Genetic variation in parasite avoidance, yet no evidence for constitutive fitness costs

Behavioral avoidance of parasites is a widespread strategy among animal hosts and in human public health. Avoidance has repercussions for both individual and population-level infection risk. Although most cases of parasite avoidance are viewed as adaptive, there is little evidence that the basic assumptions of evolution by natural selection are met. This study addresses this gap by testing whether there is a heritable variation in parasite avoidance behavior. We quantified behavioral avoidance of the bacterial parasite Serratia marcescens for 12 strains of the nematode host Caenorhabditis elegans. We found that these strains varied in their magnitude of avoidance, and we estimated the broad-sense heritability of this behavior to be in the range of 11%-26%. We then asked whether avoidance carries a constitutive fitness cost. We did not find evidence of one. Rather, strains with higher avoidance had higher fitness, measured as population growth rate. Together, these results direct future theoretical and empirical work to identify the forces maintaining genetic variation in parasite avoidance.

A diverse parasite pool can improve effectiveness of biological control constrained by genotype‐by‐genotype interactions

The outcomes of biological control programs can be highly variable, with natural enemies often failing to establish or spread in pest populations. This variability has posed a major obstacle in use of the bacterial parasite Pasteuria penetrans for biological control of Meloidogyne species, economically devastating plant-parasitic nematodes for which there are limited management options. A leading hypothesis for this variability in control is that infection is successful only for specific combinations of bacterial and nematode genotypes. Under this hypothesis, failure of biological control results from the use of P. penetrans genotypes that cannot infect local Meloidogyne genotypes. We tested this hypothesis using isofemale lines of M. arenaria derived from a single field population and multiple sources of P. penetrans from the same and nearby fields. In strong support of the hypothesis, susceptibility to infection depended on the specific combination of host line and parasite source, with lines of M. arenaria varying substantially in which P. penetrans source could infect them. In light of this result, we tested whether using a diverse pool of P. penetrans could increase infection and thereby control. We found that increasing the diversity of the P. penetrans inoculum from one to eight sources more than doubled the fraction of M. arenaria individuals susceptible to infection and reduced variation in susceptibility across host lines. Together, our results highlight genotype-by-genotype specificity as an important cause of variation in biological control and call for the maintenance of genetic diversity in natural enemy populations.

Evolution and Ecology of Parasite Avoidance

Parasite avoidance is a host defense that reduces the contact rate with parasites. We investigate avoidance as a primary driver of variation among individuals in the risk of parasitism and the evolution of host-parasite interactions. To bridge mechanistic and taxonomic divides, we define and categorize avoidance by its function and position in the sequence of host defenses. We also examine the role of avoidance in limiting epidemics and evaluate evidence for the processes that drive its evolution. Throughout, we highlight important directions to advance our conceptual and theoretical understanding of the role of avoidance in host-parasite interactions. We emphasize the need to test assumptions and quantify the effect of avoidance independent of other defenses. Importantly, many open questions may be most tractable in host systems that have not been the focus of traditional behavioral avoidance research, such as plants and invertebrates.

Bidirectional interactions between host social behaviour and parasites arise through ecological and evolutionary processes

An animal's social behaviour both influences and changes in response to its parasites. Here we consider these bidirectional links between host social behaviours and parasite infection, both those that occur from ecological vs evolutionary processes. First, we review how social behaviours of individuals and groups influence ecological patterns of parasite transmission. We then discuss how parasite infection, in turn, can alter host social interactions by changing the behaviour of both infected and uninfected individuals. Together, these ecological feedbacks between social behaviour and parasite infection can result in important epidemiological consequences. Next, we consider the ways in which host social behaviours evolve in response to parasites, highlighting constraints that arise from the need for hosts to maintain benefits of sociality while minimizing fitness costs of parasites. Finally, we consider how host social behaviours shape the population genetic structure of parasites and the evolution of key parasite traits, such as virulence. Overall, these bidirectional relationships between host social behaviours and parasites are an important yet often underappreciated component of population-level disease dynamics and host-parasite coevolution.

