Long‐term selection experiment produces breakdown of horizontal transmissibility in parasite with mixed transmission mode

Evolutionary transitions from parasitism toward beneficial or mutualistic associations may encompass a change from horizontal transmission to (strict) vertical transmission. Parasites with both vertical and horizontal transmission are amendable to study factors driving such transitions. In a long‐term experiment, microcosm populations of the protozoan Paramecium caudatum and its bacterial parasite Holospora undulata were exposed to three growth treatments, manipulating vertical transmission opportunities over ca. 800 host generations. In inoculation tests, horizontal transmission propagules produced by parasites from a “high‐growth” treatment, with elevated host division rates increasing levels of parasite vertical transmission, showed a near‐complete loss of infectivity. A similar reduction was observed for parasites from a treatment alternating between high growth and low growth (i.e., low levels of population turn‐over). Parasites from a low‐growth treatment had the highest infectivity on all host genotypes tested. Our results complement previous findings of reduced investment in horizontal transmission and increased vertical transmissibility of high‐growth parasites. We explain the loss of horizontal transmissibility by epidemiological feedbacks and resistance evolution, reducing the frequency of susceptible hosts in the population and thereby decreasing the selective advantage of horizontal transmission. This illustrates how environmental conditions may push parasites with a mixed transmission mode toward becoming vertically transmitted nonvirulent symbionts.     

Potential versus actual contribution of vertical transmission to pathogen fitness

Theory predicts that virulent parasites cannot be maintained at high prevalence if they are only vertically transmitted. However, parasites with high rates of vertical transmission that cause severe reduction in host fitness have been reported. Atkinsonella hypoxylon is a fungal pathogen capable of both vertical and horizontal transmission that drastically reduces its host''s fitness. In contrast with theoretical predictions, field and laboratory observations suggested that the primary mechanism of transmission was vertical. Using randomly amplified polymorphic DNA markers, we investigated the effective contribution of vertical and horizontal transmission to the genetic structure of three natural populations of A. hypoxylon. We found high genotypic diversity and low linkage disequilibrium, indicating that most established genotypes are derived from horizontally transmitted, sexual spores. The low contribution of vertical transmission to the parasite''s fitness despite its high potential might be due to lower establishment of cleistogamous seeds (through which vertical transmission occurs) or lower vigour of vertically transmitted fungal genotypes. Low establishment of vertically infected hosts might explain the persistence of virulent parasites with high apparent vertical transmission. Our results suggest that caution must be taken when using the potential for vertical transmission to make predictions about the evolution of parasite virulence.     

Ophiostoma quercus: An unusually diverse and globally widespread tree-infecting fungus

Ophiostoma quercus (Ascomycota, Ophiostomatales) is a globally widespread, insect-vectored fungus that colonizes a wide diversity of hardwood and conifer hosts. Although the fungus is considered to be non-pathogenic, it is closely related to the fungi that cause Dutch elm disease. We examined the global diversity of O. quercus based on a ribosomal RNA marker and three unlinked gene regions. The fungus exhibited substantial morphological diversity. In addition, O. quercus had high genetic diversity in every continent from which it was collected, although the fungus was most diverse in Eurasia. There was no evidence of geographical clustering of haplotypes based on phylogenetic and network analyses. In addition, the phylogenetic trees generated based on the different markers were non-congruent. These results suggest that O. quercus has been repeatedly moved around the globe, because of trade in wood products, and that the fungal species most likely outcrosses regularly. The high genetic diversity of the fungus, as well as its ability to utilize a wide variety of arthropod vectors and colonize a tremendous diversity of tree host species makes O. quercus truly unique among ophiostomatoid fungi.     

HOST GROWTH CONDITIONS INFLUENCE EXPERIMENTAL EVOLUTION OF LIFE HISTORY AND VIRULENCE OF A PARASITE WITH VERTICAL AND HORIZONTAL TRANSMISSION

In parasites with mixed modes of transmission, ecological conditions may determine the relative importance of vertical and horizontal transmission for parasite fitness. This may lead to differential selection pressure on the efficiency of the two modes of transmission and on parasite virulence. In populations with high birth rates, increased opportunities for vertical transmission may select for higher vertical transmissibility and possibly lower virulence. We tested this idea in experimental populations of the protozoan Paramecium caudatum and its bacterial parasite Holospora undulata. Serial dilution produced constant host population growth and frequent vertical transmission. Consistent with predictions, evolved parasites from this “high‐growth” treatment had higher fidelity of vertical transmission and lower virulence than parasites from host populations constantly kept near their carrying capacity (“low‐growth treatment”). High‐growth parasites also produced fewer, but more infectious horizontal transmission stages, suggesting the compensation of trade‐offs between vertical and horizontal transmission components in this treatment. These results illustrate how environmentally driven changes in host demography can promote evolutionary divergence of parasite life history and transmission strategies.     

