THE ORIGIN OF SPECIFICITY BY MEANS OF NATURAL SELECTION: EVOLVED AND NONHOST RESISTANCE IN HOST-PATHOGEN INTERACTIONS

Making sense of the virome in light of evolution and ecology

Understanding the patterns and drivers of viral prevalence and abundance is of key importance for understanding pathogen emergence. Over the last decade, metagenomic sequencing has exponentially expanded our knowledge of the diversity and evolution of viruses associated with all domains of life. However, as most of these 'virome' studies are primarily descriptive, our understanding of the predictors of virus prevalence, abundance and diversity, and their variation in space and time, remains limited. For example, we do not yet understand the relative importance of ecological predictors (e.g. seasonality and habitat) versus evolutionary predictors (e.g. host and virus phylogenies) in driving virus prevalence and diversity. Few studies are set up to reveal the factors that predict the virome composition of individual hosts, populations or species. In addition, most studies of virus ecology represent a snapshot of single species viromes at a single point in time and space. Fortunately, recent studies have begun to use metagenomic data to directly test hypotheses about the evolutionary and ecological factors which drive virus prevalence, sharing and diversity. By synthesizing evidence across studies, we present some over-arching ecological and evolutionary patterns in virome composition, and illustrate the need for additional work to quantify the drivers of virus prevalence and diversity.

Resistance and Tolerance to Imperfectly Specialized Parasites: Milkweed Butterflies and Their Protozoan Parasites

Understanding host specificity and cross-species transmission of parasites is crucial for predicting the risk and consequences of parasite spillover. We experimentally examined these dynamics in two closely related, sympatric, milkweed butterfly hosts: monarchs (Danaus plexippus) and queens (D. gilippus). The debilitating protozoan Ophryocystis elektroscirrha (OE) infects wild monarchs throughout their range, and similar neogregarine parasites have been reported from queens. We compared host resistance and tolerance to infection between hosts exposed to parasites of conspecific and heterospecific origin and examined whether differences in immune investment reflected variation in infection outcomes. Results showed that monarchs were highly susceptible to both conspecific and heterospecific parasites. In contrast, queens were susceptible almost exclusively to conspecific parasites. Queens showed greater tolerance to infection and greater immune defense in the form of melanization activity and concentration of encapsulating hemocytes. Additionally, monarch parasites caused higher pre-adult mortality and more wing deformities than queen parasites. Given that OE can reduce monarch abundance and migratory performance, quantifying cross-infection outcomes is important for conservation management of these two butterfly species. The greater susceptibility and costs of infection in monarchs suggest potential fitness trade-offs against resistance and tolerance to infection in migratory hosts and underscore the need to identify factors that limit hosts' adaptation to parasites.

Positive correlations in susceptibility to a diverse panel of viruses across Drosophilidae host species

Our ability to predict the emergence of novel viruses relies on there being generalisable patterns in the susceptibilities of hosts to novel infections. Studies investigating variation in susceptibility among host species have consistently shown that closely related hosts share similar susceptibilities to a given virus. However, the extent to which such phylogenetic patterns of susceptibility are correlated amongst diverse sets of viruses is unclear. Here, we investigate phylogenetic correlations in susceptibility among Drosophilidae hosts to a panel of eleven different invertebrate viruses, comprising seven unique virus species, six unique families, and both RNA and DNA viruses. The susceptibility of hosts to each pair of viruses tested was either positively correlated across host species or did not show evidence of correlation. No negative correlations, indicative of evolutionary trade-offs in host susceptibility to different viruses, were detected between any virus pairs. The strength of correlations were generally higher in viruses of the same species and family, consistent with virus phylogenetic patterns in host infectivity. Our results suggest that generalised host susceptibility can result in positive correlations, even between highly diverged viruses, while specialised interactions with individual viruses cause a stepwise decrease in correlation strength between viruses from the within-species, to the within-family, to the across-family level.

Tandem duplication of serpin genes yields functional variation and snake venom inhibitors

Tandem duplication of genes can play a critical role in the evolution of functional novelty, but our understanding is limited concerning gene duplication's role in coevolution between species. Much is known about the evolution and function of tandemly duplicated snake venom genes, however the potential of gene duplication to fuel venom resistance within prey species is poorly understood. In this study, we characterize patterns of gene duplication of the SERPINA subfamily of genes across in vertebrates and experimentally characterize functional variation in the SERPINA3-like paralogs of a wild rodent. We find the hallmarks of rapid birth-death evolution of SERPINA1-like and SERPINA3-like genes within and between rodent lineages. Next, we recombinantly expressed the 2 paralogous duplicates of SERPINA1 and 12 paralogous duplicates of SERPINA3 found in the genome of the big-eared woodrat (Neotoma macrotis), a species known to be resistant to protease-rich rattlesnake venoms. We found that two SERPINA3 paralogs inhibit snake venom serine proteases, indicating that these proteins have potential as resistance factors in SERPIN-mediated venom resistance. In addition, functional variation is apparent among paralogs, including neofunctionalization to inhibit both chymotrypsin-like and and trypsin-like proteases simultaneously for one venom-inhibiting paralog. Our results provide further evidence that the rapid evolution of SERPINA1 and SERPINA3 gene copy number across rodents has adaptive potential by producing functionally-diverse inhibitors.

Experimental adaptation to singular pathogen challenge reduces susceptibility to novel pathogens in Drosophila melanogaster

Hosts often encounter and must respond to novel pathogens in the wild, that is pathogens that they have not encountered in recent evolutionary history, and therefore are not adapted to. How hosts respond to these novel pathogens and the outcome of such infections can be shaped by the host's evolutionary history, especially by how well adapted the host is to its native pathogens, that is pathogens they have evolved with. Host adaptation to one pathogen can either increase its susceptibility to a novel pathogen, due to specialization of immune defenses and trade-offs between different arms of the immune system, or can decrease susceptibility to novel pathogens by virtue of cross-resistance. Using laboratory Drosophila melanogaster populations, we explore if hosts experimentally adapted to surviving infection challenges by a single bacterial pathogen are also better at surviving infection challenges by novel bacterial pathogens. We found that such hosts can survive infection challenges by multiple novel pathogens, with the expanse of cross-resistance determined by the identity of the native pathogen and sex of the host. Therefore, we have demonstrated that cross-resistance can evolve in host populations by virtue of adaptation to a single pathogen. This observation has important ecological consequences, especially in the modern era where spillover of novel pathogens is a common occurrence due to various factors, including climate change.

Genetic Independence of Naturally Correlated Variation in Resistance to Endemic and Novel Pathogens

The emergence of new diseases is an urgent concern, but hosts can also vary in resistance to pathogens that are novel to them, facilitating evolutionary rescue. However, little is known about the genetic source for polymorphic resistance to novel pathogens or its relationship to defences against endemic diseases. With anther-smut disease from wild plant populations, we used selection experiments and genetic analyses to show that resistances to novel and endemic pathogens are genetically independent, despite being positively correlated in nature. Moreover, novel-pathogen resistance presented a much simpler genetic basis and more rapid response to selection. We demonstrate that polymorphic resistance to a newly introduced disease is genetically determined and not an extension of defences against the related endemic pathogen, challenging the conventional view of nonhost resistance.