A need to consider the evolutionary genetics of host-symbiont mutualisms

Despite the ubiquity and importance of mutualistic interactions, we know little about the evolutionary genetics underlying their long-term persistence. As in antagonistic interactions, mutualistic symbioses are characterized by substantial levels of phenotypic and genetic diversity. In contrast to antagonistic interactions, however, we, by and large, do not understand how this variation arises, how it is maintained, nor its implications for future evolutionary change. Currently, we rely on phenotypic models to address the persistence of mutualistic symbioses, but the success of an interaction almost certainly depends heavily on genetic interactions. In this review, we argue that evolutionary genetic models could provide a framework for understanding the causes and consequences of diversity and why selection may favour processes that maintain variation in mutualistic interactions.

An experimental test of parasite adaptation to common versus rare host genotypes

A core hypothesis in coevolutionary theory proposes that parasites adapt to specifically infect common host genotypes. Under this hypothesis, parasites function as agents of negative frequency-dependent selection, favouring rare host genotypes. This parasite-mediated advantage of rarity is key to the idea that parasites maintain genetic variation and select for outcrossing in host populations. Here, we report the results of an experimental test of parasite adaptation to common versus rare host genotypes. We selected the bacterial parasite Serratia marcescens to kill Caenorhabdiis elegans hosts in uneven mixtures of host genotypes. To examine the effect of commonness itself, independent of host identity, each of four host genotypes was represented as common or rare in experimental host mixtures. After experimental selection, we evaluated a parasite line's change in virulence—the selected fitness trait—on its rare and common host genotypes. Our results were consistent with a slight advantage for rare host genotypes: on average, parasites lost virulence against rare genotypes but not against common genotypes. The response varied substantially, however, with distinct patterns across host genotype mixtures. These findings support the potential for parasites to impose negative frequency-dependent selection, while emphasizing that the cost of being common may vary with host genotype.

The evolution of parasite host range in heterogeneous host populations

Theory on the evolution of niche width argues that resource heterogeneity selects for niche breadth. For parasites, this theory predicts that parasite populations will evolve, or maintain, broader host ranges when selected in genetically diverse host populations relative to homogeneous host populations. To test this prediction, we selected the bacterial parasite Serratia marcescens to kill Caenorhabditis elegans in populations that were genetically heterogeneous (50% mix of two experimental genotypes) or homogeneous (100% of either genotype). After 20 rounds of selection, we compared the host range of selected parasites by measuring parasite fitness (i.e. virulence, the selected fitness trait) on the two focal host genotypes and on a novel host genotype. As predicted, heterogeneous host populations selected for parasites with a broader host range: these parasite populations gained or maintained virulence on all host genotypes. This result contrasted with selection in homogeneous populations of one host genotype. Here, host range contracted, with parasite populations gaining virulence on the focal host genotype and losing virulence on the novel host genotype. This pattern was not, however, repeated with selection in homogeneous populations of the second host genotype: these parasite populations did not gain virulence on the focal host genotype, nor did they lose virulence on the novel host genotype. Our results indicate that host heterogeneity can maintain broader host ranges in parasite populations. Individual host genotypes, however, vary in the degree to which they select for specialization in parasite populations.

Host heterogeneity mitigates virulence evolution

Parasites often infect genetically diverse host populations, and the evolutionary trajectories of parasite populations may be shaped by levels of host heterogeneity. Mixed genotype host populations, compared to homogeneous host populations, can reduce parasite prevalence and potentially reduce rates of parasite adaptation due to trade-offs associated with adapting to specific host genotypes. Here, we used experimental evolution to select for increased virulence in populations of the bacterial parasite Serratia marcescens exposed to either heterogeneous or homogeneous populations of Caenorhabditis elegans. We found that parasites exposed to heterogeneous host populations evolved significantly less virulence than parasites exposed to homogeneous host populations over several hundred bacterial generations. Thus, host heterogeneity impeded parasite adaptation to host populations. While we detected trade-offs in virulence evolution, parasite adaptation to two specific host genotypes also resulted in modestly increased virulence against the reciprocal host genotypes. These results suggest that parasite adaptation to heterogeneous host populations may be impeded by both trade-offs and a reduction in the efficacy of selection as different host genotypes exert different selective pressures on a parasite population.

