Modelling Fungal (Neozygites cf. Floridana) Epizootics in Local Populations of Cassava Green Mites (Mononychellus Tanajoa)

The fungus, Neozygitis cf. floridana is parasitic on the cassava green mite, Mononychellus tanajoa (Bondar) (Acari: Tetranychidae) in South America and may be considered for classical biological control of cassava green mites in Africa, where cassava is an important subsistence crop, cassava green mites are an imported pest and specific natural enemies are lacking. Spider mites generally have a viscous structure of local populations, a trait that would normally hamper the spread of a fungus that is transmitted by the contact of susceptible hosts with the halo of capilliconidia surrounding an infectious host. However, if infected mites search and settle to produce capilliconidia on sites where they are surrounded by susceptible mites before becoming infectious, then the conditions for maximal transmission in a viscous host population are met. Because the ratio between spider mites and the leaf area they occupy is constant, parasite-induced host searching behaviour leads to a constant per capita transmission rate. Hence, the transmission rate only depends on the number of infectious hosts. These assumptions on parasite-induced host search and constant host density lead to a simple, analytically tractable model that can be used to estimate the maximal capacity of the fungus to decimate local populations of the cassava green mite. By estimating the parameters of this model (host density, per capita transmission rate and duration of infected and infectious state) it was shown that the fungal pathogen can reduce the population growth of M. tanajoa, but cannot drive local mite populations to extinction. Only when the initial ratio of infectious to susceptible mites exceeds unity or the effective growth rate of the mite population is sufficiently reduced by other factors than the fungus (e.g. lower food quality of the host plant, dislodgement and death by rain and wind and predation), will the fungal pathogen be capable of decimating the cassava green mite population. Under realistic field conditions, where all of these growth-reducing factors are likely to operate, there may well be room for effective control by the parasitic fungus. This revised version was published online in November 2006 with corrections to the Cover Date.     

Hepatitis C Virus Infection in Phenotypically Distinct Huh7 Cell Lines

In 2005, the first robust hepatitis C virus (HCV) infectious cell culture system was developed based on the HCV genotype 2a JFH-1 molecular clone and the human-derived hepatoma cell line Huh7. Although much effort has been made to dissect and expand the repertoire of JFH-1-derived clones, less attention has been given to the host cell despite the intriguing facts that thus far only Huh7 cells have been found to be highly permissive for HCV infection and furthermore only a limited number of Huh7 cell lines/stocks appear to be fully permissive. As such, we compiled a panel of Huh7 lines from disparate sources and evaluated their permissiveness for HCV infection. We found that although Huh7 lines from different laboratories do vary in morphology and cell growth, the majority (8 out of 9) were highly permissive for infection, as demonstrated by robust HCV RNA and de novo infectious virion production following infection. While HCV RNA levels achieved in the 8 permissive cell lines were relatively equivalent, three Huh7 lines demonstrated higher infectious virion production suggesting these cell lines more efficiently support post-replication event(s) in the viral life cycle. Consistent with previous studies, the single Huh7 line found to be relatively resistant to infection demonstrated a block in HCV entry. These studies not only suggest that the majority of Huh7 cell lines in different laboratories are in fact highly permissive for HCV infection, but also identify phenotypically distinct Huh7 lines, which may facilitate studies investigating the cellular determinants of HCV infection.     

Spatially explicit models of Turelli-Hoffmann Wolbachia invasive wave fronts

This paper examines different mathematical models of insect dispersal and infection spread and compares these with field data. Reaction-diffusion and integro-difference equation models are used to model the spatio-temporal spread of Wolbachia in Drosophila simulans populations. The models include cytoplasmic incompatibility between infected females and uninfected males that creates a threshold density, similar to an Allee effect, preventing increase from low incidence of infection in the host population. The model builds on an earlier model (Turelli & Hoffmann, 1991) by incorporating imperfect maternal transmission. The results of simulations of the models using the same parameter values produce different dynamics for each model. These differences become very marked in the integro-difference equation models when insect dispersal patterns are assumed to be non-Gaussian. The success or failure of invasion by Wolbachia in the simulations may be attributed to the insect dispersal mechanism used in the model rather than the parameter values. As the models predict very different outcomes for the integro-difference models depending on the underlying assumptions of insect dispersal patterns, this emphasizes that good field data on real (rather than idealized) dispersal patterns need to be collected before models such as these can be used for predictive purposes.     