Repeated loss of the ability of a wild pepper disease resistance gene to function at high temperatures suggests that thermoresistance is a costly trait

Specificity in plant-pathogen gene-for-gene (GFG) interactions is determined by the recognition of pathogen proteins by the products of plant resistance (R) genes. The evolutionary dynamics of R genes in plant-virus systems is poorly understood. We analyse the evolution of the L resistance locus to tobamoviruses in the wild pepper Capsicum annuum var. glabriusculum (chiltepin), a crop relative undergoing incipient domestication. The frequency, and the genetic and phenotypic diversity, of the L locus was analysed in 41 chiltepin populations under different levels of human management over its distribution range in Mexico. The frequency of resistance was lower in Cultivated than in Wild populations. L-locus genetic diversity showed a strong spatial structure with no isolation-by-distance pattern, suggesting environment-specific selection, possibly associated with infection by the highly virulent tobamoviruses found in the surveyed regions. L alleles differed in recognition specificity and in the expression of resistance at different temperatures, broad-spectrum recognition of P0  + P1 pathotypes and expression above 32°C being ancestral traits that were repeatedly lost along L-locus evolution. Overall, loss of resistance co-occurs with incipient domestication and broad-spectrum resistance expressed at high temperatures has apparent fitness costs. These findings contribute to understand the role of fitness trade-offs in plant-virus coevolution.

Seasonal variations of intensity of avian malaria infection in the Thousand Island Lake System, China

BACKGROUND

Migratory birds play an important part in the spread of parasites, with more or less impact on resident birds. Previous studies focus on the prevalence of parasites, but changes in infection intensity over time have rarely been studied. As infection intensity can be quantified by qPCR, we measured infection intensity during different seasons, which is important for our understanding of parasite transmission mechanisms.

METHODS

Wild birds were captured at the Thousand Island Lake with mist nets and tested for avian hemosporidiosis infections using nested PCR. Parasites were identified using the MalAvi database. Then, we used qPCR to quantify the infection intensity. We analyzed the monthly trends of intensity for all species and for different migratory status, parasite genera and sexes.

RESULTS

Of 1101 individuals, 407 were infected (37.0%) of which 95 were newly identified and mainly from the genus Leucocytozoon. The total intensity trend shows peaks at the start of summer, during the breeding season of hosts and during the over-winter season. Different parasite genera show different monthly trends. Plasmodium causes high prevalence and infection intensity of winter visitors. Female hosts show significant seasonal trends of infection intensity.

CONCLUSIONS

The seasonal changes of infection intensity is consistent with the prevalence. Peaks occur early and during the breeding season and then there is a downward trend. Spring relapses and avian immunity are possible reasons that could explain this phenomenon. In our study, winter visitors have a higher prevalence and infection intensity, but they rarely share parasites with resident birds. This shows that they were infected with Plasmodium during their departure or migration and rarely transmit the disease to resident birds. The different infection patterns of different parasite species may be due to vectors or other ecological properties.

Assemblages of Plasmodium and Related Parasites in Birds with Different Migration Statuses

Migratory birds spend several months in their breeding grounds in sympatry with local resident birds and relatively shorter periods of time at stopover sites. During migration, parasites may be transmitted between migratory and resident birds. However, to what extent they share these parasites remains unclear. In this study, we compared the assemblages of haemosporidian parasites in migratory, resident, and passing birds, as well as the correlations between parasite assemblages and host phylogeny. Compared with passing birds, migratory birds were more likely to share parasites with resident birds. Shared lineages showed significantly higher prevalence rates than other lineages, indicating that common parasites are more likely to spill over from the current host to other birds. For shared lineages, the prevalence was significantly higher in resident birds than in migratory birds, suggesting that migratory birds pick up parasites at their breeding ground. Among the shared lineages, almost two-thirds presented no phylogenetic signal in their prevalence, indicating that parasite transmission among host species is weakly or not correlated with host phylogeny. Moreover, similarities between parasite assemblages are not correlated with either migration status or the phylogeny of hosts. Our results show that the prevalence, rather than host phylogeny, plays a central role in parasite transmission between migratory and resident birds in breeding grounds.

A Data-driven Horizon Scan of Bacterial Pathogens at the Wildlife-livestock Interface

Many livestock diseases rely on wildlife for the transmission or maintenance of the pathogen, and the wildlife-livestock interface represents a potential site of disease emergence for novel pathogens in livestock. Predicting which pathogen species are most likely to emerge in the future is an important challenge for infectious disease surveillance and intelligence. We used a machine learning approach to conduct a data-driven horizon scan of bacterial associations at the wildlife-livestock interface for cows, sheep, and pigs. Our model identified and ranked from 76 to 189 potential novel bacterial species that might associate with each livestock species. Wildlife reservoirs of known and novel bacteria were shared among all three species, suggesting that targeting surveillance and/or control efforts towards these reservoirs could contribute disproportionately to reducing spillover risk to livestock. By predicting pathogen-host associations at the wildlife-livestock interface, we demonstrate one way to plan for and prevent disease emergence in livestock.

Protein Cages: From Fundamentals to Advanced Applications

Proteins that self-assemble into polyhedral shell-like structures are useful molecular containers both in nature and in the laboratory. Here we review efforts to repurpose diverse protein cages, including viral capsids, ferritins, bacterial microcompartments, and designed capsules, as vaccines, drug delivery vehicles, targeted imaging agents, nanoreactors, templates for controlled materials synthesis, building blocks for higher-order architectures, and more. A deep understanding of the principles underlying the construction, function, and evolution of natural systems has been key to tailoring selective cargo encapsulation and interactions with both biological systems and synthetic materials through protein engineering and directed evolution. The ability to adapt and design increasingly sophisticated capsid structures and functions stands to benefit the fields of catalysis, materials science, and medicine.

Receptor-mediated nonhost resistance in plants

Nonhost resistance (NHR) is a plant immune response that prevents many microorganisms in the plant's environment from pathogenicity against the plant. Since successful pathogens have adapted to overcome the immune systems of their host, the durable nature of NHR has potential in the management of plant disease. At present, there is genetic and molecular evidence that the underlying molecular mechanisms of NHR are similar to the plant immune responses that occur in host plants following infection by adapted pathogens. We consider that the molecular basis of NHR is multilayered, conferred by physicochemical barriers and defense responses that are induced following molecular recognition events. Moreover, the relative contribution of each component may depend on evolutionary distances between host and nonhost plants of given pathogen species. This mini-review has focused on the current knowledge of plant NHR, especially the recognition of non-adapted pathogens by nonhost plants at the cellular level. Recent gains in understanding the roles of plasma membrane-localized pattern-recognition receptors (PRRs) and the cytoplasmic nucleotide-binding leucine-rich repeat receptors (NLRs) associated with these processes, as well as the genes involved, are summarized. Finally, we provide a theoretical perspective on the durability of receptor-mediated NHR and its practical potential as an innovative strategy for crop protection against pathogens.

