Climate change and Epichloë coenophiala association modify belowground fungal symbioses of tall fescue host

Human alteration of symbiont genetics among aboveground endophytic Epichloë coenophiala strains within tall fescue (Schedonorus arundinaceus) has led to widespread deployment of novel grass-endophyte combinations, yet little is known about their ecological consequences. In this study, clone pairs (endophyte-infected, endophyte-free) of two tall fescue genotypes received factorial combinations of increased temperature (+3 °C) and precipitation (+30% long-term annual mean) for 2 yr. We measured root arbuscular mycorrhizal fungi (AMF), dark septate endophyte (DSE) colonization, and soil AMF extraradical hyphae (ERH) length. We hypothesized that genetically distinct grass-E. coenophiala associations would differentially affect belowground fungi, and that these relationships would be climate-sensitive. Tall fescue genotype, endophyte presence, and climate treatment interactions affected AMF arbuscules, vesicles, and ERH. DSE decreased with E. coenophiala presence but increased with warming. Genetically distinct tall fescue-E. coenophiala associations may have divergent long-term impacts on other host-symbiont interactions, potentially impacting ecosystem function and response to climate change.     

Molecular evidence for host-adapted races of the fungal endophyte Epichloë bromicola after presumed host shifts

Abstract.— Host shifts of plant‐feeding insects and parasites promote adaptational changes that may result in the formation of host races, an assumed intermediate stage in sympatric speciation. Here, we report on genetically differentiated and host‐adapted races of the fungal endophyte Epichloë bromicola, which presumably emerged after a shift from the grass Bromus erectus to other Bromus hosts. Fungi of the genus Epichloë (Ascomycota) and related anamorphs of Neotyphodium are widespread endophytes of cool‐season grasses. Sexually reproducing strains sterilize the host by formation of external fruiting structures (stromata), whereas asexual strains are asymptomatic and transmitted via seeds. In E. bromicola, strains infecting B. erectus are sexual, and strains from two woodland species, B. benekenii and B. ramosus, are asexual and seed transmitted. Analyses of amplified fragment length polymorphism fingerprinting and of intron sequences of the tub2 and tef1 genes of 26 isolates from the three Bromus hosts collected at natural sites in Switzerland and nearby France demonstrated that isolates are genetically differentiated according to their host, indicating that E. bromicola does not form a single, randomly mating population. Phylogenetic analyses of sequence data did not unambiguously resolve the exact origin of asexual E. bromicola strains, but it is likely they arose from within sexual populations on B. erectus. Incongruence of trees derived from different genes may have resulted from recombination at some time in the recent history of host strains. Reciprocal inoculations of host plant seedlings showed that asexual isolates from B. benekenii and B. ramosus were incapable of infecting B. erectus, whereas the sexual isolates from B. erectus retained the assumed ancestral trait of broad compatibility with Bromus host seedlings. Because all isolates were interfertile in experimental crosses, asexual strains may not be considered independent biological species. We suggest that isolates infecting B. benekenii and B. ramosus represent long‐standing host races or incipient species that emerged after host shifts and that may evolve through host‐mediated reproductive isolation toward independent species.     

A host–microparasite model with a resistant host

The theory of heterozygote advantage is often used to explain the genetic variation found in natural populations. If a large population randomly mates and the various genotypes have the same growth and death rates, the evolution of the genotypes follows Hardy–Weinberg proportions and polymorphism results. When other environmental stresses, like predators, prey and diseases, are present, polymorphism may or may not occur depending on how the various genotypes are affected by the stress. In this paper, we use a basic host–microparasite model to demonstrate that polymorphism can occur even if one genotype suffers a higher death rate than the others in the absence of the parasite if the heterozygote has resistance or immunity to the parasite.     

Epichlo? festucae var. lolii endophyte affects host response to fungal disease progression in perennial ryegrass (Lolium perenne)

正Dear Editor,Perennial ryegrass (Lolium perenne) is a globally important forage and turf grass species that commonly forms symbiotic associations with the asexual fungal endophyte—Epichlo? festucae var. lolii. Epichlo? endophytes mutualistically interact with host plants by providing major fitness enhancements and protection from both biotic and abiotic stresses     

Persistent host–parasitoid interaction caused by host maturation variability

The heterogeneity of parasitism risk among host individuals is a key factor for stabilizing or sustaining host–parasitoid interactions. Host maturation variability, or the variation in the maturation times among host individuals, is the simplest source of such heterogeneity, but it has often been neglected in previous theoretical studies. We developed a configuration individual-based model (cIBM) of host–parasitoid interaction to investigate to what degree of host maturation variability promotes the persistence of host–parasitoid interactions. We ran simulations with various degrees of host maturation variability for different lengths of unsusceptible period. The result showed that low host maturation variability could sustain host–parasitoid dynamics when the host-unsusceptible period was short. Conversely, high levels of variability could sustain host–parasitoid dynamics when the host-unsusceptible period was about half of the total larval period. This suggests that the balance between variability and unsusceptible period is important for the persistence of host–parasitoid interaction. We conclude that maturation variability is a factor that can contribute to the sustainment of host–parasitoid interactions.     

