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Schneider P, Reece SE. The private life of malaria parasites: Strategies for sexual reproduction. Mol Biochem Parasitol 2021; 244:111375. [PMID: 34023299 PMCID: PMC8346949 DOI: 10.1016/j.molbiopara.2021.111375] [Citation(s) in RCA: 15] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2021] [Revised: 05/11/2021] [Accepted: 05/17/2021] [Indexed: 12/22/2022]
Abstract
Malaria parasites exhibit a complex lifecycle, requiring extensive asexual replication in the liver and blood of the vertebrate host, and in the haemocoel of the insect vector. Yet, they must also undergo a single round of sexual reproduction, which occurs in the vector's midgut upon uptake of a blood meal. Sexual reproduction is obligate for infection of the vector and thus, is essential for onwards transmission to new hosts. Sex in malaria parasites involves several bottlenecks in parasite number, making the stages involved attractive targets for blocking disease transmission. Malaria parasites have evolved a suite of adaptations ("strategies") to maximise the success of sexual reproduction and transmission, which could undermine transmission-blocking interventions. Yet, understanding parasite strategies may also reveal novel opportunities for such interventions. Here, we outline how evolutionary and ecological theories, developed to explain reproductive strategies in multicellular taxa, can be applied to explain two reproductive strategies (conversion rate and sex ratio) expressed by malaria parasites within the vertebrate host.
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Affiliation(s)
- Petra Schneider
- Institute of Evolutionary Biology, Institute of Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh, UK.
| | - Sarah E Reece
- Institute of Evolutionary Biology, Institute of Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh, UK
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2
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Evolutionary sex allocation theory explains sex ratios in natural Plasmodium falciparum infections. Int J Parasitol 2019; 49:601-604. [PMID: 31153899 PMCID: PMC7614805 DOI: 10.1016/j.ijpara.2019.04.001] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2018] [Revised: 03/29/2019] [Accepted: 04/05/2019] [Indexed: 01/06/2023]
Abstract
Malaria transmission is achieved by sexual stages, called gametocytes, and the proportion of gametocytes that are male versus female (sex ratio) influences transmission success. In malaria model systems, variation in gametocyte sex ratios can be explained by the predictions of evolutionary sex allocation theory. We test these predictions using natural Plasmodium falciparum infections. The predicted negative correlation between sex ratio and gametocyte density holds: the sex ratio increases when gametocyte densities decrease, and this is most apparent in single genotype infections and in the dry season. We do not observe higher gametocyte sex ratios in mixed compared with single genotype infections.
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Tadesse FG, Meerstein-Kessel L, Gonçalves BP, Drakeley C, Ranford-Cartwright L, Bousema T. Gametocyte Sex Ratio: The Key to Understanding Plasmodium falciparum Transmission? Trends Parasitol 2018; 35:226-238. [PMID: 30594415 PMCID: PMC6396025 DOI: 10.1016/j.pt.2018.12.001] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2018] [Revised: 11/30/2018] [Accepted: 12/03/2018] [Indexed: 11/25/2022]
Abstract
A mosquito needs to ingest at least one male and one female gametocyte to become infected with malaria. The sex of Plasmodium falciparum gametocytes can be determined microscopically but recent transcriptomics studies paved the way for the development of molecular methods that allow sex-ratio assessments at much lower gametocyte densities. These sex-specific gametocyte diagnostics were recently used to examine gametocyte dynamics in controlled and natural infections as well as the impact of different antimalarial drugs. It is currently unclear to what extent sex-specific gametocyte diagnostics obviate the need for mosquito feeding assays to formally assess transmission potential. Here, we review recent and historic assessments of gametocyte sex ratio in relation to host and parasite characteristics, treatment, and transmission potential. Recent RNA sequencing studies have uncovered a number of P. falciparum gametocyte sex-specific targets and provided new insights in gametocyte biology. After decades when gametocyte sex-ratio research was restricted to nonhuman malarias or in vitro experiments, molecular tools for assessing gametocyte sex ratio are now increasingly available for use in natural P. falciparum infections. Evidence that gametocyte sex ratio is influenced by total gametocyte density and antimalarial treatment, and improves predictions of transmission potential, highlight the relevance of understanding the gametocyte sex ratio during natural infections. The finding that the most widely used P. falciparum gametocyte marker Pfs25 is expressed predominantly by female gametocytes and has non-negligible levels of background expression in asexual parasites necessitates a re-evaluation of existing gametocyte data.
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Affiliation(s)
- Fitsum G Tadesse
- Radboud Institute for Health Sciences, Radboud University Medical Centre, Nijmegen, The Netherlands; Armauer Hansen Research Institute (AHRI), Addis Ababa, Ethiopia; Institute of Biotechnology, Addis Ababa University, Addis Ababa, Ethiopia; These authors contributed equally
| | - Lisette Meerstein-Kessel
- Radboud Institute for Health Sciences, Radboud University Medical Centre, Nijmegen, The Netherlands; These authors contributed equally
| | - Bronner P Gonçalves
- Department of Immunology and Infection, London School of Hygiene and Tropical Medicine, London, UK
| | - Chris Drakeley
- Department of Immunology and Infection, London School of Hygiene and Tropical Medicine, London, UK
| | - Lisa Ranford-Cartwright
- Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, UK
| | - Teun Bousema
- Radboud Institute for Health Sciences, Radboud University Medical Centre, Nijmegen, The Netherlands; Department of Immunology and Infection, London School of Hygiene and Tropical Medicine, London, UK.
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4
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Scott N, Ataide R, Wilson DP, Hellard M, Price RN, Simpson JA, Fowkes FJI. Implications of population-level immunity for the emergence of artemisinin-resistant malaria: a mathematical model. Malar J 2018; 17:279. [PMID: 30071877 PMCID: PMC6071336 DOI: 10.1186/s12936-018-2418-y] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2018] [Accepted: 07/12/2018] [Indexed: 01/12/2023] Open
Abstract
BACKGROUND Artemisinin-resistant Plasmodium falciparum has emerged in the Greater Mekong Subregion, an area of relatively low transmission, but has yet to be reported in Africa. A population-based mathematical model was used to investigate the relationship between P. falciparum prevalence, exposure-acquired immunity and time-to-emergence of artemisinin resistance. The possible implication for the emergence of resistance across Africa was assessed. METHODS The model included human and mosquito populations, two strains of malaria ("wild-type", "mutant"), three levels of human exposure-acquired immunity (none, low, high) with two types of immunity for each level (sporozoite/liver stage immunity and blood-stage/gametocyte immunity) and drug pressure based on per-capita treatment numbers. RESULTS The model predicted that artemisinin-resistant strains may circulate up to 10 years longer in high compared to low P. falciparum prevalence areas before resistance is confirmed. Decreased time-to-resistance in low prevalence areas was explained by low genetic diversity and immunity, which resulted in increased probability of selection and spread of artemisinin-resistant strains. Artemisinin resistance was estimated to be established by 2020 in areas of Africa with low (< 10%) P. falciparum prevalence, but not for 5 or 10 years later in moderate (10-25%) or high (> 25%) prevalence areas, respectively. CONCLUSIONS Areas of low transmission and low immunity give rise to a more rapid expansion of artemisinin-resistant parasites, corroborating historical observations of anti-malarial resistance emergence. Populations where control strategies are in place that reduce malaria transmission, and hence immunity, may be prone to a rapid emergence and spread of artemisinin-resistant strains and thus should be carefully monitored.
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Affiliation(s)
- Nick Scott
- Disease Elimination Program, Burnet Institute, Melbourne, VIC, 3004, Australia
- Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, VIC, 3004, Australia
| | - Ricardo Ataide
- Disease Elimination Program, Burnet Institute, Melbourne, VIC, 3004, Australia
| | - David P Wilson
- Disease Elimination Program, Burnet Institute, Melbourne, VIC, 3004, Australia
- Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, VIC, 3004, Australia
| | - Margaret Hellard
- Disease Elimination Program, Burnet Institute, Melbourne, VIC, 3004, Australia
- Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, VIC, 3004, Australia
- Department of Infectious Diseases, Alfred Hospital, Melbourne, VIC, 3004, Australia
| | - Ric N Price
- Global and Tropical Health Division, Menzies School of Health Research, Charles Darwin University, Darwin, NT, Australia
- Centre for Tropical Medicine and Global Health, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, UK
| | - Julie A Simpson
- Centre for Epidemiology and Biostatistics, Melbourne School of Population and Global Health, The University of Melbourne, Melbourne, VIC, 3010, Australia
| | - Freya J I Fowkes
- Disease Elimination Program, Burnet Institute, Melbourne, VIC, 3004, Australia.
- Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, VIC, 3004, Australia.
- Centre for Epidemiology and Biostatistics, Melbourne School of Population and Global Health, The University of Melbourne, Melbourne, VIC, 3010, Australia.
- Department of Infectious Diseases, Monash University, Melbourne, VIC, 3004, Australia.
