Identification of microsatellite markers inPlasmodium mexicanum, a lizard malaria parasite that infects nucleated erythrocytes

Drought correlates with reduced infection complexity and possibly prevalence in a decades-long study of the lizard malaria parasite Plasmodium mexicanum

Microparasites often exist as a collection of genetic 'clones' within a single host (termed multi-clonal, or complex, infections). Malaria parasites are no exception, with complex infections playing key roles in parasite ecology. Even so, we know little about what factors govern the distribution and abundance of complex infections in natural settings. Utilizing a natural dataset that spans more than 20 years, we examined the effects of drought conditions on infection complexity and prevalence in the lizard malaria parasite Plasmodium mexicanum and its vertebrate host, the western fence lizard, Sceloporus occidentalis. We analyzed data for 14,011 lizards sampled from ten sites over 34 years with an average infection rate of 16.2%. Infection complexity was assessed for 546 infected lizards sampled during the most recent 20 years. Our data illustrate significant, negative effects of drought-like conditions on infection complexity, with infection complexity expected to increase by a factor of 2.27 from the lowest to highest rainfall years. The relationship between rainfall and parasite prevalence is somewhat more ambiguous; when prevalence is modeled over the full range in years, a 50% increase in prevalence is predicted between the lowest and highest rainfall years, but this trend is not apparent or is reversed when data are analyzed over a shorter timeframe. To our knowledge, this is the first reported evidence for drought affecting the abundance of multi-clonal infections in malaria parasites. It is not yet clear what mechanism might connect drought with infection complexity, but the correlation we observed suggests that additional research on how drought influences parasite features like infection complexity, transmission rates and within-host competition may be worthwhile.

Distribution of clones among hosts for the lizard malaria parasite Plasmodium mexicanum

Background

Malaria parasites reproduce asexually, leading to the production of large numbers of genetically identical parasites, here termed a clonal line or clone. Infected hosts may harbor one or more clones, and the number of clones in a host is termed multiplicity of infection (MOI). Understanding the distribution of parasite clones among hosts can shed light on the processes shaping this distribution and is important for modeling MOI. Here, I determine whether the distribution of clones of the lizard malaria parasite Plasmodium mexicanum differ significantly from statistical distributions commonly used to model MOI and logical extensions of these models.

Methods

The number of clones per infection was assessed using four microsatellite loci with the maximum number of alleles at any one locus used as a simple estimate of MOI for each infection. I fit statistical models (Poisson, negative binomial, zero-inflated models) to data from four individual sites to determine a best fit model. I also simulated the number of alleles per locus using an unbiased estimate of MOI to determine whether the simple (but potentially biased) method I used to estimate MOI influenced model fit.

Results

The distribution of clones among hosts at individual sites differed significantly from traditional Poisson and negative binomial distributions, but not from zero-inflated modifications of these distributions. A consistent excess of two-clone infections and shortage of one-clone infections relative to all fit distributions was also observed. Any bias introduced by the simple method for estimating of MOI did not appear to qualitatively alter the results.

Conclusions

The statistical distributions used to model MOI are typically zero-truncated; truncating the Poisson or zero-inflated Poisson yield the same distribution, so the reasonable fit of the zero-inflated Poisson to the data suggests that the use of the zero-truncated Poisson in modeling is adequate. The improved fit of zero-inflated distributions relative to standard distributions may suggest that only a portion of the host population is located in areas suitable for transmission even at small sites (<1 ha). Collective transmission of clones and premunition may also contribute to deviations from standard distributions.

Do mosquitoes transmit the avian malaria-like parasite Haemoproteus? An experimental test of vector competence using mosquito saliva

Background

The life-cycle of many vector-borne pathogens includes an asexual replication phase in the vertebrate host and sexual reproduction in the insect vector. However, as only a small array of parasites can successfully develop infective phases inside an insect, few insect species are competent vectors for these pathogens. Molecular approaches have identified the potential insect vectors of blood parasites under natural conditions. However, the effectiveness of this methodology for verifying mosquito competence in the transmission of avian malaria parasites and related haemosporidians is still under debate. This is mainly because positive amplifications of parasite DNA in mosquitoes can be obtained not only from sporozoites, the infective phase of the malaria parasites that migrate to salivary glands, but also from different non-infective parasite forms in the body of the vector. Here, we assessed the vectorial capacity of the common mosquito Culex pipiens in the transmission of two parasite genera.

Methods

A total of 1,560 mosquitoes were allowed to feed on five house sparrows Passer domesticus naturally infected by Haemoproteus or co-infected by Haemoproteus/Plasmodium. A saliva sample of the mosquitoes that survived after 13 days post-exposure was taken to determine the presence of parasite DNA by PCR.