Impact of HIF-1α and hypoxia on fungal growth characteristics and fungal immunity

Human pathogenic fungi are highly adaptable to a changing environment. The ability to adjust to low oxygen conditions is crucial for colonization and infection of the host. Recently, the impact of mammalian hypoxia-inducible factor-1α (HIF-1α) on fungal immunity has emerged. In this review, the role of hypoxia and HIF-1α in fungal infections is discussed regarding the innate immune response.     

Can fungal biopesticides control malaria?

Recent research has raised the prospect of using insect fungal pathogens for the control of vector-borne diseases such as malaria. In the past, microbial control of insect pests in both medical and agricultural sectors has generally had limited success. We propose that it might now be possible to produce a cheap, safe and green tool for the control of malaria, which, in contrast to most chemical insecticides, will not eventually be rendered useless by evolution of resistance. Realizing this potential will require lateral thinking by biologists, technologists and development agencies.     

Do mites evolving in alternating host plants adapt to host switch?

A fluctuating environment may be perceived as a composition of different environments, or as an environment per se, in which it is the fluctuation itself that poses a selection pressure. If so, then organisms may adapt to this alternation. We tested this using experimental populations of spider mites that have been evolving for 45 generations in a homogeneous environment (pepper or tomato plants), or in a heterogeneous environment composed of an alternation of these two plants approximately at each generation. The performance (daily oviposition rate and juvenile survival) of individuals from these populations was tested in each of the homogeneous environments, and in two alternating environments, one every 3 days and the other between generations. To discriminate between potential genetic interactions between alleles conferring adaptation to each host plant and environmental effects of evolving in a fluctuating environment, we compared the performance of all lines with that of a cross between tomato and pepper lines. As a control, two lines within each selection regime were also crossed. We found that crosses between alternating lines and between pepper and tomato lines performed worse than crosses between lines evolving in homogeneous environments when tested in that environment. In contrast, alternating lines performed either better or similarly to lines evolving in homogeneous environments when tested in a fluctuating environment. Our results suggest that fluctuating environments are more than the juxtaposition of two environments. Hence, tests for adaptation of organisms evolving in such environments should be carried out in fluctuating conditions.     

Are tropical fungal endophytes hyperdiverse?

Fungal endophytes are ubiquitous fungi that inhabit healthy plant tissues without causing disease. Endophytes have been found in every plant species examined to date and may be important, but often overlooked, components of fungal biodiversity. In two sites in a lowland, moist tropical forest of central Panama, we quantified endophyte colonization patterns, richness, host preference, and spatial variation in healthy leaves of two co-occurring, understory tree species [ Heisteria concinna (Olacaceae) and Ouratea lucens (Ochnaceae)]. From 83 leaves, all of which were colonized by endophytes, we isolated 418 endophyte morphospecies (estimated 347 genetically distinct taxa), most of which were represented by only a single isolate (59%). Among morphospecies encountered in more than one leaf (nonsingletons), we found evidence of host preference and spatial heterogeneity using both morphospecies frequencies and presence/absence records. Based on these data, we postulate that tropical endophytes themselves may be hyperdiverse and suggest that extrapolative estimates that exclude them will markedly underestimate fungal species diversity.     

The mucormycete–host interface

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Isolation and properties of fungal β-glucosidases

Using chromatography on different matrixes, three β-glucosidases (120, 116, and 70 kDa) were isolated from enzymatic complexes of the mycelial fungi Aspergillus japonicus, Penicillium verruculosum, and Trichoderma reesei, respectively. The enzymes were identified by MALDI-TOF mass-spectrometry. Substrate specificity, kinetic parameters for hydrolysis of specific substrates, ability to catalyze the transglucosidation reaction, dependence of the enzymatic activity on pH and temperature, stability of the enzymes at different temperatures, adsorption ability on insoluble cellulose, and the influence of glucose on catalytic properties of the enzymes were investigated. According to the substrate specificity, the enzymes were shown to belong to two groups: i) β-glucosidase of A. japonicus exhibiting high specific activity to the low molecular weight substrates cellobiose and pNPG (the specific activity towards cellobiose was higher than towards pNPG) and low activity towards polysaccharide substrates (β-glucan from barley and laminarin); ii) β-glucosidases from P. verruculosum and T. reesei exhibiting relatively high activity to polysaccharide substrates and lower activity to low molecular weight substrates (activity to cellobiose was lower than to pNPG).     

Does endocytosis occur in fungal hyphae?

The evidence and arguments for and against the occurrence of endocytosis in fungal hyphae are summarized. The balance of evidence is in favour of the existence of endocytosis. This is supported by an analysis of the recently sequenced Neurospora genome which strongly suggests that this fungus possesses the complex protein machinery required to conduct endocytosis.     

Parasite regulation by host hormones: an old mechanism of host exploitation?

