Trade-off between virulence and vertical transmission and the maintenance of a virulent plant pathogen

Emerging evidence of seed transmission of begomoviruses: implications in global circulation and disease outbreak

Begomoviruses (family Geminiviridae) are known for causing devastating diseases in fruit, fibre, pulse, and vegetable crops throughout the world. Begomoviruses are transmitted in the field exclusively through insect vector whitefly (Bemisia tabaci), and the frequent outbreaks of begomoviruses are attributed largely due to the abundance of whitefly in the agri-ecosystem. Begomoviruses being phloem-borne were known not be transmitted through seeds of the infected plants. The recent findings of seed transmission of begomoviruses brought out a new dimension of begomovirus perpetuation and dissemination. The first convincing evidence of seed transmission of begomoviruses was known in 2015 for sweet potato leaf curl virus followed by several begomoviruses, like bhendi yellow vein mosaic virus, bitter gourd yellow mosaic virus, dolichos yellow mosaic virus, mungbean yellow mosaic virus, mungbean yellow mosaic India virus, pepper yellow leaf curl Indonesia virus, tomato leaf curl New Delhi virus, tomato yellow leaf curl virus, tomato yellow leaf curl Sardinia virus, and okra yellow mosaic Mexico virus. These studies brought out two perspectives of seed-borne nature of begomoviruses: (i) the presence of begomovirus in the seed tissues derived from the infected plants but no expression of disease symptoms in the progeny seedlings and (ii) the seed infection successfully transmitted the virus to cause disease to the progeny seedlings. It seems that the seed transmission of begomovirus is a feature of a specific combination of host-genotype and virus strain, rather than a universal phenomenon. This review comprehensively describes the seed transmitted begomoviruses reported in the last 9 years and the possible mechanism of seed transmission. An emphasis is placed on the experimental results that proved the seed transmission of various begomoviruses, factors affecting seed transmission and impact of begomovirus seed transmission on virus circulation, outbreak of the disease, and management strategies.

Resource availability for the mosquito Aedes aegypti affects the transmission mode evolution of a microsporidian parasite

Ecological conditions may greatly affect the relative importance of vertical and horizontal transmission, in particular for parasites with a mixed mode of transmission. Resource availability is one important environmental factor, affecting host growth and fecundity, but also the parasite’s own development. The consequences for the potential of vertical and horizontal transmission and for the evolution of transmission mode are largely unknown. We let the mixed-mode microsporidian parasite Edhazardia aedis evolve on its mosquito host Aedes aegypti under high-food or low-food conditions, representing permissive and restricted conditions. These alter the timing of development of infected larvae and thereby the probabilities for the parasites to enter the vertical or horizontal transmission pathways. After 10 generations, evolved parasites were assayed under the two food levels. There was an ecological trade-off between transmission modes, mediated by nutrient effects on host development, resulting in a higher vertical transmission (VT) potential under high-food and a higher horizontal transmission (HT) potential under low-food test conditions. Evolution under high food increased the VT potential of the parasite, particularly if it was tested at low food. This involved higher probability of carrying binucleate spores for the emerging females, greater fecundity and a longer life compared to parasites that were tested in the same conditions but had evolved under low food. The changes are related to the developmental regulation and switch in the production of two spore types, affecting investment in VT or HT. In contrast, the HT potential remained relatively unaffected by the parasite’s evolutionary history, suggesting that, within our experiential design, the VT mode evolved independently of the HT mode. Our work illustrates the possible links between resource availability, within-host developmental processes and the evolution of parasite transmission investment. Future work, theoretical and experimental, should scale up from within-host to between-host levels, including eco-evolutionary and epidemiological dynamics.

Within-Host Multiplication and Speed of Colonization as Infection Traits Associated with Plant Virus Vertical Transmission