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Simam J, Rono M, Ngoi J, Nyonda M, Mok S, Marsh K, Bozdech Z, Mackinnon M. Gene copy number variation in natural populations of Plasmodium falciparum in Eastern Africa. BMC Genomics 2018; 19:372. [PMID: 29783949 PMCID: PMC5963192 DOI: 10.1186/s12864-018-4689-7] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2017] [Accepted: 04/17/2018] [Indexed: 01/05/2023] Open
Abstract
BACKGROUND Gene copy number variants (CNVs), which consist of deletions and amplifications of single or sets of contiguous genes, contribute to the great diversity in the Plasmodium falciparum genome. In vitro studies in the laboratory have revealed their important role in parasite fitness phenotypes such as red cell invasion, transmissibility and cytoadherence. Studies of natural parasite populations indicate that CNVs are also common in the field and thus may facilitate adaptation of the parasite to its local environment. RESULTS In a survey of 183 fresh field isolates from three populations in Eastern Africa with different malaria transmission intensities, we identified 94 CNV loci using microarrays. All CNVs had low population frequencies (minor allele frequency < 5%) but each parasite isolate carried an average of 8 CNVs. Nine CNVs showed high levels of population differentiation (FST > 0.3) and nine exhibited significant clines in population frequency across a gradient in transmission intensity. The clearest example of this was a large deletion on chromosome 9 previously reported only in laboratory-adapted isolates. This deletion was present in 33% of isolates from a population with low and highly seasonal malaria transmission, and in < 9% of isolates from populations with higher transmission. Subsets of CNVs were strongly correlated in their population frequencies, implying co-selection. CONCLUSIONS These results support the hypothesis that CNVs are the target of selection in natural populations of P. falciparum. Their environment-specific patterns observed here imply an important role for them in conferring adaptability to the parasite thus enabling it to persist in its highly diverse ecological environment.
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Affiliation(s)
| | - Martin Rono
- KEMRI-Wellcome Trust Research Program, Kilifi, Kenya.,Centre for Tropical Medicine and Global Health, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, UK.,Pwani University Bioscience Research Centre, Pwani University, Kilifi, Kenya
| | - Joyce Ngoi
- KEMRI-Wellcome Trust Research Program, Kilifi, Kenya
| | - Mary Nyonda
- Department of Microbiology and Molecular Medicine, Medical Faculty, University of Geneva, Geneva, Switzerland
| | - Sachel Mok
- Department of Microbiology and Immunology, Columbia University, New York, USA
| | - Kevin Marsh
- Centre for Tropical Medicine and Global Health, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, UK
| | - Zbynek Bozdech
- School of Biological Sciences, Nanyang Technological University, Singapore, Singapore
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Ataíde R, Powell R, Moore K, McLean A, Phyo AP, Nair S, White M, Anderson TJ, Beeson JG, Simpson JA, Nosten F, Fowkes FJI. Declining Transmission and Immunity to Malaria and Emerging Artemisinin Resistance in Thailand: A Longitudinal Study. J Infect Dis 2017; 216:723-731. [PMID: 28934435 PMCID: PMC5853569 DOI: 10.1093/infdis/jix371] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
Background Reductions in malaria transmission decrease naturally acquired immunity, which may influence the emergence of Plasmodium falciparum artemisinin-resistant phenotypes and genotypes over time. Methods Antibodies specific for P. falciparum antigens were determined in uncomplicated hyperparasitemic malaria patients over a 10-year period of declining malaria transmission and emerging artemisinin resistance in northwestern Thailand. We investigated the association between antibody levels and both parasite clearance time (PCt½) and artemisinin resistance–associated kelch13 genotypes over time. Results Immunity to P. falciparum declined prior to 2004, preceding the emergence of artemisinin resistance-associated genotypes and phenotypes (maximum mean change in antibody level per year: anti-MSP142 = −0.17; 95% confidence interval [CI] = −.31 to −.04; P = .01). In this period of declining immunity, and in the absence of kelch13 mutations, PCt½ increased. Between 2007 and 2011, levels of antibodies fluctuated, and higher antibody levels were associated with faster PCt½ (maximum yearly change in PCt½, in hours: EBA140rII = −0.39; 95% CI = −.61 to −.17; P < .001). Conclusions Understanding the impact of changing transmission and immunity on the emergence of artemisinin resistance is important particularly as increased malaria control and elimination activities may enhance immunological conditions for the expansion of artemisinin-resistant P. falciparum.
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Affiliation(s)
| | | | | | | | - Aung Pyae Phyo
- Shoklo Malaria Research Unit, Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Mae Sot,Thailand
| | - Shalini Nair
- Texas Biomedical Research Institute, San Antonio
| | - Marina White
- Texas Biomedical Research Institute, San Antonio
| | | | | | - Julie A Simpson
- Centre for Epidemiology and Biostatistics, Melbourne School of Population and Global Health, University of Melbourne
| | - Francois Nosten
- Shoklo Malaria Research Unit, Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University, Mae Sot, Thailand.,Centre for Tropical Medicine and Global Health, Nuffield Department of Medicine Research, University of Oxford, United Kingdom
| | - Freya J I Fowkes
- Disease Elimination Program, Burnet Institute.,Centre for Epidemiology and Biostatistics, Melbourne School of Population and Global Health, University of Melbourne.,Department of Epidemiology and Preventive Medicine, Department of Infectious Diseases, Monash University, Melbourne, Australia
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7
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White NJ. Why is it that antimalarial drug treatments do not always work? ANNALS OF TROPICAL MEDICINE AND PARASITOLOGY 2016. [DOI: 10.1080/00034983.1998.11813303] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
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Evolutionary implications for the determination of gametocyte sex ratios under fecundity variation for the malaria parasite. J Theor Biol 2016; 408:260-273. [DOI: 10.1016/j.jtbi.2016.08.015] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2016] [Revised: 08/02/2016] [Accepted: 08/10/2016] [Indexed: 11/23/2022]
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Abstract
Mathematical modelling provides an effective way to challenge conventional wisdom about
parasite evolution and investigate why parasites ‘do what they do’ within the host. Models
can reveal when intuition cannot explain observed patterns, when more complicated biology
must be considered, and when experimental and statistical methods are likely to mislead.
We describe how models of within-host infection dynamics can refine experimental design,
and focus on the case study of malaria to highlight how integration between models and
data can guide understanding of parasite fitness in three areas: (1) the adaptive
significance of chronic infections; (2) the potential for tradeoffs between virulence and
transmission; and (3) the implications of within-vector dynamics. We emphasize that models
are often useful when they highlight unexpected patterns in parasite evolution, revealing
instead why intuition yields the wrong answer and what combination of theory and data are
needed to advance understanding.
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Abstract
Malaria remains one of the leading causes of death worldwide, despite decades of public health efforts. The recent commitment by many endemic countries to eliminate malaria marks a shift away from programs aimed at controlling disease burden towards one that emphasizes reducing transmission of the most virulent human malaria parasite, Plasmodium falciparum. Gametocytes, the only developmental stage of malaria parasites able to infect mosquitoes, have remained understudied, as they occur in low numbers, do not cause disease, and are difficult to detect in vivo by conventional methods. Here, we review the transmission biology of P. falciparum gametocytes, featuring important recent discoveries of genes affecting parasite commitment to gametocyte formation, microvesicles enabling parasites to communicate with each other, and the anatomical site where immature gametocytes develop. We propose potential parasite targets for future intervention and highlight remaining knowledge gaps.
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Affiliation(s)
- Sandra K. Nilsson
- Department of Immunology and Infectious Diseases, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, United States of America
| | - Lauren M. Childs
- Centre for Communicable Disease Dynamics and Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, United States of America
| | - Caroline Buckee
- Centre for Communicable Disease Dynamics and Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, United States of America
- * E-mail: (CB); (MM)
| | - Matthias Marti
- Department of Immunology and Infectious Diseases, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, United States of America
- * E-mail: (CB); (MM)
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Neal AT, Taylor PD. Local mate competition and transmission bottlenecks: A new model for understanding malaria parasite and other sex ratios. J Theor Biol 2014; 363:381-9. [DOI: 10.1016/j.jtbi.2014.08.037] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2014] [Revised: 08/08/2014] [Accepted: 08/19/2014] [Indexed: 01/22/2023]
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12
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Carter LM, Schneider P, Reece SE. Information use and plasticity in the reproductive decisions of malaria parasites. Malar J 2014; 13:115. [PMID: 24670151 PMCID: PMC3986881 DOI: 10.1186/1475-2875-13-115] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2013] [Accepted: 03/23/2014] [Indexed: 01/13/2023] Open
Abstract
BACKGROUND Investment in the production of transmissible stages (gametocytes) and their sex ratio are malaria parasite traits that underpin mosquito infectivity and are therefore central to epidemiology. Malaria parasites adjust their levels of investment into gametocytes and sex ratio in response to changes in the in-host environment (including red blood cell resource availability, host immune responses, competition from con-specific genotypes in mixed infections, and drug treatment). This plasticity appears to be adaptive (strategic) because parasites prioritize investment (in sexual versus asexual stages and male versus female stages) in manners predicted to maximize fitness. However, the information, or 'cues' that parasites use to detect environmental changes and make appropriate decisions about investment into gametocytes and their sex ratio are unknown. METHODS Single genotype Plasmodium chabaudi infections were exposed to 'cue' treatments consisting of intact or lysed uninfected red blood cells, lysed parasitized RBCs of the same clone or an unrelated clone, and an unmanipulated control. Infection dynamics (proportion of reticulocytes, red blood cell and asexual stage parasite densities) were monitored, and changes in gametocyte investment and sex ratio in response to cue treatments, applied either pre- or post-peak of infection were examined. RESULTS AND CONCLUSIONS A significant reduction in gametocyte density was observed in response to the presence of lysed parasite material and a borderline significant increase in sex ratio (proportion of male gametocytes) upon exposure to lysed red blood cells (both uninfected and infected) was observed. Furthermore, the changes in gametocyte density and sex ratio in response to these cues depend on the age of infection. Demonstrating that variation in gametocyte investment and sex ratio observed during infections are a result of parasite strategies (rather than the footprint of host physiology), provides a foundation to investigate the fitness consequences of plasticity and explore whether drugs could be developed to trick parasites into making suboptimal decisions.