Density-dependent dispersal in host-parasitoid assemblages

Most spatial population models assume constant rates of dispersal. However, in a given community, dispersal may not only depend on the density of conspecifics, i.e. density‐dependent dispersal, but also on the density of other species, a phenomenon we term ‘community‐dependent dispersal’. We co‐vary the densities of both the beetle host Callosobruchus chinensis and its parasitoid wasp, Anisopteromalus calandrae, in a laboratory study and record the proportions of each species that disperse within a two‐hour period. The parasitoid in these systems exhibits community‐dependent dispersal – dispersing more frequently when parasitoid density is high and larval host density is low. This supported our prediction that individuals should disperse according to competition for available resources. However, in this study the host's dispersal was independent of density. We suggest that this may be due to less intense selection acting on host dispersal strategies than on the parasitoid. We consider some possible consequences of community‐dependent dispersal for a number of spatial population processes. A well‐known host‐parasitoid metapopulation model is expanded so that it includes a greater range of dispersal functions. When the model is parameterised with the parasitoid community‐dependent dispersal function observed in the empirical study, similar population dynamics are obtained as when fixed‐rate dispersal functions are applied. The importance of dispersal functions for invasions of both competitive and host‐parasitoid systems is also considered. The model results demonstrate that understanding how individuals disperse in response to different species’ population densities is important in determining the rate of spread of an invasion. We suggest that more empirical studies are needed to establish what determines dispersal rate and distance in a range of species, combined with theoretical studies investigating the role of the dispersal function in determining spatial population processes.     

Ecology and evolution of symbiosis in metapopulations

We present a model for symbionts in plant host metapopulation. Symbionts are assumed not only to form a systemic infection throughout the host and pass into the host seeds, but also to reproduce and infect new plants by spores. Thus, we study a metapopulation of qualitatively identical patches coupled through seeds and spores dispersal. Symbionts that are only vertically inherited cannot persist in such a uniform environment if they lower the host's fitness. They have to be beneficial in order to coexist with the host if they are not perfectly transmitted to the seeds; but evolution selects for 100% fidelity of infection inheritance. In this model we want to see how mixed strategies (both vertical and horizontal infection) affect the coexistence of uninfected and infected plants at equilibrium; also, what would evolution do for the host, for the symbionts and for their association. We present a detailed classification of the possible equilibria with examples. The stability of the steady states is rigorously proved for the first time in a metapopulation set-up.     

Flagellin phase‐dependent swimming on epithelial cell surfaces contributes to productive Salmonella gut colonisation

The flagellum is a sophisticated nanomachine and an important virulence factor of many pathogenic bacteria. Flagellar motility enables directed movements towards host cells in a chemotactic process, and near‐surface swimming on cell surfaces is crucial for selection of permissive entry sites. The long external flagellar filament is made of tens of thousands subunits of a single protein, flagellin, and many Salmonella serovars alternate expression of antigenically distinct flagellin proteins, FliC and FljB. However, the role of the different flagellin variants during gut colonisation and host cell invasion remains elusive. Here, we demonstrate that flagella made of different flagellin variants display structural differences and affect Salmonella's swimming behaviour on host cell surfaces. We observed a distinct advantage of bacteria expressing FliC‐flagella to identify target sites on host cell surfaces and to invade epithelial cells. FliC‐expressing bacteria outcompeted FljB‐expressing bacteria for intestinal tissue colonisation in the gastroenteritis and typhoid murine infection models. Intracellular survival and responses of the host immune system were not altered. We conclude that structural properties of flagella modulate the swimming behaviour on host cell surfaces, which facilitates the search for invasion sites and might constitute a general mechanism for productive host cell invasion of flagellated bacteria.     

Dispersal increases local transmission of avian malarial parasites

The relationships between dispersal and local transmission rate of parasites are essential to understanding host–parasite coevolution and the emergence and spread of novel disease threats. Here we show that year‐round transmission, as opposed to summer transmission, has repeatedly evolved in malarial parasites (genera Plasmodium and Haemoproteus) of a migratory bird. Year‐round transmission allows parasites to spread in sympatric host's wintering areas, and hence to colonize distantly located host's breeding areas connected by host‐migration movements. Widespread parasites had higher local prevalence, revealing increased transmission, than geographically restricted parasites. Our results show a positive relationship between dispersal and local transmission of malarial parasites that is apparently mediated by frequent evolutionary changes in parasite transmission dynamics, which has important implications for the ecology and evolution of infectious diseases.     

