Endophytic Xylaria (Xylariaceae) among liverworts and angiosperms: phylogenetics, distribution, and symbiosis

Nuclear ribosomal 18S and internal transcribed spacer (ITS) sequence data were used to identify endophytic fungi cultured from six species of liverworts collected in Jamaica and North Carolina. Comparisons with other published fungal sequences and phylogenetic analyses yielded the following conclusions: (1) the endophytes belong to the ascomycete families Xylariaceae, Hypocreaceae, and Ophiostomataceae, and (2) liverwort endophytes in the genus Xylaria are closely related to each other and to endophytes isolated from angiosperms in China, Puerto Rico, and Europe. Liverwort endophytes are expected to be foragers or endophytic specialists, although little is known about the role of these fungi in symbioses. Features that may indicate a mutualistic role for these endophytes are discussed.     

Ants inhabiting myrmecophytic ferns regulate the distribution of lianas on emergent trees in a Bornean tropical rainforest

Little is known about the spatial distribution of lianas on emergent trees in tropical rainforests and the factors affecting this distribution. The present study investigated the effects of an arboreal ant species, Crematogaster difformis, which forms myrmecophytic symbioses with two epiphytic ferns, Lecanopteris sp. and Platycerium sp., on the spatial distribution of lianas associated with emergent trees. Living lianas were placed onto trunk surfaces inside and outside the territories of the ants in the canopy, to examine their ability to remove them. The number of leaves pruned by the ants was significantly higher on lianas inside than outside their territories. The spatial overlap of the distributions of lianas and the two ferns on emergent trees were then examined. The frequency of liana colonization of tree crowns was found to be significantly lower on trees with than without ferns. Under the natural conditions, C. difformis workers were observed biting and pruning the lianas. These results suggest that C. difformis regulates the distribution of lianas on emergent trees.     

Shifts in Symbiotic Endophyte Communities of a Foundational Salt Marsh Grass following Oil Exposure from the Deepwater Horizon Oil Spill

Symbiotic associations can be disrupted by disturbance or by changing environmental conditions. Endophytes are fungal and bacterial symbionts of plants that can affect performance. As in more widely known symbioses, acute or chronic stressor exposure might trigger disassociation of endophytes from host plants. We tested this hypothesis by examining the effects of oil exposure following the Deepwater Horizon (DWH) oil spill on endophyte diversity and abundance in Spartina alterniflora – the foundational plant in northern Gulf coast salt marshes affected by the spill. We compared bacterial and fungal endophytes isolated from plants in reference areas to isolates from plants collected in areas with residual oil that has persisted for more than three years after the DWH spill. DNA sequence-based estimates showed that oil exposure shifted endophyte diversity and community structure. Plants from oiled areas exhibited near total loss of leaf fungal endophytes. Root fungal endophytes exhibited a more modest decline and little change was observed in endophytic bacterial diversity or abundance, though a shift towards hydrocarbon metabolizers was found in plants from oiled sites. These results show that plant-endophyte symbioses can be disrupted by stressor exposure, and indicate that symbiont community disassembly in marsh plants is an enduring outcome of the DWH spill.     

Population studies of native grass-endophyte symbioses provide clues for the roles of host jumps and hybridization in driving their evolution

