Parasite Population Size Research Articles

Measuring changes in Plasmodium falciparum census population size in response to sequential malaria control interventions.

Here we introduce a new endpoint ″census population size″ to evaluate the epidemiology and control of Plasmodium falciparum infections, where the parasite, rather than the infected human host, is the unit of measurement. To calculate census population size, we rely on a definition of parasite variation known as multiplicity of infection (MOI var ), based on the hyper-diversity of the var multigene family. We present a Bayesian approach to estimate MOI var from sequencing and counting the number of unique DBLα tags (or DBLα types) of var genes, and derive from it census population size by summation of MOI var in the human population. We track changes in this parasite population size and structure through sequential malaria interventions by indoor residual spraying (IRS) and seasonal malaria chemoprevention (SMC) from 2012 to 2017 in an area of high-seasonal malaria transmission in northern Ghana. Following IRS, which reduced transmission intensity by > 90% and decreased parasite prevalence by ~40-50%, significant reductions in var diversity, MOI var , and population size were observed in ~2,000 humans across all ages. These changes, consistent with the loss of diverse parasite genomes, were short lived and 32-months after IRS was discontinued and SMC was introduced, var diversity and population size rebounded in all age groups except for the younger children (1-5 years) targeted by SMC. Despite major perturbations from IRS and SMC interventions, the parasite population remained very large and retained the var population genetic characteristics of a high-transmission system (high var diversity; low var repertoire similarity) demonstrating the resilience of P. falciparum to short-term interventions in high-burden countries of sub-Saharan Africa.

Genetic infrapopulation sizes in blood parasites: a pilot quantification of the bottleneck in louse fly vectors

IntroductionBottleneck events are crucial for the strength of genetic drift, selection and speed of evolution. They are believed to play a particularly prominent role for parasitic infrapopulations, inhabiting single host individuals, which are often established by very few parasite individuals during transmission. In vector-borne pathogens, the bottlenecking effects can even be serialized through repeated filtering of parasitic stages at different tissues and organs of the vector. Using qPCR we aimed to quantify the number of potentially transmittable sporozoites of the hemosporidian blood parasite Haemoproteus columbae in the specialized vector louse flies Pseudolynchia canariensis which transmit these parasites between house pigeon hosts Columba livia.ResultsBased on qPCR measurements of organ-derived DNA of individual louse flies, we estimate that the midgut of these vectors contains on average 20 parasites, the hindgut and other intestines ca. 50 parasites and the salivary glands ca. 5 parasite cells. Nearly one third of all vector individuals appeared to lack parasite DNA, despite having only infected hosts as blood meal sources. The magnitude of parasite numbers in midgut and salivary glands tended to correlate positively.DiscussionOur results indicate, potential severe bottlenecking of parasite populations during individual transmission events and a probable effect of individual vector immunity on this variable. However, this may be partly alleviated by the coloniality of house pigeons, the frequency of louse flies and their daily feeding events in most populations, leading to repeated transmission opportunities, decreased quasi-vertical transmission between parents and offspring and probable panmixia of Haemoproteus columbae lineages. Many of these mechanisms might not apply in other host-vector systems. We propose several additional molecular and microscopical tools to improve the accuracy of estimating parasite population sizes in vectors and call for more estimations in different vector species to better understand the co-evolution between malaria-like blood parasites and their avian and insect hosts.

Genomic malaria surveillance of antenatal care users detects reduced transmission following elimination interventions in Mozambique

Routine sampling of pregnant women at first antenatal care (ANC) visits could make Plasmodium falciparum genomic surveillance more cost-efficient and convenient in sub-Saharan Africa. We compare the genetic structure of parasite populations sampled from 289 first ANC users and 93 children from the community in Mozambique between 2015 and 2019. Samples are amplicon sequenced targeting 165 microhaplotypes and 15 drug resistance genes. Metrics of genetic diversity and relatedness, as well as the prevalence of drug resistance markers, are consistent between the two populations. In an area targeted for elimination, intra-host genetic diversity declines in both populations (p = 0.002-0.007), while for the ANC population, population genetic diversity is also lower (p = 0.0004), and genetic relatedness between infections is higher (p = 0.002) than control areas, indicating a recent reduction in the parasite population size. These results highlight the added value of genomic surveillance at ANC clinics to inform about changes in transmission beyond epidemiological data.