种间竞争梯度对白车轴草与其病原菌白车轴草单孢莠菌相互作用的影响

白车轴草（Trifolium repens)在与其病原菌白车轴草单孢锈菌（Uromyces trifolii-repentis)的长期相互作用中分化形成了抗病型（Resistance)无性系和易感型（Susceptibility)无性系。该研究工作旨在了解：1）在种间竞争不断增强的环境梯度中,抗型无性系和易感型无性系的生长表现有何区别？2）在同样的实验条件下,分别对抗病型无性系和易感型无性系进行接种感染后,两者的生长表现又有何区别？在一严重感病的白车轴草自然种群中,分别标定了17个抗病型无性系和14个易感型无性系,从各元性系的匍匐茎上分别剪取长5cm,具两个叶两个腋芽的小段作实验材料。接种孢子采自同一植物种群植株的病叶。实验在一个处在次生演替阶段的草本植物群落上进行,在该群落的种间竞争梯度上选定3个分别具有弱、中等和强种间竞争的生境（Habitat)建立实验站,每站设置等量的实验和对照组。结果显示：1）随着环境中种间竞争强度的增加,无论接种与否,抗病型和易感型白车轴草的叶生长量均凝减;2）在弱或强种间竞争环境条件下,无论接种与否,易感型无性系的生长均好于抗抗病型无性系;但在中等间竞争强度的环境条件下,对照实验中易感型无性系的生长显著好于抗病型无性系,而接种实验中的结果相反。这一研究结果表明,种间竞争强度可能是影响给定生境中寄主植物抗病型和易感型遗传型比率的重要因子之一,这一影响还将随着病原菌存在而否而发生改变。     

Sex biases in dispersal and philopatry: insights from a meta-analysis based on capture–mark–recapture studies of damselflies

Sex-biased dispersal is well known for birds and mammals, typically by females and males, respectively. Little is known about general patterns of sex-biased dispersal in other animal taxa. We reviewed return rates for a model group of invertebrates (damselflies) and explored putative costs and benefits of dispersal by males and females. We used published capture–mark–recapture data and examined whether a sex bias existed in likelihood of recapture at least once, at both emergence and/or breeding sites. We assessed whether this metric of likelihood of recapture was indicative of dispersal or philopatry, and whether any emerging pattern(s) were consistent across damselfly families. Using a meta-analysis, we found a higher likelihood of recapture at least once for males than for females at both natal sites and breeding sites, which seemed attributable to higher female-biased dispersal, although female-biased mortality cannot be discounted particularly for some species. Sex biases in dispersal among damselflies may be understood based on sex differences in maturation rate and foraging behaviour, both of which should affect the costs and benefits of dispersing. This hypothesis may be useful for explaining patterns of dispersal in other animal taxa.     

Evolution of dispersal in metacommunities of interacting species

Theoretical studies on the evolution of dispersal in metacommunities are rare despite empirical evidence suggesting that interspecific interactions can modify dispersal behaviour of organisms. To understand the role of species interactions for dispersal evolution, we utilize an individual‐based model of a metacommunity where local population dynamics follows a stochastic version of the Nicholson–Bailey model and dispersal probability is an evolving trait. Our results show that in comparison with a neutral system (commensalism), parasitism promotes dispersal of hosts and parasites, while mutualism tends to reduce dispersal in both partners. Search efficiency of guests (only in the case of parasitism), dispersal mortality and external extinction risk can influence the evolution of dispersal of all partners. In systems composed of two host and two guest species, lower dispersal probabilities evolve under parasitism as well as mutualism than in one host and one guest species systems. This is because of frequency‐dependent modulations of dispersal benefits emerging in such systems for all partners.     

Fungal parasitism in freshwater calanoid population: ecological consequences and possible mechanisms involved in the infection process

This paper reports the results of a study on a zoosporic fungus (Saprolegniaceae) parasitizing populations of Eudiaptomus intermedius (Copepoda, Calanoida) in mountain lakes located in the Northern Apennines, Italy. The dynamics of this infectious process was previously described in one of the lakes of the study area. The virulence is particularly high at the end of the calanoid reproductive phase, when the fungal hyphae penetrate the host eggs provoking their degeneration. Here new data are presented and the phenology of the host–parasite interaction is comparatively analyzed in different years and in different sites. The occurrence of the parasite was assessed at a regional scale. Possible mechanisms involved in the pathogen dispersal and in the host recognition are proposed, and the ecological impact of the fungal parasitism on the calanoid populations is discussed.     

