI: 10.1371/journal.pone.0015872] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2010] [Accepted: 11/25/2010] [Indexed: 11/20/2022] Open
Abstract
The trade-off hypothesis proposes that the evolution of pathogens' virulence is shaped by a link between virulence and contagiousness. This link is often assumed to come from the fact that pathogens are contagious only if they can reach high parasitic load in the infected host. In this paper we present an experimental test of the hypothesis that selection on fast replication can affect virulence. In a serial passage experiment, we selected 80 lines of the bacterial insect-pathogen Xenorhabdus nematophila to multiply fast in an artificial culture medium. This selection resulted in shortened lag phase in our selected bacteria. We then injected these bacteria into insects and observed an increase in virulence. This could be taken as a sign that virulence in Xenorhabdus is linked to fast multiplication. But we found, among the selected lineages, either no link or a positive correlation between lag duration and virulence: the most virulent bacteria were the last to start multiplying. We then surveyed phenotypes that are under the control of the flhDC super regulon, which has been shown to be involved in Xenorhabdus virulence. We found that, in one treatment, the flhDC regulon has evolved rapidly, but that the changes we observed were not connected to virulence. All together, these results indicate that virulence is, in Xenorhabdus as in many other pathogens, a multifactorial trait. Being able to grow fast is one way to be virulent. But other ways exist which renders the evolution of virulence hard to predict.
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Affiliation(s)
- Élodie Chapuis
- The Center for Biology and Management of Populations, Montpellier, France
| | - Sylvie Pagès
- Laboratoire EMIP, UMR INRA UM2 1133, Université Montpellier 2, Montpellier, France
| | - Vanya Emelianoff
- The Center for Biology and Management of Populations, Montpellier, France
| | - Alain Givaudan
- Laboratoire EMIP, UMR INRA UM2 1133, Université Montpellier 2, Montpellier, France
| | - Jean-Baptiste Ferdy
- Laboratoire Évolution et Diversité Biologique UMR CNRS UPS 5174, Université Toulouse 3, Toulouse, France
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523
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de Roode JC, de Castillejo CLF, Faits T, Alizon S. Virulence evolution in response to anti-infection resistance: toxic food plants can select for virulent parasites of monarch butterflies. J Evol Biol 2011; 24:712-22. [PMID: 21261772 DOI: 10.1111/j.1420-9101.2010.02213.x] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
Host resistance to parasites can come in two main forms: hosts may either reduce the probability of parasite infection (anti-infection resistance) or reduce parasite growth after infection has occurred (anti-growth resistance). Both resistance mechanisms are often imperfect, meaning that they do not fully prevent or clear infections. Theoretical work has suggested that imperfect anti-growth resistance can select for higher parasite virulence by favouring faster-growing and more virulent parasites that overcome this resistance. In contrast, imperfect anti-infection resistance is thought not to select for increased parasite virulence, because it is assumed that it reduces the number of hosts that become infected, but not the fitness of parasites in successfully infected hosts. Here, we develop a theoretical model to show that anti-infection resistance can in fact select for higher virulence when such resistance reduces the effective parasite dose that enters a host. Our model is based on a monarch butterfly-parasite system in which larval food plants confer resistance to the monarch host. We carried out an experiment and showed that this environmental resistance is most likely a form of anti-infection resistance, through which toxic food plants reduce the effective dose of parasites that initiates an infection. We used these results to build a mathematical model to investigate the evolutionary consequences of food plant-induced resistance. Our model shows that when the effective infectious dose is reduced, parasites can compensate by evolving a higher per-parasite growth rate, and consequently a higher intrinsic virulence. Our results are relevant to many insect host-parasite systems, in which larval food plants often confer imperfect anti-infection resistance. Our results also suggest that - for parasites where the infectious dose affects the within-host dynamics - vaccines that reduce the effective infectious dose can select for increased parasite virulence.
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Affiliation(s)
- J C de Roode
- Biology Department, Emory University, Atlanta, GA 30322, USA.
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524
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Rankin DJ, Rocha EPC, Brown SP. What traits are carried on mobile genetic elements, and why? Heredity (Edinb) 2011; 106:1-10. [PMID: 20332804 PMCID: PMC3183850 DOI: 10.1038/hdy.2010.24] [Citation(s) in RCA: 201] [Impact Index Per Article: 15.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2009] [Revised: 01/28/2010] [Accepted: 02/02/2010] [Indexed: 01/22/2023] Open
Abstract
Although similar to any other organism, prokaryotes can transfer genes vertically from mother cell to daughter cell, they can also exchange certain genes horizontally. Genes can move within and between genomes at fast rates because of mobile genetic elements (MGEs). Although mobile elements are fundamentally self-interested entities, and thus replicate for their own gain, they frequently carry genes beneficial for their hosts and/or the neighbours of their hosts. Many genes that are carried by mobile elements code for traits that are expressed outside of the cell. Such traits are involved in bacterial sociality, such as the production of public goods, which benefit a cell's neighbours, or the production of bacteriocins, which harm a cell's neighbours. In this study we review the patterns that are emerging in the types of genes carried by mobile elements, and discuss the evolutionary and ecological conditions under which mobile elements evolve to carry their peculiar mix of parasitic, beneficial and cooperative genes.