Bats host the most virulent-but not the most dangerous-zoonotic viruses

SignificanceThe clear need to mitigate zoonotic risk has fueled increased viral discovery in specific reservoir host taxa. We show that a combination of viral and reservoir traits can predict zoonotic virus virulence and transmissibility in humans, supporting the hypothesis that bats harbor exceptionally virulent zoonoses. However, pandemic prevention requires thinking beyond zoonotic capacity, virulence, and transmissibility to consider collective "burden" on human health. For this, viral discovery targeting specific reservoirs may be inefficient as death burden correlates with viral, not reservoir, traits, and depends on context-specific epidemiological dynamics across and beyond the human-animal interface. These findings suggest that longitudinal studies of viral dynamics in reservoir and spillover host populations may offer the most effective strategy for mitigating zoonotic risk.

Bats host the most virulent-but not the most dangerous-zoonotic viruses

SignificanceThe clear need to mitigate zoonotic risk has fueled increased viral discovery in specific reservoir host taxa. We show that a combination of viral and reservoir traits can predict zoonotic virus virulence and transmissibility in humans, supporting the hypothesis that bats harbor exceptionally virulent zoonoses. However, pandemic prevention requires thinking beyond zoonotic capacity, virulence, and transmissibility to consider collective "burden" on human health. For this, viral discovery targeting specific reservoirs may be inefficient as death burden correlates with viral, not reservoir, traits, and depends on context-specific epidemiological dynamics across and beyond the human-animal interface. These findings suggest that longitudinal studies of viral dynamics in reservoir and spillover host populations may offer the most effective strategy for mitigating zoonotic risk.

Antagonistic Coevolution Limits the Range of Host Defense in C. elegans Populations

Host populations often evolve defenses against parasites due to the significant fitness costs imposed by infection. However, adaptation to a specific parasite may alter the effectiveness of the host’s defenses in general. Consequently, the specificity of host defense may be influenced by a host population’s evolutionary history with parasites. Further, the degree of reciprocal change within an interaction may profoundly alter the range of host defense, given that antagonistic coevolutionary interactions are predicted to favor defense against specific parasite genotypes. Here, we examined the effect of host evolutionary history on host defense range by assessing the mortality rates of Caenorhabditis elegans host populations exposed to an array of Serratia marcescens bacterial parasite strains. Importantly, each of the host populations were derived from the same genetic background but have different experimental evolution histories with parasites. Each of these histories (exposure to either heat-killed, fixed genotype, or coevolving parasites) carries a different level of evolutionary reciprocity. Overall, we observed an effect of host evolutionary history in that previously coevolved host populations were generally the most susceptible to novel parasite strains. This data demonstrates that host evolutionary history can have a significant impact on host defense, and that host-parasite coevolution can increase host susceptibility to novel parasites.

Susceptibility reversed: modified plant susceptibility genes for resistance to bacteria

Plants have evolved complex defence mechanisms to avoid invasion of potential pathogens. Despite this, adapted pathogens deploy effector proteins to manipulate host susceptibility (S) genes, rendering plant defences ineffective. The identification and mutation of plant S genes exploited by bacterial pathogens are important for the generation of crops with durable and broad-spectrum resistance. Application of mutant S genes in the breeding of resistant crops is limited because of potential pleiotropy. New genome editing techniques open up new possibilities for the modification of S genes. In this review, we focus on S genes manipulated by bacteria and propose ways for their identification and precise modification. Finally, we propose that genes coding for transporter proteins represent a new group of S genes.

The ghost of hosts past: impacts of host extinction on parasite specificity

A growing body of research is focused on the extinction of parasite species in response to host endangerment and declines. Beyond the loss of parasite species richness, host extinction can impact apparent parasite host specificity, as measured by host richness or the phylogenetic distances among hosts. Such impacts on the distribution of parasites across the host phylogeny can have knock-on effects that may reshape the adaptation of both hosts and parasites, ultimately shifting the evolutionary landscape underlying the potential for emergence and the evolution of virulence across hosts. Here, we examine how the reshaping of host phylogenies through extinction may impact the host specificity of parasites, and offer examples from historical extinctions, present-day endangerment, and future projections of biodiversity loss. We suggest that an improved understanding of the impact of host extinction on contemporary host-parasite interactions may shed light on core aspects of disease ecology, including comparative studies of host specificity, virulence evolution in multi-host parasite systems, and future trajectories for host and parasite biodiversity. This article is part of the theme issue 'Infectious disease macroecology: parasite diversity and dynamics across the globe'.

Digest: Evolutionary dynamics of specialization of a fungal pathogen

Pathogen specialization may not always arise due to an adaptive trade-off between infecting one or many species. Bruns et al. characterized the potential host range of four lineages of the fungal pathogen Microbotryum on three Dianthus species. They found that both generalists and specialists co-occurred in nature and no clear fitness benefit is found for either. They suggested that specialization may be related to the specific geographic distribution of these plants.

Resistance Correlations Influence Infection by Foreign Pathogens

AbstractReciprocal selection promotes the specificity of host-pathogen associations and resistance polymorphisms in response to disease. However, plants and animals also vary in response to pathogen species not previously encountered in nature, with potential effects on new disease emergence. Using anther smut disease, we show that resistance (measured as infection rates) to foreign pathogens can be correlated with standing variation in resistance to an endemic pathogen. In Silene vulgaris, genetic variation in resistance to its endemic anther smut pathogen correlated positively with resistance variation to an anther smut pathogen from another host, but the relationship was negative between anther smut and a necrotrophic pathogen. We present models describing the genetic basis for assessing resistance relationships between endemic and foreign pathogens and for quantifying infection probabilities on foreign pathogen introduction. We show that even when the foreign pathogen has a lower average infection ability than the endemic pathogen, infection outcomes are determined by the sign and strength of the regression of the host's genetic variation in infection rates by a foreign pathogen on variation in infection rates by an endemic pathogen as well as by resistance allele frequencies. Given that preinvasion equilibria of resistance are determined by factors including resistance costs, we show that protection against foreign pathogens afforded by positively correlated resistances can be lessened or even result in elevated infection risk at the population level, depending on local dynamics. Therefore, a pathogen's emergence potential could be influenced not only by its average infection rate but also by resistance variation resulting from prior selection imposed by endemic diseases.