Genetic diversity and molecular evolution of Plasmodium vivax Duffy Binding Protein and Merozoite Surface Protein-1 in northwestern Thailand.

The local diversity and population structure of malaria parasites vary across different regions of the world, reflecting variations in transmission intensity, host immunity, and vector species. This study aimed to use amplicon sequencing to investigate the genotypic patterns and population structure of P. vivax isolates from a highly endemic province of Thailand in recent years. Amplicon deep sequencing was performed on 70 samples for the 42-kDa region of pvmsp1 and domain II of pvdbp. Unique haplotypes were identified and a network constructed to illustrate genetic relatedness in northwestern Thailand. Based on this dataset of 70 samples collected between 2015 and 2021, 16 and 40 unique haplotypes were identified in pvdbpII and pvmsp142kDa, respectively. Nucleotide diversity was higher in pvmsp142kDa than in pvdbpII (π=0.027 and 0.012), as was haplotype diversity (Hd=0.962 and 0.849). pvmsp142kDa also showed a higher recombination rate and higher levels of genetic differentiation (Fst) in northwestern Thailand versus other regions (0.2761-0.4881). These data together suggested that the genetic diversity of P. vivax in northwestern Thailand at these two studied loci evolved under a balancing selection, most likely host immunity. The lower genetic diversity of pvdbpII may reflect its stronger functional constrain. In addition, despite the balancing selection, a decrease in genetic diversity was observed. Hd of pvdbpII decreased from 0.874 in 2015-2016 to 0.778 in 2018-2021; π of pvmsp142kDa decreased from 0.030 to 0.022 over the same period. Thus, the control activities must have had a strong impact on the parasite population size. The findings from this study provide an understanding of P. vivax population structure and the evolutionary force on vaccine candidates. They also established a new baseline for tracking future changes in P. vivax diversity in the most malarious area of Thailand.

Ectoparasites population dynamics are affected by host body size but not host density or water temperature in a 32‐year long time series

Host density, host body size and ambient temperature have all been positively associated with increases in parasite infection. However, the relative importance of these factors in shaping long‐term parasite population dynamics in wild host populations is unknown due to the absence of long‐term studies. Here, we examine long‐term drivers of gill lice (Copepoda) infections in Arctic charr (Salmonidae) over 32 years. We predicted that host density and body size and water temperature would all positively affect parasite population size and population growth rate. Our results show that fish size was the main driver of gill lice infections in Arctic charr. In addition, Arctic charr became infected at smaller sizes and with more parasites in years of higher brown trout population size. Negative intraguild interactions between brown trout and Arctic charr appear to drive smaller Arctic charr to seek refuge in deeper areas of the lake, thus increasing infection risk. There was no effect of host density on the force of infection, and the relationship between Arctic charr density and parasite mean abundance was negative, possibly due to an encounter‐dilution effect. The population densities of host and parasite fluctuated independently of one another. Water temperature had negligible effects on the temporal dynamics of the gill lice population. Understanding long‐term drivers of parasite population dynamics is key for research and management. In fish farms, artificially high densities of hosts lead to vast increases in the transmission of parasitic copepods. However, in wild fish populations fluctuating at natural densities, the surface area available for copepodid attachment might be more important than the density of available hosts.

Novel genomic approaches to study antagonistic coevolution between hosts and parasites.