Novel Technique for Isolating Microstructures Present in Shake Cultures of the Fungus Ceratocystis ulmi

Microstructures found in shake cultures of Ceratocystis ulmi, the fungus causing Dutch elm disease, have been isolated by a novel technique using the effect of bubbling gas through the culture filtrate.     

Restoration of threatened species: a noble cause for transgenic trees

Some of the first applications of transgenic trees in North America may be for the conservation or restoration of threatened forest trees that have been devastated by fungal pathogens or insect pests. In some cases, where resistance has yet to be found in the natural population of a tree species, incorporating genes from other organisms may offer the only hope for restoration. In others, transgenics may play a role as part of an integrated approach, along with conventional breeding or biocontrol agents. American chestnut (Castanea dentata) was wiped out as a canopy species by a fungal disease accidentally introduced into the United States around 1900. Similarly, American elm (Ulmus americana) virtually disappeared as a favored street tree from Northeastern U.S. cities after the introduction of the Dutch elm disease fungus in the 1940s. In both cases, progress has been made toward restoration via conventional techniques such as selection and propagation of tolerant cultivars (American elm) or breeding with a related resistant species (American chestnut). Recently, progress has also been made with development of systems for engineering antifungal candidate genes into these “heritage trees.” An Agrobacterium-leaf disk system has been used to produce transgenic American elm trees engineered with an antimicrobial peptide gene that may enhance resistance to Dutch elm disease. Two gene transfer systems have been developed for American chestnut using Agrobacterium-mediated transformation of embryogenic cultures, setting the stage for the first tests of potential antifungal genes for their ability to confer resistance to the chestnut blight fungus. Despite the promise of transgenic approaches for restoration of these heritage trees, a number of technical, environmental, economic, and ethical questions remain to be addressed before such trees can be deployed, and the debate around these questions may be quite different from that associated with transgenic trees developed for other purposes.     

The role of adaptations in two-strain competition for sylvatic Trypanosoma cruzi transmission

This study presents a continuous-time model for the sylvatic transmission dynamics of two strains of Trypanosoma cruzi enzootic in North America, in order to study the role that adaptations of each strain to distinct modes of transmission (classical stercorarian transmission on the one hand, and vertical and oral transmission on the other) may play in the competition between the two strains. A deterministic model incorporating contact process saturation predicts competitive exclusion, and reproductive numbers for the infection provide a framework for evaluating the competition in terms of adaptive trade-off between distinct transmission modes. Results highlight the importance of oral transmission in mediating the competition between horizontal (stercorarian) and vertical transmission; its presence as a competing contact process advantages vertical transmission even without adaptation to oral transmission, but such adaptation appears necessary to explain the persistence of (vertically-adapted) T. cruzi IV in raccoons and woodrats in the southeastern United States.     

The role of adaptations in two-strain competition for sylvatic Trypanosoma cruzi transmission

This study presents a continuous-time model for the sylvatic transmission dynamics of two strains of Trypanosoma cruzi enzootic in North America, in order to study the role that adaptations of each strain to distinct modes of transmission (classical stercorarian transmission on the one hand, and vertical and oral transmission on the other) may play in the competition between the two strains. A deterministic model incorporating contact process saturation predicts competitive exclusion, and reproductive numbers for the infection provide a framework for evaluating the competition in terms of adaptive trade-off between distinct transmission modes. Results highlight the importance of oral transmission in mediating the competition between horizontal (stercorarian) and vertical transmission; its presence as a competing contact process advantages vertical transmission even without adaptation to oral transmission, but such adaptation appears necessary to explain the persistence of (vertically-adapted) T. cruzi IV in raccoons and woodrats in the southeastern United States.     