Although vertical transmission from parents to offspring through seeds is an important fitness component of many plant viruses, very little is known about the factors affecting this process. Viruses reach the seed by direct invasion of the embryo and/or by infection of the ovules or the pollen. Thus, it can be expected that the efficiency of seed transmission would be determined by (i) virus within-host multiplication and movement, (ii) the ability of the virus to invade gametic tissues, (iii) plant seed production upon infection, and (iv) seed survival in the presence of the virus. However, these predictions have seldom been experimentally tested. To address this question, we challenged 18 Arabidopsis thaliana accessions with Turnip mosaic virus and Cucumber mosaic virus Using these plant-virus interactions, we analyzed the relationship between the effect of virus infection on rosette and inflorescence weights; short-, medium-, and long-term seed survival; virulence; the number of seeds produced per plant; virus within-host speed of movement; virus accumulation in the rosette and inflorescence; and efficiency of seed transmission measured as a percentage and as the total number of infected seeds. Our results indicate that the best estimators of percent seed transmission are the within-host speed of movement and multiplication in the inflorescence. Together with these two infection traits, virulence and the number of seeds produced per infected plant were also associated with the number of infected seeds. Our results provide support for theoretical predictions and contribute to an understanding of the determinants of a process central to plant-virus interactions.IMPORTANCE One of the major factors contributing to plant virus long-distance dispersal is the global trade of seeds. This is because more than 25% of plant viruses can infect seeds, which are the main mode of germplasm exchange/storage, and start new epidemics in areas where they were not previously present. Despite the relevance of this process for virus epidemiology and disease emergence, the infection traits associated with the efficiency of virus seed transmission are largely unknown. Using turnip mosaic and cucumber mosaic viruses and their natural host Arabidopsis thaliana as model systems, we have identified the within-host speed of virus colonization and multiplication in the reproductive structures as the main determinants of the efficiency of seed transmission. These results contribute to shedding light on the mechanisms by which plant viruses disperse and optimize their fitness and may help in the design of more-efficient strategies to prevent seed infection.

Anthropogenic influences on emergence of vector-borne plant viruses: the persistent problem of Potato virus Y

Potato virus Y (PVY) has reemerged as a serious impediment to seed potato production, responsible for reduced yields and tuber quality, as well as the majority of seed lot rejections by certification programs due to excessive virus incidence. This has led to seed shortages, especially in cultivars highly susceptible to infection. While seed certification programs have been effective at managing many virus diseases below economic thresholds, PVY has rapidly evolved in recent decades to become a complex of strains that evade many certification and farm management practices. The evolution of PVY strains is naturally occurring, but several human influences can be linked to the rapid change in PVY populations affecting the potato crop. Here we highlight the recent history and current status of PVY in potatoes and suggest some approaches for managing the virus moving forward.

Host genotype and environment affect the trade-off between horizontal and vertical transmission of the parasite Edhazardia aedis

Background

If a parasite is able to transmit horizontally or vertically, which transmission mode will it choose? We investigated how the growth conditions and the genotype of the mosquito Aedes aegypti affect the transmission mode of the parasite Edhazardia aedis.

Results

In poor conditions the parasites were more likely to be transmitted horizontally, whereas in favourable conditions they were more likely to be transmitted vertically. Unfavourable conditions delayed emergence, giving the parasite more time to produce its horizontally transmitted stage; in more favourable conditions mosquitoes have greater reproductive success, increasing the effectiveness of vertical transmission. In addition, the parasite’s ability to transmit vertically was influenced by the genetic background of the host (i.e., its full-sib family), giving a genetic correlation between the host’s life-history and which of the parasite’s transmission mode it enables. In particular, genotypes with large bodies (and therefore high fecundity) were more likely to enable vertical transmission than genotypes with small bodies. This led to a trade-off among the host’s families (which can be interpreted as a genetic correlation) for the parasite’s transmission mode.

Conclusions

Since horizontal transmission is linked to higher virulence than vertical transmission, the host’s contribution to transmission mode has important consequences for the evolution of parasites with mixed-mode transmission.

The evolution of transmission mode

This article reviews research on the evolutionary mechanisms leading to different transmission modes. Such modes are often under genetic control of the host or the pathogen, and often in conflict with each other via trade-offs. Transmission modes may vary among pathogen strains and among host populations. Evolutionary changes in transmission mode have been inferred through experimental and phylogenetic studies, including changes in transmission associated with host shifts and with evolution of the unusually complex life cycles of many parasites. Understanding the forces that determine the evolution of particular transmission modes presents a fascinating medley of problems for which there is a lack of good data and often a lack of conceptual understanding or appropriate methodologies. Our best information comes from studies that have been focused on the vertical versus horizontal transmission dichotomy. With other kinds of transitions, theoretical approaches combining epidemiology and population genetics are providing guidelines for determining when and how rapidly new transmission modes may evolve, but these are still in need of empirical investigation and application to particular cases. Obtaining such knowledge is a matter of urgency in relation to extant disease threats.This article is part of the themed issue 'Opening the black box: re-examining the ecology and evolution of parasite transmission'.