Host-parasite coevolution is ubiquitous, shaping genetic and phenotypic diversity and the evolutionary trajectory of interacting species. With the advances of high throughput sequencing technologies applicable to model and non-model organisms alike, it is now feasible to study in greater detail (a) the genetic underpinnings of coevolution, (b) the speed and type of dynamics at coevolving loci, and (c) the genomic consequences of coevolution. This review focuses on three recently developed approaches that leverage information from host and parasite full genome data simultaneously to pinpoint coevolving loci and draw inference on the coevolutionary history. First, co-genome-wide association study (co-GWAS) methods allow pinpointing the loci underlying host-parasite interactions. These methods focus on detecting associations between genetic variants and the outcome of experimental infection tests or on correlations between genomes of naturally infected hosts and their infecting parasites. Second, extensions to population genomics methods can detect genes under coevolution and infer the coevolutionary history, such as fitness costs. Third, correlations between host and parasite population size in time are indicative of coevolution, and polymorphism levels across independent spatially distributed populations of hosts and parasites can reveal coevolutionary loci and infer coevolutionary history. We describe the principles of these three approaches and discuss their advantages and limitations based on coevolutionary theory. We present recommendations for their application to various host (prokaryotes, fungi, plants, and animals) and parasite (viruses, bacteria, fungi, and macroparasites) species. We conclude by pointing out methodological and theoretical gaps to be filled to extract maximum information from full genome data and thereby to shed light on the molecular underpinnings of coevolution.

Impact of bile salts on coevolutionary dynamics between the gut bacterium Escherichia coli and its lytic phage PP01

Although host and parasites are typically embedded in complex abiotic and biotic environments our understanding of how environmental variation impacts on host-parasite interactions, including antagonistic coevolution (AC) is poorly understood. Nonetheless, previous studies using bacteria and bacteriophages have shown that variation in just one abiotic parameter can have profound effects not only on the type of AC dynamics observed but also the time-frames over which AC interactions can persist. Here, we investigated the effect of an important component of the abiotic human gut environment, bile salts, on AC dynamics between the bacterium Escherichia coli and the lytic phage PP01 in an in vitro model system. In the absence of bile salts E.coli and PP01 coevolved in a manner that is consistent with a directional arms race dynamic (ARD), with bacteria and phages evolving increasing resistance and infectivity ranges through time. However, in the presence of bile salts, evidence of directional coevolution was weaker and more variable across replicate communities. These effects may be explained, in part, by the negative effect of bile salts on both host and parasite population sizes; lower population sizes for both bacteria and phages will reduce encounter rates which in turn could mitigate the benefits of generalism in both host and parasite resistance and infectivity ranges that are observed for ARDs. The negative effect of bile salts on phage population size may also be partially independent of host population size as bile salts was found to negatively impact phage viability in the absence of bacteria, as well as reducing phage adsorption efficiency. Differences in bacterial morphological diversity between treatments were also noted, with the emergence of mucoid colonies in both bile salts and non-bile salts treatments but only in the presence of phages. These data contribute to the growing body of knowledge on how environmental variation can impact on interactions between hosts and parasites. More specifically, these results are particularly relevant to our understanding of how bacteria-phage interactions may be affected by different abiotic factors relevant to the complex environment of the human gut and have clear implications for the development of phage as therapeutics to target members of the gut microbiota and/or intestinal pathogens.

Indirect ecological effects interact with community genetic effects in a host-parasite system and dramatically reduce parasite burden.

Community genetic (CG) effects and ecological factors create a complex set of interactions that are key drivers of evolutionary dynamics in ecological systems. To date, most studies investigating trait variation have focused on either effects of intraspecific genetic variation or on genotype by environment (GxE) interactions in isolation. Poorly investigated but very important are the interactions between CGs and indirect ecological effects (IEEs) that are caused by plant-soil interactions. Here, we tested how CGs in a cabbage host and its aphid parasite depended on the ecological conditions under which the host was grown. We established microcosms of different cabbage cultivars and aphid genotypes on soils inoculated with samples of other soils previously trained with onion. We hypothesized that such IEEs will have significantly different outcomes for ecosystems than predicted from simpler CG or GxE studies. Our analysis demonstrated a large IEE that differed by context and aphid genotype causing reduced parasite population sizes by up to 90%. The IEE is induced by insect-repellent properties and the microbiome of the onion. Our results highlight the importance of interacting IEEs and CGs for ecosystems dynamics showing that IEEs offer sustainable solutions by dramatically reducing parasite burden on cash crops.