Assessment of Associations between Malaria Parasites and Avian Hosts-A Combination of Classic System and Modern Molecular Approach

Simple Summary

Throughout history, frequent outbreaks of diseases in humans have occurred following transmission from animals. While some diseases can jump between birds and mammals, others are stuck to closely related species. Understanding the mechanisms of host–parasite associations will enable us to predict the outbreaks of diseases and will therefore be important to society and ecological health. For decades, scientists have attempted to reveal how host–parasite associations are formed and persist. The key is to assess the ability of the parasite to infect and reproduce within the host without killing the host. Related studies have faced numerous challenges, but technical advances are providing solutions and are gradually broadening our understanding. In this review, I use bird malaria and related blood parasites as a model system and summarize the important advances in techniques and perspectives and how they provide new approaches for understanding the evolution of host–parasite associations to further predict disease outbreaks.

Abstract

Avian malaria and related haemosporidian parasites are responsible for fitness loss and mortality in susceptible bird species. This group of globally distributed parasites has long been used as a classical system for investigating host–parasite associations. The association between a parasite and its hosts can be assessed by the prevalence in the host population and infection intensity in a host individual, which, respectively, reflect the ability of the parasite to infect the host and reproduce within the host. However, the latter has long been poorly investigated due to numerous challenges, such as lack of general molecular markers and limited sensitivity of traditional methods, especially when analysing naturally infected birds. The recent development of genetic databases, together with novel molecular methodologies, has shed light on this long-standing problem. Real-time quantitative PCR has enabled more accurate quantification of avian haemosporidian parasites, and digital droplet PCR further improved experimental sensitivity and repeatability of quantification. In recent decades, parallel studies have been carried out all over the world, providing great opportunities for exploring the adaptation of haemosporidian parasites to different hosts and the variations across time and space, and further investigating the coevolutionary history between parasites and their hosts. I hereby review the most important milestones in diagnosis techniques of avian haemosporidian parasites and illustrate how they provide new insights for understanding host–parasite associations.

The evolutionary impact of population size, mutation rate and virulence on pathogen niche width

Understanding the evolution of pathogen niche width is important for predicting disease spread and the probability that pathogens can emerge in novel hosts. Findings from previous theoretical studies often suggest that pathogens will evolve to be specialists in specific host environments. However, several of these studies make unrealistic assumptions regarding demographic stochasticity and the ability of pathogens to select their hosts. Here, an individual-based model was used to predict how population size, virulence and pathogen mutation rate affects the evolution niche specialism in pathogens. Pathogen specialism evolved regardless of virulence or populations size; thus, the findings of this study are somewhat consistent with those of previous work. However, because specialist pathogens had only a weak selective advantage over generalist pathogens, high mutation rates caused random trait variation to accumulate, preventing the evolution of specialism. Mutation rate varies greatly across different species and strains of pathogen. By showing that high mutation rates may prevent pathogen specialism evolving, this study highlights an intrinsic pathogen trait that may influence the evolution of pathogen niche width.

Host Susceptibility Modulates Escovopsis Pathogenic Potential in the Fungiculture of Higher Attine Ants

Health and disease emerge from intricate interactions between genotypes, phenotypes, and environmental features. The outcomes of such interactions are context-dependent, existing as a dynamic continuum ranging from benefits to damage. In host-microbial interactions, both the host and environmental conditions modulate the pathogenic potential of a microorganism. Microbial interactions are the core of the agricultural systems of ants in the subtribe Attina, which cultivate basidiomycete fungi for food. The fungiculture environment harbors a diverse microbial community, including fungi in the genus Escovopsis that has been studied as damage-causing agent. Here, we consider the ant colony as a host and investigate to what extent its health impacts the dynamics and outcomes of host-Escovopsis interactions. We found that different ant fungal cultivars vary in susceptibility to the same Escovopsis strains in plate-assays interactions. In subcolony-Escovopsis interactions, while healthy subcolonies gradually recover from infection with different concentrations of Escovopsis conidia, insecticide-treated subcolonies evidenced traits of infection and died within 7 days. The opportunistic nature of Escovopsis infections indicates that diseases in attine fungiculture are a consequence of host susceptibility, rather than the effect of a single microbial agent. By addressing the host susceptibility as a major modulator of Escovopsis pathogenesis, our findings expand the understanding of disease dynamics within attine colonies.

How Do Pathogens Evolve Novel Virulence Activities?

This article is part of the Top 10 Unanswered Questions in MPMI invited review series.We consider the state of knowledge on pathogen evolution of novel virulence activities, broadly defined as anything that increases pathogen fitness with the consequence of causing disease in either the qualitative or quantitative senses, including adaptation of pathogens to host immunity and physiology, host species, genotypes, or tissues, or the environment. The evolution of novel virulence activities as an adaptive trait is based on the selection exerted by hosts on variants that have been generated de novo or arrived from elsewhere. In addition, the biotic and abiotic environment a pathogen experiences beyond the host may influence pathogen virulence activities. We consider host-pathogen evolution, host range expansion, and external factors that can mediate pathogen evolution. We then discuss the mechanisms by which pathogens generate and recombine the genetic variation that leads to novel virulence activities, including DNA point mutation, transposable element activity, gene duplication and neofunctionalization, and genetic exchange. In summary, if there is an (epi)genetic mechanism that can create variation in the genome, it will be used by pathogens to evolve virulence factors. Our knowledge of virulence evolution has been biased by pathogen evolution in response to major gene resistance, leaving other virulence activities underexplored. Understanding the key driving forces that give rise to novel virulence activities and the integration of evolutionary concepts and methods with mechanistic research on plant-microbe interactions can help inform crop protection.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

From generalist to specialists: Variation in the host range and performance of anther-smut pathogens on Dianthus

Determining the processes that drive the evolution of pathogen host range can inform our understanding of disease dynamics and the potential for host shifts. In natural populations, patterns of host range could be driven by genetically based differences in pathogen infectivity or ecological differences in host availability. In northwestern Italy, four reproductively isolated lineages of the fungal plant-pathogen Microbotryum have been shown to co-occur on several species in the genus Dianthus. We carried out cross-inoculation experiments to determine whether patterns of realized host range in these four lineages were driven by differences in infectivity and to test whether there was evidence of a trade-off between host range and within-host reproduction. We found strong concordance between field patterns of host range and pathogen infectivity on different Dianthus species using experimental inoculation, indicating that infection ability is a major driving force of host range. However, we found no evidence of a trade-off between the ability to infect a wider range of host species and spore production on a shared host.

Phylogenetically diverse diets favor more complex venoms in North American pitvipers

The role of natural selection in the evolution of trait complexity can be characterized by testing hypothesized links between complex forms and their functions across species. Predatory venoms are composed of multiple proteins that collectively function to incapacitate prey. Venom complexity fluctuates over evolutionary timescales, with apparent increases and decreases in complexity, and yet the causes of this variation are unclear. We tested alternative hypotheses linking venom complexity and ecological sources of selection from diet in the largest clade of front-fanged venomous snakes in North America: the rattlesnakes, copperheads, cantils, and cottonmouths. We generated independent transcriptomic and proteomic measures of venom complexity and collated several natural history studies to quantify dietary variation. We then constructed genome-scale phylogenies for these snakes for comparative analyses. Strikingly, prey phylogenetic diversity was more strongly correlated to venom complexity than was overall prey species diversity, specifically implicating prey species' divergence, rather than the number of lineages alone, in the evolution of complexity. Prey phylogenetic diversity further predicted transcriptomic complexity of three of the four largest gene families in viper venom, showing that complexity evolution is a concerted response among many independent gene families. We suggest that the phylogenetic diversity of prey measures functionally relevant divergence in the targets of venom, a claim supported by sequence diversity in the coagulation cascade targets of venom. Our results support the general concept that the diversity of species in an ecological community is more important than their overall number in determining evolutionary patterns in predator trait complexity.