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Affiliation(s)
- Lucy M Carter
- Institute of Evolutionary Biology, School of Biological Sciences, Ashworth Laboratories, University of Edinburgh, Edinburgh, UK
| | - Petra Schneider
- Institute of Evolutionary Biology, School of Biological Sciences, Ashworth Laboratories, University of Edinburgh, Edinburgh, UK
| | - Sarah E Reece
- Institute of Evolutionary Biology, School of Biological Sciences, Ashworth Laboratories, University of Edinburgh, Edinburgh, UK
- Centre for Immunity, Infection & Evolution, Institutes of Evolution, Immunology and Infection Research, School of Biological Sciences, Ashworth Laboratories, University of Edinburgh, Edinburgh, UK
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Neal AT, Schall JJ. TESTING SEX RATIO THEORY WITH THE MALARIA PARASITEPLASMODIUM MEXICANUMIN NATURAL AND EXPERIMENTAL INFECTIONS. Evolution 2014; 68:1071-81. [DOI: 10.1111/evo.12334] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2013] [Accepted: 12/05/2013] [Indexed: 11/29/2022]
Affiliation(s)
- Allison T. Neal
- Department of Biology; University of Vermont; Burlington Vermont 05405
| | - Jos. J. Schall
- Department of Biology; University of Vermont; Burlington Vermont 05405
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Male fecundity and optimal gametocyte sex ratios for Plasmodium falciparum during incomplete fertilization. J Theor Biol 2012; 307:183-92. [DOI: 10.1016/j.jtbi.2012.05.021] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2011] [Revised: 05/11/2012] [Accepted: 05/21/2012] [Indexed: 11/19/2022]
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Gbotosho GO, Sowunmi A, Happi CT, Okuboyejo TM. Plasmodium falciparum gametocyte carriage, sex ratios and asexual parasite rates in Nigerian children before and after a treatment protocol policy change instituting the use of artemisinin-based combination therapies. Mem Inst Oswaldo Cruz 2012; 106:685-90. [PMID: 22012222 DOI: 10.1590/s0074-02762011000600007] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2011] [Accepted: 07/19/2011] [Indexed: 11/22/2022] Open
Abstract
The effects of artemisinin-based combination therapies (ACTs) on transmission of Plasmodium falciparum were evaluated after a policy change instituting the use of ACTs in an endemic area. P. falciparum gametocyte carriage, sex ratios and inbreeding rates were examined in 2,585 children at presentation with acute falciparum malaria during a 10-year period from 2001-2010. Asexual parasite rates were also evaluated from 2003-2010 in 10,615 children before and after the policy change. Gametocyte carriage declined significantly from 12.4% in 2001 to 3.6% in 2010 (χ2 for trend = 44.3, p < 0.0001), but sex ratios and inbreeding rates remained unchanged. Additionally, overall parasite rates remained unchanged before and after the policy change (47.2% vs. 45.4%), but these rates declined significantly from 2003-2010 (χ2 for trend 35.4, p < 0.0001). Chloroquine (CQ) and artemether-lumefantrine (AL) were used as prototype drugs before and after the policy change, respectively. AL significantly shortened the duration of male gametocyte carriage in individual patients after treatment began compared with CQ (log rank statistic = 7.92, p = 0.005). ACTs reduced the rate of gametocyte carriage in children with acute falciparum infections at presentation and shortened the duration of male gametocyte carriage after treatment. However, parasite population sex ratios, inbreeding rates and overall parasite rate were unaffected.
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Affiliation(s)
- Grace Olusola Gbotosho
- Department of Pharmacology and Therapeutics and Malaria Research Laboratories, Institute for Medical Research and Training, University of Ibadan, Ibadan, Nigeria
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Gbotosho GO, Sowunmi A, Okuboyejo TM, Happi CT, Michael OS, Folarin OA, Adewoye EO. Plasmodium falciparum gametocyte carriage, emergence, clearance and population sex ratios in anaemic and non-anaemic malarious children. Mem Inst Oswaldo Cruz 2012; 106:562-9. [PMID: 21894377 DOI: 10.1590/s0074-02762011000500008] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2010] [Accepted: 06/06/2011] [Indexed: 11/22/2022] Open
Abstract
Anaemia in falciparum malaria is associated with an increased risk of gametocyte carriage, but its effects on transmission have not been extensively evaluated in malarious children. Plasmodium falciparum gametocyte carriage, emergence, clearance, population sex ratios (SR) (defined as the proportion of gametocytes that are male), inbreeding rates and temporal changes in SR were evaluated in 840 malarious children. Gametocyte carriage pre-treatment was at a level of 8.1%. Anaemia at enrolment was an independent risk factor for gametocyte carriage post-treatment. The emergence of gametocytes seven days post-treatment was significantly more frequent in anaemic children (7/106 vs. 10/696, p = 0.002). In the initially detected gametocytes, the proportion of children with a male-biased SR (MBSR) (> 0.5) was significantly higher in anaemic children (6/7 vs. 3/10, p = 0.027). Pre-treatment SR and estimated inbreeding rates (proportion of a mother's daughters fertilised by her sons) were similar in anaemic and non-anaemic children. Pre-treatment SR became more female-biased in non-anaemic children following treatment. However, in anaemic children, SR became male-biased. Anaemia was shown to significantly increase gametocyte emergence and may significantly alter the SR of emerging gametocytes. If MBSR is more infective to mosquitoes at low gametocytaemia, then these findings may have significant implications for malaria control efforts in endemic settings where malaria-associated anaemia is common.
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Affiliation(s)
- Grace Olusola Gbotosho
- Department of Pharmacology and Therapeutics, Institute for Medical Research and Training, University of Ibadan, Ibadan, Nigeria
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Male gametocyte fecundity and sex ratio of a malaria parasite, Plasmodium mexicanum. Parasitology 2011; 138:1203-10. [DOI: 10.1017/s0031182011000941] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
SUMMARYEvolutionary theory predicts that the sex ratio of Plasmodium gametocytes will be determined by the number of gametes produced per male gametocyte (male fecundity), parasite clonal diversity and any factor that reduces male gametes' ability to find and combine with female gametes. Despite the importance of male gametocyte fecundity for sex ratio theory as applied to malaria parasites, few data are available on gamete production by male gametocytes. In this study, exflagellating gametes, a measure of male fecundity, were counted for 866 gametocytes from 26 natural infections of the lizard malaria parasite, Plasmodium mexicanum. The maximum male fecundity observed was 8, but most gametocytes produced 2–3 gametes, a value consistent with the typical sex ratio observed for P. mexicanum. Male gametocytes in infections with higher gametocytaemia had lower fecundity. Male fecundity was not correlated with gametocyte size, but differed among infections, suggesting genetic variation for fecundity. Fecundity and sex ratio were correlated (more female gametocytes with higher fecundity) as predicted by theory. Results agree with evolutionary theory, but also suggest a possible tradeoff between production time and fecundity, which could explain the low fecundity of this species, the variation among infections, and the correlation with gametocytaemia.
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Abstract
The protozoan parasites belonging to the genus Plasmodium have a complex life cycle in which the asexual multiplication of parasites in the vertebrate host alternates with an obligate sexual reproduction in the mosquito. Gametocytes (male and female) produced in the vertebrate host are responsible for transmitting parasites to mosquitoes. Although our understanding of the biology and genetics of sexual differentiation in Plasmodium is expanding, the most basic questions concerning molecular mechanisms of sexual differentiation and sex determination still remain unanswered. Recently, insight into the control of this complex process in P. falciparum and P. berghei has come from studying parasite mutants with aberrant capacities for gametocyte production. Here, Cheryl-Ann Lobo and Nirbhay Kumar review these analyses in P. falciparum.