Asexual parasites and their extraordinary host ranges

In diverse parasite taxa, from scale insects to root-knot nematodes, asexual lineages have exceptionally large host ranges, larger than those of their sexual relatives. Phylogenetic comparative studies of parasite taxa indicate that increases in host range and geographic range increase the probability of establishment of asexual lineages. At first pass, this convergence of traits appears counter-intuitive: intimate, antagonistic association with an enormous range of host taxa correlates with asexual reproduction, which should limit genetic variation within populations. Why would narrow host ranges favor sexual parasites and large host ranges favor asexual parasites? To take on this problem I link theory on ecological specialization to the two predominant hypotheses for the evolution of sex. I argue that both hypotheses predict a positive association between host range and the probability of invasion of asexual parasites, mediated either by variation in population size or in the strength of antagonistic coevolution. I also review hypotheses on colonization and the evolution of niche breadth in asexual lineages. I emphasize parasite taxa, with their diversity of reproductive modes and ecological strategies, as valuable assets in the hunt for solutions to the classic problems of the evolution of sex and geographic parthenogenesis.

Periodic, Parasite-Mediated Selection For and Against Sex

Asexual lineages should rapidly replace sexual populations. Why sex then? The Red Queen hypothesis proposes that parasite-mediated selection against common host genotypes could counteract the per capita birth rate advantage of asexuals. Under the Red Queen hypothesis, fluctuations in parasite-mediated selection can drive fluctuations in the asexual population, leading to the coexistence of sexual and asexual reproduction. Does shifting selection by parasites drive fluctuations in the fitness and frequency of asexuals in nature? Combining long-term field data with mesocosm experiments, we detected a shift in the direction of parasite selection in the snail Potamopyrgus antipodarum and its coevolving parasite, Microphallus sp. In the early 2000s, asexuals were more infected than sexuals. A decade later, the asexuals had declined in frequency and were less infected than sexuals. Over time, the mean infection prevalence of asexuals equaled that of sexuals but varied far more. This variation in asexual infection prevalence suggests the potential for parasite-mediated fluctuations in asexual fitness. Accordingly, we detected fitness consequences of the shift in parasite selection: when they were less infected than sexuals, asexuals increased in frequency in the field and in paired mesocosms that isolated the effect of parasites. The match between field and experiment argues that coevolving parasites drive temporal change in the relative fitness and frequency of asexuals, potentially promoting the coexistence of reproductive modes in P. antipodarum.

Bloody-minded parasites and sex: the effects of fluctuating virulence

Asexual lineages can grow at a faster rate than sexual lineages. Why then is sexual reproduction so widespread? Much empirical evidence supports the Red Queen hypothesis. Under this hypothesis, coevolving parasites favour sexual reproduction by adapting to infect common asexual clones and driving them down in frequency. One limitation, however, seems to challenge the generality of the Red Queen: in theoretical models, parasites must be very virulent to maintain sex. Moreover, experiments show virulence to be unstable, readily shifting in response to environmental conditions. Does variation in virulence further limit the ability of coevolving parasites to maintain sex? To address this question, we simulated temporal variation in virulence and evaluated the outcome of competition between sexual and asexual females. We found that variation in virulence did not limit the ability of coevolving parasites to maintain sex. In fact, relatively high variation in virulence promoted parasite-mediated maintenance of sex. With sufficient variation, sexual females persisted even when mean virulence fell well below the threshold virulence required to maintain sex under constant conditions. We conclude that natural variation in virulence does not limit the relevance of the Red Queen hypothesis for natural populations; on the contrary, it could expand the range of conditions over which coevolving parasites can maintain sex.