Fungal endophytes in the genera Epichloë and Neotyphodium, collectively termed the epichloae, have fascinated biologists for decades. These intriguing fungi, also referred to as ‘class 1 or clavicipitaceous endophytes’, spend the large majority, or even their entire life cycle, within the tissues of their cool‐season grass hosts without eliciting any symptoms of infection. While all epichloae reside within the intercellular spaces of aboveground vegetative grass tissues, the species at the symbiotic extreme are known as Neotyphodium, and the intimacy of their interaction extends to the reproductive (flowering) stage. At this point, fungal filaments (hyphae) nondestructively invade the developing ovaries of their host and are incorporated into perfectly viable, healthy seeds. Thus, these endophytes live solely within the tissues of their host plants and are transmitted maternally from generation to generation. A second life history characteristic of interest is that while all Epichloë and some Neotyphodium species are haploid, a great many of the strictly seed‐transmitted Neotyphodium spp. are interspecific hybrids. This phenomenon may be critical for the success of these symbioses over longer spans of evolutionary time and will be discussed in greater detail below. A third characteristic, and one of the primary reasons these grass endophytes have received so much attention over the last three decades, is the strong mutualistic nature these relationships often exhibit. In exchange for photosynthetically derived carbon, the endophytes protect their cool‐season grass hosts from grazing herbivores and a variety of abiotic stresses. It has been hypothesized that these three biological phenomena are related ( Schardl & Craven 2003 ), perhaps with the former two driving the third, and it is here that the recent article in Molecular Ecology entitled ‘Genetic diversity in epichloid endophytes of Hordelymus europaeus suggests repeated host jumps and interspecific hybridizations’, by Oberhofer & Leuchtmann (2012) , provides critical clues to linking these traits together. While the large majority of studies have focused on documenting the ever‐increasing list of mutualistic qualities attributed to these fungi, very few have taken an exhaustive population‐level approach to document plant and endophyte genotypes within a naturally occurring system ( Faeth et al. 2010 ; Jani et al. 2010 ; Tintjer & Rudgers 2006 ). Such information is crucial to more fully elucidate the factors shaping grass‐endophyte symbioses and those often driving these relationships to mutualistic extremes.     

Two fungal symbioses collide: endophytic fungi are not welcome in leaf-cutting ant gardens

Interactions among the component members of different symbioses are not well studied. For example, leaf-cutting ants maintain an obligate symbiosis with their fungal garden, while the leaf material they provide to their garden is usually filled with endophytic fungi. The ants and their cultivar may interact with hundreds of endophytic fungal species, yet little is known about these interactions. Experimental manipulations showed that (i) ants spend more time cutting leaves from a tropical vine, Merremia umbellata, with high versus low endophyte densities, (ii) ants reduce the amount of endophytic fungi in leaves before planting them in their gardens, (iii) the ants'' fungal cultivar inhibits the growth of most endophytes tested. Moreover, the inhibition by the ants'' cultivar was relatively greater for more rapidly growing endophyte strains that could potentially out-compete or overtake the garden. Our results suggest that endophytes are not welcome in the garden, and that the ants and their cultivar combine ant hygiene behaviour with fungal inhibition to reduce endophyte activity in the nest.     

An endophyte‐rich diet increases ant predation on a specialist herbivorous insect

1. All plants form symbioses with microfungi, known as endophytes, which live within plant tissues. Numerous studies have documented endophyte–herbivore antagonism in grass systems, but plant–endophyte–insect interactions are highly variable for forbs and woody plants. 2. The net effect of endophytes on insect herbivory may be modified by their interactions with higher trophic levels, such as predators. Including these multitrophic dynamics may explain some of the variability among endophyte studies of non‐grass plants, which are currently based exclusively on bitrophic studies. 3. The abundance of natural foliar endophytes in a Neotropical vine was manipulated and beetles were fed high or low endophyte diets. Experimental assays assessed whether dietary endophyte load affected beetle growth, leaf consumption, and susceptibility to ant predation. 4. Beetles feeding on high‐ versus low‐endophyte plants had almost identical growth and leaf consumption rates. 5. In a field bioassay, however, it was discovered that feeding on an endophyte‐rich diet increased a beetle's odds of capture by predatory ants nine‐fold. 6. Endophytes could thus provide an indirect, enemy‐mediated form of plant defence that operates even against specialist herbivores. We argue that a multitrophic approach is necessary to untangle the potentially diverse types of endophyte defence among plants.     