Results

Overall, 31.2% mosquito’s head-thorax and 5.8% saliva samples analysed showed positive amplifications for avian malaria parasites. In contrast to Haemoproteus DNA, which was not found in either the body parts or the saliva, Plasmodium DNA was detected in both the head-thorax and the saliva of mosquitoes. Parasites isolated from mosquitoes feeding on the same bird corresponded to the same Plasmodium lineage.

Conclusions

Our experiment provides good evidence for the competence of Cx. pipiens in the transmission of Plasmodium but not of Haemoproteus. Molecular analyses of saliva are an effective method for testing the vector competence of mosquitoes and other insects in the transmission of vector-borne pathogens.

Apparent vector-mediated parent-to-offspring transmission in an avian malaria-like parasite

Parasite transmission strategies strongly impact host-parasite co-evolution and virulence. However, studies of vector-borne parasites such as avian malaria have neglected the potential effects of host relatedness on the exchange of parasites. To test whether extended parental care in the presence of vectors increases the probability of transmission from parents to offspring, we used high-throughput sequencing to develop microsatellites for malaria-like Leucocytozoon parasites of a wild raptor population. We show that host siblings carry genetically more similar parasites than unrelated chicks both within and across years. Moreover, chicks of mothers of the same plumage morph carried more similar parasites than nestlings whose mothers were of different morphs, consistent with matrilineal transmission of morph-specific parasite strains. Ours is the first evidence of an association between host relatedness and parasite genetic similarity, consistent with vector-mediated parent-to-offspring transmission. The conditions for such 'quasi-vertical' transmission may be common and could suppress the evolution of pathogen virulence.

Dynamics of clonal diversity in natural infections of the malaria parasite Plasmodium mexicanum in its free-ranging lizard host

Within mixed-genotype infections of malaria parasites (Plasmodium), the number of genetic clones present is associated with variation in important life history traits of the infection, including virulence. Although the number of clones present is important, how the proportion of those clones varies over time is poorly known. Clonal proportions of the lizard malaria parasite, Plasmodium mexicanum, were assessed in naturally infected free-ranging lizards followed in a mark-recapture program over as long as two warm seasons, the typical life span of the lizard. Clonal proportions were determined by amplifying two microsatellite markers, a method previously verified for accuracy. Most blood samples had been stored for over a decade, so a verification test determined that these samples had not degraded. Although the environment experienced by the parasite (its host) varies over the seasons and transmission occurs over the entire warm season, 68% of infections were stable over time, harboring a single clone (37% of infections) or multiple clones changing only 1-12% maximum comparing any two samples (31% of infections). The maximum change seen in any infection (comparing any two sample periods) was only 30%. A new clone entered three infections (only once successfully), and a clone was lost in only three infections. These results mirror those seen for a previous study of experimentally induced infections that showed little change in relative proportions over time. The results of this study, the first look at how clonal proportions vary over time for any malaria parasite of a nonhuman vertebrate host for natural infections, were surprising because experimental studies show clones of P. mexicanum appear to interact, yet relative proportions of clones typically remain constant over time.

Establishment efficiency among clones of the malaria parasite, Plasmodium mexicanum, for mixed-clone infections in its natural lizard host

Within genetically diverse infections of malaria parasites ( Plasmodium spp.), the relative proportions of genetic clones in the vertebrate host's blood can influence clonal competition, transmission success, gametocyte sex ratio, and virulence. Clonal proportions depend on establishment success of each clone when they enter a new host and on subsequent differences in rates of asexual replication and clearance. Both of these life history traits could be influenced by clone genotype. To assess genetic (clonal) influences on both establishment success and later changes in relative proportion for the lizard malaria parasite Plasmodium mexicanum , 7 naturally infected fence lizards harboring a single clone of P. mexicanum served as donors to initiate replicate experimental infections containing each of the clones and combinations of 2 clones. Measured were relative establishment success of each clone, change in relative proportions over time, and rate of increase of parasite density and total parasitemia. Relative clonal proportions were determined using microsatellite markers. Rates of increase in the parasitemia and degree of change in relative proportions were not correlated, so both rapidly and slowly growing infections could show either little or substantial change in clonal proportions over time. There was a significant clone effect on establishment efficiency but not on later changes in relative proportions. These results argue for a combination of genetic and environmental (host) effects on the success of P. mexicanum clones in genetically complex infections. The maintenance of genetic variation for establishment success, but not subsequent replication rate or shifts in relative proportion, suggests trade-offs between these traits during life history evolution of malaria parasites.

Microsatellite analysis of malaria parasites

Microsatellites have been increasingly used to investigate the population structure of malaria parasites, to map genetic loci contributing to phenotypes such as drug resistance and virulence in laboratory crosses and genome-wide association studies and to distinguish between treatment failures and new infections in clinical trials. Here, we provide optimized protocols for genotyping highly polymorphic microsatellites sampled from across the genomes of the human malaria parasites Plasmodium falciparum and P. vivax that have been extensively used in research laboratories worldwide.