A Model for Evolutionary Ecology of Disease: The Case for Caenorhabditis Nematodes and Their Natural Parasites

Many of the outstanding questions in disease ecology and evolution call for combining observation of natural host-parasite populations with experimental dissection of interactions in the field and the laboratory. The "rewilding" of model systems holds great promise for this endeavor. Here, we highlight the potential for development of the nematode Caenorhabditis elegans and its close relatives as a model for the study of disease ecology and evolution. This powerful laboratory model was disassociated from its natural habitat in the 1960s. Today, studies are uncovering that lost natural history, with several natural parasites described since 2008. Studies of these natural Caenorhabditis-parasite interactions can reap the benefits of the vast array of experimental and genetic tools developed for this laboratory model. In this review, we introduce the natural parasites of C. elegans characterized thus far and discuss resources available to study them, including experimental (co)evolution, cryopreservation, behavioral assays, and genomic tools. Throughout, we present avenues of research that are interesting and feasible to address with caenorhabditid nematodes and their natural parasites, ranging from the maintenance of outcrossing to the community dynamics of host-associated microbes. In combining natural relevance with the experimental power of a laboratory supermodel, these fledgling host-parasite systems can take on fundamental questions in evolutionary ecology of disease.

The two-fold cost of sex: experimental evidence from a natural system

Over four decades ago, John Maynard Smith showed that a mutation causing asexual reproduction should rapidly spread in a dioecious sexual population. His reasoning was that the per‐capita birth rate of an asexual population would exceed that of a sexual population, because asexual females do not invest in sons. Hence, there is a cost of sexual reproduction that Maynard Smith called the “cost of males.” Assuming all else is otherwise equal among sexual and asexual females, the cost is expected to be two‐fold in outcrossing populations with separate sexes and equal sex ratios. Maynard Smith's model led to one of the most interesting questions in evolutionary biology: why is there sex? There are, however, no direct estimates of the proposed cost of sex. Here, we measured the increase in frequency of asexual snails in natural, mixed population of sexual and asexual snails in large outdoor mesocosms. We then extended Maynard Smith's model to predict the change in frequency of asexuals for any cost of sex and for any initial frequency of asexuals. Consistent with the “all‐else equal” assumption, we found that the increase in frequency of asexual snails closely matched that predicted under a two‐fold cost.

Tenofovir Alafenamide

OBJECTIVE

To review the pharmacology, efficacy, safety, and place in therapy for tenofovir alafenamide (TAF).

DATA SOURCES

A search using PubMed was conducted (2004 to May 2016) using the following keywords: tenofovir alafenamide, TAF, and GS-7340. Articles were evaluated for content, and bibliographies were reviewed. Data available exclusively as abstracts from major infectious diseases and HIV conferences were also evaluated for inclusion.

STUDY SELECTION AND DATA EXTRACTION

Studies included were in vitro investigations; phase I, II, and III clinical trials; and pharmacokinetic and pharmacodynamic evaluations.

DATA SYNTHESIS

Similar to tenofovir disoproxil fumarate (TDF), TAF is a prodrug of tenofovir but results in significantly higher intracellular tenofovir concentrations and lower serum levels. As a result, TAF is expected to have efficacy similar to that of TDF while reducing tenofovir-associated nephrotoxicity and bone mineral density losses. Clinical trials evaluating the safety and efficacy of TAF-containing antiretroviral regimens have confirmed these expectations, consistently demonstrating similar virological suppression compared with TDF-containing regimens as well as significant improvements in markers of kidney function and bone health. Three combination products containing TAF were approved by the United States Food and Drug Administration for the management of HIV-1 infection. The first of these was a single tablet regimen containing elvitegravir, cobicistat, emtricitabine, and TAF which is now a recommended regimen in clinical practice guidelines for initial treatment in antiretroviral-naïve patients.