2004 SIVB Congress Symposium Proceeding: Biotechnological progress in dealing with dutch elm disease

Summary Applications of biotechnological tools, including genetic modification aimed at combating Dutch elm disease, are described. In vitro shoot cultures of Ulmus procera (English elm) SR4, U. glabra, U. americana, and U. parvifolia have been established and used as source material in genetic transfer experiments. Biolistic transformation of U. procera leaf material with cauliflower mosaic virus 35S-promoted constructs resulted in transient gusA (β-glucuronidase) expression. Subsequently, regenerant U. procera have been obtained following transformation of stem pieces with wild-type tumor-inducing or root-inducing plasmid-harboring Agrobacterium strains. Genetically modified elms expressing gusA, gfp (green fluorescent protein), and nptII (neomycin phosphotransferase II) genes have been produced using disarmed vectors. Regenerant English elms have been produced following transformation with anti-fungal genes, transferred to soil, and are currently being tested for their ability to resist the Dutch elm disease fungus Ophiostoma novo-ulmi. Future prospects for fighting this fungal wilt using biotechnological tools are discussed.     

Rapid Evolutionary Changes in a Globally Invading Fungal Pathogen (Dutch Elm Disease)

Two enormously destructive pandemics of Dutch elm disease occurred in the 20th century, resulting in the death of a majority of mature elms across much of the northern hemisphere. The first pandemic, caused by Ophiostoma ulmi, occurred as this pathogen spread across Europe, North America and Southwest and Central Asia during the 1920s–1940s. The current pandemic is caused by another Ophiostoma species, O. novo-ulmi. Since the 1940s, O. novo-ulmi has been spreading into the regions previously affected by O. ulmi. It has also spread as two distinct subspecies, termed subsp. americana and subsp. novo-ulmi. This sequence of events has resulted in competitive interactions between these previously geographically isolated pathogens. This article summarizes the biological properties of the Dutch elm disease pathogens and their history of spread. It reviews the remarkable series of genetic events that have occurred during their migrations; including the emergence of genetic clones, the spread of deleterious fungal viruses within the pathogen clones, and the rapid and continuing evolution of O. novo-ulmi via horizontal gene flow. The wider role of horizontal gene flow in the evolutionary potential of migratory plant pathogens is discussed.     

The pathogen causing Dutch elm disease makes host trees attract insect vectors

Dutch elm disease is caused by the fungal pathogen Ophiostoma novo-ulmi which is transmitted by the native elm bark beetle, Hylurgopinus rufipes. We have found that four semiochemicals (the monoterpene (-)-beta-pinene and the sesquiterpenes (-)-alpha-cubebene, (+)-spiroaxa-5,7-diene and (+)-delta-cadinene) from diseased American elms, Ulmus americana, synergistically attract H. rufipes, and that sesquiterpene emission is upregulated in elm trees inoculated with O. novo-ulmi. The fungus thus manipulates host trees to enhance their apparency to foraging beetles, a strategy that increases the probability of transportation of the pathogen to new hosts.     

An empirical study of the evolution of virulence under both horizontal and vertical transmission

According to current thinking, a parasite's transmission mode will be a major determinant of virulence, defined as the harm induced by parasites to their hosts. With horizontal transmission, virulence will increase as a byproduct of a trade-off between fitness gained through increased among-host transmission (infectivity) and fitness lost through increased virulence. With vertical transmission, virulence will decrease because a parasite's reproductive potential will be maximized only by decreasing harm to the host, allowing parasite transmission to more host offspring. To test both predictions, we transmitted barley stripe mosaic virus (BSMV) horizontally and then vertically in its host, barley (Hordeum vulgare). After four generations of horizontal transmission, we observed a nearly twofold increase in horizontal infectivity and nearly tripled virulence. After three generations of subsequent vertical transmission, we observed a modest (16%) increase in vertical transmissibility and a large (40%) reduction in virulence. Increased horizontal transmission is often due to increased pathogen replication which, in turn, causes increased virulence. However, we found no correlation between within-host virus concentration and virulence, indicating that the observed changes in virulence were not due to changes in viral titer. Finally, horizontally transmitted BSMV had reduced vertical transmission and vertically transmitted BSMV had reduced horizontal infectivity. These two observations suggest that, in nature, in different host populations with varying opportunities for horizontal and vertical transmission, different viral strains may be favored.     