Ergot Alkaloids of the Family Clavicipitaceae

Ergot alkaloids are highly diverse in structure, exhibit diverse effects on animals, and are produced by diverse fungi in the phylum Ascomycota, including pathogens and mutualistic symbionts of plants. These mycotoxins are best known from the fungal family Clavicipitaceae and are named for the ergot fungi that, through millennia, have contaminated grains and caused mass poisonings, with effects ranging from dry gangrene to convulsions and death. However, they are also useful sources of pharmaceuticals for a variety of medical purposes. More than a half-century of research has brought us extensive knowledge of ergot-alkaloid biosynthetic pathways from common early steps to several taxon-specific branches. Furthermore, a recent flurry of genome sequencing has revealed the genomic processes underlying ergot-alkaloid diversification. In this review, we discuss the evolution of ergot-alkaloid biosynthesis genes and gene clusters, including roles of gene recruitment, duplication and neofunctionalization, as well as gene loss, in diversifying structures of clavines, lysergic acid amides, and complex ergopeptines. Also reviewed are prospects for manipulating ergot-alkaloid profiles to enhance suitability of endophytes for forage grasses.

Geographic Variation in Festuca rubra L. Ploidy Levels and Systemic Fungal Endophyte Frequencies

Polyploidy and symbiotic Epichloë fungal endophytes are common and heritable characteristics that can facilitate environmental range expansion in grasses. Here we examined geographic patterns of polyploidy and the frequency of fungal endophyte colonized plants in 29 Festuca rubra L. populations from eight geographic sites across latitudes from Spain to northernmost Finland and Greenland. Ploidy seemed to be positively and negatively correlated with latitude and productivity, respectively. However, the correlations were nonlinear; 84% of the plants were hexaploids (2n = 6x = 42), and the positive correlation between ploidy level and latitude is the result of only four populations skewing the data. In the southernmost end of the gradient 86% of the plants were tetraploids (2n = 4x = 28), whereas in the northernmost end of the gradient one population had only octoploid plants (2n = 8x = 56). Endophytes were detected in 22 out of the 29 populations. Endophyte frequencies varied among geographic sites, and populations and habitats within geographic sites irrespective of ploidy, latitude or productivity. The highest overall endophyte frequencies were found in the southernmost end of the gradient, Spain, where 69% of plants harbored endophytes. In northern Finland, endophytes were detected in 30% of grasses but endophyte frequencies varied among populations from 0% to 75%, being higher in meadows compared to riverbanks. The endophytes were detected in 36%, 30% and 27% of the plants in Faroe Islands, Iceland and Switzerland, respectively. Practically all examined plants collected from southern Finland and Greenland were endophyte-free, whereas in other geographic sites endophyte frequencies were highly variable among populations. Common to all populations with high endophyte frequencies is heavy vertebrate grazing. We propose that the detected endophyte frequencies and ploidy levels mirror past distribution history of F. rubra after the last glaciation period, and local adaptations to past or prevailing selection forces such as vertebrate grazing.

Endophytic Epichloë species and their grass hosts: from evolution to applications

The closely linked fitness of the Epichloë symbiont and the host grass is presumed to align the coevolution of the species towards specialization and mutually beneficial cooperation. Ecological observations demonstrating that Epichloë-grass symbioses can modulate grassland ecosystems via both above- and belowground ecosystem processes support this. In many cases the detected ecological importance of Epichloë species is directly or indirectly linked to defensive mutualism attributable to alkaloids of fungal-origin. Now, modern genetic and molecular techniques enable the precise studies on evolutionary origin of endophytic Epichloë species, their coevolution with host grasses and identification the genetic variation that explains phenotypic diversity in ecologically relevant characteristics of Epichloë-grass associations. Here we briefly review the most recent findings in these areas of research using the present knowledge of the genetic variation that explains the biosynthetic pathways driving the diversity of alkaloids produced by the endophyte. These findings underscore the importance of genetic interplay between the fungus and the host in shaping their coevolution and ecological role in both natural grass ecosystems, and in the agricultural arena.