Influence of insecticide treatments on ant-hemiptera associations in tropical plantations

In this survey conducted in southern Cameroon, we compared ant-Hemiptera associations on plantations treated with insecticides, on plantations 2 years after insecticide treatments ceased, and on control lots that never received insecticide treatments. By eliminating arboreal-nesting ants, insecticides favored the presence of "ecologically dominant" ground-nesting, arboreal-foraging species that occupied the tree crowns. The reinstallation of arboreal ants was slow as 2 yr after insecticide treatment ceased differences with the control lots were significant. This intermediary period also illustrated that arboreal ants can found and develop colonies on trees occupied by ground-nesting species. Certain arboreal species were more frequent during this intermediary period than on the control lots, showing that the period of installation in the trees was followed by competition between arboreal ants. We confirm that ground-nesting ants tend a wide range of hemipteran families, including well known agricultural pests, whereas arboreal ants, particularly dominant species, were mostly associated with Coccidae and Stictococcidae that do not pose problems to the supporting trees. A tree effect was also noted for both ant and hemipteran distribution. We concluded that because of insecticide use, ground-nesting ants pose problems through their associated Hemiptera. On the contrary, dominant arboreal ants, strong predators, benefit their supporting trees by excluding ground-nesting species and tending mostly nonpest Hemiptera. Nevertheless, certain of them, carpenter species or species likely to tend Pseudococcidae, have to be eliminated through integrated management.     

The sudden emergence of pathogenicity in insect-fungus symbioses threatens naive forest ecosystems

Invasive symbioses between wood-boring insects and fungi are emerging as a new and currently uncontrollable threat to forest ecosystems, as well as fruit and timber industries throughout the world. The bark and ambrosia beetles (Curculionidae: Scolytinae and Platypodinae) constitute the large majority of these pests, and are accompanied by a diverse community of fungal symbionts. Increasingly, some invasive symbioses are shifting from non-pathogenic saprotrophy in native ranges to a prolific tree-killing in invaded ranges, and are causing significant damage. In this paper, we review the current understanding of invasive insect-fungus symbioses. We then ask why some symbioses that evolved as non-pathogenic saprotrophs, turn into major tree-killers in non-native regions. We argue that a purely pathology-centred view of the guild is not sufficient for explaining the lethal encounters between exotic symbionts and naive trees. Instead, we propose several testable hypotheses that, if correct, lead to the conclusion that the sudden emergence of pathogenicity is a new evolutionary phenomenon with global biogeographical dynamics. To date, evidence suggests that virulence of the symbioses in invaded ranges is often triggered when several factors coincide: (i) invasion into territories with naive trees, (ii) the ability of the fungus to either overcome resistance of the naive host or trigger a suicidal over-reaction, and (iii) an 'olfactory mismatch' in the insect whereby a subset of live trees is perceived as dead and suitable for colonization. We suggest that individual cases of tree mortality caused by invasive insect-fungus symbionts should no longer be studied separately, but in a global, biogeographically and phylogenetically explicit comparative framework.     

Interaction between growth environment and host progeny shape fungal endophytic assemblages in transplanted Fagus sylvatica

Fungal endophytes are an integral part of the leaf microbiome of forest trees. Most of these endophytes are horizontally transmitted, however little is known about their assembly drivers. Endophytic assemblages differ in composition according to geography and host individuals. In addition, climate and genetic diversity are also reported to lead to host plant adaptation. To determine the impact of the host progeny and respective adaptation to environmental conditions on endophytic assemblages, we designed a transplantation experiment in beech trees (Fagus sylvatica). Beech nuts were collected from distant geographical regions and germinated in a common greenhouse. One-year-old beech seedlings were transplanted to the different locations and the leaf-endophytic assemblages were characterized in the second growth season after planting by cultivation-independent metabarcoding. The chlorophyll and flavonoid content of the respective leaves were also measured. The results revealed host progeny effects in shaping leaf-endophytic fungal assemblages, that might be concealed by major geographical effects. We hypothesise and discuss possible interactions of different assembly drivers.     

Endophytic fungi in forest trees: are they mutualists?