Recent experimental evidence suggests that parasites can not only evade immune responses actively but also exploit the hormonal microenvironment within the host to favor their establishment, growth and reproduction. The benefit for parasites of hormonal exploitation is so great that they have evolved structures similar to the steroid and protein hormone receptors expressed in upper vertebrates that can bind to the hormonal metabolites synthesized by the host. This strategy is exemplified by two parasites that respond to adrenal steroids and sexual steroids, respectively: Schistosoma mansoni and Taenia crassiceps. Understanding how the host endocrine system can, under certain circumstances, favor the establishment of a parasite, and characterizing the parasite hormone receptors that are involved might aid the design of hormonal analogs and drugs that affect the parasite exclusively.     

What makes a host profitable? Parasites balance host nutritive resources against immunity

Numerous host qualities can modulate parasite fitness, and among these, host nutritive resources and immunity are of prime importance. Indeed, parasite fitness increases with the amount of nutritive resources extracted from the host body and decreases with host immune response. To maximize fitness, parasites have therefore to balance these two host components. Yet, because host nutritive resources and immunity both increase with host body condition, it is unclear whether parasites perform better on hosts in prime, intermediate, or poor condition. We investigated blood meal size and survival of the ectoparasitic louse fly Crataerina melbae in relation to body condition and cutaneous immune response of their Alpine swift (Apus melba) nestling hosts. Louse flies took a smaller blood meal and lived a shorter period of time when feeding on nestlings that were experimentally food deprived or had their cutaneous immune response boosted with methionine. Consistent with these results, louse fly survival was the highest when feeding on nonexperimental nestlings in intermediate body condition. Our findings emphasize that although hosts in poor condition had a reduced immunocompetence, parasites may have avoided them because individuals in poor condition did not provide adequate resources. These findings highlight the fact that giving host immunocompetence primary consideration can result in a biased appraisal of host-parasite interactions.     

The effect of host mycorrhizal status on host plant–parasitic plant interactions

Two pot experiments were conducted to examine three-level interactions between host plants, mycorrhizal fungi and parasitic plants. In a greenhouse experiment, Poa annua plants were grown in the presence or absence of an AM fungus (either Glomus lamellosum V43a or G. mosseae BEG29) and in the presence or absence of a root hemiparasitic plant (Odontites vulgaris). In a laboratory experiment, mycorrhizal infection (Glomus claroideum BEG31) of Trifolium pratense host plants (mycorrhizal versus non-mycorrhizal) was combined with hemiparasite infection (Rhinanthus serotinus) of the host (parasitized versus non-parasitized). Infection with the two species of Glomus had no significant effect on the growth of P. annua, while hemiparasite infection caused a significant reduction in host biomass. Mycorrhizal status of P. annua hosts (i.e. presence/absence of AM fungus) affected neither the biomass nor the number of flowers produced by the attached O. vulgaris plants. Infection with G. claroideum BEG31 greatly increased the biomass of T. pratense, but hemiparasite infection had no effect. The hemiparasitic R. serotinus plants attached to mycorrhizal hosts had higher biomass and produced more flowers than plants growing with non-mycorrhizal hosts. Roots of T. pratense were colonized by the AM fungus to an extent independent of the presence or absence of the hemiparasite. Our results confirm earlier findings that the mycorrhizal status of a host plant can affect the performance of an attached root hemiparasite. However, improvement of the performance of the parasitic plant following attachment to a mycorrhizal host depends on the extent to which the AM fungi is able to enhance the growth of the host. Accepted: 23 February 2001     

Does the host matter? Variable influence of host traits on parasitism rates

Parasitism of mammals is ubiquitous, but the processes driving parasite aggregation on hosts are poorly understood, as each system seems to show unique correlations between parasitism and host traits such as sex, age, size and body mass. Genetic diversity is also posited to influence susceptibility to parasitism, and provides a quantifiable measure of an intrinsic unchanging host property, but this link has not been well established. A lack of consistency in host traits predicting parasite heterogeneity may derive from the contribution of environmental factors to parasite aggregation. To evaluate this question, a large dataset was leveraged to explore the relationship between unchanging, intrinsic host traits (heterozygosity and sex), variable host traits (age, length and body mass), and extrinsic factors (sampling date/year and population) and flea presence/absence, abundance and intensity on two species of social burrowing mammal, the black-tailed prairie dog (Cynomys ludovicianus) and the Gunnison’s prairie dog (Cynomys gunnisoni). Prairie dogs experience frequent parasitism by fleas, but the distribution of fleas among individuals is highly skewed. In these systems, intrinsic host traits were nuanced in how they predicted flea aggregation on individual prairie dogs, with sex unimportant to parasitism rates and heterozygosity increasing the probability of infection and influencing the number of fleas in divergent ways. Variable host traits interacted with each other and with environmental or geographic stochasticity to influence flea aggregation. Length and age tended to increase parasitism, whereas the effects of body mass and condition were mediated by date and other host traits to produce both positive and negative effects on parasitism. This finding suggests that the factors affecting ectoparasite infection on individuals are complex, even within species. Importantly, there was no correlation between the number of fleas on an individual in one year and the number of fleas on the same individual the next year, supporting the idea that flea aggregation is not driven by unchanging, intrinsic characteristics of the host. Rather, these findings indicate that host traits influence parasitism in nuanced ways, including interactions with environmental characteristics and stochastic factors.