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Affiliation(s)
- D J Rankin
- Department of Biochemistry, University of Zürich, Zürich, Switzerland.
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525
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López‐Villavicencio M, Courjol F, Gibson AK, Hood ME, Jonot O, Shykoff JA, Giraud T. COMPETITION, COOPERATION AMONG KIN, AND VIRULENCE IN MULTIPLE INFECTIONS. Evolution 2010; 65:1357-66. [DOI: 10.1111/j.1558-5646.2010.01207.x] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Affiliation(s)
- Manuela López‐Villavicencio
- Origine, Structure, Evolution de la Diversité, UMR 7205 CNRS‐MNHN, Muséum national d’histoire naturelle, CP39, 57 rue Cuvier, 75231 Paris Cedex 05, France
- E‐mail:
| | - Flavie Courjol
- Ecologie, Systématique et Evolution, UMR 8079, Bâtiment 360, Université Paris‐Sud, F‐91405 Orsay cedex, France; UMR 8079, Bâtiment 360, CNRS, F‐91405 Orsay cedex; France
- E‐mail:
| | - Amanda K. Gibson
- Ecologie, Systématique et Evolution, UMR 8079, Bâtiment 360, Université Paris‐Sud, F‐91405 Orsay cedex, France; UMR 8079, Bâtiment 360, CNRS, F‐91405 Orsay cedex; France
- E‐mail:
| | - Michael E. Hood
- Department of Biology, McGuire Life Sciences Building, Amherst College, Rts 9 and 116, Amherst, Massachusetts 01002‐5000
- E‐mail:
| | - Odile Jonot
- Ecologie, Systématique et Evolution, UMR 8079, Bâtiment 360, Université Paris‐Sud, F‐91405 Orsay cedex, France; UMR 8079, Bâtiment 360, CNRS, F‐91405 Orsay cedex; France
- E‐mail:
| | - Jacqui A. Shykoff
- Ecologie, Systématique et Evolution, UMR 8079, Bâtiment 360, Université Paris‐Sud, F‐91405 Orsay cedex, France; UMR 8079, Bâtiment 360, CNRS, F‐91405 Orsay cedex; France
- E‐mail:
| | - Tatiana Giraud
- Ecologie, Systématique et Evolution, UMR 8079, Bâtiment 360, Université Paris‐Sud, F‐91405 Orsay cedex, France; UMR 8079, Bâtiment 360, CNRS, F‐91405 Orsay cedex; France
- E‐mail:
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526
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Smith J. Superinfection drives virulence evolution in experimental populations of bacteria and plasmids. Evolution 2010; 65:831-41. [PMID: 21054359 DOI: 10.1111/j.1558-5646.2010.01178.x] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A prominent hypothesis proposes that pathogen virulence evolves in large part due to a trade-off between infectiousness and damage to hosts. Other explanations emphasize how virulence evolves in response to competition among pathogens within hosts. Given the proliferation of theoretical possibilities, what best predicts how virulence evolves in real biological systems? Here, I show that virulence evolution in experimental populations of bacteria and self-transmissible plasmids is best explained by within-host competition. Plasmids evolved to severely reduce the fitness of their hosts even in the absence of uninfected cells. This result is inconsistent with the trade-off hypothesis, which predicts that under these conditions vertically transmitted pathogens would evolve to be less virulent. Plasmid virulence was strongly correlated with the ability to superinfect cells containing competing plasmid genotypes, suggesting a key role for within-host competition. When virulent genotypes became common, hosts evolved resistance to plasmid infection. These results show that the trade-off hypothesis can incorrectly predict virulence evolution when within-host interactions are neglected. They also show that symbioses between bacteria and plasmids can evolve to be surprisingly antagonistic.
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Affiliation(s)
- Jeff Smith
- Department of Biology, Emory University, Atlanta, Georgia 30322, USA.
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527
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Ulrich Y, Sadd BM, Schmid-Hempel P. Strain filtering and transmission of a mixed infection in a social insect. J Evol Biol 2010; 24:354-62. [PMID: 21091570 DOI: 10.1111/j.1420-9101.2010.02172.x] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Mixed-genotype infections have attracted considerable attention as drivers of pathogen evolution. However, experimental approaches often overlook essential features of natural host-parasite interactions, such as host heterogeneity, or the effects of between-host selection during transmission. Here, following inoculation of a mixed infection, we analyse the success of different strains of a trypanosome parasite throughout the colony cycle of its bumblebee host. We find that most colonies efficiently filter the circulating infection before it reaches the new queens, the only offspring that carry infections to the next season. A few colonies with a poor filtering ability thus contributed disproportionately to the parasite population in the next season. High strain diversity but not high infection intensity within colony was associated with an increased probability of transmission of the infection to new queens. Interestingly, the representation of the different strains changed dramatically over time, so that long-term parasite success could not be predicted from short-term observations. These findings highlight the shaping of within-colony parasite diversity through filtering as a crucial determinant of year-to-year pathogen transmission and emphasize the importance of host ecology and heterogeneity for disease dynamics.
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Affiliation(s)
- Y Ulrich
- ETH Zürich, Institute of Integrative Biology, Zürich, Switzerland.