Forest trees form symbiotic associations with endophytic fungi which live inside healthy tissues as quiescent microthalli. All forest trees in temperate zones host endophytic fungi. The species diversity of endophyte communities can be high. Some tree species host more than 100 species in one tissue type, but communities are usually dominated by a few host-specific species. The endophyte communities in angiosperms are frequently dominated by species of Diaporthales and those in gymnosperms by species of Helotiales. Divergence of angiosperms and gymnosperms coincides with the divergence of the Diaporthales and the Helotiales in the late Carboniferous about 300 million years (Ma) ago, indicating that the Diaporthalean and Helotialean ancestors of tree endophytes had been associated, respectively, with angiosperms and gymnosperms since ≥300 Ma. Consequently, dominant tree endophytes have been evolving with their hosts for millions of years. High virulence of such endophytes can be excluded. Some are, however, opportunists and can cause disease after the host has been weakened by some other factor. Mutualism of tree endophytes is often assumed, but evidence is mostly circumstantial. The sheer impossibility of producing endophyte-free control trees impedes proof of mutualism. Some tree endophytes exhibit either a pathogenic or a putatively mutualistic behaviour depending on the situation. The lifestyle (mutualism, commensalism, parasitism) of most tree endophytes is, however, not known. They are just there in the tissue and resume growth at the onset of natural senescence of the host tissue on which they eventually sporulate. Density of colonization of conifer needles by endophytes increases with needle age. It is postulated that the needles die as soon as colonization density reaches a threshold value. Normally, the threshold is not reached before the onset of natural senescence. The threshold value may, however, be reached earlier under some adverse conditions, e.g. lack of light in dense stands. As a consequence, endophytes kill the needles prematurely. Needle endophytes could, thus, be useful to eliminate “parasitic” needle mass, i.e. needles which consume more than they produce.     

Cyclitol metabolism is a central feature of Burkholderia leaf symbionts

The symbioses between plants of the Rubiaceae and Primulaceae families with Burkholderia bacteria represent unique and intimate plant–bacterial relationships. Many of these interactions have been identified through PCR-dependent typing methods, but there is little information available about their functional and ecological roles. We assembled 17 new endophyte genomes representing endophytes from 13 plant species, including those of two previously unknown associations. Genomes of leaf endophytes belonging to Burkholderia s.l. show extensive signs of genome reduction, albeit to varying degrees. Except for one endophyte, none of the bacterial symbionts could be isolated on standard microbiological media. Despite their taxonomic diversity, all endophyte genomes contained gene clusters linked to the production of specialized metabolites, including genes linked to cyclitol sugar analog metabolism and in one instance non-ribosomal peptide synthesis. These genes and gene clusters are unique within Burkholderia s.l. and are likely horizontally acquired. We propose that the acquisition of secondary metabolite gene clusters through horizontal gene transfer is a prerequisite for the evolution of a stable association between these endophytes and their hosts.     

Wild trees in the Amazon basin harbor a great diversity of beneficial endosymbiotic fungi: is this evidence of protective mutualism?

It has been shown that the disappearance of, or drastic changes in, ancestral and indigenous (or native) endosymbiotic microbiota can lead to many adverse health consequences. However, the effects of changes in beneficial endosymbionts in plants are poorly known (except for mycorrhizal and rhizobial associations). We sampled and compared endophytes from hundreds of trees belonging to the economically important genus Hevea, the source of natural rubber, in their native range in the Amazon basin and in plantations. We also conducted antagonism tests to determine the potential effects that some of these endophytes may have on selected plant pathogenic fungi. The natural and indigenous endosymbiotic mycota of the rubber tree (Hevea) contains a high diversity of beneficial fungi that may protect against pathogens (protective mutualism). In contrast, plantation trees have a reduced and different diversity of these beneficial fungi. We propose that abundance, and not just presence, of competitive fungal strains and species (i.e., Trichoderma and Tolypocladium) create a protective effect against pathogens in wild trees. This study provides support for the importance of mutualistic endosymbionts in plant health and ecosystem resilience, and calls for awareness of their potential loss by human-related activities.     

Seed hairs of poplar trees as natural airborne pollen trap for allergenic pollen grains

The present article deals with the efficacy of seed hairs of poplar trees (Populus spp.) as a potent natural airborne pollen trap. Different species of Populus are commonly found planted along the streets in the cities of North China. The seed hairs and pericarp of poplar trees were collected from the trees and on the ground in Beijing Botanical Garden of Chinese Academy of Sciences and around Miyun Reservoir during May 2005 for pollen analysis. Different pollen spectra are recorded from different samples and are characterised by dominant occurrence of pollen grains of arboreal and anemophilous plants. In addition, pollen grains of non‐arboreal plants including grasses are also found trapped. Among the 46 trapped pollen grains, 26 are known to be allergenic. This study suggests that poplar seed hairs possibly make people feel uncomfortable due to the presence of allergenic pollen trapped in the hairs.     