CONCLUSIONS

TAF is a novel nucleotide reverse transcriptase inhibitor for the treatment of HIV-1 infection that has efficacy similar to that of TDF and improved safety compared with TDF.

Within-population covariation between sexual reproduction and susceptibility to local parasites

Evolutionary biology has yet to reconcile the ubiquity of sex with its costs relative to asexual reproduction. Here, we test the hypothesis that coevolving parasites maintain sex in their hosts. Specifically, we examined the distributions of sexual reproduction and susceptibility to local parasites within a single population of freshwater snails (Potamopyrgus antipodarum). Susceptibility to local trematode parasites (Microphallus sp.) is a relative measure of the strength of coevolutionary selection in this system. Thus, if coevolving parasites maintain sex, sexual snails should be common where susceptibility is high. We tested this prediction in a mixed population of sexual and asexual snails by measuring the susceptibility of snails from multiple sites in a lake. Consistent with the prediction, the frequency of sexual snails was tightly and positively correlated with susceptibility to local parasites. Strikingly, in just two years, asexual females increased in frequency at sites where susceptibility declined. We also found that the frequency of sexual females covaries more strongly with susceptibility than with the prevalence of Microphallus infection in the field. In linking susceptibility to the frequency of sexual hosts, our results directly implicate spatial variation in coevolutionary selection in driving the geographic mosaic of sex.

Fine-Scale Spatial Covariation between Infection Prevalence and Susceptibility in a Natural Population

The prevalence of infection varies dramatically on a fine spatial scale. Many evolutionary hypotheses are founded on the assumption that this variation is due to host genetics, such that sites with a high frequency of alleles conferring susceptibility are associated with higher infection prevalence. This assumption is largely untested and may be compromised at finer spatial scales where gene flow between sites is high. We put this assumption to the test in a natural snail-trematode interaction in which host susceptibility is known to have a strong genetic basis. A decade of field sampling revealed substantial spatial variation in infection prevalence between 13 sites around a small lake. Laboratory assays replicated over 3 years demonstrate striking variation in host susceptibility among sites in spite of high levels of gene flow between sites. We find that mean susceptibility can explain more than one-third of the observed variation in mean infection prevalence among sites. We estimate that variation in susceptibility and exposure together can explain the majority of variation in prevalence. Overall, our findings in this natural host-parasite system argue that spatial variation in infection prevalence covaries strongly with variation in the distribution of genetically based susceptibility, even at a fine spatial scale.

Assortative mating and self-fertilization differ in their contributions to reinforcement, cascade speciation, and diversification

Cascade speciation and reinforcement can evolve rapidly when traits are pleiotropic and act as both signal/cue in nonrandom mating. Here, we examine the contribution of two key traits-assortative mating and self-fertilization-to reinforcement and (by extension) cascade speciation. First, using a population genetic model of reinforcement we find that both assortative mating and self-fertilization can make independent contributions to increased reproductive isolation, consistent with reinforcement. Self-fertilization primarily evolves due to its 2-fold transmission advantage when inbreeding depression (d) is lower (d < 0.45) but evolves as a function of the cost of hybridization under higher inbreeding depression (0.45 < d < 0.48). When both traits can evolve simultaneously, increased self-fertilization often prohibits the evolution of assortative mating. We infer that, under specific conditions, mating system transitions are more likely to lead to increased reproductive isolation and initiate cascade speciation, than assortative mating. Based on the results of our simulations, we hypothesized that transitions to self-fertilization could contribute to clade-wide diversification if reinforcement or cascade speciation is common. We tested this hypothesis with comparative data from two different groups. Consistent with our hypothesis, there was a trend towards uniparental reproduction being associated with increased diversification rate in the Nematode phylum. For the plant genus Mimulus, however, self-fertilization was associated with reduced diversification. Reinforcement driving speciation via transitions to self-fertilization might be short lived or unsustainable across macroevolutionary scales in some systems (some plants), but not others (such as nematodes), potentially due to differences in susceptibility to inbreeding depression and/or the ability to transition between reproductive modes.