Relationship between Soil Fungi and Seedling Density in the Vicinity of Adult Conspecifics in an Arid Desert Forest

Research Highlights: 1. Soil fungi have a higher influence on seedling density compared to soil environmental factors; 2. Host-specific pathogens and beneficial fungi affect seeding density via different influencing mechanisms. Background and Objectives: The growth and development of seedlings are the key processes that affect forest regeneration and maintain community dynamics. However, the influencing factors of seedling growth around their adult conspecifics are not clear in arid desert forests. Probing the intrinsic relations among soil fungi, soil environmental factors (pH, water content, salinity, and nutrition), and seedling density will improve our understanding of forest development and provide a theoretical basis for forest management and protection. Materials and Methods: Four experimental plot types, depending on the distance to adult conspecifics, were set in an arid desert forest. Soil environmental factors, the diversity and composition of the soil fungal community, and the seedlings’ density and height were measured in the four experimental plot types, and their mutual relations were analyzed. Results: Seedling density as well as the diversity and composition of the soil fungal community varied significantly among the four plot types (p < 0.05). Soil environmental factors, especially soil salinity, pH, and soil water content, had significant influences on the seedling density and diversity and composition of the soil fungal community. The contribution of soil fungi (72.61%) to the variation in seedling density was much higher than the soil environmental factors (27.39%). The contribution of detrimental fungi to the variation in seedling density was higher than the beneficial fungi. Conclusions: Soil fungi mostly affected the distribution of seedling density in the vicinity of adult conspecifics in an arid desert forest. The distribution of seedling density in the vicinity of adults was mainly influenced by the detrimental fungi, while the adults in the periphery area was mainly influenced by the beneficial fungi.

What is the Molecular Basis of Nonhost Resistance?

This article is part of the Top 10 Unanswered Questions in MPMI invited review series.Nonhost resistance is typically considered the ability of a plant species to repel all attempts of a pathogen species to colonize it and reproduce on it. Based on this common definition, nonhost resistance is presumed to be very durable and, thus, of great interest for its potential use in agriculture. Despite considerable research efforts, the molecular basis of this type of plant immunity remains nebulous. We here stress the fact that "nonhost resistance" is a phenomenological rather than a mechanistic concept that comprises more facets than typically considered. We further argue that nonhost resistance essentially relies on the very same genes and pathways as other types of plant immunity, of which some may act as bottlenecks for particular pathogens on a given plant species or under certain conditions. Thus, in our view, the frequently used term "nonhost genes" is misleading and should be avoided. Depending on the plant-pathogen combination, nonhost resistance may involve the recognition of pathogen effectors by host immune sensor proteins, which might give rise to host shifts or host range expansions due to evolutionary-conditioned gains and losses in respective armories. Thus, the extent of nonhost resistance also defines pathogen host ranges. In some instances, immune-related genes can be transferred across plant species to boost defense, resulting in augmented disease resistance. We discuss future routes for deepening our understanding of nonhost resistance and argue that the confusing term "nonhost resistance" should be used more cautiously in the light of a holistic view of plant immunity.

Evolution of behavioural resistance in host-pathogen systems

Behavioural resistance to parasites is widespread in animals, yet little is known about the evolutionary dynamics that have shaped these strategies. We show that theory developed for the evolution of physiological parasite resistance can only be applied to behavioural resistance under limited circumstances. We find that accounting explicitly for the behavioural processes, including the detectability of infected individuals, leads to novel dynamics that are strongly dependent on the nature of the costs and benefits of social interactions. As with physiological resistance, evolutionary dynamics of behavioural resistance can also lead to mixed strategies that balance these costs and benefits.

Higher host plant specialization of root-associated endophytes than mycorrhizal fungi along an arctic elevational gradient

How community-level specialization differs among groups of organisms, and changes along environmental gradients, is fundamental to understanding the mechanisms influencing ecological communities. In this paper, we investigate the specialization of root-associated fungi for plant species, asking whether the level of specialization varies with elevation. For this, we applied DNA barcoding based on the ITS region to root samples of five plant species equivalently sampled along an elevational gradient at a high arctic site. To assess whether the level of specialization changed with elevation and whether the observed patterns varied between mycorrhizal and endophytic fungi, we applied a joint species distribution modeling approach. Our results show that host plant specialization is not environmentally constrained in arctic root-associated fungal communities, since there was no evidence for changing specialization with elevation, even if the composition of root-associated fungal communities changed substantially. However, the level of specialization for particular plant species differed among fungal groups, root-associated endophytic fungal communities being highly specialized on particular host species, and mycorrhizal fungi showing almost no signs of specialization. Our results suggest that plant identity affects associated mycorrhizal and endophytic fungi differently, highlighting the need of considering both endophytic and mycorrhizal fungi when studying specialization in root-associated fungal communities.

A new protocol for absolute quantification of haemosporidian parasites in raptors and comparison with current assays

BACKGROUND

Accurate quantification of infection intensity is essential to estimate infection patterns of avian haemosporidian parasites in order to understand the evolution of host-parasite associations. Traditional microscopy is cost-effective but requires high-quality blood smears and considerable experience, while the widely used semi-quantitative qPCR methods are mostly employed with ideal, laboratory-based golden samples and standard curves, which may limit the comparison of parasitemia from different laboratories.

METHODS

Here we present a digital droplet PCR (ddPCR) protocol for absolute quantification of avian haemosporidians in raptors. Novel primers were designed that target a conserved fragment of a rRNA region of the mitochondrial genome of the parasites. Sensitivity and repeatability were evaluated compared to qPCR and other assays.

RESULTS

This ddPCR assay enables accurate quantification of haemosporidian parasites belonging to Plasmodium, Haemoproteus and Leucocytozoon with minimum infection quantities of 10^-5 (i.e. one parasite copy in 10⁵ host genomes) without the use of standard curves. Quantities assessed by ddPCR were more accurate than qPCR using the same primers through reduction of non-specific amplification in low-intensity samples. The ddPCR technique was more consistent among technical duplicates and reactions, especially when infection intensities were low, and this technique demonstrated equal sensitivity with high correspondence (R² = 0.97) compared to the widely used qPCR assay. Both ddPCR and qPCR identified more positive samples than the standard nested PCR protocol for the cytb gene used for barcoding avian haemosporidians.