Impact of Bacterial-Fungal Interactions on the Colonization of the Endosphere

Research on different endophyte taxa and the related scientific disciplines have largely developed separately, and comprehensive community-level studies on bacterial and fungal interactions and their importance are lacking. Here, we discuss the transmission modes of bacteria and fungi and the nature of their interactions in the endosphere at both the molecular and physiological level. Mixed-community biofilms in the endosphere may have a role in protecting endophytes against encountered stresses, such as from plant defense systems. However, transmission from static (in biofilms) to free-living (planktonic) forms may be crucial for the exploration of new habitable spaces in plants. Important features previously recognized as plant-microbe interactions or antagonism in endophyte genomes and metagenomes are proposed to have essential roles in the modulation of endophyte communities.

Vertical transmission selects for reduced virulence in a plant virus and for increased resistance in the host

For the last three decades, evolutionary biologists have sought to understand which factors modulate the evolution of parasite virulence. Although theory has identified several of these modulators, their effect has seldom been analysed experimentally. We investigated the role of two such major factors—the mode of transmission, and host adaptation in response to parasite evolution—in the evolution of virulence of the plant virus Cucumber mosaic virus (CMV) in its natural host Arabidopsis thaliana. To do so, we serially passaged three CMV strains under strict vertical and strict horizontal transmission, alternating both modes of transmission. We quantified seed (vertical) transmission rate, virus accumulation, effect on plant growth and virulence of evolved and non-evolved viruses in the original plants and in plants derived after five passages of vertical transmission. Our results indicated that vertical passaging led to adaptation of the virus to greater vertical transmission, which was associated with reductions of virus accumulation and virulence. On the other hand, horizontal serial passages did not significantly modify virus accumulation and virulence. The observed increases in CMV seed transmission, and reductions in virus accumulation and virulence in vertically passaged viruses were due also to reciprocal host adaptation during vertical passages, which additionally reduced virulence and multiplication of vertically passaged viruses. This result is consistent with plant-virus co-evolution. Host adaptation to vertically passaged viruses was traded-off against reduced resistance to the non-evolved viruses. Thus, we provide evidence of the key role that the interplay between mode of transmission and host-parasite co-evolution has in determining the evolution of virulence.

Virulence is a key property of parasites, and is linked to the emergence of new diseases and to the reduction of ecosystem biodiversity. Consequently, scientists have devoted a great effort to build theoretical models that predict which factors may modulate virulence evolution. However, whether (and how) these factors affect virulence evolution has been seldom analysed experimentally. Using the plant virus Cucumber mosaic virus (CMV) and its natural host Arabidopsis thaliana, we studied the role of two such factors: the mode of transmission, and host adaptation in response to parasite evolution. We serially passaged CMV under strict vertical and strict horizontal transmission, and a combination of both. Subsequently, we analysed differences in CMV seed (vertical) transmission rate, accumulation and virulence between evolved and non-evolved viruses. We also compared whether these differences varied in original plants and in plants evolved during vertical passaging. Vertical passaging increased CMV seed transmission, and reduced accumulation and virulence, while horizontal passaging had no effect. Changes during vertical passaging were determined also by reciprocal host adaptation, which additionally reduced virulence and accumulation of vertically transmitted viruses. Hence, we provide evidence that the interplay between the transmission mode and host-parasite co-evolution is central in determining virulence evolution.

Epichloë spp. associated with grasses: new insights on life cycles, dissemination and evolution

Epichloë species with their asexual states are specialized fungi associated with cool-season grasses. they grow endophytically in tissues of aerial parts of host plants to form systemic and mostly asymptomatic associations. Their life cycles may involve vertical transmission through host seeds and/or horizontal transmission from one plant to other plants of the same species through fungal propagules. Vertical transmission has been well studied, but comparatively little research has been done on horizontal dissemination. The goal of this review is to provide new insights on modes of dissemination of systemic grass endophytes. The review addresses recent progress in research on (i) the process of growth of Epichloë endophytes in the host plant tissues, (ii) the types and development of reproductive structures of the endophyte, (iii) the role of the reproductive structures in endophyte dissemination and host plant infection processes and (iv) some ecological and evolutionary implications of their modes of dissemination. Research in the Epichloë grass endophytes has accelerated in the past 25 y and has demonstrated the enormous complexity in endophyte-grass symbioses. There still remain large gaps in our understanding of the role and functions of these fungi in agricultural systems and understanding the functions, ecology and evolution of these endophytes in natural grass populations.