The evolution of reduced antagonism--A role for host-parasite coevolution

Why do some host-parasite interactions become less antagonistic over evolutionary time? Vertical transmission can select for reduced antagonism. Vertical transmission also promotes coevolution between hosts and parasites. Therefore, we hypothesized that coevolution itself may underlie transitions to reduced antagonism. To test the coevolution hypothesis, we selected for reduced antagonism between the host Caenorhabditis elegans and its parasite Serratia marcescens. This parasite is horizontally transmitted, which allowed us to study coevolution independently of vertical transmission. After 20 generations, we observed a response to selection when coevolution was possible: reduced antagonism evolved in the copassaged treatment. Reduced antagonism, however, did not evolve when hosts or parasites were independently selected without coevolution. In addition, we found strong local adaptation for reduced antagonism between replicate host/parasite lines in the copassaged treatment. Taken together, these results strongly suggest that coevolution was critical to the rapid evolution of reduced antagonism.

A novel Sarcocystis neurona genotype XIII is associated with severe encephalitis in an unexpectedly broad range of marine mammals from the northeastern Pacific Ocean

Sarcocystis neurona is an important cause of protozoal encephalitis among marine mammals in the northeastern Pacific Ocean. To characterise the genetic type of S. neurona in this region, samples from 227 stranded marine mammals, most with clinical or pathological evidence of protozoal disease, were tested for the presence of coccidian parasites using a nested PCR assay. The frequency of S. neurona infection was 60% (136/227) among pinnipeds and cetaceans, including seven marine mammal species not previously known to be susceptible to infection by this parasite. Eight S. neurona fetal infections identified this coccidian parasite as capable of being transmitted transplacentally. Thirty-seven S. neurona-positive samples were multilocus sequence genotyped using three genetic markers: SnSAG1-5-6, SnSAG3 and SnSAG4. A novel genotype, referred to as Type XIII within the S. neurona population genetic structure, has emerged recently in the northeastern Pacific Ocean and is significantly associated with an increased severity of protozoal encephalitis and mortality among multiple stranded marine mammal species.

Life-history strategy defends against disease and may select against physiological resistance

Host ecological traits may limit exposure to infectious disease, thereby generating the wide variation in disease incidence observed between host populations or species. The exclusion of disease by ecological traits may then allow selection to act against physiological defenses when they are costly to maintain in the absence of disease. This study investigates ecological resistance in the Silene-Microbotryum pathosystem. An estimated 80% of perennial Silene species host the anther-smut disease while no annuals harbor the disease in nature. Artificial inoculations of annual and perennial Silene plants, obtained from both natural and horticultural populations, demonstrate that the absence of disease in annuals is not explained by elevated physiological resistance. The annual habit is thus a powerful form of ecological defense against anther smut. Moreover, the higher susceptibility of annual species to anther smut relative to perennials supports the hypothesis of a loss of costly physiological resistance under ecological protection. The observation in annuals that physiological susceptibility is correlated with lower rates of flowering (i.e., lower fitness) suggests that variation in physiological resistance is costly in the absence of disease, even in a naїve Silene species. The absence of disease in natural populations of annuals combined with their high physiological susceptibility attest to the strength of host ecology in shaping the distribution of disease and to the dynamic nature of disease resistance.

Why so many unknown genes? Partitioning orphans from a representative transcriptome of the lone star tick Amblyomma americanum

Background

Genomic resources within the phylum Arthropoda are largely limited to the true insects but are beginning to include unexplored subphyla, such as the Crustacea and Chelicerata. Investigations of these understudied taxa uncover high frequencies of orphan genes, which lack detectable sequence homology to genes in pre-existing databases. The ticks (Acari: Chelicerata) are one such understudied taxon for which genomic resources are urgently needed. Ticks are obligate blood-feeders that vector major diseases of humans, domesticated animals, and wildlife. In analyzing a transcriptome of the lone star tick Amblyomma americanum, one of the most abundant disease vectors in the United States, we find a high representation of unannotated sequences. We apply a general framework for quantifying the origin and true representation of unannotated sequences in a dataset and for evaluating the biological significance of orphan genes.