Evolutionary origins and ecological consequences of endophyte symbiosis with grasses

Over the past 20 yr much has been learned about a unique symbiotic interaction between fungal endophytes and grasses. The fungi (Clavicipitaceae, Ascomycota) grow intercellularly and systemically in aboveground plant parts. Vertically transmitted asexual endophytes forming asymptomatic infections of cool-season grasses have been repeatedly derived from sexual species that abort host inflorescences. The phylogenetic distribution of seed-transmitted endophytes is strongly suggestive of cocladogenesis with their hosts. Molecular evidence indicates that many seed-transmitted endophytes are interspecific hybrids. Superinfection may result in hyphal fusion and parasexual recombination. Most endophytes produce one or more alkaloid classes that likely play some role in defending the host plant against pests. Hybridization may have led to the proliferation of alkaloid-production genes among asexual endophytes, favoring hybrids. The ergot alkaloid ergovaline, lolitrems, and lolines are produced by only a single sexual species, Epichlo? festucae, but they are common in seed-transmitted endophytes, suggesting that E. festucae contributed genes for their synthesis. Asexual hybrids may also be favored by the counteracting of the accumulation of deleterious mutations (Muller's rachet). Endophyte infection can provide other benefits, such as enhanced drought tolerance, photosynthetic rate, and growth. Estimates of infection frequency have revealed variable levels of infection with especially high prevalence in the subfamily Pooideae. Longitudinal studies suggest that the prevalence of seed-transmitted endophytes can increase rapidly over time. In field experiments, infected tall fescue suppressed other grasses and forbs relative to uninfected fescue and supported lower consumer populations. Unlike other widespread plant/microbial symbioses based on the acquisition of mineral resources, grass/endophyte associations are based primarily on protection of the host from biotic and abiotic stresses.     

Fungal endophyte–grass symbioses are rare in the California floristic province and other regions with Mediterranean-influenced climates

Research on agronomic grasses has shown that Class 1 fungal endophytes (Neotyphodium/Epichloë; Clavicipitaceae) can have profound effects on host plant fitness. However, in natural systems, even basic ecological knowledge of most endophyte symbioses is lacking. Here, I describe the distribution and abundance of endophytes across 36 native (or naturalized) grasses in a previously unsurveyed region, the California Floristic Province. Symbiosis was generally low: 8.33 % of species and 18.75 % of genera hosted endophytes. I then compared the proportions of symbiotic species and genera found in California and other Mediterranean regions to the proportions found in non-Mediterranean regions. Surveys of Mediterranean-influenced regions showed significantly lower proportions of species (～66 % lower) and genera (～65 % lower) hosting endophyte than surveys of non-Mediterranean regions. This pattern suggests that selection in Mediterranean climates may not favor endophyte symbioses.     

Novel and co-evolved associations between insects and microorganisms as drivers of forest pestilence

Some of the most devastating diseases of trees involve associations between forest insects and microorganisms. Although a small number of native insect-microorganism symbioses can cause tree mortality, the majority of associations with tree health implications involve one or more exotic organisms. Here, we divide damaging symbioses between forest insects and microorganisms into four categories based on the native/exotic status of the species involved: (1) insect and microorganism are native; (2) insect is native, microorganism is exotic; (3) insect is exotic, microorganism is native; and (4) insect and microorganism are both exotic. For each category, we describe several well-researched examples of forest insect symbioses and discuss some of the consequences of the types of interactions within each category. We then discuss priorities for research on forest insect symbioses that could help to further elucidate patterns in the complexity of such interactions in the context of invasion biology. We argue that a nuanced understanding of insect-pathogen relationships is lacking, even for the few well-studied examples. Because novel associations between insects, microorganisms, and trees are increasing with globalization, such symbioses and their potential to negatively impact forest ecosystems demand focused research in the future.     