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528
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Jackson RW, Johnson LJ, Clarke SR, Arnold DL. Bacterial pathogen evolution: breaking news. Trends Genet 2010; 27:32-40. [PMID: 21047697 DOI: 10.1016/j.tig.2010.10.001] [Citation(s) in RCA: 45] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2010] [Revised: 09/21/2010] [Accepted: 10/07/2010] [Indexed: 02/04/2023]
Abstract
The immense social and economic impact of bacterial pathogens, from drug-resistant infections in hospitals to the devastation of agricultural resources, has resulted in major investment to understand the causes and consequences of pathogen evolution. Recent genome sequencing projects have provided insight into the evolution of bacterial genome structures; revealing the impact of mobile DNA on genome restructuring and pathogenicity. Sequencing of multiple genomes of related strains has enabled the delineation of pathogen evolution and facilitated the tracking of bacterial pathogens globally. Other recent theoretical and empirical studies have shown that pathogen evolution is significantly influenced by ecological factors, such as the distribution of hosts within the environment and the effects of co-infection. We suggest that the time is ripe for experimentalists to use genomics in conjunction with evolutionary ecology experiments to further understanding of how bacterial pathogens evolve.
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Affiliation(s)
- Robert W Jackson
- School of Biological Sciences, University of Reading, Whiteknights, Reading, RG6 6AJ, UK.
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529
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Eshelman CM, Vouk R, Stewart JL, Halsne E, Lindsey HA, Schneider S, Gualu M, Dean AM, Kerr B. Unrestricted migration favours virulent pathogens in experimental metapopulations: evolutionary genetics of a rapacious life history. Philos Trans R Soc Lond B Biol Sci 2010; 365:2503-13. [PMID: 20643740 DOI: 10.1098/rstb.2010.0066] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Understanding pathogen infectivity and virulence requires combining insights from epidemiology, ecology, evolution and genetics. Although theoretical work in these fields has identified population structure as important for pathogen life-history evolution, experimental tests are scarce. Here, we explore the impact of population structure on life-history evolution in phage T4, a viral pathogen of Escherichia coli. The host-pathogen system is propagated as a metapopulation in which migration between subpopulations is either spatially restricted or unrestricted. Restricted migration favours pathogens with low infectivity and low virulence. Unrestricted migration favours pathogens that enter and exit their hosts quickly, although they are less productive owing to rapid extirpation of the host population. The rise of such 'rapacious' phage produces a 'tragedy of the commons', in which better competitors lower productivity. We have now identified a genetic basis for a rapacious life history. Mutations at a single locus (rI) cause increased virulence and are sufficient to account for a negative relationship between phage competitive ability and productivity. A higher frequency of rI mutants under unrestricted migration signifies the evolution of rapaciousness in this treatment. Conversely, spatially restricted migration favours a more 'prudent' pathogen strategy, in which the tragedy of the commons is averted. As our results illustrate, profound epidemiological and ecological consequences of life-history evolution in a pathogen can have a simple genetic cause.
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Affiliation(s)
- Christal M Eshelman
- Department of Biology, University of Washington, Box 351800, Seattle, WA 98195, USA
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530
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CLÉMENT JAJ, MAGALON H, PELLÉ R, MARQUER B, ANDRIVON D. Alteration of pathogenicity-linked life-history traits by resistance of its host Solanum tuberosum impacts sexual reproduction of the plant pathogenic oomycete Phytophthora infestans. J Evol Biol 2010; 23:2668-76. [DOI: 10.1111/j.1420-9101.2010.02150.x] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
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531
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532
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McLean AK, Luciani F, Tanaka MM. Trade-offs in resource allocation in the intracellular life-cycle of hepatitis C virus. J Theor Biol 2010; 267:565-72. [PMID: 20883700 DOI: 10.1016/j.jtbi.2010.09.031] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2010] [Revised: 09/22/2010] [Accepted: 09/22/2010] [Indexed: 12/23/2022]
Abstract
Positive sense single-stranded RNA viruses undergo three mutually exclusive processes to replicate within a cell. These are translation to produce proteins, replication to produce RNA viral genomes, and packaging to form virions. The allocation of newly synthesised viral genomes to these processes, which can be regarded as life-history traits, may be subject to natural selection for efficient reproduction. Here, we develop a mathematical model of the process of intracellular viral replication to study alternative strategies for the allocation and reallocation of viral genomes to these processes. We explore four cases of the model: (1) Free Movement, in which viral genomes can freely be allocated and reallocated among translation, replication and packaging; (2) Unidirectional Reallocation, in which allocation occurs freely but reallocation can only proceed from translation to replication to packaging; (3) Conveyor Belt, in which viral genomes are first allocated to translation, then passed on to replication and finally to packaging; and (4) Permanent Allocation in which new genomes are allocated to the three processes but not reallocated between them. We apply this model to hepatitis C virus and study changes in the production of virus as the rates of allocation and reallocation are varied. We find that high viral production occurs when allocation and reallocation of the genome are weighted towards the translation and replication processes. The replication process in particular is favoured. The most productive strategy is a form of the Free Movement model in which genomes are allocated entirely to the replication-translation cycle while allowing some genomes to be packaged through reallocation.
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Affiliation(s)
- Alison K McLean
- School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney, NSW 2052, Australia.