CONCLUSIONS

We developed a novel ddPCR assay enabling accurate quantification of avian haemosporidians without golden samples or standard curves. This assay can be used as a robust method for investigating infection patterns in different host-parasite assemblages and can facilitate the comparison of results from different laboratories.

Evolution of division of labour in mutualistic symbiosis

Mutualistic symbiosis can be regarded as interspecific division of labour, which can improve the productivity of metabolites and services but deteriorate the ability to live without partners. Interestingly, even in environmentally acquired symbiosis, involved species often rely exclusively on the partners despite the lethal risk of missing partners. To examine this paradoxical evolution, we explored the coevolutionary dynamics in symbiotic species for the amount of investment in producing their essential metabolites, which symbiotic species can share. Our study has shown that, even if obtaining partners is difficult, 'perfect division of labour' (PDL) can be maintained evolutionarily, where each species perfectly specializes in producing one of the essential metabolites so that every member entirely depends on the others for survival, i.e. in exchange for losing the ability of living alone. Moreover, the coevolutionary dynamics shows multistability with other states including a state without any specialization. It can cause evolutionary hysteresis: once PDL has been achieved evolutionarily when obtaining partners was relatively easy, it is not reverted even if obtaining partners becomes difficult later. Our study suggests that obligate mutualism with a high degree of mutual specialization can evolve and be maintained easier than previously thought.

Tissue tropism and transmission ecology predict virulence of human RNA viruses

Novel infectious diseases continue to emerge within human populations. Predictive studies have begun to identify pathogen traits associated with emergence. However, emerging pathogens vary widely in virulence, a key determinant of their ultimate risk to public health. Here, we use structured literature searches to review the virulence of each of the 214 known human-infective RNA virus species. We then use a machine learning framework to determine whether viral virulence can be predicted by ecological traits, including human-to-human transmissibility, transmission routes, tissue tropisms, and host range. Using severity of clinical disease as a measurement of virulence, we identified potential risk factors using predictive classification tree and random forest ensemble models. The random forest approach predicted literature-assigned disease severity of test data with mean accuracy of 89.4% compared to a null accuracy of 74.2%. In addition to viral taxonomy, the ability to cause systemic infection was the strongest predictor of severe disease. Further notable predictors of severe disease included having neural and/or renal tropism, direct contact or respiratory transmission, and limited (0 < R₀ ≤ 1) human-to-human transmissibility. We present a novel, to our knowledge, comparative perspective on the virulence of all currently known human RNA virus species. The risk factors identified may provide novel perspectives in understanding the evolution of virulence and elucidating molecular virulence mechanisms. These risk factors could also improve planning and preparedness in public health strategies as part of a predictive framework for novel human infections.

Comparative analysis using machine learning shows that specificity of tissue tropism and transmission biology can act as predictive risk factors for the virulence of human RNA viruses.

Persistence of single species of symbionts across multiple closely-related host species

Some symbiont species are highly host-specific, inhabiting only one or a very few host species, and typically have limited dispersal abilities. When they do occur on multiple host species, populations of such symbionts are expected to become genetically structured across these different host species, and this may eventually lead to new symbiont species over evolutionary timescales. However, a low number of dispersal events of symbionts between host species across time might be enough to prevent population structure and species divergence. Overall, processes of evolutionary divergence and the species status of most putative multi-host symbiont systems are yet to be investigated. Here, we used DNA metabarcoding data of 6,023 feather mites (a total of 2,225 OTU representative sequences) from 147 infracommunities (i.e., the assemblage consisting of all mites of different species collected from the same bird host individual) to investigate patterns of population genetic structure and species status of three different putative multi-host feather mite species Proctophyllodes macedo Vitzthum, 1922, Proctophyllodes motacillae Gaud, 1953, and Trouessartia jedliczkai (Zimmerman, 1894), each of which inhabits a variable number of different closely related wagtail host species (genus Motacilla). We show that mite populations from different host species represent a single species. This pattern was found in all the mite species, suggesting that each of these species is a multi-host species in which dispersal of mites among host species prevents species divergence. Also, we found evidence of limited evolutionary divergence manifested by a low but significant level of population genetic structure among symbiont populations inhabiting different host species. Our study agrees with previous studies showing a higher than expected colonization opportunities in host-specific symbionts. Indeed, our results support that these dispersal events would allow the persistence of multi-host species even in symbionts with limited dispersal capabilities, though additional factors such as the geographical structure of some bird populations may also play a role.

An interaction between host and microbe genotypes determines colonization success of a key bumble bee gut microbiota member

There has been a proliferation of studies demonstrating an organism's health is influenced by its microbiota. However, factors influencing beneficial microbe colonization and the evolution of these relationships remain understudied relative to host-pathogen interactions. Vertically transmitted beneficial microbes are predicted to show high levels of specificity in colonization, including genotype matching, which may transpire through coevolution. We investigate how host and bacterial genotypes influence colonization of a core coevolved microbiota member in bumble bees. The hindgut colonizing Snodgrassella alvi confers direct benefits, but, as an early colonizer, also facilitates the further development of a healthy microbiota. Due to predominantly vertical transmission promoting tight evolution between colonization factors of bacteria and host lineages, we predict that genotype-by-genotype interactions will determine successful colonization. Germ-free adult bees from seven bumble bee colonies (host genotypic units) were inoculated with one of six genetically distinct strains of S. alvi. Subsequent colonization within host and microbe genotypes combinations ranged from 0 to 100%, and an interaction between host and microbe genotypes determined colonization success. This novel finding of a genotype-by-genotype interaction determining colonization in an animal host-beneficial microbe system has implications for the ecological and evolutionary dynamics of host and microbe, including associated host-fitness benefits.

Resistance in marine cyanobacteria differs against specialist and generalist cyanophages

Long-term coexistence between unicellular cyanobacteria and their lytic viruses (cyanophages) in the oceans is thought to be due to the presence of sensitive cells in which cyanophages reproduce, ultimately killing the cell, while other cyanobacteria survive due to resistance to infection. Here, we investigated resistance in marine cyanobacteria from the genera Synechococcus and Prochlorococcus and compared modes of resistance against specialist and generalist cyanophages belonging to the T7-like and T4-like cyanophage families. Resistance was extracellular in most interactions against specialist cyanophages irrespective of the phage family, preventing entry into the cell. In contrast, resistance was intracellular in practically all interactions against generalist T4-like cyanophages. The stage of intracellular arrest was interaction-specific, halting at various stages of the infection cycle. Incomplete infection cycles proceeded to various degrees of phage genome transcription and translation as well as phage genome replication in numerous interactions. In a particularly intriguing case, intracellular capsid assembly was observed, but the phage genome was not packaged. The cyanobacteria survived the encounter despite late-stage infection and partial genome degradation. We hypothesize that this is tolerated due to genome polyploidy, which we found for certain strains of both Synechococcus and Prochlorococcus Our findings unveil a heavy cost of promiscuous entry of generalist phages into nonhost cells that is rarely paid by specialist phages and suggests the presence of unknown mechanisms of intracellular resistance in the marine unicellular cyanobacteria. Furthermore, these findings indicate that the range for virus-mediated horizontal gene transfer extends beyond hosts to nonhost cyanobacterial cells.