Microsatellite loci over a thirty-three year period for a malaria parasite (Plasmodium mexicanum): bottleneck in effective population size and effect on allele frequencies

Changes in population allele frequencies may be driven by several forces, including selection and drift, and are revealed only by sampling over many generations. Such studies, however, are rare for protist parasites. Microsatellite allele frequencies for 4 loci were followed in a population of Plasmodium mexicanum, a malaria parasite of lizards in California USA at 1 site from 1978 to 2010. Rapid turnover of the lizards indicates the parasite was studied for a minimum of 33 transmission cycles and possibly twice that number. Sample sizes ranged from 841 to 956 scored parasite clones per locus. DNA was extracted from frozen dried blood and blood removed from stained blood smears from the earliest years, and a verification study demonstrated DNA from the blood smears provided valid genetic data. Parasite prevalence and effective population size (Ne) dropped after 2000, remaining lower for the next decade. For 2 loci, allele frequencies appeared stable for the first 2 decades of the study, but changed more rapidly after the decline in prevalence. Allele frequencies changed more gradually for the other 2 loci. Genetic drift could account for changes in allele frequencies, especially after the drop in prevalence and Ne, but the force of selection could also have driven the observed patterns.

Male gametocyte fecundity and sex ratio of a malaria parasite, Plasmodium mexicanum

Evolutionary 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.

Prevalence, diversity, and interaction patterns of avian haemosporidians in a four-year study of blackcaps in a migratory divide

Migratory birds contribute to the movement of avian parasites between distant locations, thereby influencing parasite distribution and ecology. Here we analyse the prevalence, diversity and interaction patterns of Haemosporida parasites infecting Blackcap (Sylvia atricapilla) populations in a recently established migratory divide of southwestern Germany across 4 years. We hypothesize that the temporal and spatial isolation provided by 2 sympatric Blackcap breeding populations (migratory divide) might modify ecological interactions and thus create differences in the structure of the parasite community according to migratory route. We used a fragment of the mitochondrial DNA cytochrome b gene to determine haemosporidian haplotypes. We detected an overall infection prevalence of 70.3% (348 out of 495 blackcaps sampled from 2006 to 2009), and prevalence rates were significantly different among years and seasons. We observed a total of 27 parasite haplotypes infecting blackcaps, from them 6 new rare Haemoproteus haplotypes were found in 2 mixed infections. H. parabelopolskyi haplotypes SYAT01 (35.7%) and SYAT02 (20.8%) comprised most of the infections. An association analysis suggests that SYAT01 and SYAT02 are interacting negatively, implying that they are either competing directly for host resources, or indirectly by eliciting a cross-immune response. Molecular data show no clear difference between the parasite communities infecting blackcaps with different migratory routes, despite some temporal and spatial isolation between the two sympatric blackcap populations.

Relative clonal proportions over time in mixed-genotype infections of the lizard malaria parasite Plasmodium mexicanum

Vertebrate hosts of malaria parasites (Plasmodium) often harbour two or more genetically distinct clones of a single species, and interaction among these co-existing clones can play an important role in Plasmodium biology. However, how relative clonal proportions vary over time in a host is still poorly known. Experimental mixed-clone infections of the lizard malaria parasite, Plasmodium mexicanum, were followed in its natural host, the western fence lizard using microsatellite markers to determine the relative proportions of two to five co-existing clones over time (2-3 months). Results for two markers, and two PCR primer pairs for one of those, matched very closely, supporting the efficacy of the method. Of the 54 infections, 67% displayed stable relative clonal proportions, with the others showing a shift in proportions, usually with one clone outpacing the others. Infections with rapidly increasing or slowly increasing parasitemia were stable, showing that all clones within these infections reproduced at the same rapid or slow rate. Replicate infections containing the same clones did not always reveal the same growth rate, final parasitemia or dominant clone; thus there was no clone effect for these life history measures. The rate of increase in parasitemia was not associated with stable versus unstable relative proportions, but infections with four to five clones were more likely to be unstable than those with two to three clones. This rare look into events in genetically complex Plasmodium infections suggests that parasite clones may be interacting in complex and unexpected ways.

Gametocyte sex ratio in single-clone infections of the malaria parasite Plasmodium mexicanum

Sex 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.