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533
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Rankin DJ, Bichsel M, Wagner A. Mobile DNA can drive lineage extinction in prokaryotic populations. J Evol Biol 2010; 23:2422-31. [PMID: 20860700 DOI: 10.1111/j.1420-9101.2010.02106.x] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
Natural selection ultimately acts on genes and other DNA sequences. Adaptations that are good for the gene can have adverse effects at higher levels of organization, including the individual or the population. Mobile genetic elements illustrate this principle well, because they can self-replicate within a genome at a cost to their host. As they are costly and can be transmitted horizontally, mobile elements can be seen as genomic parasites. It has been suggested that mobile elements may cause the extinction of their host populations. In organisms with very large populations, such as most bacteria, individual selection is highly effective in purging genomes of deleterious elements, suggesting that extinction is unlikely. Here we investigate the conditions under which mobile DNA can drive bacterial lineages to extinction. We use a range of epidemiological and ecological models to show that harmful mobile DNA can invade, and drive populations to extinction, provided their transmission rate is high and that mobile element-induced mortality is not too high. Population extinction becomes more likely when there are more elements in the population. Even if elements are costly, extinction can still occur because of the combined effect of horizontal gene transfer, a mortality induced by mobile elements. Our study highlights the potential of mobile DNA to be selected at the population level, as well as at the individual level.
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Affiliation(s)
- D J Rankin
- Department of Biochemistry, University of Zürich, Zürich, Switzerland.
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534
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Abstract
Coevolutionary interactions, such as those between host and parasite, predator and prey, or plant and pollinator, evolve subject to the genes of both interactors. It is clear, for example, that the evolution of pollination strategies can only be understood with knowledge of both the pollinator and the pollinated. Studies of the evolution of virulence, the reduction in host fitness due to infection, have nonetheless tended to focus on parasite evolution. Host-centric approaches have also been proposed—for example, under the rubric of “tolerance”, the ability of hosts to minimize virulence without necessarily minimizing parasite density. Within the tolerance framework, however, there is room for more comprehensive measures of host fitness traits, and for fuller consideration of the consequences of coevolution. For example, the evolution of tolerance can result in changed selection on parasite populations, which should provoke parasite evolution despite the fact that tolerance is not directly antagonistic to parasite fitness. As a result, consideration of the potential for parasite counter-adaptation to host tolerance—whether evolved or medially manipulated—is essential to the emergence of a cohesive theory of biotic partnerships and robust disease control strategies.
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Affiliation(s)
- Tom J Little
- Institute of Evolutionary Biology, Centre for Immunity, Infection and Evolution, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom.
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535
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BADETS M, MORRISON C, VERNEAU O. Alternative parasite development in transmission strategies: how time flies! J Evol Biol 2010; 23:2151-2162. [DOI: 10.1111/j.1420-9101.2010.02078.x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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536
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Carval D, Ferriere R. A unified model for the coevolution of resistance, tolerance, and virulence. Evolution 2010; 64:2988-3009. [PMID: 20497218 DOI: 10.1111/j.1558-5646.2010.01035.x] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
We present a general host-parasite model that unifies previous theory by investigating the coevolution of virulence, resistance, and tolerance, with respect to multiple physiological, epidemiological, and environmental parameters. Four sets of new predictions emerge. First, compared to virulence coevolving with resistance or tolerance, three-trait coevolution promotes more virulence and less tolerance, and broadens conditions under which pure defenses evolve. Second, the cost and efficiency of virulence and the epidemiological rates are the key factors of virulence coevolving with resistance and tolerance. Maximum virulence evolves for intermediate infection rate, at which coevolved levels of resistance and tolerance are both high. The influence of host and parasite background mortalities is strong on the evolution of defenses and weak on the coevolution of virulence. Third, evolutionary correlations between defenses can switch sign along single-parameter gradients. The evolutionary trade-off between resistance and tolerance may coevolve with virulence that either increases or decreases monotonically, depending on the underlying parameter gradient. Fourth, despite global attractiveness and stability of coevolutionary equilibria, not-so-rare and not-so-small mutations can beget large variation in virulence and defenses around equilibrium, in the form of transient "evolutionary spikes." Implications for evolutionary management of infections are discussed and directions for future research are outlined.
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Affiliation(s)
- Dominique Carval
- Laboratoire Ecologie & Evolution, CNRS UMR 7625, Université Paris 6, 7 quai Saint-Bernard, Paris Cedex 05, France.
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537
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Abstract
There has been a renewed controversy on the processes that determine evolution in spatially structured populations. Recent theoretical and empirical studies have suggested that parasites should be expected to be more “prudent” (less harmful and slower transmitting) when infection occurs locally. Using a novel approach based on spatial moment equations, we show that the evolution of parasites in spatially structured host populations is determined by the interplay of genetic and demographic spatial structuring, which in turn depends on the details of the ecological dynamics. This allows a detailed understanding of the roles of epidemiology, demography and network topology. Demographic turnover is needed for local interactions to select for prudence in the susceptible-infected models that have been the focus of previous studies. In diseases with little demographic turnover (as typical of many human diseases), we show that only parasites causing diseases with long-lived immunity are likely to be prudent in space. We further demonstrate why, at intermediate parasite dispersal, virulence can evolve to higher levels than predicted by non-spatial theory.