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Sex and Death: The Effects of Innate Immune Factors on the Sexual Reproduction of Malaria Parasites. PLoS Pathog 2011; 7:e1001309. [PMID: 21408620 PMCID: PMC3048364 DOI: 10.1371/journal.ppat.1001309] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2010] [Accepted: 02/01/2011] [Indexed: 12/31/2022] Open
Abstract
Malaria parasites must undergo a round of sexual reproduction in the blood meal of a mosquito vector to be transmitted between hosts. Developing a transmission-blocking intervention to prevent parasites from mating is a major goal of biomedicine, but its effectiveness could be compromised if parasites can compensate by simply adjusting their sex allocation strategies. Recently, the application of evolutionary theory for sex allocation has been supported by experiments demonstrating that malaria parasites adjust their sex ratios in response to infection genetic diversity, precisely as predicted. Theory also predicts that parasites should adjust sex allocation in response to host immunity. Whilst data are supportive, the assumptions underlying this prediction – that host immune responses have differential effects on the mating ability of males and females – have not yet been tested. Here, we combine experimental work with theoretical models in order to investigate whether the development and fertility of male and female parasites is affected by innate immune factors and develop new theory to predict how parasites' sex allocation strategies should evolve in response to the observed effects. Specifically, we demonstrate that reactive nitrogen species impair gametogenesis of males only, but reduce the fertility of both male and female gametes. In contrast, tumour necrosis factor-α does not influence gametogenesis in either sex but impairs zygote development. Therefore, our experiments demonstrate that immune factors have complex effects on each sex, ranging from reducing the ability of gametocytes to develop into gametes, to affecting the viability of offspring. We incorporate these results into theory to predict how the evolutionary trajectories of parasite sex ratio strategies are shaped by sex differences in gamete production, fertility and offspring development. We show that medical interventions targeting offspring development are more likely to be ‘evolution-proof’ than interventions directed at killing males or females. Given the drive to develop medical interventions that interfere with parasite mating, our data and theoretical models have important implications. Malaria and related parasites cause some of the most serious infectious diseases of humans, domestic animals and wildlife. To be transmitted, these parasites produce male and female sexual stages that differentiate into gametes and mate when taken up in a mosquito blood meal. Despite the need to develop a transmission-blocking intervention, remarkably little is understood about the evolution of parasite mating strategies. However, recent research demonstrates that producing the right ratio of male to female stages is central to mating success. Evolutionary theory predicts that sex ratios are adjusted in line with a variety of factors that affect mating success, including host immunity. We test this theory by investigating whether ubiquitous immune factors differentially affect the production and fertility of males and females. Our experiments demonstrate that immune factors have complex, sex-specific effects, from reducing gamete production to affecting offspring viability. We use these results to generate theory predicting how such effects shape the evolutionary trajectories of parasite sex ratio strategies. Given the drive to develop medical interventions that prevent transmission by blocking parasite mating, our results have important implications. Specifically, we suggest that medical interventions targeting offspring development are more likely to be ‘evolution-proof’ than interventions with sex-specific effects.
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Anderson TJC, Williams JT, Nair S, Sudimack D, Barends M, Jaidee A, Price RN, Nosten F. Inferred relatedness and heritability in malaria parasites. Proc Biol Sci 2010; 277:2531-40. [PMID: 20392725 PMCID: PMC2894920 DOI: 10.1098/rspb.2010.0196] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2010] [Accepted: 03/22/2010] [Indexed: 12/22/2022] Open
Abstract
Malaria parasites vary in phenotypic traits of biomedical or biological interest such as growth rate, virulence, sex ratio and drug resistance, and there is considerable interest in identifying the genes that underlie this variation. An important first step is to determine trait heritability (H(2)). We evaluate two approaches to measuring H(2) in natural parasite populations using relatedness inferred from genetic marker data. We collected single-clone Plasmodium falciparum infections from 185 patients from the Thailand-Burma border, monitored parasite clearance following treatment with artemisinin combination therapy (ACT), measured resistance to six antimalarial drugs and genotyped parasites using 335 microsatellites. We found strong relatedness structure. There were 27 groups of two to eight clonally identical (CI) parasites, and 74 per cent of parasites showed significant relatedness to one or more other parasites. Initially, we used matrices of allele sharing and variance components (VC) methods to estimate H(2). Inhibitory concentrations (IC(50)) for six drugs showed significant H(2) (0.24 to 0.79, p = 0.06 to 2.85 x 10(-9)), demonstrating that this study design has adequate power. However, a phenotype of current interest--parasite clearance following ACT--showed no detectable heritability (H(2) = 0-0.09, ns) in this population. The existence of CI parasites allows the use of a simple ANOVA approach for quantifying H(2), analogous to that used in human twin studies. This gave similar results to the VC method and requires considerably less genotyping information. We conclude (i) that H(2) can be effectively measured in malaria parasite populations using minimal genotype data, allowing rational design of genome-wide association studies; and (ii) while drug response (IC(50)) shows significant H(2), parasite clearance following ACT was not heritable in the population studied.
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Affiliation(s)
- Tim J C Anderson
- Southwest Foundation for Biomedical Research, San Antonio, TX, USA.
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Gametocyte sex ratio in single-clone infections of the malaria parasite Plasmodium mexicanum. Parasitology 2010; 137:1851-9. [DOI: 10.1017/s0031182010000909] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
SUMMARYSex ratio theory predicts that malaria parasites should bias gametocyte production toward female cells in single-clone infections because they will experience complete inbreeding of parasite gametes within the vector. A higher proportion of male gametocytes is favoured under conditions that reduce success of male gametes at reaching females such as low gametocyte density or attack of the immune system later in the infection. Recent experimental studies reveal genetic variation for gametocyte sex ratio in single-clone infections. We examined these issues with a study of experimental single-clone infections for the lizard malaria parasite Plasmodium mexicanum in its natural host. Gametocyte sex ratios of replicate single-clone infections were determined over a period of 3–4 months. Sex ratios were generally female biased, but not as strongly as expected under simple sex ratio theory. Gametocyte density was not related to sex ratio, and male gametocytes did not become more common later in infections. The apparent surplus of male gametocytes could be explained if male fecundity is low in this parasite, or if rapid clotting of the lizard blood reduces male gamete mobility. There was also a significant clone effect on sex ratio, suggesting genetic variation for some life-history trait, possibly male fecundity.
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Sowunmi A, Gbotosho GO, Happi CT, Folarin OA, Balogun ST. Population structure of Plasmodium falciparum gametocyte sex ratios in malarious children in an endemic area. Parasitol Int 2009; 58:438-43. [DOI: 10.1016/j.parint.2009.08.007] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2009] [Revised: 08/23/2009] [Accepted: 08/24/2009] [Indexed: 10/20/2022]
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Mitri C, Thiery I, Bourgouin C, Paul REL. Density-dependent impact of the human malaria parasite Plasmodium falciparum gametocyte sex ratio on mosquito infection rates. Proc Biol Sci 2009; 276:3721-3726. [PMID: 19656795 PMCID: PMC2817308 DOI: 10.1098/rspb.2009.0962] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2009] [Accepted: 07/16/2009] [Indexed: 11/15/2023] Open
Abstract
Malaria parasites produce male and female life cycle stages (gametocytes) that must fertilize to achieve successful colonization of the mosquito. Gametocyte sex ratios have been shown to be under strong selection pressure both as an adaptive response to a worsening blood environment for transmission and according to the number of co-infecting clones in the vertebrate. Evidence for an impact of sex ratio on the transmission success of Plasmodium falciparum has, however, been more controversial. Theoretical models of fertilization predict that increasingly male sex ratios will be favoured at low gametocyte densities to ensure fertilization. Here, we analyse in vitro transmission studies of P. falciparum to Anopheles gambiae mosquitoes and test this prediction. We find that there is a discernible effect of sex ratio on transmission but which is dependent upon the gametocyte density. While increasingly male sex ratios do give higher transmission success at low gametocyte densities, they reduce success at higher densities. This therefore provides empirical confirmation that sex ratio has an immediate impact on transmission success and that it is density-dependent. Identifying the signals used by the parasite to alter its sex ratio is essential to determine the success of transmission-blocking vaccines that aim to impede the fertilization process.
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Affiliation(s)
- C Mitri
- Institut Pasteur, Center for the Production and Infection of Anopheles, 25 rue du Dr. Roux, Paris, France
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Schall JJ. Do malaria parasites follow the algebra of sex ratio theory? Trends Parasitol 2009; 25:120-3. [DOI: 10.1016/j.pt.2008.12.006] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2008] [Revised: 11/28/2008] [Accepted: 12/05/2008] [Indexed: 10/21/2022]
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Reece SE, Ramiro RS, Nussey DH. Plastic parasites: sophisticated strategies for survival and reproduction? Evol Appl 2009; 2:11-23. [PMID: 20305703 PMCID: PMC2836026 DOI: 10.1111/j.1752-4571.2008.00060.x] [Citation(s) in RCA: 86] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2008] [Accepted: 11/26/2008] [Indexed: 11/28/2022] Open
Abstract
Adaptive phenotypic plasticity in life history traits, behaviours, and strategies is ubiquitous in biological systems. It is driven by variation in selection pressures across environmental gradients and operates under constraints imposed by trade-offs. Phenotypic plasticity has been thoroughly documented for multicellular taxa, such as insects, birds and mammals, and in many cases the underlying selective pressures are well understood. Whilst unicellular parasites face many of the same selective pressures and trade-offs, plasticity in their phenotypic traits has been largely overlooked and remains poorly understood. Here, we demonstrate that evolutionary theory, developed to explain variation observed in the life-history traits of multicellular organisms, can be applied to parasites. Though our message is general - we can expect the life-histories of all parasites to have evolved phenotypic plasticity - we focus our discussion on malaria parasites. We use an evolutionary framework to explain the trade-offs that parasites face and how plasticity in their life history traits will be expressed according to changes in their in-host environment. Testing whether variation in parasites traits is adaptive will provide new and fundamental insights into the basic biology of parasites, their epidemiology and the processes of disease during individual infections.