Similar pathogen targets in Arabidopsis thaliana and homo sapiens protein networks

We study the behavior of pathogens on host protein networks for humans and Arabidopsis - noting striking similarities. Specifically, we preform [Formula: see text]-shell decomposition analysis on these networks - which groups the proteins into various "shells" based on network structure. We observe that shells with a higher average degree are more highly targeted (with a power-law relationship) and that highly targeted nodes lie in shells closer to the inner-core of the network. Additionally, we also note that the inner core of the network is significantly under-targeted. We show that these core proteins may have a role in intra-cellular communication and hypothesize that they are less attacked to ensure survival of the host. This may explain why certain high-degree proteins are not significantly attacked.

Importance of autoinfection to the epidemiology of polycyclic foliar disease

Autoinfection (infection resulting from inoculum produced on the same host unit) can result in strongly clustered disease at the local scale. In contrast, much epidemiological theory incorporates the simplification of spatially random or uniform infection. Earlier studies suggested only low to moderate levels of autoinfection, especially when the host unit is small. However, several studies published within the last 5 years suggest that autoinfection rates may be substantially higher than previously indicated. I discuss the potential importance of accounting for high autoinfection rates in example epidemiological processes that occur at different spatial scales: microbial interactions on the phylloplane, temporal disease progression in plant populations, and spatiotemporal disease spread at the landscape scale. Accounting for high autoinfection rates can have important qualitative and quantitative consequences for epidemiological processes, and further studies of autoinfection will contribute significantly to our understanding of epidemics.

Competition favours reduced cost of plasmids to host bacteria

Conjugative plasmids of Gram-negative bacteria have both vertical and horizontal modes of transmission: they are segregated to daughter cells during division, and transferred between hosts by plasmid-encoded conjugative machinery. Despite maintaining horizontal mobility, many plasmids carry fertility inhibition (fin) systems that repress their own conjugative transfer. To assess the ecological basis of self-transfer repression, we compared the invasion of bacterial populations by fin(+) and fin(-) variants of the plasmid R1 using a computational model and co-culture competitions. We observed that the fin(+) variant had a modest cost to the host (measured by reduction in growth rate), while the fin(-) variant incurred a larger cost. In simulations and empirical competitions the fin(-) plasmid invaded cultures quickly, but was subsequently displaced by the fin(+) plasmid. This indicated a competitive advantage to reducing horizontal transmission and allowing increased host replication. Computational simulations predicted that the advantage associated with reduced cost to the host would be maintained over a wide range of environmental conditions and plasmid costs. We infer that vertical transmission in concert with competitive exclusion favour decreased horizontal mobility of plasmids. Similar dynamics may exert evolutionary pressure on parasites, such as temperate bacteriophages and vertically transmitted animal viruses, to limit their rates of horizontal transfer.

Variation in horizontal and vertical transmission of the endophyte Epichloë elymi infecting the grass Elymus hystrix

Systemic fungal endophytes (Clavicipitaceae) of grasses reproduce sexually when the fungus forms stromata and contagious ascospores, or asexually by vertical transmission of hyphae into seeds and seedlings. Vertical transmission is predicted to favor reduced virulence compared with horizontal transmission in systems with both types of transmission. Here, variation in vertical and horizontal transmission and its potential heritability in a host grass-endophyte interaction, Elymus hystrix infected with Epichloë elymi, were examined in natural populations and two common garden experiments using field-collected host tillers and seed progeny of maternal plants with known infection phenotypes. Transmission mode exhibited year-to-year variation in field and common garden environments. In the common garden there were consistent differences among maternal plant families in stroma production and significant correlations between stroma production in the common garden and in natural populations. Transmission mode differed among maternal families, spanning a continuum from pure vertical transmission to a high proportion of stroma production and horizontal transmission potential. Vertical transmission to seeds occurred at high rates in all maternal families regardless of their stroma production. Observed patterns of variation indicate that endophyte transmission mode and correlated changes in virulence can respond to selection by biotic and abiotic factors.