A hybrid machine model of rice blast fungus,Magnaporthe grisea

The fungus, Magnaporthe grisea (Rice blast fungus) is a major agricultural problem affecting rice and related food crops. The way that the fungus invades the host plant and propagates itself is a very important scientific problem and recent advances in research into the genetic basis of these processes can be used to build a simple partial model using hybrid computational modelling techniques. The possible potential benefits of doing this include the use of computer simulation and automated analysis through techniques such as model checking to understand the complex behaviour of such systems. The example is a metaphor for the process of trying to integrate and understand much of the vast amounts of genomic and other data that is being produced in current molecular biology research.     

Larval experience induces adult aversion to rearing host plants: a novel behaviour contrary to Hopkins' host selection principle

1. Hopkins' host selection principle (HSP) states that insects should prefer foliage from their rearing host plant over that of an alternative host. 2. The current study tested whether eastern spruce budworm, Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae), that were laid and developed on, respectively, resistant and susceptible white spruce Picea glauca (Moench) Voss, showed differences in their feeding and oviposition preferences for these two hosts. 3. The data revealed that previous experience of spruce budworm on a host tree type does not influence the host acceptance and feeding behaviour of later larval stages. However, adult budworm reared on resistant white spruce needles preferentially selected susceptible white spruce needles as the host for their progeny, whereas those reared on susceptible needles showed no preference. 4. To the authors' knowledge, this is the first report of an insect showing an oviposition preference for the non‐rearing host plant. This would tend to increase mixing between insects from susceptible and resistant trees. The present results thus argue against Hopkins' HSP and suggest that learned aversion to resistant foliage experienced by larvae is carried into the adult stage.     

Comparative phylogeography of a vulnerable bat and its ectoparasite reveals dispersal of a non-mobile parasite among distinct evolutionarily significant units of the host

Knowledge about phylogeographical structuring and genetic diversity is of key importance for the conservation of endangered species. Comparative phylogeography of a host and its parasite has the potential to reveal cryptic dispersal and behaviour in both species, and can thus be used to guide conservation management. In this study, we investigate the phylogeographic structure of the Bechstein’s bat, Myotis bechsteinii, and its ectoparasitic bat fly, Basilia nana, at 12 sites across their entire distribution. For both species, a mitochondrial sequence fragment (ND1 and COI respectively) and nuclear microsatellite genotypes (14 and 10 loci respectively) were generated and used to compare the phylogeography of host and parasite. Our findings confirm the presence of three distinct genetic subpopulations of the Bechstein’s bat in (1) Europe, (2) the Caucasus and (3) Iran, which remain isolated from one another. The genetic distinctiveness of host populations in the Caucasus region and Iran emphasize that these populations must be managed as distinct evolutionarily significant units. This phylogeographical pattern is however not reflected in its parasite, B. nana, which shows evidence for more recent dispersal between host subpopulations. The discordant genetic pattern between host and parasite suggest that despite the long-term genetic isolation of the different host subpopulations, long-range dispersal of the parasite has occurred more recently, either as the result of secondary contact in the primary host or via secondary host species. This indicates that a novel pathogenic threat to one host subpopulation may be able to disperse, and thus have important consequences for all subpopulations.     

Synchrony in brown trout, Salmo trutta, population dynamics: a 'Moran effect' on early-life stages

Synchrony among populations (i.e. spatial covariation in temporal fluctuations of population size or growth rate) is a common feature to many animals. Both large-scale autocorrelated climatic factors (the 'Moran effect') and dispersal between populations are candidates to explain synchrony, although their relative influence is difficult to assess. Only a few investigations have reported patterns of synchrony among freshwater populations, and even fewer directly related these patterns to an environmental variable. In the present study, we analysed the spatio-temporal patterns of fluctuation of 57 brown trout populations widespread across France, each sampled continuously during 5 years. We compared the respective influence of connectivity and stream distance within basins (i.e. that potentially allow a basin-scale dispersal) and environmental factors (hydrological and air temperature variables, available for 37 sites) on the synchrony of brown trout cohort densities (0+, 1+ and adults). A series of Mantel tests revealed that the degree of synchrony was not related to connectivity or stream distance between sites, suggesting no effect of dispersal at the basin-scale. The degree of synchrony among sites for the 0+ fish was significantly related to the degree of hydrological synchrony (based on high flows during the emergence period). For all three age classes, the synchrony in the temperature patterns did not explain synchrony in trout dynamics. Our results allow us to discuss the respective influence of dispersal and climatic factors on the spatio-temporal patterns of trout dynamics at the basin scale.     