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 Epichlo? 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.     

A simple model for the dynamics of a host-parasite-hyperparasite interaction

Hyperparasites can play a crucial role in the control of a host-parasite interaction if they are successfully established in the community. We investigated the specific traits of the hyperparasite and those of the release event which allow a successful regulation of primary parasite populations. This study has been motivated by the case study of chestnut-Cryphonectria parasitica-Cryphonectria Hypovirus interaction. We use a model of SIR/SIS type which assumes a limited diffusion of the parasite. Our model emphasizes the thresholds for invasion linked to the ecological specificities of both the pathogen and the hyperparasite (transmission rates and virulence) and to the initial conditions of the system (population sizes of the different categories). The predictions are consistent with data on the observed spread of the virus. "Mild" strains of the hyperparasite, characterized by a high vertical transmission rate and low virulence, are more prone to establish than "severe" strains. It also demonstrates that the horizontal transmission of the virus, which is controlled by a vegetative incompatibility system in the fungus, is not the unique constraint for the virus establishment. This study may contribute to theoretical and practical aspects of the biological control of plant diseases with a hyperparasite and to the ecology of biological invasions.     

Development of the Dutch elm disease epidemic in southern England, 1971-6

The current epidemic of Dutch elm disease was studied by recording the fate of individual hedgerow elms (Ulmus procera) in five plots in the West Midlands, and by analysing data from successive Forestry Commission surveys of non-woodland elms in 234 plots in southern England. Ninty-five percent of the individual trees died between May 1972 and September 1975. The average infection rate (r) was found to be 1 -35 during the period when the proportion of disease, x, increased from 0–16 to 0–42. In the plots of the main survey the average infection rate was 0–65 and the cumulative loss increased from 6 to 62% between 1971 and 1976, with little evidence that the course of the epidemic was influenced by variations in the weather from year to year. These infection rates are as high as those recorded in Dutch elm disease epidemics elsewhere in the world. The infection rate in English elm was higher than in either the wych elm or the heterogeneous ‘smooth-leaved elm’. The study of English elm in four geographical areas of southern Britain showed that there was an initial drop in infection rate until x = 0–12, when a steady infection rate obtained in all four areas, ranging from 0–56 in the Midlands to 0–76 in the south-east. It is concluded that the epidemic is likely to continue at a high rate until most non-woodland elm have died. Most trees which survive are likely to be smooth-leaved elm in East Anglia. Few communities in southern England have been able to practice vigorous sanitation control programmes, but data from two, in East Sussex and Brighton, are analysed and the effect on disease progress discussed.     

Interactions between frequency-dependent and vertical transmission in host-parasite systems.

We investigate host-pathogen dynamics and conditions for coexistence in two models incorporating frequency-dependent horizontal transmission in conjunction with vertical transmission. The first model combines frequency-dependent and uniparental vertical transmission, while the second addresses parasites transmitted vertically via both parents. For the first model, we ask how the addition of vertical transmission changes the coexistence criteria for parasites transmitted by a frequency-dependent horizontal route, and show that vertical transmission significantly broadens the conditions for parasite invasion. Host-parasite coexistence is further affected by the form of density-dependent host regulation. Numerical analyses demonstrate that within a host population, a parasite strain with horizontal frequency-dependent transmission can be driven to extinction by a parasite strain that is additionally transmitted vertically for a wide range of parameters. Although models of asexual host populations predict that vertical transmission alone cannot maintain a parasite over time, analysis of our second model shows that vertical transmission via both male and female parents can maintain a parasite at a stable equilibrium. These results correspond with the frequent co-occurrence of vertical with sexual transmission in nature and suggest that these transmission modes can lead to host-pathogen coexistence for a wide range of systems involving hosts with high reproductive rates.     

Sap flow-based quantitative indication of progression of Dutch elm disease after inoculation with Ophiostoma novo-ulmi

Key message

The rate of progression of Dutch elm disease can be continuously and quantitatively estimated from sap flow measurements.