Evolution of virulence in Fusarium oxysporum f. sp. vasinfectum using serial passage assays through susceptible cotton

Fifty strains of Fusarium oxysporum, recovered from rhizosphere soil around native Gossypium species and found to be mildly virulent on cotton (Gossypium hirsutum), were used to assay the propensity for evolution of virulence using serial passage assays through cotton. Only one lineage A strain, 2613, successfully completed 10 successive passages, while all others lost the ability to cause foliar disease symptoms at various stages during this process. Based on 46 amplified fragment length polymorphism (AFLP) markers generated with four EcoRI x MseI primer combinations, mutants were identified in offspring isolates from strain 2613 regardless of whether serial passages occurred in cotton or on water agar, suggesting the occurrence of spontaneous mutations. Significantly increased virulence was observed in the offspring isolates generated on cotton, while no increasing virulence was found in those obtained on water agar, suggesting that the evolution of virulence in F. oxysporum f. sp. vasinfectum is associated with the presence of cotton. No clear correlation was observed between the AFLP mutations and increased virulence in this study.

Male-biased sex-ratio distortion caused by Octosporea bayeri, a vertically and horizontally-transmitted parasite of Daphnia magna

Female-biased sex-ratio distortion is often observed in hosts infected with vertically-transmitted microsporidian parasites. This bias is assumed to benefit the spread of the parasite, because male offspring usually do not transmit the parasite further. The present study reports on sex-ratio distortion in a host-parasite system with both horizontal and vertical parasite transmission: the microsporidium Octosporea bayeri and its host, the planktonic cladoceran Daphnia magna. In laboratory and field experiments, we found an overall higher proportion of male offspring in infected than in uninfected hosts. In young males, there was no parasite effect on sperm production, but, later in life, infected males produced significantly less sperm than uninfected controls. This shows that infected males are fertile. As males are unlikely to transmit the parasite vertically, an increase in male production could be advantageous to the host during phases of sexual reproduction, because infected mothers may obtain uninfected grandchildren through their sons. Life-table experiments showed that, overall, sons harboured more parasite spores than their sisters, although they reached a smaller body size and died earlier. Male production may thus be beneficial for the parasite when horizontal transmission has a large pay-off as males may contribute more effectively to parasite spread than females.

Transmission as a predictor of ecological host specificity with a focus on vertical transmission of microsporidia

Consideration of vertical transmission is particularly important for understanding the life cycles of entomopathogens that are naturally occurring in invertebrate populations, are a problem in beneficial insect colonies, or are under consideration as classical biological control agents. Empirical studies generally corroborate the evolutionary hypothesis that virulence should be relatively low for pathogen species that utilize vertical transmission as one mechanism for maintenance in the host population. Nevertheless, many entomopathogens with significant effects on host populations are vertically as well as horizontally transmitted. In addition to gaining a better understanding of pathogen-host interactions and population dynamics, studies of the host range and specificity of putative biological control agents can benefit by using transmission studies to better predict ecological host specificity from physiological data. Horizontal transmission requires a tightly organized host-pathogen relationship to succeed, but still involves, albeit restricted by host behavior and pathogen dosage, the physiological susceptibility of the nontarget host. Vertical transmission studies can provide increased stringency for determining the ecological host specificity of a species and may be one very accurate predictor of the ability of a pathogen to successfully host-switch when introduced into a naïve population.

Spatially structured superinfection and the evolution of disease virulence

When pathogen strains differing in virulence compete for hosts, spatial structuring of disease transmission can govern both evolved levels of virulence and patterns in strain coexistence. We develop a spatially detailed model of superinfection, a form of contest competition between pathogen strains; the probability of superinfection depends explicitly on the difference in levels of virulence. We apply methods of adaptive dynamics to address the interplay of spatial dynamics and evolution. The mean-field approximation predicts evolution to criticality; any small increase in virulence capable of dynamical persistence is favored. Both pair approximation and simulation of the detailed model indicate that spatial structure constrains disease virulence. Increased spatial clustering reduces the maximal virulence capable of single-strain persistence and, more importantly, reduces the convergent-stable virulence level under strain competition. The spatially detailed model predicts that increasing the probability of superinfection, for given difference in virulence, increases the likelihood of between-strain coexistence. When strains differing in virulence can coexist ecologically, our results may suggest policies for managing diseases with localized transmission. Comparing equilibrium densities from the pair approximation, we find that introducing a more virulent strain into a host population infected by a less virulent strain can sometimes reduce total host mortality and increase global host density.