Disease propagation in connected host populations with density-dependent dynamics: the case of the Feline Leukemia Virus

Spatial heterogeneity is a strong determinant of host-parasite relationships, however local-scale mechanisms are often not elucidated. Generally speaking, in many circumstances dispersal is expected to increase disease persistence. We consider the case when host populations show density-dependent dynamics and are connected through the dispersal of individuals. Taking the domestic cats (Felis catus)--Feline Leukemia Virus (FeLV) as a toy model of host-microparasite system, we predict the disease dynamics when two host populations with distinct or similar structures are connected together and to the surrounding environment by dispersal. Our model brings qualitatively different predictions from one-population models. First, as expected, biologically realistic rates of dispersal may allow FeLV to persist in sets of populations where the virus would have gone extinct otherwise, but a reverse outcome is also possible: eradication of FeLV from a small population by connexion to a larger population where it is not persistent. Second, overall prevalence as well as depression of host population size due to infection are both enhanced by dispersal, even at low dispersal rates when disease persistence is not achieved in the two populations. This unexpected prediction is probably due to the combination of dispersal with density-dependent population dynamics. Third, the dispersal of non-infectious cats has more influence on virus prevalence than the dispersal of infectious. Finally, prevalence and depression of host population size are both related to the rate of dispersion, to the health status of individuals dispersing and to the dynamics of host populations.     

Response of Simian Virus 40-Transformed Cell Lines and Cell Hybrids to Superinfection with Simian Virus 40 and Its Deoxyribonucleic Acid

Whereas normal human and monkey cells were susceptible both to intact simian virus 40 (SV40) and to SV40 deoxyribonucleic acid (DNA), human and monkey cells transformed by SV40 were incapable of producing infectious virus after exposure to SV40, but displayed susceptibility to SV40 DNA. On the other hand, mouse and hamster cells, either normal or SV40-transformed, were resistant both to the virus and to SV40 DNA. Hybrids between permissive and nonpermissive parental cells revealed a complex response: whereas most hybrids tested were resistant, three of them produced a small amount of infectious virus upon challenge with SV40 DNA. All were resistant to whole virus challenge. The persistence of infectious SV40 DNA in permissive and nonpermissive cells up to 96 hr after infection was ascertained by cell fusion. The decay kinetics proved to be quite different in permissive and nonpermissive cells. Adsorption of SV40 varied widely among the different cell lines. Very low adsorption of SV40 was detected in nonsusceptible cells with the exception of the mKS-BU100 cell line. A strong increase in SV40 adsorption was produced by pretreating cells with polyoma virus. In spite of this increased adsorption, the resistance displayed by SV40-transformed cells to superinfection with the virus was maintained.     

Emergence of oscillations and spatio-temporal coherence states in a continuum-model of excitatory and inhibitory neurons

A neural field model of the reaction-diffusion type for the emergence of oscillatory phenomena in visual cortices is proposed. To investigate the joint spatio-temporal oscillatory dynamics in a continuous distribution of excitatory and inhibitory neurons, the coupling among oscillators is modelled as a diffusion process, combined with non-linear point interactions. The model exhibits cooperative activation properties in both time and space, by reacting to volleys of activations at multiple cortical sites with ordered spatio-temporal oscillatory states, similar to those found in the physiological experiments on slow-wave field potentials. The possible use of the resulting spatial distributions of coherent states, as a flexible medium to establish feature association, is discussed.     

Studying and approximating spatio-temporal models for epidemic spread and control

A class of simple spatio-temporal stochastic models for the spread and control of plant disease is investigated. We consider a lattice-based susceptible-infected model in which the infection of a host occurs through two distinct processes: a background infective challenge representing primary infection from external sources, and a short-range interaction representing the secondary infection of susceptibles by infectives within the population. Recent data-modelling studies have suggested that the above model may describe the spread of aphid-borne virus diseases in orchards. In addition, we extend the model to represent the effects of different control strategies involving replantation (or recovery). The Contact Process is a particular case of this model. The behaviour of the model has been studied using Cellular-Automata simulations. An alternative approach is to formulate a set of deterministic differential equations that captures the essential dynamics of the stochastic system. Approximate solutions to this set of equations, describing the time evolution over the whole parameter range, have been obtained using the pairwise approximation (PA) as well as the most commonly used mean-field approximation (MF). Comparison with simulation results shows that PA is significantly superior to MF, predicting accurately both transient and long-run, stationary behaviour over relevant parts of the parameter space. The conditions for the validity of the approximations to the present model and extensions thereof are discussed.