Characterizing the growth of Sarcoptes scabiei infrapopulations.

During the course of parasitic disease infestations, parasite population sizes change at both individual host (infrapopulation) and host population (metapopulation) levels. However, most studies only report epidemiological values for specific locations and times. In this study we analysed the dynamics of several Sarcoptes scabiei infrapopulations from experimentally infested Iberian ibex, Capra pyrenaica. We obtained mite counts by digesting small skin biopsies, which we compared with indices obtained from histopathological analyses performed on adjacent skin biopsies. We obtained the finite growth rate and the daily growth rate for the mite infrapopulations: mean ± SE = 11.53 ± 10.17 and 0.10 ± 0.08 mites/day, respectively. Mite counts derived from skin sample digestion did not correlate with the histological mite indices obtained from adjacent skin biopsies. At a metapopulational level, both indices of mite abundance were modelled using GLMMs and the factors influencing their variation are analysed and discussed. Our results suggest that mites are not distributed uniformly over the whole area of the skin lesion. Therefore, direct diagnoses of mange and mite counts could be inaccurate if only small skin samples are used.

Substantial population structure of Plasmodium vivax in Thailand facilitates identification of the sources of residual transmission.

BackgroundPlasmodium vivax transmission in Thailand has been substantially reduced over the past 10 years, yet it remains highly endemic along international borders. Understanding the genetic relationship of residual parasite populations can help track the origins of the parasites that are reintroduced into malaria-free regions within the country.Methodology/ResultsA total of 127 P. vivax isolates were genotyped from two western provinces (Tak and Kanchanaburi) and one eastern province (Ubon Ratchathani) of Thailand using 10 microsatellite markers. Genetic diversity was high, but recent clonal expansion was detected in all three provinces. Substantial population structure and genetic differentiation of parasites among provinces suggest limited gene flow among these sites. There was no haplotype sharing among the three sites, and a reduced panel of four microsatellite markers was sufficient to assign the parasites to their provincial origins.Conclusion/SignificanceSignificant parasite genetic differentiation between provinces shows successful interruption of parasite spread within Thailand, but high diversity along international borders implies a substantial parasite population size in these regions. The provincial origin of P. vivax cases can be reliably determined by genotyping four microsatellite markers, which should be useful for monitoring parasite reintroduction after malaria elimination.

Uneven abundances determine nestedness in climbing plant-host interaction networks

Nestedness is a common pattern in interaction networks. However, its ecological and evolutionary meaning is under debate. Evidence shows that nestedness in mutualistic networks may be just a consequence of the species–abundance distribution. This has been questioned as abundance itself could be influenced by differences in generalism between species. Host-parasite networks in plant communities also show nested patterns, but their relationship with abundance has been seldom addressed. Importantly, an assessment of the potentially different effect of the number of interacting species (i.e. generalism levels) on the size of parasite and host populations can help understanding the role of abundance in determining both generalism and nestedness. Here we show that nestedness follows abundance expectations in an interaction network of climbing plants (i.e. structural parasites) and their tree and shrub hosts. Our results also point to a direct effect of abundance on both nestedness and generalism levels because species degree does not deviate from abundance expectations for both climbing plants and their hosts. Further, we found a similar level of discordance between generalization (a generalism measure independent of species abundance) and abundance for both parties. Our findings provide evidence that the factors underlying uneven abundance distributions can induce nestedness in interaction networks. We stress the importance of neutral processes related to species dominance as major determinants of nestedness in host-parasite networks.