ACEPHALINE GREGARINE PARASITES (MONOCYSTIS SP.) ARE NOT TRANSMITTED SEXUALLY AMONG THEIR LUMBRICID EARTHWORM HOSTS

The precise transmission mode(s) of acephaline gregarines in their earthworm hosts has long been questioned, yet a rigorous experimental evaluation of sexual transmission is currently lacking. That Monocystis sp., a common gregarine parasite of the earthworm Lumbricus terrestris, infects the sexual organs of its host is suggestive of sexual transmission. Considering the divergent evolutionary consequences of various modes of transmission, excluding or proving sexual transmission in this host-parasite system is critical to fully understanding it. We cultured uninfected earthworms from cocoons and subsequently mated them to either an infected or uninfected partner (from the wild). We then compared these individuals with an orally infected group, which were infected using a newly developed gavage (oral injection) method. Our data have unambiguously established that (1) horizontal sexual transmission does not play a significant role in the transmission of Monocystis sp., and (2) oral transmission through the soil is likely the principal mode of transmission between earthworms. This finding is important to models of mate-choice because infection avoidance does not appear to drive mating decisions. Finally, we further report a successful and relatively simple method to obtain infection-free individuals, which can subsequently be infected via oral gavage and used in empirical studies.

Phenotypic plasticity of host-parasite interactions in response to the route of infection

The microsporidium Octosporea bayeri can infect its host, the planktonic crustacean Daphnia magna, vertically and horizontally. The two routes differ greatly in the way the parasite leaves the harbouring host (transmission) and in the way it enters a new, susceptible host (infection). Infections resulting from each route may thus vary in the way they affect host and parasite life-histories and, subsequently, host and parasite fitness. We conducted a life-table experiment to compare D. magna infected with O. bayeri either horizontally or vertically, using three different parasite isolates. Both the infection route and the parasite isolate had significant effects on host life-history. Hosts matured at different ages depending on the parasite isolate, and at a size that varied with infection route. The frequency of host sterility and the host's life-time reproductive success were affected by both the infection route and the parasite isolate. The infection route also affected parasite life-history. The production of parasite spores was much higher in vertically than in horizontally infected hosts. We found a trade-off between the production of spores (the parasite's horizontal fitness component) and the production of infected host offspring (the parasite's vertical fitness component). This study shows that hosts and parasites can react plastically to different routes of infection, suggesting that ecological factors that may influence the relative importance of horizontal and vertical transmission can shape the evolution of host and parasite life histories, and, consequently, the evolution of virulence.

Within-host dynamics of a microsporidium with horizontal and vertical transmission: Octosporea bayeri in Daphnia magna

The fresh-water crustacean Daphnia magna may acquire an infection with the microsporidium Octosporea bayeri either by ingesting spores from the water (horizontally), or directly from its mother (vertically). Due to differences in the time and mechanisms of transmission, horizontal and vertical infections may lead to differences in the growth of the parasite within the host. This may influence parasite virulence, transmission to new hosts, and, consequently, epidemiology and evolution. Here we describe the within-host dynamics of 3 spore-types of O. bayeri from infections that were acquired either horizontally or vertically. In all treatments the number of spores increased exponentially until spore density reached a plateau, suggesting density-dependent within-host growth. The spore types seen differ in their growth dynamics, suggesting different roles in the parasite life-cycle. Horizontally-infected hosts harboured significantly fewer spores than vertically-infected hosts. Further, host survival was affected by infection route, with mortality being higher in horizontal infections than in vertical infections. Our results suggest that different routes of infection have an immediate effect on within-host parasite growth and thus on parasite fitness and epidemiology.

A general theory for the evolutionary dynamics of virulence

Most theory on the evolution of virulence is based on a game-theoretic approach. One potential shortcoming of this approach is that it does not allow the prediction of the evolutionary dynamics of virulence. Such dynamics are of interest for several reasons: for experimental tests of theory, for the development of useful virulence management protocols, and for understanding virulence evolution in situations where the epidemiological dynamics never reach equilibrium and/or when evolutionary change occurs on a timescale comparable to that of the epidemiological dynamics. Here we present a general theory similar to that of quantitative genetics in evolutionary biology that allows for the easy construction of models that include both within-host mutation as well as superinfection and that is capable of predicting both the short- and long-term evolution of virulence. We illustrate the generality and intuitive appeal of the theory through a series of examples showing how it can lead to transparent interpretations of the selective forces governing virulence evolution. It also leads to novel predictions that are not possible using the game-theoretic approach. The general theory can be used to model the evolution of other pathogen traits as well.