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Affiliation(s)
- Sébastien Lion
- School of Biological Sciences, Royal Holloway University of London, Egham TW200EX, UK.
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538
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Intensive Farming: Evolutionary Implications for Parasites and Pathogens. Evol Biol 2010; 37:59-67. [PMID: 21151485 PMCID: PMC2987527 DOI: 10.1007/s11692-010-9089-0] [Citation(s) in RCA: 94] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2010] [Accepted: 07/13/2010] [Indexed: 11/29/2022]
Abstract
An increasing number of scientists have recently raised concerns about the threat posed by human intervention on the evolution of parasites and disease agents. New parasites (including pathogens) keep emerging and parasites which previously were considered to be ‘under control’ are re-emerging, sometimes in highly virulent forms. This re-emergence may be parasite evolution, driven by human activity, including ecological changes related to modern agricultural practices. Intensive farming creates conditions for parasite growth and transmission drastically different from what parasites experience in wild host populations and may therefore alter selection on various traits, such as life-history traits and virulence. Although recent epidemic outbreaks highlight the risks associated with intensive farming practices, most work has focused on reducing the short-term economic losses imposed by parasites, such as application of chemotherapy. Most of the research on parasite evolution has been conducted using laboratory model systems, often unrelated to economically important systems. Here, we review the possible evolutionary consequences of intensive farming by relating current knowledge of the evolution of parasite life-history and virulence with specific conditions experienced by parasites on farms. We show that intensive farming practices are likely to select for fast-growing, early-transmitted, and hence probably more virulent parasites. As an illustration, we consider the case of the fish farming industry, a branch of intensive farming which has dramatically expanded recently and present evidence that supports the idea that intensive farming conditions increase parasite virulence. We suggest that more studies should focus on the impact of intensive farming on parasite evolution in order to build currently lacking, but necessary bridges between academia and decision-makers.
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539
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Fitness and virulence of a bacterial endoparasite in an environmentally stressed crustacean host. Parasitology 2010; 138:122-31. [DOI: 10.1017/s0031182010000995] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
SUMMARYHost-parasite interactions are shaped by the co-evolutionary arms race of parasite virulence, transmission success as well as host resistance and recovery. The virulence and fitness of parasites may depend on host condition, which is mediated, for instance, by host energy constraints. Here, we investigated to what extent stress imposed by predation threat and environmental pollutants influences host-parasite interactions. We challenged the crustacean hostDaphnia magnawith the sterilizing bacterial endoparasitePasteuria ramosaand simultaneously exposed the host to fish kairomones, the pesticide carbaryl or both stressors. While parasite virulence, measured as impact on host mortality and sterilization, increased markedly after short-term pesticide exposure, it was not influenced by predation threat. Parasite fitness, measured in terms of produced transmission stages, decreased both in fish and pesticide treatments. This effect was much stronger under predation threat than carbaryl exposure, and was attributable to reduced somatic growth of the host, presumably resulting in fewer resources for parasite development. While the indirect impact of both stressors on spore loads provides evidence for host condition-dependent parasite fitness, the finding of increased virulence only under carbaryl exposure indicates a stronger physiological impact of the neurotoxic chemical compared with the effect of a non-toxic fish kairomone.
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540
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Meyer SE, Stewart TE, Clement S. The quick and the deadly: growth vs virulence in a seed bank pathogen. THE NEW PHYTOLOGIST 2010; 187:209-216. [PMID: 20406404 DOI: 10.1111/j.1469-8137.2010.03255.x] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
*We studied the relationship between virulence (ability to kill nondormant Bromus tectorum seeds) and mycelial growth index in the necrotrophic seed pathogen Pyrenophora semeniperda. Seed pathosystems involving necrotrophs differ from those commonly treated in traditional evolution-of-virulence models in that host death increases pathogen fitness by preventing germination, thereby increasing available resources. Because fast-germinating, nondormant B. tectorum seeds commonly escape mortality, we expected virulence to be positively correlated with mycelial growth index. *We performed seed inoculations using conidia from 78 pathogen isolates and scored subsequent mortality. For a subset of 40 of these isolates, representing a range of virulence phenotypes, we measured mycelial growth index. *Virulence varied over a wide range (3-43% seed mortality) and was significantly negatively correlated with mycelial growth index (R(2) = 0.632). More virulent isolates grew more slowly than less virulent isolates. *We concluded that there is an apparent tradeoff between virulence and growth in this pathogen, probably because the production of toxins necessary for necrotrophic pathogenesis competes with metabolic processes associated with growth. Variation in both virulence and growth rate in this pathosystem may be maintained in part by seasonal variation in the relative abundance of rapidly germinating vs dormant host seeds available to the pathogen.