Results

Expressed sequence tags (ESTs) were derived from different life stages and populations of A. americanum and combined with ESTs available from GenBank to produce 14,310 ESTs, over twice the number previously available. The vast majority (71%) has no sequence homology to proteins archived in UniProtKB. We show that poor sequence or assembly quality is not a major contributor to this high representation by orphan genes. Moreover, most unannotated sequences are functional: a microarray experiment demonstrates that 59% of functional ESTs are unannotated. Lastly, we attempt to further annotate our EST dataset using genomic datasets from other members of the Acari, including Ixodes scapularis, four other tick species and the mite Tetranychus urticae. We find low homology with these species, consistent with significant divergence within this subclass.

Conclusions

We conclude that the abundance of orphan genes in A. americanum likely results from 1) taxonomic isolation stemming from divergence within the tick lineage and limited genomic resources for ticks and 2) lineage-specific genes needing functional genomic studies to evaluate their association with the unique biology of ticks. The EST sequences described here will contribute substantially to the development of tick genomics. Moreover, the framework provided for the evaluation of orphan genes can guide analyses of future transcriptome sequencing projects.

Sibling competition arena: selfing and a competition arena can combine to constitute a barrier to gene flow in sympatry

Closely related species coexisting in sympatry provide critical insight into the mechanisms underlying speciation and the maintenance of genetic divergence. Selfing may promote reproductive isolation by facilitating local adaptation, causing reduced hybrid fitness in parental environments. Here, we propose a novel mechanism by which selfing can further impair interspecific gene flow: selfing may act to ensure that nonhybrid progeny systematically co-occur whenever hybrid genotypes are produced. Under a competition arena, the fitness differentials between nonhybrid and hybrid progeny are then magnified, preventing development of interspecific hybrids. We investigate whether this "sibling competition arena" can explain the coexistence in sympatry of closely related species of the plant fungal pathogens (Microbotryum) causing anther-smut disease. The probabilities of intrapromycelial mating (automixis), outcrossing, and sibling competition were manipulated in artificial inoculations to evaluate their contribution to reproductive isolation. We report that both intrapromycelial selfing and sibling competition significantly reduced rates of hybrid infection beyond that expected based solely upon selfing rates and noncompetitive fitness differentials between hybrid and nonhybrid progeny. Our results thus suggest that selfing and a sibling competition arena can combine to constitute a barrier to gene flow and diminish selection for additional barriers to gene flow in sympatry.

Population genetics of Toxoplasma gondii: new perspectives from parasite genotypes in wildlife

Toxoplasma gondii, a zoonotic protozoal parasite, is well-known for its global distribution and its ability to infect virtually all warm-blooded vertebrates. Nonetheless, attempts to describe the population structure of T. gondii have been primarily limited to samples isolated from humans and domesticated animals. More recent studies, however, have made efforts to characterize T. gondii isolates from a wider range of host species and geographic locales. These findings have dramatically changed our perception of the extent of genetic diversity in T. gondii and the relative roles of sexual recombination and clonal propagation in the parasite's lifecycle. In particular, identification of novel, disease-causing T. gondii strains in wildlife has raised concerns from both a conservation and public health perspective as to whether distinct domestic and sylvatic parasite gene pools exist. If so, overlap of these cycles may represent regions of high probability of disease emergence. Here, we attempt to answer these key questions by reviewing recent studies of T. gondii infections in wildlife, highlighting those which have advanced our understanding of the genetic diversity and population biology of this important zoonotic pathogen.