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Abstract
Studies of sex allocation provide some of the best evidence for Darwinian adaptation in nature. A new study of malaria parasites provides striking support for this cornerstone of evolutionary biology, with important implications for both evolutionary and medical biology.
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Affiliation(s)
- Sarah C L Knowles
- Edward Grey Institute, Dept of Zoology, University of Oxford, Oxford OX1 3PS, UK.
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Babiker HA, Schneider P, Reece SE. Gametocytes: insights gained during a decade of molecular monitoring. Trends Parasitol 2008; 24:525-30. [PMID: 18801702 PMCID: PMC2764380 DOI: 10.1016/j.pt.2008.08.001] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2008] [Revised: 07/30/2008] [Accepted: 08/04/2008] [Indexed: 11/19/2022]
Abstract
In vertebrate hosts, malaria parasites produce specialized male and female sexual stages (gametocytes). Soon after being taken up by a mosquito, gametocytes rapidly produce gametes and, once mated, they infect their vector and can be transmitted to new hosts. Despite being the parasite stages that were first identified (over a century ago), gametocytes have remained elusive, and basic questions remain concerning their biology. However, the postgenomic era has substantiated information on the specialized molecular machinery of gametocytogenesis and expedited the development of molecular tools to detect and quantify gametocytes. The application of such highly sensitive and specific tools has opened up novel approaches and provided new insights into gametocyte biology. Here, we review the discoveries made during the past decade, highlight unanswered questions and suggest new directions.
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Affiliation(s)
- Hamza A Babiker
- Biochemistry Department, Faculty of Medicine, Sultan Qaboos University, Alkhod, PO Box 35, Muscat, Code 123, Oman, UK.
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Reece SE, Drew DR, Gardner A. Sex ratio adjustment and kin discrimination in malaria parasites. Nature 2008; 453:609-14. [PMID: 18509435 PMCID: PMC3807728 DOI: 10.1038/nature06954] [Citation(s) in RCA: 175] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/29/2008] [Accepted: 03/28/2008] [Indexed: 11/09/2022]
Abstract
Malaria parasites and related Apicomplexans are the causative agents of the some of the most serious infectious diseases of humans, companion animals, livestock and wildlife. These parasites must undergo sexual reproduction to transmit from vertebrate hosts to vectors, and their sex ratios are consistently female-biased. Sex allocation theory, a cornerstone of evolutionary biology, is remarkably successful at explaining female-biased sex ratios in multicellular taxa, but has proved controversial when applied to malaria parasites. Here we show that, as predicted by theory, sex ratio is an important fitness-determining trait and Plasmodium chabaudi parasites adjust their sex allocation in response to the presence of unrelated conspecifics. This suggests that P. chabaudi parasites use kin discrimination to evaluate the genetic diversity of their infections, and they adjust their behaviour in response to environmental cues. Malaria parasites provide a novel way to test evolutionary theory, and support the generality and power of a darwinian approach.
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Affiliation(s)
- Sarah E Reece
- Institute of Evolutionary Biology, Ashworth Laboratories, School of Biological Science, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK.
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Drew DR, Reece SE. Development of reverse-transcription PCR techniques to analyse the density and sex ratio of gametocytes in genetically diverse Plasmodium chabaudi infections. Mol Biochem Parasitol 2007; 156:199-209. [PMID: 17889948 PMCID: PMC3818572 DOI: 10.1016/j.molbiopara.2007.08.004] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2007] [Revised: 08/07/2007] [Accepted: 08/10/2007] [Indexed: 11/29/2022]
Abstract
We have developed cross-genotype and genotype-specific quantitative reverse-transcription PCR (qRT-PCR) assays to detect and quantify the number of parasites, transmission stages (gametocytes) and male gametocytes in blood stage Plasmodium chabaudi infections. Our cross-genotype assays are reliable, repeatable and generate counts that correlate strongly (R(2)s>90%) with counts expected from blood smears. Our genotype-specific assays can distinguish and quantify different stages of genetically distinct parasite clones (genotypes) in mixed infections and are as sensitive as our cross-genotype assays. Using these assays we show that gametocyte density and gametocyte sex ratios vary during infections for two genetically distinct parasite lines (genotypes) and present the first data to reveal how sex ratio is affected when each genotype experiences competition in mixed-genotype infections. Successful infection of mosquito vectors depends on both gametocyte density and their sex ratio and we discuss the implications of competition in genetically diverse infections for transmission success.
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Affiliation(s)
| | - Sarah E. Reece
- Corresponding author. Tel.: +44 131 650 5547; fax: +44 131 650 6564.
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Paul RE, Bonnet S, Boudin C, Tchuinkam T, Robert V. Age-structured gametocyte allocation links immunity to epidemiology in malaria parasites. Malar J 2007; 6:123. [PMID: 17850648 PMCID: PMC2040156 DOI: 10.1186/1475-2875-6-123] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2007] [Accepted: 09/12/2007] [Indexed: 11/16/2022] Open
Abstract
BACKGROUND Despite a long history of attempts to model malaria epidemiology, the over-riding conclusion is that a detailed understanding of host-parasite interactions leading to immunity is required. It is still not known what governs the duration of an infection and how within-human parasite dynamics relate to malaria epidemiology. PRESENTATION OF THE HYPOTHESIS Immunity to Plasmodium falciparum develops slowly and requires repeated exposure to the parasite, which thus generates age-structure in the host-parasite interaction. An age-structured degree of immunity would present the parasite with humans of highly variable quality. Evolutionary theory suggests that natural selection will mould adaptive phenotypes that are more precise (less variant) in "high quality" habitats, where lifetime reproductive success is best. Variability in malaria parasite gametocyte density is predicted to be less variable in those age groups who best infect mosquitoes. Thus, the extent to which variation in gametocyte density is a simple parasite phenotype reflecting the complex within-host parasite dynamics is addressed. TESTING THE HYPOTHESIS Gametocyte densities and corresponding infectiousness to mosquitoes from published data sets and studies in both rural and urban Cameroon are analysed. The mean and variation in gametocyte density according to age group are considered and compared with transmission success (proportion of mosquitoes infected). Across a wide range of settings endemic for malaria, the age group that infected most mosquitoes had the least variation in gametocyte density, i.e. there was a significant relationship between the variance rather than the mean gametocyte density and age-specific parasite transmission success. In these settings, the acquisition of immunity over time was evident as a decrease in asexual parasite densities with age. By contrast, in an urban setting, there were no such age-structured relationships either with variation in gametocyte density or asexual parasite density. IMPLICATIONS OF THE HYPOTHESIS Gametocyte production is seemingly predicted by evolutionary theory, insofar as a reproductive phenotype (gametocyte density) is most precisely expressed (i.e. is most invariant) in the most infectious human age group. This human age group would thus be expected to be the habitat most suitable for the parasite. Comprehension of the immuno-epidemiology of malaria, a requisite for any vaccine strategies, remains poor. Immunological characterization of the human population stratified by parasite gametocyte allocation would be a step forward in identifying the salient immunological pathways of what makes a human a good habitat.