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Affiliation(s)
- Susan E Meyer
- US Forest Service, Rocky Mountain Research Station, Shrub Sciences Laboratory, 735 North 500 East, Provo, UT 84606, USA
| | - Thomas E Stewart
- Department of Plant and Wildlife Science, Brigham Young University, Provo, UT 84601, USA
| | - Suzette Clement
- US Forest Service, Rocky Mountain Research Station, Shrub Sciences Laboratory, 735 North 500 East, Provo, UT 84606, USA
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541
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Froissart R, Doumayrou J, Vuillaume F, Alizon S, Michalakis Y. The virulence-transmission trade-off in vector-borne plant viruses: a review of (non-)existing studies. Philos Trans R Soc Lond B Biol Sci 2010; 365:1907-18. [PMID: 20478886 PMCID: PMC2880117 DOI: 10.1098/rstb.2010.0068] [Citation(s) in RCA: 104] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
The adaptive hypothesis invoked to explain why parasites harm their hosts is known as the trade-off hypothesis, which states that increased parasite transmission comes at the cost of shorter infection duration. This correlation arises because both transmission and disease-induced mortality (i.e. virulence) are increasing functions of parasite within-host density. There is, however, a glaring lack of empirical data to support this hypothesis. Here, we review empirical investigations reporting to what extent within-host viral accumulation determines the transmission rate and the virulence of vector-borne plant viruses. Studies suggest that the correlation between within-plant viral accumulation and transmission rate of natural isolates is positive. Unfortunately, results on the correlation between viral accumulation and virulence are very scarce. We found only very few appropriate studies testing such a correlation, themselves limited by the fact that they use symptoms as a proxy for virulence and are based on very few viral genotypes. Overall, the available evidence does not allow us to confirm or refute the existence of a transmission-virulence trade-off for vector-borne plant viruses. We discuss the type of data that should be collected and how theoretical models can help us refine testable predictions of virulence evolution.
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Affiliation(s)
- R Froissart
- Laboratoire Génétique & évolution des maladies infectieuses (GEMI), UMR 2724 CNRS IRD, 911 avenue Agropolis, 34394 Montpellier, France.
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542
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Within-host competition drives selection for the capsule virulence determinant of Streptococcus pneumoniae. Curr Biol 2010; 20:1222-6. [PMID: 20619820 PMCID: PMC2913241 DOI: 10.1016/j.cub.2010.05.051] [Citation(s) in RCA: 82] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2009] [Revised: 05/11/2010] [Accepted: 05/13/2010] [Indexed: 11/22/2022]
Abstract
For many opportunistic pathogens, it is unclear why their virulence determinants and expression of pathogenic behavior have evolved when damage or death of their host offers no obvious selective advantage to microbial growth or survival [1–3]. Many pathogens initiate interactions with their host on mucosal surfaces and must compete with other members of the microflora for the same niche. Here we explore whether competitive interactions between microbes promote the acquisition of virulence characteristics. During model murine nasal colonization, Haemophilus influenzae outcompetes another member of the local flora, Streptococcus pneumoniae, by recruiting neutrophils and stimulating the killing of complement-opsonized pneumococci [4]. For S. pneumoniae, resistance to opsonophagocytic killing is determined by its polysaccharide capsule [5, 6]. Although there are many capsule types among different S. pneumoniae isolates that allow for efficient colonization, virulent pneumococci express capsules that confer resistance to opsonophagocytic clearance. Modeling of interspecies interaction predicts that these more virulent S. pneumoniae will prevail during competition with H. influenzae, even if production of a capsule is otherwise costly. Experimental colonization studies confirmed the increased survival of the more virulent S. pneumoniae type during competition. Our findings demonstrate that competition between microbes during their commensal state may underlie selection for characteristics that allow invasive disease.
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543
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Wargo AR, Garver KA, Kurath G. Virulence correlates with fitness in vivo for two M group genotypes of Infectious hematopoietic necrosis virus (IHNV). Virology 2010; 404:51-8. [PMID: 20494388 DOI: 10.1016/j.virol.2010.04.023] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2010] [Revised: 03/17/2010] [Accepted: 04/22/2010] [Indexed: 11/18/2022]
Abstract
The nature of the association between viral fitness and virulence remains elusive in vertebrate virus systems, partly due to a lack of in vivo experiments using statistically sufficient numbers of replicate hosts. We examined the relationship between virulence and fitness in Infectious hematopoietic necrosis virus (IHNV), in vivo, in intact living rainbow trout. Trout were infected with a high or low virulence genotype of M genogroup IHNV, or a mixture of the two genotypes, so as to calculate relative fitness and the effect of a competition environment on fitness. Fitness was measured as total viral load in the host at time of peak viral density, quantified by genotype-specific quantitative RT-PCR (qRT-PCR). The more virulent IHNV genotype reached higher densities in both single and mixed infections. There was no effect of competition on the performance of either genotype. Our results suggest a positive link between IHNV genotype fitness and virulence.
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Affiliation(s)
- Andrew R Wargo
- Western Fisheries Research Center, Seattle, WA 98115-5016, USA.