An elaborated SIR model for haemonchosis in sheep in South Africa under a targeted selective anthelmintic treatment regime

Infection with the abomasal nematode Haemonchus contortus is responsible for considerable production loss in small ruminants globally, and especially in warm, summer-rainfall regions. Previous attempts to predict infection levels have followed the traditional framework for macroparasite models, i.e. tracking parasite population sizes as a function of host and climatic factors. Targeted treatment strategies, in which patho-physiological indices are used to identify the individuals most affected by parasites, could provide a foundation for alternative, incidence-based epidemiological models. In this paper, an elaboration of the classic susceptible-infected-recovered (SIR) model framework for microparasites was adapted to haemonchosis and used to predict disease in Merino sheep on a commercial farm in South Africa. Incidence was monitored over a single grazing season using the FAMACHA scoring system for conjunctival mucosal coloration, which indicates high burdens of H. contortus, and used to fit the model by estimating transmission parameters. The model predicted force of infection (FOI) between sequential FAMACHA monitoring events in groups of dry, pregnant and lactating ewes, and related FOI to factors including climate (temperature, rainfall and rainfall entropy), using a random effects model with reproductive status group as the cluster variable. Temperature and rainfall in the seven days prior to monitoring significantly predicted the interval FOI (p≤0.002), while rainfall entropy did not (p=0.289). Differences across the three groups accounted for approximately 90% of the variability in the interval FOI over the period of investigation. Maintained FOI during targeted treatment of cases of haemonchosis suggests strong underlying transmission from sub-clinically infected animals, and/or limited impact on pre-existing pasture contamination by removal of clinical worm burdens later in the grazing season. The model has the potential to contribute to the sustainable management of H. contortus by predicting periods of heightened risk, and hence to focus and optimise limited resources for monitoring and treatment. SIR-type model frameworks are an alternative to classic abundance-based compartmental models of macroparasite epidemiology, and could be useful where incidence data are available. Significant challenges remain, however, in the ability to calibrate such models to field data at spatial and temporal scales that are useful for decision support at farm level.

Inequalities in body size among mermithid nematodes parasitizing earwigs.

Variation among body sizes of adult parasitic worms determines the relative genetic contribution of individuals to the next generation as it affects the effective parasite population size. Here, we investigate inequalities in body size and how they are affected by intensity of infection in Mermis nigrescens (Mermithidae: Nematoda) parasitizing the European earwig Forficula auricularia in New Zealand. Among a population of pre-adult worms prior to their emergence from the host, we observed only modest inequalities in body length; however, among worms sharing the same individual host, inequalities in body sizes decreased with increasing intensity of infection. Thus, the more worms occurred in a host, the more the second-longest, third-longest and even fourth-longest worms approached the longest worm in body length. This pattern, also known from another mermithid species, suggests that worms sharing the same host may have infected it roughly simultaneously, when the host encountered a clump of eggs in the environment. Thus, the life history and mode of infection of the parasite may explain the modest inequalities in the sizes achieved by pre-adult worms, which are lower than those reported for endoparasitic helminths of vertebrates.

Plasmodium copy number variation scan: gene copy numbers evaluation in haploid genomes.

BackgroundIn eukaryotic genomes, deletion or amplification rates have been estimated to be a thousand more frequent than single nucleotide variation. In Plasmodium falciparum, relatively few transcription factors have been identified, and the regulation of transcription is seemingly largely influenced by gene amplification events. Thus copy number variation (CNV) is a major mechanism enabling parasite genomes to adapt to new environmental changes.MethodsCurrently, the detection of CNVs is based on quantitative PCR (qPCR), which is significantly limited by the relatively small number of genes that can be analysed at any one time. Technological advances that facilitate whole-genome sequencing, such as next generation sequencing (NGS) enable deeper analyses of the genomic variation to be performed. Because the characteristics of Plasmodium CNVs need special consideration in algorithms and strategies for which classical CNV detection programs are not suited a dedicated algorithm to detect CNVs across the entire exome of P. falciparum was developed. This algorithm is based on a custom read depth strategy through NGS data and called PlasmoCNVScan.ResultsThe analysis of CNV identification on three genes known to have different levels of amplification and which are located either in the nuclear, apicoplast or mitochondrial genomes is presented. The results are correlated with the qPCR experiments, usually used for identification of locus specific amplification/deletion.ConclusionsThis tool will facilitate the study of P. falciparum genomic adaptation in response to ecological changes: drug pressure, decreased transmission, reduction of the parasite population size (transition to pre-elimination endemic area).