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Affiliation(s)
- Richard E Paul
- Laboratoire d'Entomologie Médicale, Institut Pasteur de Dakar, 36 Avenue Pasteur, BP 220 Dakar, Sénégal
- Laboratoire de Génétique de la réponse aux infections chez l'homme, Institut Pasteur, 28 rue de Dr. Roux, F-75724, Paris cedex 15, France
| | - Sarah Bonnet
- Laboratoire IRD de Recherche sur le Paludisme, Organisation de Coordination pour la lutte Contre les Endemies en Afrique Centrale, P.O. Box 288, Yaounde, Cameroon
- Ecole Nationale Vétérinaire de Nantes, Service de parasitologie, UMR ENVN/INRA 1034 Interactions Hôte-Parasite-Milieu, Atlanpole-La Chantrerie, B.P. 40706, 44307 Nantes cedex 03, France
| | - Christian Boudin
- UR Paludisme Afro-tropical, Institut de Recherche pour le Développement, B.P.1386 Dakar, Sénégal
| | - Timoleon Tchuinkam
- Laboratoire IRD de Recherche sur le Paludisme, Organisation de Coordination pour la lutte Contre les Endemies en Afrique Centrale, P.O. Box 288, Yaounde, Cameroon
| | - Vincent Robert
- UR Paludisme Afro-tropical, Institut de Recherche pour le Développement, B.P.1386 Dakar, Sénégal
- Institut de Recherche pour le Développement/Muséum National d'Histoire Naturelle, 61 rue Buffon, case courrier 52, 75231 Paris cedex 05, France
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Vardo AM, Schall JJ. Clonal diversity of a lizard malaria parasite, Plasmodium mexicanum, in its vertebrate host, the western fence lizard: role of variation in transmission intensity over time and space. Mol Ecol 2007; 16:2712-20. [PMID: 17594442 DOI: 10.1111/j.1365-294x.2007.03355.x] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Within the vertebrate host, infections of a malaria parasite (Plasmodium) could include a single genotype of cells (single-clone infections) or two to several genotypes (multiclone infections). Clonal diversity of infection plays an important role in the biology of the parasite, including its life history, virulence, and transmission. We determined the clonal diversity of Plasmodium mexicanum, a lizard malaria parasite at a study region in northern California, using variable microsatellite markers, the first such study for any malaria parasite of lizards or birds (the most common hosts for Plasmodium species). Multiclonal infections are common (50-88% of infections among samples), and measures of genetic diversity for the metapopulation (expected heterozygosity, number of alleles per locus, allele length variation, and effective population size) all indicated a substantial overall genetic diversity. Comparing years with high prevalence (1996-1998 = 25-32% lizards infected), and years with low prevalence (2001-2005 = 6-12%) found fewer alleles in samples taken from the low-prevalence years, but no reduction in overall diversity (H = 0.64-0.90 among loci). In most cases, rare alleles appeared to be lost as prevalence declined. For sites chronically experiencing low transmission intensity (prevalence approximately 1%), overall diversity was also high (H = 0.79-0.91), but there were fewer multiclonal infections. Theory predicts an apparent excess in expected heterozygosity follows a genetic bottleneck. Evidence for such a distortion in genetic diversity was observed after the drop in parasite prevalence under the infinite alleles mutation model but not for the stepwise mutation model. The results are similar to those reported for the human malaria parasite, Plasmodium falciparum, worldwide, and support the conclusion that malaria parasites maintain high genetic diversity in host populations despite the potential for loss in alleles during the transmission cycle or during periods/locations when transmission intensity is low.
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Affiliation(s)
- A M Vardo
- Department of Biology, University of Vermont, Burlington, VT 05405, USA.
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Annan Z, Durand P, Ayala FJ, Arnathau C, Awono-Ambene P, Simard F, Razakandrainibe FG, Koella JC, Fontenille D, Renaud F. Population genetic structure of Plasmodium falciparum in the two main African vectors, Anopheles gambiae and Anopheles funestus. Proc Natl Acad Sci U S A 2007; 104:7987-92. [PMID: 17470800 PMCID: PMC1876559 DOI: 10.1073/pnas.0702715104] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We investigated patterns of genetic diversity of Plasmodium falciparum associated with its two main African vectors: Anopheles gambiae and Anopheles funestus. We dissected 10,296 wild-caught mosquitoes from three tropical sites, two in Cameroon (Simbock and Tibati, separated by 320 km) and one in Kenya (Rota, >2,000 km from the other two sites). We assayed seven microsatellite loci in 746 oocysts from 183 infected mosquito guts. Genetic polymorphism was very high in parasites isolated from both vector species. The expected heterozygosity (H(E)) was 0.79 in both species; the observed heterozygosities (H(O)) were 0.32 in A. funestus and 0.42 in A. gambiae, indicating considerable inbreeding within both vector species. Mean selfing (s) between genetically identical gametes was s = 0.33. Differences in the rate of inbreeding were statistically insignificant among sites and between the two vector species. As expected, because of the high rate of inbreeding, linkage disequilibrium was very high; it was significant for all 21 loci pairs in A. gambiae and for 15 of 21 pairs in A. funestus, although only two pairwise comparisons were between loci on the same chromosome. Overall, the genetic population structure of P. falciparum, as evaluated by F statistics, was predominantly clonal rather than panmictic, a population structure that facilitates the spread of antimalarial drug and vaccine resistance and thus may impair the effectiveness of malaria control efforts.
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Affiliation(s)
- Zeinab Annan
- *Génétique et Evolution des Maladies Infectieuses, Unité Mixte de Recherche-Institut de Recherche pour le Développement/Centre National de la Recherche Scientifique 2724, B.P. 64501, 34394 Montpellier Cedex 5, France
- Laboratoire de Lutte Contre les Insectes Nuisibles, Unité de Recherche 016-Institut de Recherche pour le Développement, B.P. 64501, 34394 Montpellier Cedex 5, France
| | - Patrick Durand
- *Génétique et Evolution des Maladies Infectieuses, Unité Mixte de Recherche-Institut de Recherche pour le Développement/Centre National de la Recherche Scientifique 2724, B.P. 64501, 34394 Montpellier Cedex 5, France
| | - Francisco J. Ayala
- Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697
- To whom correspondence should be addressed at:
Department of Ecology and Evolutionary Biology, University of California, 321 Steinhaus Hall, Irvine, CA 92697-2525. E-mail:
| | - Céline Arnathau
- *Génétique et Evolution des Maladies Infectieuses, Unité Mixte de Recherche-Institut de Recherche pour le Développement/Centre National de la Recherche Scientifique 2724, B.P. 64501, 34394 Montpellier Cedex 5, France
| | - Parfait Awono-Ambene
- Laboratoire de Recherche sur le Paludisme-Institut de Recherche pour le Développement, Organisation de Coordination pour la Lutte Contre les Endémies en Afrique Centrale, B.P. 288, Yaoundé, Cameroon
| | - Frédéric Simard
- Laboratoire de Recherche sur le Paludisme-Institut de Recherche pour le Développement, Organisation de Coordination pour la Lutte Contre les Endémies en Afrique Centrale, B.P. 288, Yaoundé, Cameroon
| | - Fabien G. Razakandrainibe
- Laboratoire de Parasitologie Evolutive, Unité Mixte de Recherche-Centre National de la Recherche Scientifique 7103, Université Pierre et Marie Curie, 75252 Paris, France; and
| | - Jacob C. Koella
- **Division of Biology, Imperial College London, Silwood Park, Ascot, Berkshire SL5 7PY, United Kingdom
| | - Didier Fontenille
- Laboratoire de Lutte Contre les Insectes Nuisibles, Unité de Recherche 016-Institut de Recherche pour le Développement, B.P. 64501, 34394 Montpellier Cedex 5, France
| | - François Renaud
- *Génétique et Evolution des Maladies Infectieuses, Unité Mixte de Recherche-Institut de Recherche pour le Développement/Centre National de la Recherche Scientifique 2724, B.P. 64501, 34394 Montpellier Cedex 5, France
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Abstract
Theileria parva is a tick-borne intracellular protozoan of cattle, with obligate sequential differentiation stages in lymphocytes and erythrocytes. Immunity is mediated by cytotoxic T lymphocytes (CTL) that target and clear parasitized lymphocytes but allow persistence of infected erythrocytes, which are required for transmission to the tick. The life cycle of T. parva is haploid with the exception of a brief diploid stage in the tick vector during which sexual recombination occurs. There is evidence for antigenic diversity in field parasite populations, although broad immunity can be acquired following exposure to a limited number of strains. The CTL response in individual animals is tightly focused and its specificity is strongly influenced by major histocompatibility complex (MHC) phenotype. This review discusses the issue of how CTL immunity is likely to impact on parasite population structure in the light of available information on diversity of the parasite and its ability to recombine.
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Affiliation(s)
- D J McKeever
- Department of Veterinary Clinical Studies, University of Edinburgh, Easter Bush Veterinary Centre, Roslin, Midlothian, UK.
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Shutler D, Reece SE, Mullie A, Billingsley PF, Read AF. Rodent malaria parasites Plasmodium chabaudi and P. vinckei do not increase their rates of gametocytogenesis in response to mosquito probing. Proc Biol Sci 2005; 272:2397-402. [PMID: 16243686 PMCID: PMC1559963 DOI: 10.1098/rspb.2005.3232] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Several vector-borne infectious agents facultatively alter their life history strategies in response to local vector densities. Some evidence suggests that malaria parasites invest more heavily in transmission stage production (gametocytogenesis) when vectors are present. Such a strategy could rapidly increase malaria transmission rates, particularly when adult mosquitoes begin to appear after dry seasons. However, in contrast to a recent experiment with a rodent malaria (Plasmodium chabaudi), we found no change in gametocytogenesis in either P. chabaudi or in another rodent malaria, P. vinckei, when their mouse hosts were exposed to mosquitoes. Positive results in the earlier study may have been because mosquito-feeding caused anaemia in hosts, a known promoter of gametocytogenesis. The substantial evidence that malaria and a variety of other parasites facultatively alter transmission strategies in response to a variety of environmental influences makes our results surprising.
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Affiliation(s)
- Dave Shutler
- Acadia University, Department of Biology, Wolfville, NS, Canada B4P 2R6.