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544
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Reece SE, Ali E, Schneider P, Babiker HA. Stress, drugs and the evolution of reproductive restraint in malaria parasites. Proc Biol Sci 2010; 277:3123-9. [PMID: 20484242 PMCID: PMC2982055 DOI: 10.1098/rspb.2010.0564] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
Life-history theory predicts that sexually reproducing organisms have evolved to resolve resource-allocation trade-offs between growth/survival versus reproduction, and current versus future reproduction. Malaria parasites replicate asexually in their vertebrate hosts, but must reproduce sexually to infect vectors and be transmitted to new hosts. As different specialized stages are required for these functions, the division of resources between these life-history components is a fundamental evolutionary problem. Here, we test how drug-sensitive and drug-resistant isolates of the human malaria parasite Plasmodium falciparum resolve the trade-off between in-host replication and between-host transmission when exposed to treatment with anti-malarial drugs. Previous studies have shown that parasites increase their investment in sexual stages when exposed to stressful conditions, such as drugs. However, we demonstrate that sensitive parasites facultatively decrease their investment in sexual stages when exposed to drugs. In contrast to previous studies, we tested parasites from a region where treatment with anti-malarial drugs is common and transmission is seasonal. We hypothesize that when exposed to drugs, parasites invest in their survival and future transmission by diverting resources from reproduction to replication. Furthermore, as drug-resistant parasites did not adjust their investment when exposed to drugs, we suggest that parasites respond to changes in their proliferation (state) rather the presence of drugs.
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Affiliation(s)
- Sarah E. Reece
- Centre for Immunity, Infection and Evolution, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, UK
- Institutes of Evolution, Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, UK
| | - Eltayeb Ali
- Sudan Atomic Energy Commission, PO Box 3001, Khartoum, Sudan
- Department of Biochemistry, Faculty of Medicine, University of Khartoum, Khartoum, Sudan
| | - Petra Schneider
- Centre for Immunity, Infection and Evolution, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, UK
- Institutes of Evolution, Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, UK
| | - Hamza A. Babiker
- Institutes of Evolution, Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, UK
- Biochemistry Department, Faculty of Medicine, Sultan Qaboos University, Alkhod, PO Box 35, Muscat, Oman
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545
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Hurford A, Cownden D, Day T. Next-generation tools for evolutionary invasion analyses. J R Soc Interface 2010; 7:561-71. [PMID: 19955121 PMCID: PMC2842787 DOI: 10.1098/rsif.2009.0448] [Citation(s) in RCA: 73] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2009] [Accepted: 11/11/2009] [Indexed: 11/12/2022] Open
Abstract
Evolutionary invasion analysis is a powerful technique for modelling in evolutionary biology. The general approach is to derive an expression for the growth rate of a mutant allele encoding some novel phenotype, and then to use this expression to predict long-term evolutionary outcomes. Mathematically, such 'invasion fitness' expressions are most often derived using standard linear stability analyses from dynamical systems theory. Interestingly, there is a mathematically equivalent approach to such stability analyses that is often employed in mathematical epidemiology, and that is based on so-called 'next-generation' matrices. Although this next-generation matrix approach has sometimes also been used in evolutionary invasion analyses, it is not yet common in this area despite the fact that it can sometimes greatly simplify calculations. The aim of this article is to bring the approach to a wider evolutionary audience in two ways. First, we review the next-generation matrix approach and provide a novel, and easily intuited, interpretation of how this approach relates to more standard techniques. Second, we illustrate next-generation methods in evolutionary invasion analysis through a series of informative examples. Although focusing primarily on evolutionary invasion analysis, we provide several insights that apply to biological modelling in general.
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Affiliation(s)
- Amy Hurford
- Department of Mathematics and Statistics, Queen's University, Ontario, Canada.
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546
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Thébaud G, Chadœuf J, Morelli MJ, McCauley JW, Haydon DT. The relationship between mutation frequency and replication strategy in positive-sense single-stranded RNA viruses. Proc Biol Sci 2010; 277:809-17. [PMID: 19906671 PMCID: PMC2842737 DOI: 10.1098/rspb.2009.1247] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2009] [Accepted: 10/22/2009] [Indexed: 01/21/2023] Open
Abstract
For positive-sense single-stranded RNA virus genomes, there is a trade-off between the mutually exclusive tasks of transcription, translation and encapsidation. The replication strategy that maximizes the intracellular growth rate of the virus requires iterative genome transcription from positive to negative, and back to positive sense. However, RNA viruses experience high mutation rates, and the proportion of genomes with lethal mutations increases with the number of replication cycles. Thus, intracellular mutant frequency will depend on the replication strategy. Introducing apparently realistic mutation rates into a model of viral replication demonstrates that strategies that maximize viral growth rate could result in an average of 26 mutations per genome by the time plausible numbers of positive strands have been generated, and that virus viability could be as low as 0.1 per cent. At high mutation rates or when a high proportion of mutations are deleterious, the optimal strategy shifts towards synthesizing more negative strands per positive strand, and in extremis towards a 'stamping-machine' replication mode where all the encapsidated genomes come from only two transcriptional steps. We conclude that if viral mutation rates are as high as current estimates suggest, either mutation frequency must be considerably higher than generally anticipated and the proportion of viable viruses produced extremely small, or replication strategies cannot be optimized to maximize viral growth rate. Mechanistic models linking mutation frequency to replication mechanisms coupled with data generated through new deep-sequencing technologies could play an important role in improving the estimates of viral mutation rate.