Climate Change and Livestock Production: A Review with Special Attention to Health Aspects

This review work was conducted to explore the likely impacts of climate change on livestock health. Currently, the world is facing a number of challenges, of which climate change is a priority area. Climate change affects livestock health through several pathways involving both direct and indirect effect. The direct effects being most likely pronounced for diseases that are vector- borne, soil associated, water or flood associated, rodent associated, or air temperature/humidity associated and sensitive to climate. Furthermore, Climate change influences the emergence and proliferation of disease hosts or vectors and pathogens and their breeding, development and disease transmission. Consequently, it affects distributions and host–parasite relationships and its assemblages to new areas. Climate factors also influences habitat suitability, distribution, and abundance; intensity and temporal pattern of vector activity. Pathogens and parasites that are sensitive to moist or dry conditions may be affected by changes to precipitation and soil moisture. Higher temperatures resulting from climate change may increase the rate of development of certain pathogens or parasites that have one or more life cycle stages outside their animal host. This may shorten generation times and, possibly, increase the total number of generations per year, leading to higher pathogen/ parasite population sizes. Mammalian cellular immunity can be suppressed following heightened exposure to ultraviolet. In particular, there is depression of the number of T helper 1 lymphocytes, the cells involved in the immune response to intracellular pathogens. Therefore, successful adaptations may be shown as better way of coping with the negative consequences of climate change on livestock health.

Local diversity reduces infection risk across multiple freshwater host‐parasite associations

Summary In many host–parasite systems, infection risk can be reduced by high local biodiversity, though the mitigating effects of diversity are context dependent and not universal. In aquatic ecosystems, local fauna can reduce the transmission success of parasite free‐swimming infective stages by preying on them, acting as decoy hosts, or physically interfering with transmission. However, most prior research has focused on the effect of a single non‐host organism at a time and/or has been performed under simplified and artificial conditions. Here, using data on 11 trematode species sampled in different New Zealand lakes, we test whether local biodiversity affects infection risk in target second intermediate hosts, as well as total parasite population size (number of parasites per m2), under natural conditions. We considered four components of local biodiversity: total biomass of non‐host fish species, diversity (Simpson index) of non‐host benthic invertebrates, total density of zooplankton and macrophyte biomass. Our analyses also accounted for host density, a known determinant of parasite prevalence, intensity of infection and total parasite population density. The only influence of local biodiversity we detected was a negative effect of the diversity of non‐host benthic invertebrates on the prevalence achieved by trematodes in their second intermediate hosts: that is, the proportion of individual hosts that are infected. Interestingly, this effect was discernible in all 11 trematode species considered here, even if very weak within some species. Our findings suggest that higher non‐host benthic diversity may generally decrease infection risk for target hosts including snails, arthropods and fish. However, reduced infection success did not automatically mean smaller overall parasite population size, as other factors can maintain the parasite population in the face of high local diversity of non‐hosts.

Effect of disease prevalence and spatial heterogeneity on polymorphism maintenance in host–parasite interactions

Plants and their parasites co‐evolve at key genes of interactions following the so‐called gene‐for‐gene (GFG) relationship. Previous models of co‐evolution assume (i) single infinitely large populations of hosts and parasites and (ii) costs of resistance and infectivity. The effects of three biologically realistic characteristics of plant and parasite populations on polymorphism maintenance at GFG loci were investigated. First, two components of the cost of resistance were disentangled: the cost of harbouring the resistance allele itself, and the cost of triggering resistance when encountering a parasite. Secondly, it was assumed that plants encounter parasites depending on fixed disease prevalence in time. Thirdly, finite sizes of host and parasite populations were introduced, assuming genetic drift and mutation. In a single population, statistical polymorphism in either host or parasite can be obtained in the finite population size model if there is no cost of harbouring the resistance allele and disease prevalence is low. On the other hand, long‐term polymorphism can be maintained by heterogeneity in disease prevalence and costs of resistance in a spatially structured population with two demes linked by migration. More precisely, the trench warfare co‐evolutionary dynamics occurs when assuming large host and parasite population sizes, and large differences between demes for disease prevalence or costs of triggering resistance. Moreover, the resistance allele does not need to harbour a fitness cost in itself for long‐term stable polymorphism to occur in the co‐evolutionary models. This observation may explain the lack of empirical evidence of high costs of carrying resistance alleles.