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37
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Martínez-de la Puente J, Merino S, Tomás G, Moreno J, Morales J, Lobato E. Are multiple gametocyte infections in malarial parasites an adaptation to ensure fertility? Parasitology 2005; 132:23-8. [PMID: 16393350 DOI: 10.1017/s0031182005008711] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/21/2005] [Revised: 06/20/2005] [Accepted: 07/13/2005] [Indexed: 11/07/2022]
Abstract
Multiple infections, those by more than one parasite in the same erythrocyte, may be adaptive for the malarial parasite as a means to ensure fertility. Alternatively they may simply be the consequence of a non-adaptive process forcing several parasites to compete for resources in one host cell. Avian hosts infected withHaemoproteuswere medicated with primaquine or injected with saline solution and the density of infection and number, maturity and sex of mature multiple infections counted. Multiple infections depend on density of infection, and maturity is attained rarely and usually by gametocytes of the same sex. The role of multiple infections for fertility insurance is not supported by these results.
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Affiliation(s)
- J Martínez-de la Puente
- Departamento de Ecología Evolutiva, Museo Nacional de Ciencias Naturales-CSIC, J. Gutiérrez Abascal 2, E-28006 Madrid, Spain
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38
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Gardner A, Allsop DJ, Charnov EL, West SA. A Dimensionless Invariant for Relative Size at Sex Change in Animals: Explanation and Implications. Am Nat 2005; 165:551-66. [PMID: 15795852 DOI: 10.1086/429526] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2004] [Accepted: 12/16/2004] [Indexed: 11/03/2022]
Abstract
Recent comparative studies across sex-changing animals have found that the relative size and age at sex change are strikingly invariant. In particular, 91%-97% of the variation in size at sex change across species can be explained by the simple rule that individuals change sex when they reach 72% of their maximum body size. However, this degree of invariance is surprising and has proved controversial. In particular, it is not clear why this result should hold, given that there is considerable biological variation across species in factors that can influence the evolutionarily stable timing of sex change. Our overall aim here is to explain this result and determine the implications for other life-history variables. Specifically, we use a combination of approaches to formalize and make explicit previous analytical theory in this area, examine the robustness of the empirical invariance result, and carry out sensitivity analyses to determine what the empirical data imply about the mean value and variation in several key life-history variables.
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Affiliation(s)
- Andy Gardner
- Institute of Evolutionary Biology, University of Edinburgh, Edinburgh EH9 3JT, United Kingdom.
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39
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Abstract
The suggestion that a clonal population structure may typify Plasmodium populations has proved highly controversial. For the most part, existing population genetic data from wild populations contradict the idea and are consistent with randomly interbreeding populations. In this article, Andrew Read and Koren Day point out that these data could also be consistent with population subdivision and frequent nonrandom mating, which current sampling methods would be incapable of detecting.
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Affiliation(s)
- A F Read
- Department of Zoology, University of Oxford, Oxford, UK
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40
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Taylor LH, Read AF. Why so few transmission stages? Reproductive restraint by malaria parasites. ACTA ACUST UNITED AC 2005; 13:135-40. [PMID: 15275099 DOI: 10.1016/s0169-4758(97)89810-9] [Citation(s) in RCA: 126] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Vast numbers of malaria parasites exist in a population: perhaps 10(10) in just one vertebrate host. Yet gametocytes, the only stage capable of transmission, usually constitute just a few percent or even less of the circulating parasites. Why? Parasite fitness should be intimately linked with transmission probability and infectiousness rises with gametocyte density. Here, Louise Taylor and Andrew Read propose several testable hypotheses that might explain why natural selection has not favoured variants producing more transmission stages.
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Affiliation(s)
- L H Taylor
- Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh, UK.
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41
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Reiczigel J, Lang Z, Rózsa L, Tóthmérész B. PROPERTIES OF CROWDING INDICES AND STATISTICAL TOOLS TO ANALYZE PARASITE CROWDING DATA. J Parasitol 2005; 91:245-52. [PMID: 15986595 DOI: 10.1645/ge-281r1] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022] Open
Abstract
Crowding, i.e., the size of the infrapopulation inhabiting an individual host, is a major component of parasites' environment, which often influences both morphological and life-history characters (the so-called density-dependent characters) in different parasite taxa. Although crowding equals intensity in case of a single parasite individual, mean intensity of the host population does not define mean crowding of the parasite population. Crowding indices are notoriously hard to handle statistically because of the inherently large number of nonindependent values in data. In this study, we aim to investigate the apparently paradox features of crowding indices and to make some proposals and also to introduce statistical methods to calculate confidence intervals and 1-sample and 2-sample tests for mean crowding. All methods described in this study are supported by the freely distributed statistical software Quantitative Parasitology.
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Affiliation(s)
- Jenö Reiczigel
- Department of Biomathematics and Informatics, Faculty of Veterinary Science, Szent István University, P.O. Box 2, Budapest H-1400, Hungary.
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42
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Reece SE, Duncan AB, West SA, Read AF. Host cell preference and variable transmission strategies in malaria parasites. Proc Biol Sci 2005; 272:511-7. [PMID: 15799947 PMCID: PMC1578707 DOI: 10.1098/rspb.2004.2972] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Malaria and other haemosporin parasites must undergo a round of sexual reproduction in their insect vector in order to produce stages that can be transmitted to vertebrate hosts. Consequently, it is crucial that parasites produce the sex ratio (proportion of male sexual stages) that will maximize the number of fertilizations and thus, transmission to new vertebrate hosts. There is some evidence to show that, consistent with evolutionary theory, the sex ratios of malaria parasites are negatively correlated to their inbreeding rate. However, recent theory has shown that when fertilization success is compromised, parasites should respond by increasing their investment in sexual stages or by producing a less female biased sex ratio than predicted by their inbreeding rate alone. Here, we show that two species of rodent malaria,
Plasmodium chabaudi
and
Plasmodium vinckei petteri
, adopt different strategies in response to host anaemia, a factor thought to compromise transmission success:
P. chabaudi
increases investment in sexual stages, whereas
P. vinckei
produces a less female biased sex ratio. We suggest that these different transmission strategies may be due to marked species differences in host cell preference.
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Affiliation(s)
- Sarah E Reece
- Institutes of Evolution, Immunology and Infection Research, School of Biological Sciences, Ashworth Laboratories, University of Edinburgh, Edinburgh EH9 3JT, UK.
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43
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Merino S, Tomás G, Moreno J, Sanz JJ, Arriero E, Folgueira C. Changes in Haemoproteus sex ratios: fertility insurance or differential sex lifespan? Proc Biol Sci 2004; 271:1605-9. [PMID: 15306307 PMCID: PMC1691765 DOI: 10.1098/rspb.2004.2764] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
There is little direct evidence of the fitness effects of changes in malaria gametocyte sex ratio. Gametocyte sex ratios in haemospororin parasites (phylum Apicomplexa) are usually female skewed. However, in some cases and especially in Haemoproteus parasites, less female-biased and even male-biased sex ratios are encountered. The 'fertility insurance hypothesis' tries to explain these biases as an evolutionary strategy to facilitate gamete encounter. Thus, the hypothesis predicts that, if there is a reduction in gametocyte density (intensity of infection) or other factors preventing gametes from meeting, a change to a higher proportion of male gametocytes may be favoured. By contrast, a change in sex ratio may be caused by other non-adaptive mechanisms, for example differential survival of the gametocytes of each sex. We study within-host changes in Haemoproteus majoris sex ratios following an experimental reduction in the density of the parasites in the blood in a breeding population of blue tits (Parus caeruleus). Medication with the antimalarial drug primaquine induced a significant reduction in Haemoproteus gametocyte infection intensity in two different breeding seasons and under two different doses of medication. Sex ratios became male skewed following the experimental treatment in agreement with the predictions of the 'fertility insurance' hypothesis. Also in support of the hypothesis, a significant change towards male-biased sex ratios emerged for non-medicated birds in one year, probably owing to the natural immune reduction of the density of the parasites in the blood. The alternative possibility that changes are caused by different lifespans of gametocytes is not supported by changes in sex ratios in control hosts, where new production and release of gametocytes occur.
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Affiliation(s)
- Santiago Merino
- Museo Nacional de Ciencias Naturales, CSIC, C/ José Gutiérrez Abascal, 2, E-28006 Madrid, Spain.
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44
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Talman AM, Domarle O, McKenzie FE, Ariey F, Robert V. Gametocytogenesis: the puberty of Plasmodium falciparum. Malar J 2004; 3:24. [PMID: 15253774 PMCID: PMC497046 DOI: 10.1186/1475-2875-3-24] [Citation(s) in RCA: 136] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2004] [Accepted: 07/14/2004] [Indexed: 11/16/2022] Open
Abstract
The protozoan Plasmodium falciparum has a complex life cycle in which asexual multiplication in the vertebrate host alternates with an obligate sexual reproduction in the anopheline mosquito. Apart from the apparent recombination advantages conferred by sex, P. falciparum has evolved a remarkable biology and adaptive phenotypes to insure its transmission despite the dangers of sex. This review mainly focuses on the current knowledge on commitment to sexual development, gametocytogenesis and the evolutionary significance of various aspects of gametocyte biology. It goes further than pure biology to look at the strategies used to improve successful transmission. Although gametocytes are inevitable stages for transmission and provide a potential target to fight malaria, they have received less attention than the pathogenic asexual stages. There is a need for research on gametocytes, which are a fascinating stage, responsible to a large extent for the success of P. falciparum.