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Affiliation(s)
- Gaël Thébaud
- Institut National de la Recherche Agronomique (INRA), UMR BGPI, Cirad TA A-54/K, Campus de Baillarguet, 34398 Montpellier cedex 5, France
| | - Joël Chadœuf
- INRA, UR546 Biostatistique et Processus Spatiaux, Domaine Saint-Paul, 84914 Avignon, France
| | - Marco J. Morelli
- Boyd Orr Centre for Population and Ecosystem Health, Faculty of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK
| | - John W. McCauley
- The National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK
| | - Daniel T. Haydon
- Boyd Orr Centre for Population and Ecosystem Health, Faculty of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK
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547
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Lee C, Inouye B. Mutualism between Consumers and Their Shared Resource Can Promote Competitive Coexistence. Am Nat 2010; 175:277-88. [DOI: 10.1086/650370] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
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548
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Abstract
Most models of virulence evolution assume that transmission and virulence are constant during an infection. In many viral (HIV and influenza), bacterial (TB) and prion (BSE and CWD) systems, disease-induced mortality occurs long after the host becomes infectious. Therefore, we constructed a model with two infected classes that differ in transmission rate and virulence in order to understand how the evolutionarily stable strategy (ESS) depends on the relative difference in transmission and virulence between classes, on the transition rate between classes and on the recovery rate from the second class. We find that ESS virulence decreases when expressed early in the infection or when transmission occurs late in an infection. When virulence occurred relatively equally in each class and there was disease recovery, ESS virulence increased with increased transition rate. In contrast, ESS virulence first increased and then decreased with transition rate when there was little virulence early in the infection and a rapid recovery rate. This model predicts that ESS virulence is highly dependent on the timing of transmission and pathology after infection; thus, pathogen evolution may either increase or decrease virulence after emergence in a new host.
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Affiliation(s)
- Erik E Osnas
- Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA.
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549
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Lanzas C, Ayscue P, Ivanek R, Gröhn YT. Model or meal? Farm animal populations as models for infectious diseases of humans. Nat Rev Microbiol 2010; 8:139-48. [PMID: 20040917 PMCID: PMC7097165 DOI: 10.1038/nrmicro2268] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
In recent decades, theory addressing the processes that underlie the dynamics of infectious diseases has progressed considerably. Unfortunately, the availability of empirical data to evaluate these theories has not grown at the same pace. Although laboratory animals have been widely used as models at the organism level, they have been less appropriate for addressing issues at the population level. However, farm animal populations can provide empirical models to study infectious diseases at the population level.
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Affiliation(s)
- Cristina Lanzas
- Department of Population Medicine and Diagnostic Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853, USA.
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550
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Luciani F, Alizon S. The evolutionary dynamics of a rapidly mutating virus within and between hosts: the case of hepatitis C virus. PLoS Comput Biol 2009; 5:e1000565. [PMID: 19911046 PMCID: PMC2768904 DOI: 10.1371/journal.pcbi.1000565] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2009] [Accepted: 10/15/2009] [Indexed: 01/27/2023] Open
Abstract
Many pathogens associated with chronic infections evolve so rapidly that strains found late in an infection have little in common with the initial strain. This raises questions at different levels of analysis because rapid within-host evolution affects the course of an infection, but it can also affect the possibility for natural selection to act at the between-host level. We present a nested approach that incorporates within-host evolutionary dynamics of a rapidly mutating virus (hepatitis C virus) targeted by a cellular cross-reactive immune response, into an epidemiological perspective. The viral trait we follow is the replication rate of the strain initiating the infection. We find that, even for rapidly evolving viruses, the replication rate of the initial strain has a strong effect on the fitness of an infection. Moreover, infections caused by slowly replicating viruses have the highest infection fitness (i.e., lead to more secondary infections), but strains with higher replication rates tend to dominate within a host in the long-term. We also study the effect of cross-reactive immunity and viral mutation rate on infection life history traits. For instance, because of the stochastic nature of our approach, we can identify factors affecting the outcome of the infection (acute or chronic infections). Finally, we show that anti-viral treatments modify the value of the optimal initial replication rate and that the timing of the treatment administration can have public health consequences due to within-host evolution. Our results support the idea that natural selection can act on the replication rate of rapidly evolving viruses at the between-host level. It also provides a mechanistic description of within-host constraints, such as cross-reactive immunity, and shows how these constraints affect the infection fitness. This model raises questions that can be tested experimentally and underlines the necessity to consider the evolution of quantitative traits to understand the outcome and the fitness of an infection. Rapidly mutating viruses, such as hepatitis C virus, can escape host immunity by generating new strains that avoid the immune system. Existing data support the idea that such within-host evolution affects the outcome of the infection. Few theoretical models address this question and most follow viral diversity or qualitative traits, such as drug resistance. Here, we study the evolution of two virus quantitative traits—the replication rate and the ability to be recognised by the immune response—during an infection. We develop an epidemiological framework where transmission events are driven by within-host dynamics. We find that the replication rate of the virus that initially infects the host has a strong influence on the epidemiological success of the disease. Furthermore, we show that the cross-reactive immune response is key to determining the outcome of the infection (acute or chronic). Finally, we show that the timing of the start of an anti-viral treatment has a strong effect on viral evolution, which impacts the efficiency of the treatment. Our analysis suggests a new mechanism to explain infection outcomes and proposes testable predictions that can drive future experimental approaches.
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Affiliation(s)
- Fabio Luciani
- Centre for Infection and Inflammation Research (CIIR), School of Medical Sciences, University of New South Wales, Sydney, Australia
- * E-mail: (FL); (SA)
| | - Samuel Alizon
- Institut für Integrative Biologie, ETH, Zürich, Switzerland
- * E-mail: (FL); (SA)
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