Effects of host phylogeny, habitat and spatial proximity on host specificity and diversity of pathogenic and mycorrhizal fungi in a subtropical forest

Soil plant-pathogenic (PF) and mycorrhizal fungi (MF) are both important in maintaining plant diversity, for example via host-specialized effects. However, empirical knowledge on the degree of host specificity and possible factors affecting the fungal assemblages is lacking. We identified PF and MF in fine roots of 519 individuals across 45 subtropical tree species in southern China in order to quantify the importance of host phylogeny (including via its effects on functional traits), habitat and space in determining fungal communities. We also compared host specificity in PF and MF at different host-phylogenetic scales. In both PF and MF, host phylogeny independently accounted for > 19% of the variation in fungal richness and composition, whereas environmental and spatial factors each explained no more than 4% of the variation. Over 77% of the variation explained by phylogeny was attributable to covariation in plant functional traits. Host specificity was phylogenetically scale-dependent, being stronger in PF than in MF at low host-phylogenetic scales (e.g. within genus) but similar at larger scales. Our study suggests that host-phylogenetic effects dominate the assembly of both PF and MF communities, resulting from phylogenetically clustered plant traits. The scale-dependent host specificity implies that PF were specialized at lower-level and MF at higher-level host taxa.

Disease mortality in domesticated animals is predicted by host evolutionary relationships

Infectious diseases of domesticated animals impact human well-being via food insecurity, loss of livelihoods, and human infections. While much research has focused on parasites that infect single host species, most parasites of domesticated mammals infect multiple species. The impact of multihost parasites varies across hosts; some rarely result in death, whereas others are nearly always fatal. Despite their high ecological and societal costs, we currently lack theory for predicting the lethality of multihost parasites. Here, using a global dataset of >4,000 case-fatality rates for 65 infectious diseases (caused by microparasites and macroparasites) and 12 domesticated host species, we show that the average evolutionary distance from an infected host to other mammal host species is a strong predictor of disease-induced mortality. We find that as parasites infect species outside of their documented phylogenetic host range, they are more likely to result in lethal infections, with the odds of death doubling for each additional 10 million years of evolutionary distance. Our results for domesticated animal diseases reveal patterns in the evolution of highly lethal parasites that are difficult to observe in the wild and further suggest that the severity of infectious diseases may be predicted from evolutionary relationships among hosts.

Phage-Derived Antibacterials: Harnessing the Simplicity, Plasticity, and Diversity of Phages

Despite the successful use of antibacterials, the emergence of multidrug-resistant bacteria has become a serious threat to global healthcare. In this era of antibacterial crisis, bacteriophages (phages) are being explored as an antibacterial treatment option since they possess a number of advantages over conventional antibacterials, especially in terms of specificity and biosafety; phages specifically lyse target bacteria while not affecting normal and/or beneficial bacteria and display little or no toxicity in that they are mainly composed of proteins and nucleic acids, which consequently significantly reduces the time and cost involved in antibacterial development. However, these benefits also create potential issues regarding antibacterial spectra and host immunity; the antibacterial spectra being very narrow when compared to those of chemicals, with the phage materials making it possible to trigger host immune responses, which ultimately disarm antibacterial efficacy upon successive treatments. In addition, phages play a major role in horizontal gene transfer between bacterial populations, which poses serious concerns for the potential of disastrous consequences regarding antibiotic resistance. Fortunately, however, recent advancements in synthetic biology tools and the speedy development of phage genome resources have allowed for research on methods to circumvent the potentially disadvantageous aspects of phages. These novel developments empower research which goes far beyond traditional phage therapy approaches, opening up a new chapter for phage applications with new antibacterial platforms. Herein, we not only highlight the most recent synthetic phage engineering and phage product engineering studies, but also discuss a new proof-of-concept for phage-inspired antibacterial design based on the studies undertaken by our group.

Genes involved in nonhost disease resistance as a key to engineer durable resistance in crops

Most potential pathogens fail to establish virulence for a plethora of plants found in nature. This intrinsic property to resist pathogen virulence displayed by organisms without triggering canonical resistance (R) genes has been termed nonhost resistance (NHR). While host resistance involves recognition of pathogen elicitors such as avirulence factors by bona fide R proteins, mechanism of NHR seems less obvious, often involving more than one gene. We can generally describe NHR in two steps: 1) pre-invasive resistance, either passive or active, which can restrict the pathogen from entering the host, and 2) post-invasive resistance, an active defense response that often results in hypersensitive response like programmed cell death and reactive oxygen species accumulation. While PAMP-triggered-immunity (PTI) is generally effective against nonhost pathogens, effector-triggered-immunity (ETI) can be effective against both host and nonhost pathogens. Prolonged interactions between adapted pathogens and their resistant host plants results in co-evolution, which can lead to new pathogen strains that can be virulent and cause disease on supposedly resistant host. In this context, engineering durable resistance by manipulating genes involved in NHR is an attractive approach for sustainable agriculture. Several genes involved in NHR have been characterized for their role in plant defense. In this review, we report genes involved in NHR identified to date and highlight a few examples where genes involved in NHR have been used to confer resistance in crop plants against economically important diseases.

The structure of temperate phage-bacteria infection networks changes with the phylogenetic distance of the host bacteria

With their ability to integrate into the bacterial chromosome and thereby transfer virulence or drug-resistance genes across bacterial species, temperate phage play a key role in bacterial evolution. Thus, it is paramount to understand who infects whom to be able to predict the movement of DNA across the prokaryotic world and ultimately the emergence of novel (drug-resistant) pathogens. We empirically investigated lytic infection patterns among Vibrio spp. from distinct phylogenetic clades and their derived temperate phage. We found that across distantly related clades, infections occur preferentially within modules of the same clade. However, when the genetic distance of the host bacteria decreases, these clade-specific infections disappear. This indicates that the structure of temperate phage-bacteria infection networks changes with the phylogenetic distance of the host bacteria.