Uniform vertical transmission and selection in a host–symbiont system. Non-random symbiont distribution generates apparent differential selection

We investigate the survival, prevalence, and distribution of a symbiont in its host population when the forces of vertical transmission (beta) and symbiont-induced selection (s) are uniform (invariant across host genotypes). We use host-symbiont disequilibria to quantify the role played by non-random associations between host genotypes and the symbiont in altering host genetic structure. Results show a larger part of the parameter space permits symbiont survival under mutualism (beta>/=0.25) than parasitism (beta>0.5). The nonlinear interaction between beta and s determines symbiont survival and prevalence at equilibrium; initial symbiont prevalence is a factor only in a small number of parameter combinations. The symbiont's non-random distribution generates apparent differential selection, when selective differences across host genotypes and alleles exist under uniform selection. The direction of change in host allele frequencies is determined exclusively by the signs of s and the allelic disequilibrium. Disequilibria cannot be created or maintained, and heterozygote disequilibrium changes sign in a greater number of runs and at higher magnitudes than homozygote disequilibria. This investigation increases our understanding of the interactions between vertical transmission and selection, and their effect on the coevolutionary dynamics and final states of interacting species under different selection regimes.

The persistence of vertically transmitted fungi in grass metapopulations

Theory predicts that (i) vertical transmission of parasites (i.e. when they are passed directly from a host to its offspring) selects for benign association with the host and that (ii) vertically transmitted parasites that lower their hosts' fitness cannot persist if they are not able to infect horizontally (i.e. contagiously) other host individuals in the population. In this paper, we develop a mathematical model to examine whether mutualism is a prerequisite for persistence of exclusively vertically transmitted (from maternal plant to offspring via seeds) fungal endophytes in structured grass metapopulations. Interestingly, endophyte survival does not require plant mutualism, even in a metapopulation consisting of qualitatively identical patches, if vertical transmission of the fungus is perfect, i.e. if all established seedlings in offspring of the endophyte-infected plant are infected. In more realistic situations, when the metapopulation consists of qualitatively different patches, endophyte-infected plants may persist at the metapopulation level even if the vertical transmission is imperfect (due to hyphae inviability or failure to grow into all seeds) and the endophyte decreases the host grass fitness in certain environments. These results have biological importance because they (i) question the requirement of a mutualistic nature in exclusively vertically transmitted symbionts and (ii) emphasize the importance of habitat diversity in relation to symbiont success in vertical transmission.

Horizontal versus vertical transmission of parasites in a stochastic spatial model

A number of pathogens may be transmitted from parent to child at or before birth (vertically) or from one individual to another by contact (horizontally). A natural deterministic and non-spatial model, introduced by Lipsitch et al. [Proc. Roy. Soc. London Ser. B 260 (1995) 3211 shows that an epidemic is possible if the vertical transmission or the horizontal transmission is high enough. In contrast, we introduce a stochastic spatial model that shows that, on a particular graph, if the vertical transmission is not high enough, then the infected individuals disappear even for very high horizontal transmission. This illustrates the fact that introducing space may greatly change the qualitative behavior of a model.

Symbiont survival and host-symbiont disequilibria under differential vertical transmission

Interspecific genetic interactions in host-symbiont systems raise intriguing coevolutionary questions and may influence the effectiveness of public health and management policies. Here we present an analytical and numerical investigation of the effects of host genetic heterogeneity in the rate of vertical transmission of a symbiont. We consider the baseline case with a monomorphic symbiont and a single diallelic locus in its diploid host, where vertical transmission is the sole force. Our analysis introduces interspecific disequilibria to quantify nonrandom associations between host genotypes and alleles and symbiont presence/absence. The transient and equilibrium behavior is examined in simulations with randomly generated initial conditions and transmission parameters. Compared to the case where vertical transmission rates are uniform across host genotypes, differential transmission (i) increases average symbiont survival from 50% to almost 60%, (ii) dramatically reduces the minimum average transmission rate for symbiont survival from 0.5 to 0.008, and (iii) readily creates permanent host-symbiont disequilibria de novo, whereas uniform transmission can neither create nor maintain such associations. On average, heterozygotes are slightly more likely to carry and maintain the symbiont in the population and are more randomly associated with the symbiont. Results show that simple evolutionary forces can create substantial nonrandom associations between two species.