Nestling sex predicts susceptibility to parasitism and influences parasite population size within avian broods

Vertebrate hosts differ in their level of parasite susceptibility and infestation. In avian broods, variation in susceptibility of nestlings to ectoparasites may be associated with non‐uniform distributions of parasites among brood mates, with parasites concentrating feeding on the most vulnerable hosts. The presence of a highly susceptible nestling in a brood can benefit the remaining young by reducing the parasite pressure they experience; however, from a parasite’s perspective, broods with fewer susceptible hosts may provide effectively fewer resources than broods of the same size containing a greater abundance of susceptible hosts, and this could limit the number of parasites that a host brood can sustain. To test whether variation in number of susceptible hosts affects the number of parasites in bird nests, we first examined the role of host sex and induced immunity (via methionine supplementation) on susceptibility of mountain bluebirds Sialia currucoides to parasitism by blow flies Protocalliphora spp. We then assessed the effect of variation in number of susceptible hosts on the number of parasites inhabiting the nest. Only females showed a benefit of methionine supplementation, gaining mass more rapidly following supplementation compared to males. This suggests that females are more susceptible to parasites in this system; this was further supported by parasite feeding trials, in which parasites extracted larger blood meals from female than male hosts. Finally, the abundance of parasites in nests was predicted by brood sex ratio: broods containing more female young harboured more parasites. Hence, within‐brood variation in host susceptibility to parasites can not only influence the costs of parasitism for individual nestlings, but may also have consequences for the size of parasite populations within nests. If patterns of maternal investment affect the abundance of nest‐dwelling parasites, these interactions may be important for understanding fitness consequences of maternal resource allocation in many vertebrate hosts.

Plasmodium falciparum populations from northeastern Myanmar display high levels of genetic diversity at multiple antigenic loci

Levels of genetic diversity of the malaria parasites and multiclonal infections are correlated with transmission intensity. In order to monitor the effect of strengthened malaria control efforts in recent years at the China–Myanmar border area, we followed the temporal dynamics of genetic diversity of three polymorphic antigenic markers msp1, msp2, and glurp in the Plasmodium falciparum populations. Despite reduced malaria prevalence in the region, parasite populations exhibited high levels of genetic diversity. Genotyping 258 clinical samples collected in four years detected a total of 22 PCR size alleles. Multiclonal infections were detected in 45.7% of the patient samples, giving a minimum multiplicity of infection of 1.41. The majority of alleles experienced significant temporal fluctuations through the years. Haplotype diversity based on the three-locus genotypes ranged from the lowest in 2009 at 0.33 to the highest in 2010 at 0.80. Sequencing of msp1 fragments from 36 random samples of five allele size groups detected 13 different sequences, revealing an additional layer of genetic complexity. This study suggests that despite reduced prevalence of malaria infections in this region, the parasite population size and transmission intensity remained high enough to allow effective genetic recombination of the parasites and continued maintenance of genetic diversity.

Specialized nursery pollination mutualisms as evolutionary traps stabilized by antagonistic traits

We examine the conditions for the transition from antagonism to mutualism between plants and their specialists nursery pollinators in a reference case which is the Trollius europaeus– Chiastocheta interaction. The mechanistic model we developed shows that a specialization of T. europaeus on Chiastocheta could be the result of an attempt to escape over-exploitation by closing its flower. The pressure for such an escape increases with the parasite's frequency and its pollination efficiency but decreases in the presence of alternative pollinators. The resulting specialization is a priori an unstable one, leading either to strong evolutionary oscillations, or to evolutionary suicide due to over-exploitation of the plants. It becomes stable if the plants develop a defense mechanism to regulate their parasite's population size and limit seed-exploitation. The development of a counter-measure by the latter can destabilize the mutualism depending on the costs linked to such a trait. On the other hand, we find that a specialization on a purely mutualistic basis would require a preexisting high diversity of flower-opening within the population.