Polyparasitism is associated with increased disease severity in Toxoplasma gondii-infected marine sentinel species

In 1995, one of the largest outbreaks of human toxoplasmosis occurred in the Pacific Northwest region of North America. Genetic typing identified a novel Toxoplasma gondii strain linked to the outbreak, in which a wide spectrum of human disease was observed. For this globally-distributed, water-borne zoonosis, strain type is one variable influencing disease, but the inability of strain type to consistently explain variations in disease severity suggests that parasite genotype alone does not determine the outcome of infection. We investigated polyparasitism (infection with multiple parasite species) as a modulator of disease severity by examining the association of concomitant infection of T. gondii and the related parasite Sarcocystis neurona with protozoal disease in wild marine mammals from the Pacific Northwest. These hosts ostensibly serve as sentinels for the detection of terrestrial parasites implicated in water-borne epidemics of humans and wildlife in this endemic region. Marine mammals (151 stranded and 10 healthy individuals) sampled over 6 years were assessed for protozoal infection using multi-locus PCR-DNA sequencing directly from host tissues. Genetic analyses uncovered a high prevalence and diversity of protozoa, with 147/161 (91%) of our sampled population infected. From 2004 to 2009, the relative frequency of S. neurona infections increased dramatically, surpassing that of T. gondii. The majority of T. gondii infections were by genotypes bearing Type I lineage alleles, though strain genotype was not associated with disease severity. Significantly, polyparasitism with S. neurona and T. gondii was common (42%) and was associated with higher mortality and more severe protozoal encephalitis. Our finding of widespread polyparasitism among marine mammals indicates pervasive contamination of waterways by zoonotic agents. Furthermore, the significant association of concomitant infection with mortality and protozoal encephalitis identifies polyparasitism as an important factor contributing to disease severity in marine mammals.

Severity of toxoplasmosis, a water-borne zoonosis, varies widely from chronic and benign to acutely fatal. Here, we investigate polyparasitism (infection with multiple parasite species) as one factor governing the spectrum of disease in Toxoplasma gondii infections. This study utilized wild marine mammals as sentinels to detect contamination of waterways by T. gondii and a similar protozoan, Sarcocystis neurona, which have been linked to water-borne outbreaks in humans and wildlife along North America's Pacific Coast. Using genetic tools, we found high rates of protozoal infection, predominantly concomitant infections, in animals inhabiting major waterways of the Pacific Northwest. These dual infections of T. gondii and S. neurona were more frequently associated with mortality and protozoal encephalitis than single infections, indicating a role for polyparasitism in disease severity. Finally, rare T. gondii genotypes linked to a major human outbreak in the Pacific Northwest were abundant in marine mammals of the region, emphasizing wildlife as relevant sentinels for evaluation of human health risks. Our data implicate polyparasitism as a critical factor associated with the severity of protozoal disease. We also identify the need for vigilant surveillance of public waterways to prevent fecal contamination recurrently threatening human and wildlife health along the Pacific coast.

Distribution of the anther-smut pathogen Microbotryum on species of the Caryophyllaceae

*Understanding disease distributions is of fundamental and applied importance, yet few studies benefit from integrating broad sampling with ecological and phylogenetic data. Here, anther-smut disease, caused by the fungus Microbotryum, was assessed using herbarium specimens of Silene and allied genera of the Caryophyllaceae. *A total of 42,000 herbarium specimens were examined, and plant geographical distributions and morphological and life history characteristics were tested as correlates of disease occurrence. Phylogenetic comparative methods were used to determine the association between disease and plant life-span. *Disease was found on 391 herbarium specimens from 114 species and all continents with native Silene. Anther smut occurred exclusively on perennial plants, consistent with the pathogen requiring living hosts to overwinter. The disease was estimated to occur in 80% of perennial species of Silene and allied genera. The correlation between plant life-span and disease was highly significant while controlling for the plant phylogeny, but the disease was not correlated with differences in floral morphology. *Using resources available in natural history collections, this study illustrates how disease distribution can be determined, not by restriction to a clade of susceptible hosts or to a limited geographical region, but by association with host life-span, a trait that has undergone frequent evolutionary transitions.