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Affiliation(s)
- Arthur M Talman
- Groupe de Recherche sur le Paludisme, Institut Pasteur de Madagascar, B.P.1274 Antananarivo 101, Madagascar
- Department of Biological Sciences, Imperial College London, Exhibition Road, SW7 2AZ London, UK
| | - Olivier Domarle
- Groupe de Recherche sur le Paludisme, Institut Pasteur de Madagascar, B.P.1274 Antananarivo 101, Madagascar
| | - F Ellis McKenzie
- Fogarty International Centre, National Institutes of Health, Bethesda, MD 20892, USA
| | - Frédéric Ariey
- Groupe de Recherche sur le Paludisme, Institut Pasteur de Madagascar, B.P.1274 Antananarivo 101, Madagascar
| | - Vincent Robert
- Groupe de Recherche sur le Paludisme, Institut Pasteur de Madagascar, B.P.1274 Antananarivo 101, Madagascar
- UR 77 Paludisme Afro-tropical, Institut de Recherche pour le Développement, Madagascar
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45
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Gouagna LC, Ferguson HM, Okech BA, Killeen GF, Kabiru EW, Beier JC, Githure JI, Yan G. Plasmodium falciparummalaria disease manifestations in humans and transmission toAnopheles gambiae: a field study in Western Kenya. Parasitology 2004; 128:235-43. [PMID: 15074873 DOI: 10.1017/s003118200300444x] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Transmission of the malaria parasitePlasmodiumis influenced by many different host, vector and parasite factors. Here we conducted a field study at Mbita, an area of endemic malaria in Western Kenya, to test whether parasite transmission to mosquitoes is influenced by the severity of malaria infection in its human host at the time when gametocytes, the transmission forms, are present in the peripheral blood. We examined the infectivity of 81Plasmodium falciparumgametocyte carriers to mosquitoes. Of these, 21 were patients with fever and other malaria-related symptoms, and 60 were recruited among apparently healthy volunteers. Laboratory-rearedAnopheles gambiaes.s. (local strain) were experimentally infected with blood from these gametocyte carriers by membrane-feeding. The severity of the clinical symptoms was greater in febrile patients. These symptomatic patients had higher asexual parasitaemia and lower gametocyte densities (P=0·05) than healthy volunteers. Ookinete development occurred in only 6 out of the 21 symptomatic patients, of which only 33·3% successfully yielded oocysts. The oocyst prevalence was only 0·6% in the 546 mosquitoes that were fed on blood from this symptomatic group, with mean oocyst intensity of 0·2 (range 0–2) oocysts per mosquito. In contrast, a higher proportion (76·7%) of healthy gametocyte carriers yielded ookinetes, generating an oocyst rate of 12% in the 1332 mosquitoes that fed on them (mean intensity of 6·3, range: 1–105 oocysts per mosquito). Statistical analysis indicated that the increased infectivity of asymptomatic gametocyte carriers was not simply due to their greater gametocyte abundance, but also to the higher level of infectivity of their gametocytes, possibly due to lower parasite mortality within mosquitoes fed on blood from healthy hosts. These results suggest that blood factors and/or conditions correlated with illness reduceP. falciparumgametocyte infectivity.
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Affiliation(s)
- L C Gouagna
- Human Health Division, Mbita Point Research and Training Station, International Centre of Insect Physiology an2d Ecology, P.O. Box 30772, Nairobi, Kenya.
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46
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Gaillard FO, Boudin C, Chau NP, Robert V, Pichon G. Togetherness among Plasmodium falciparum gametocytes: interpretation through simulation and consequences for malaria transmission. Parasitology 2004; 127:427-35. [PMID: 14653532 DOI: 10.1017/s0031182003004025] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Previous experimental gametocyte infections of Anopheles arabiensis on 3 volunteers naturally infected with Plasmodium falciparum were conducted in Senegal. They showed that gametocyte counts in the mosquitoes are, like macroparasite intakes, heterogeneous (overdispersed). They followed a negative binomial distribution, the overdispersion coefficient seeming constant (k = 3.1). To try to explain this heterogeneity, we used an individual-based model (IBM), simulating the behaviour of gametocytes in the human blood circulation and their ingestion by mosquitoes. The hypothesis was that there exists a clustering of the gametocytes in the capillaries. From a series of simulations, in the case of clustering the following results were obtained: (i) the distribution of the gametocytes ingested by the mosquitoes followed a negative binomial, (ii) the k coefficient significantly increased with the density of circulating gametocytes. To validate this model result, 2 more experiments were conducted in Cameroon. Pooled experiments showed a distinct density dependency of the k-values. The simulation results and the experimental results were thus in agreement and suggested that an aggregation process at the microscopic level might produce the density-dependent overdispersion at the macroscopic level. Simulations also suggested that the clustering of gametocytes might facilitate fertilization of gametes.
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Affiliation(s)
- F O Gaillard
- Institut de Recherche pour le Développement (IRD), UR 79, Geodes, Bondy, France
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47
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Osgood SM, Schall JJ. Gametocyte sex ratio of a malaria parasite: response to experimental manipulation of parasite clonal diversity. Parasitology 2004; 128:23-9. [PMID: 15002900 DOI: 10.1017/s0031182003004207] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Sex ratio theory posits that the adaptive proportion of male to female gametocytes of a malaria parasite within the vertebrate host depends on the degree of inbreeding within the vector. Gametocyte sex ratio could be phenotypically flexible, being altered based on the infection's clonal diversity, and thus likely inbreeding. This idea was tested by manipulating the clonal diversity of infections of Plasmodium mexicanum in its lizard host, Sceloporus occidentalis. Naive lizards were inoculated with infected blood from a single donor or 3 pooled donors. Donors varied in their gametocyte sex ratios (17–46% male), and sex ratio theory allowed estimation of the clonal diversity within donor and recipient infections. Phenotypic plasticity would produce a correlation between donor and recipient infections for infections initiated from a single donor, and a less female-biased gametocyte sex ratio in recipients that received a mixed blood inoculum (with predicted higher clonal diversity) than recipients receiving blood from a single donor. Neither pattern was observed. Gametocyte sex ratio of most infections ranged from 35 to 42% male, expected if clonal diversity was high for all infections. Alternative explanations are suggested for the observed variation of gametocyte sex ratio among P. mexicanum infections.
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Affiliation(s)
- S M Osgood
- Department of Biology, University of Vermont, Burlington, Vermont 05405, USA
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48
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Reece SE, Duncan AB, West SA, Read AF. Sex ratios in the rodent malaria parasite, Plasmodium chabaudi. Parasitology 2003; 127:419-25. [PMID: 14653531 DOI: 10.1017/s0031182003004013] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
The sex ratios of malaria and related Apicomplexan parasites play a major role in transmission success. Here, we address 2 fundamental issues in the sex ratios of the rodent malaria parasite, Plasmodium chabaudi. First we test the accuracy of empirical methods for estimating sex ratios in malaria parasites, and show that sex ratios made with standard thin smears may overestimate the proportion of female gametocytes. Secondly, we test whether the mortality rate differs between male and female gametocytes, as assumed by sex ratio theory. Conventional application of sex ratio theory to malaria parasites assumes that the primary sex ratio can be accurately determined from mature gametocytes circulating in the peripheral circulation. We stopped gametocyte production with chloroquine in order to study a cohort of gametocytes in vitro. The mortality rate was significantly higher for female gametocytes, with an average half-life of 8 h for female gametocytes and 16 h for male gametocytes.
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Affiliation(s)
- S E Reece
- Institute of Cell, Animal and Population Biology, Ashworth Laboratories, West Mains Road, University of Edinburgh, Edinburgh EH9 37T, UK.
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49
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50
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Gardner A, Reece SE, West SA. Even more extreme fertility insurance and the sex ratios of protozoan blood parasites. J Theor Biol 2003; 223:515-21. [PMID: 12875828 DOI: 10.1016/s0022-5193(03)00142-5] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Theory developed for malaria and other protozoan parasites predicts that the evolutionarily stable gametocyte sex ratio (z*; proportion of gametocytes that are male) should be related to the inbreeding rate (f) by the equation z*=(1-f)/2. Although this equation has been applied with some success, it has been suggested that in some cases a less female biased sex ratio can be favoured to ensure female gametes are fertilized. Such fertility insurance can arise in response to two factors: (i) low numbers of gametes produced per gametocyte and (ii) the gametes of only a limited number of gametocytes being able to interact. However, previous theoretical studies have considered the influence of these two forms of fertility insurance separately. We use a stochastic analytical model to address this problem, and examine the consequences of when these two types of fertility insurance are allowed to occur simultaneously. Our results show that interactions between the two types of fertility insurance reduce the extent of female bias predicted in the sex ratio, suggesting that fertility insurance may be more important than has previously been assumed.
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Affiliation(s)
- A Gardner
- Institute of Cell, Animal and Population Biology, Ashworth Laboratories, University of Edinburgh, Edinburgh EH9 3JT, UK.
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