Risks Posed by Intercrops and Weeds as Alternative Hosts to Xanthomonas campestris pv. musacearum in Banana Fields

Alternative host plants are important in the survival and perpetuation of several crop pathogens and have been suspected to play a role in the survival of Xanthomonas campestris pv. musacearum (Xcm) and perpetuation of Xanthomonas wilt (XW) disease of banana and enset. This study determined the potential risk posed by two weeds (Canna spp. and wild sorghum) and common banana intercrops (maize, millet, sorghum, taro, and sugarcane) as alternative hosts to Xcm. The study employed screenhouse experiments, laboratory procedures and diagnosis of banana fields in XW-affected landscapes. Typical XW symptoms were only observed in artificially inoculated Canna sp., with an incidence of 96%. Leaf lesions characteristic of xanthomonads occurred on millet (50%) and sorghum (35%), though the plants recovered. No symptoms occurred in maize, sugarcane, taro or wild sorghum. However, Xcm was recovered from all these plant species, with higher recoveries in Canna sp. (47%), millet (27%), sugarcane (27%), and wild sorghum (25%). Only isolates recovered from Canna sp., millet, sorghum and wild sorghum caused disease in banana plantlets. The presence and incidence of XW on-farm was positively associated with the presence of susceptible ABB Musa genotypes and negatively with number of banana cultivars on farm and household access to training on XW management. Only 0.02% of field sampled Canna spp. plants had Xcm. Risk posed by Canna spp. on-farm could be limited to tool transmission as it has persistent floral bracts that prevent insect-mediated infections. Given the high susceptibility, perennial nature and propagation through rhizomes of Canna sp., it could pose a moderate-high risk, thus warranting some attention in the management of XW disease. Sugarcane could offer a low-moderate risk due to its perennial nature and propagation through rhizomes while risk from maize, millet, and sorghum was deemed zero-low due to their annual nature, wind-mediated mode of pollination and propagation through seed. Understanding the interactions of a crop pathogen with other plants is thus important when diversifying agroecosystems. The study findings also suggest other factors such as cultivar composition and management of the disease at farm and landscape level to be important in the perpetuation of XW disease.

Escovopsis kreiselii specialization to its native hosts in the fungiculture of the lower attine ant Mycetophylax morschi

Parasite-host associations are widespread in nature and the fungus-growing ants are considered model organisms to study such interactions. These insects cultivate basidiomycetous fungi for food, which are threatened by mycotrophic fungi in the genus Escovopsis. Although recently described from colonies of the lower attine ant Mycetophylax morschi, the biology and pathogenicity of Escovopsis kreiselii are unknown. Herein, we evaluated the interaction of E. kreiselii with fungi cultivated by M. morschi (native hosts) and with a fungus cultivated by another attine ant species (non-native host). In addition, we examined the physical interactions between hypha of E. kreiselii and hypha from its native hosts using scanning electron microscopy. Escovopsis kreiselii inhibited the growth of fungal cultivars by 24% or more (with exception of one isolate), when compared to the fungal cultivars growing alone. Escovopsis kreiselii is attracted towards its native hosts through chemotaxis and inhibition occurs when there is physical contact with the hyphae of the fungal cultivar. As reported for Escovopsis parasites associated with leafcutter ants (higher attines), E. kreiselii growth increased in the presence of its native hosts, even before contact between both fungi occurred. In interactions with the fungal cultivar that is not naturally infected by E. kreiselii (non-native host), it caused inhibition but not at the same magnitude as in native hosts. Multiple lines of evidence suggest that E. kreiselii is an antagonist of the fungus cultivated by M. morschi and can chemically recognize such fungus.

Bidirectional backcrosses between wild and cultivated lettuce identify loci involved in nonhost resistance to downy mildew

KEY MESSAGE

The nonhost resistance of wild lettuce to lettuce downy mildew seems explained by four components of a putative set of epistatic genes. The commonplace observation that plants are immune to most potential pathogens is known as nonhost resistance (NHR). The genetic basis of NHR is poorly understood. Inheritance studies of NHR require crosses of nonhost species with a host, but these crosses are usually unsuccessful. The plant-pathosystem of lettuce and downy mildew, Bremia lactucae, provides a rare opportunity to study the inheritance of NHR, because the nonhost wild lettuce species Lactuca saligna is sufficiently cross-compatible with the cultivated host Lactuca sativa. Our previous studies on NHR in one L. saligna accession led to the hypothesis that multi-locus epistatic interactions might explain NHR. Here, we studied NHR at the species level in nine accessions. Besides the commonly used approach of studying a target trait from a wild donor species in a cultivar genetic background, we also explored the opposite, complementary approach of cultivar introgression in a wild species background. This bidirectional approach encompassed (1) nonhost into host introgression: identification of L. saligna derived chromosome regions that were overrepresented in highly resistant BC1 plants (F1 × L. sativa), (2) host into nonhost introgression: identification of L. sativa derived chromosome regions that were overrepresented in BC1 inbred lines (F1 × L. saligna) with relatively high infection levels. We demonstrated that NHR is based on resistance factors from L. saligna and the genetic dose for NHR differs between accessions. NHR seemed explained by combinations of epistatic genes on three or four chromosome segments, of which one chromosome segment was validated by the host into nonhost approach.

Mapping resistance to powdery mildew in barley reveals a large-effect nonhost resistance QTL

Resistance factors against non-adapted powdery mildews were mapped in barley. Some QTLs seem effective only to non-adapted mildews, while others also play a role in defense against the adapted form. The durability and effectiveness of nonhost resistance suggests promising practical applications for crop breeding, relying upon elucidation of key aspects of this type of resistance. We investigated which genetic factors determine the nonhost status of barley (Hordeum vulgare L.) to powdery mildews (Blumeria graminis). We set out to verify whether genes involved in nonhost resistance have a wide effectiveness spectrum, and whether nonhost resistance genes confer resistance to the barley adapted powdery mildew. Two barley lines, SusBgt_SC and SusBgt_DC, with some susceptibility to the wheat powdery mildew B. graminis f.sp. tritici (Bgt) were crossed with cv Vada to generate two mapping populations. Each population was assessed for level of infection against four B. graminis ff.spp, and QTL mapping analyses were performed. Our results demonstrate polygenic inheritance for nonhost resistance, with some QTLs effective only to non-adapted mildews, while others play a role against adapted and non-adapted forms. Histology analyses of nonhost interaction show that most penetration attempts are stopped in association with papillae, and also suggest independent layers of defence at haustorium establishment and conidiophore formation. Nonhost resistance of barley to powdery mildew relies mostly on non-hypersensitive mechanisms. A large-effect nonhost resistance QTL mapped to a 1.4 cM interval is suitable for map-based cloning.

Ecological Complexity in Plant Virus Host Range Evolution

The host range of a plant virus is the number of species in which it can reproduce. Most studies of plant virus host range evolution have focused on the genetics of host-pathogen interactions. However, the distribution and abundance of plant viruses and their hosts do not always overlap, and these spatial and temporal discontinuities in plant virus-host interactions can result in various ecological processes that shape host range evolution. Recent work shows that the distributions of pathogenic and resistant genotypes, vectors, and other resources supporting transmission vary widely in the environment, producing both expected and unanticipated patterns. The distributions of all of these factors are influenced further by competitive effects, natural enemies, anthropogenic disturbance, the abiotic environment, and herbivory to mention some. We suggest the need for further development of approaches that (i) explicitly consider resource use and the abiotic and biotic factors that affect the strategies by which viruses exploit resources; and (ii) are sensitive across scales. Host range and habitat specificity will largely determine which phyla are most likely to be new hosts, but predicting which host and when it is likely to be infected is enormously challenging because it is unclear how environmental heterogeneity affects the interactions of viruses and hosts.