Abstract

Response of sap flow to inoculation with Ophiostoma novo-ulmi, a causal agent which causes vascular mycosis called Dutch elm disease, was studied in a field experiment comprised of 4-year-old wych elm trees (Ulmus glabra). Sap flow was measured on inoculated trees using the trunk heat balance method with external heating (EMS 62, Czech Republic) throughout the experiment. The first detectable symptoms of reduction in sap flow occurred 6 days after inoculation and all inoculated trees died within 16 days. Our experiment confirmed the ability of O. novo-ulmi to quickly kill young elm trees. The disease progressed faster than in previous experiments utilizing O. ulmi. To the best of our knowledge, this is the first experiment using sap flow measurements on trees inoculated by O. novo-ulmi. The trunk heat balance sap flow method is an effective non-invasive tool for continuous quantitative monitoring of the progression of vascular tree diseases, and show increased potential for field and greenhouse studies on changes in xylem hydraulic conductivity in a wide range of broadleaved and coniferous tree species.     

Leaf trait dissimilarities between Dutch elm hybrids with a contrasting tolerance to Dutch elm disease

Background and Aims

Previous studies have shown that Ophiostoma novo-ulmi, the causative agent of Dutch elm disease (DED), is able to colonize remote areas in infected plants of Ulmus such as the leaf midrib and secondary veins. The objective of this study was to compare the performances in leaf traits between two Dutch elm hybrids ‘Groeneveld’ and ‘Dodoens’ which possess a contrasting tolerance to DED. Trait linkages were also tested with leaf mass per area (LMA) and with the reduced Young''s modulus of elasticity (MOE) as a result of structural, developmental or functional linkages.

Methods

Measurements and comparisons were made of leaf growth traits, primary xylem density components, gas exchange variables and chlorophyll a fluorescence yields between mature plants of ‘Groeneveld’ and ‘Dodoens’ grown under field conditions. A recently developed atomic force microscopy technique, PeakForce quantitative nanomechanical mapping, was used to reveal nanomechanical properties of the cell walls of tracheary elements such as MOE, adhesion and dissipation.

Key Results

‘Dodoens’ had significantly higher values for LMA, leaf tissue thickness variables, tracheary element lumen area (A), relative hydraulic conductivity (RC), gas exchange variables and chlorophyll a fluorescence yields. ‘Groeneveld’ had stiffer cell walls of tracheary elements, and higher values for water-use efficiency and leaf water potential. Leaves with a large carbon and nutrient investment in LMA tended to have a greater leaf thickness and a higher net photosynthetic rate, but LMA was independent of RC. Significant linkages were also found between the MOE and some vascular traits such as RC, A and the number of tracheary elements per unit area.

Conclusions

Strong dissimilarities in leaf trait performances were observed between the examined Dutch elm hybrids. Both hybrids were clearly separated from each other in the multivariate leaf trait space. Leaf growth, vascular and gas exchange traits in the infected plants of ‘Dodoens’ were unaffected by the DED fungus. ‘Dodoens’ proved to be a valuable elm germplasm for further breeding strategies.     

TRADEOFF BETWEEN HORIZONTAL AND VERTICAL MODES OF TRANSMISSION IN BACTERIAL PLASMIDS

It has been hypothesized that there is a fundamental conflict between horizontal (infectious) and vertical (intergenerational) modes of parasite transmission. Activities of a parasite that increase its rate of infectious transmission are presumed to reduce its host's fitness. This reduction in host fitness impedes vertical transmission of the parasite and causes a tradeoff between horizontal and vertical transmission. Given this tradeoff, and assuming no multiple infections (no within-host competition among parasites), a simple model predicts that the density of uninfected hosts in the environment should determine the optimum balance between modes of parasite transmission. When susceptible hosts are abundant, selection should favor increased rates of horizontal transfer, even at the expense of reduced vertical transmission. Conversely, when hosts are rare, selection should favor increased vertical transmission even at the expense of lower horizontal transfer. We tested the tradeoff hypothesis and these evolutionary predictions using conjugative plasmids and the bacteria that they infect. Plasmids were allowed to evolve for 500 generations in environments with different densities of susceptible hosts. The plasmid's rate of horizontal transfer by conjugation increased at the expense of host fitness, indicating a tradeoff between horizontal and vertical transmission. Also, reductions in conjugation rate repeatedly coincided with the loss of a particular plasmid-encoded antibiotic resistance gene. However, susceptible host density had no significant effect on the evolution of horizontal versus vertical modes of plasmid transmission. We consider several possible explanations for the failure to observe such an effect.