Clonal diversity of a malaria parasite, Plasmodium mexicanum, and its transmission success from its vertebrate-to-insect host

Infections of the lizard malaria parasite Plasmodium mexicanum are often genetically complex within their fence lizard host (Sceloporus occidentalis) harbouring two or more clones of parasite. The role of clonal diversity in transmission success was studied for P. mexicanum by feeding its sandfly vectors (Lutzomyia vexator and Lutzomyia stewarti) on experimentally infected lizards. Experimental infections consisted of one, two, three or more clones, assessed using three microsatellite markers. After 5days, vectors were dissected to assess infection status, oocyst burden and genetic composition of the oocysts. A high proportion (92%) of sandflies became infected and carried high oocyst burdens (mean of 56 oocysts) with no influence of clonal diversity on these two measures of transmission success. Gametocytemia was positively correlated with transmission success and the more common vector (L. vexator) developed more oocysts on midguts. A high proportion ( approximately 74%) of all alleles detected in the lizard blood was found in infected vectors. The relative proportion of clones within mixed infections, determined by peak heights on pherograms produced by the genetic analyser instrument, was very similar for the lizard's blood and infections in the vectors. These results demonstrate that P. mexicanum achieves high transmission success, with most clones making the transition from vertebrate-to-insect host, and thus explains in part the high genetic diversity of the parasite among all hosts at the study site.

Clonal diversity alters the infection dynamics of a malaria parasite (Plasmodium mexicanum) in its vertebrate host

Ecological and evolutionary theory predicts that genetic diversity of microparasites within infected hosts will influence the parasite replication rate, parasitemia, transmission strategy, and virulence. We manipulated clonal diversity (number of genotypes) of the malaria parasite, Plasmodium mexicanum, in its natural lizard host and measured important features of the infection dynamics, the first such study for any natural Plasmodium-host association. Hosts harboring either a single P. mexicanum clone or various combinations of clones (scored via three microsatellite markers) were established. Production of asexually replicating stages (meronts) and maximal meront parasitemia did not differ by clonal diversity, nor did timing of first production of transmission stages (gametocytes). However, mean rate of gametocyte increase and maximal gametocyte parasitemia were greater for hosts with mixed-clone infections. Characteristics of infections were more variable in hosts with mixed-clone infections than with single-clone infections except for first production of gametocytes. One or more of the parasite reproductive traits were extreme in 20 of 52 hosts with mixed-clone infections. This was not associated with specific clones, but diversity itself. The overall pattern from studies of clonal diversity for human, rodent, and now reptile malaria parasites confirms that the genetic diversity of infections in the vertebrate host is of central importance for the ecology of Plasmodium.

Detecting number of clones, and their relative abundance, of a malaria parasite (Plasmodium mexicanum) infecting its vertebrate host

Microsatellites, short tandem repeats of nucleotides in the genome, are useful markers to detect clonal diversity within Plasmodium infections. However, accuracy in determining number of clones and their relative proportions based on standard genetic analyzer instruments is poorly known. DNA extracted from lizards infected with a malaria parasite, Plasmodium mexicanum, provided template to genotype the parasite based on three microsatellite markers. Replicate genotyping of the same natural infections demonstrated strong repeatability of data from the instrument. Mixing DNA extracted from several infected lizards simulated mixed-clone infections with known clonal diversity and relative proportions of clones (N = 56 simulations). The instrument readily detected at least four alleles (clones), even when DNA concentrations among clones differed up to tenfold, but alleles of similar size can be missed because they fall within the "stutter" artifact, and rarely does an allele fail to be detected. For simulations of infections that changed their relative proportions over time, changes in relative peak heights on the instrument output closely followed the known changes in relative proportions. Such data are useful for a broad range of studies on the ecology of malaria parasites.

Plastic parasites: sophisticated strategies for survival and reproduction?

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.

Clonal diversity within infections and the virulence of a malaria parasite, Plasmodium mexicanum

Both verbal and mathematical models of parasite virulence predict that genetic diversity of microparasite infections will influence the level of costs suffered by the host. We tested this idea by manipulating the number of co-existing clones of Plasmodium mexicanum in its natural vertebrate host, the fence lizard Sceloporus occidentalis. We established replicate infections of P. mexicanum made up of 1, 2, 3, or >3 clones (scored using 3 microsatellite loci) to observe the influence of clone number on several measures of parasite virulence. Clonal diversity did not affect body growth or production of immature erythrocytes. Blood haemoglobin concentration was highest for the most genetically complex infections (equal to that of non-infected lizards), and blood glucose levels and rate of blood clotting was highest for the most diverse infections (with greater glucose and more rapid clotting than non-infected animals). Neither specific clones nor parasitaemia were associated with virulence. In this first experiment that manipulated the clonal diversity of a natural Plasmodium-host system, the cost of infection with 1 or 2 clones of P. mexicanum was similar to that previously reported for infected lizards, but the most complex infections had either no cost or could be beneficial for the host.

Gametocytes: insights gained during a decade of molecular monitoring

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.

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

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.