An Ant Mosaic Revisited: Dominant Ant Species Disassemble Arboreal Ant Communities but Co-Occur Randomly

The spatial distributions of many tropical arboreal ant species are often arranged in a mosaic such that dominant species have mutually exclusive distributions among trees. These dominant species can also mediate the structure of the rest of the arboreal ant community. Little attention has been paid to how diet might shape the effects of dominant species on one another and the rest of the ant community. Here, we take advantage of new information on the diets of many tropical arboreal ant species to examine the intra- and inter-guild effects of dominant species on the spatial distribution of one another and the rest of the tropical arboreal ant community in a cocoa farm in Bahia, Brazil. Using null model analyses, we found that all ant species, regardless of dominance status or guild membership, co-occur much less than expected by chance. Surprisingly, the suite of five dominant species showed random co-occurrence patterns, suggesting that interspecific competition did not shape their distribution among cocoa trees. Across all species, there was no evidence that competition shaped co-occurrence patterns within guilds. Co-occurrence patterns of subordinant species were random on trees with dominant species, but highly nonrandom on trees without dominant species, suggesting that dominant species disassemble tropical arboreal ant communities. Taken together, our results highlight the often complex nature of interactions that structure species-rich tropical arboreal ant assemblages.     

Symbiotic control of canopy dominance in subtropical and tropical forests

Subtropical and tropical forests in Asia often comprise canopy dominant trees that form symbioses with ectomycorrhizal fungi, and species-rich understorey trees that form symbioses with arbuscular mycorrhizal fungi. We propose a virtuous phosphorus acquisition hypothesis to explain this distinct structure. The hypothesis is based on (i) seedlings being rapidly colonised by ectomycorrhizal fungi from established mycelial networks that generates positive feedback and resistance to pathogens, (ii) ectomycorrhizal fungi having evolved a suite of morphological, physiological, and molecular traits to enable them to capture phosphorus from a diversity of chemical forms, including organic forms, and (iii) allocation of photosynthate carbon from adult host plants to provide the energy needed to undertake these processes.     

Biological nitrogen fixation in non-leguminous field crops: Facilitating the evolution of an effective association between Azospirillum and wheat

Recent advances towards achieving significant nitrogen fixation by diazotrophs in symbioses with cereals are reviewed, referring to the literature on the evolution of effective symbioses involving rhizobia and Frankia as microsymbionts. Data indicating that strains of Acetobacter and Herbaspirillum colonizing specific cultivars of sugarcane as endophytes make a significant contribution to the nitrogen economy of this crop improves the prospects that similar associative systems may be developed for other gramineous species such as rice and wheat. By contrast, the transfer of nodulation genes similar to those in legumes or Parasponia to achieve nodulation in crops like rice and wheat is considered to be a more ambitious and distant goal. Progress in developing an effective associative system for cereals has been materially assisted by the development of genetic tools based on the application of lacZ and gusA fusions with the promoters of genes associated with nitrogen fixation. These reporter genes have provided clear evidence that crack-entry at the points of emergence of lateral roots or of 2,4-D induced para-nodules is the most significant route of endophytic colonization. Furthermore, using the laboratory model of para-nodulated wheat, there is now evidence that the ability of azospirilla and other nitrogen fixing bacteria to colonize extensively as endophytes can be genetically controlled. The most successful strain of Azospirillum brasilense (Sp7-S) for endophytic colonization and nitrogen fixation in wheat seedlings is a mutant with reduced exopolysaccharide production. Most other strains of azospirilla do not colonize as endophytes and it is concluded that though these are poorly adapted to providing nitrogen for the host plant, they are well adapted for survival and persistence in soil. A research program combining the study of endophytic colonization by azospirilla with an examination of the factors controlling the effectiveness of association (oxygen tolerance and nitrogen transfer) is now being pursued. It is proposed that a process of facilitated evolution of para-nodulated wheat involving the stepwise genetic improvement of both the prospective microsymbionts and the cereal host will eventually lead to effective nitrogen-fixing associations. In the attempt to achieve this goal, continued study of the endophytes occurring naturally in sugar cane and other grasses (e.g. Azoarcus sp.) should be of assistance.