Keith Clay

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, Epichloë 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 (Mullers 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.

Soil pathogens and spatial patterns of seedling mortality in a temperate tree.

The Janzen–Connell hypothesis proposes that host-specific, distance- and/or density-dependent predators and herbivores maintain high tree diversity in tropical forests. Negative feedback between plant and soil communities could be a more effective mechanism promoting species coexistence because soil pathogens can increase rapidly in the presence of their host, causing conditions unfavourable for local conspecific recruitment. Here we show that a soil pathogen leads to patterns of seedling mortality in a temperate tree (Prunus serotina) as predicted by the Janzen–Connell hypothesis. In the field, the mean distance to parent of seedling cohorts shifted away from maternal trees over a period of 3 years. Seedlings were grown in soil collected 0–5 m or 25–30 m from Prunus trees. Sterilization of soil collected beneath trees improved seedling survival relative to unsterilized soil, whereas sterilization of distant soil did not affect survival. Pythium spp., isolated from roots of dying seedlings and used to inoculate healthy seedlings, decreased survival by 65% relative to controls. Our results provide the most complete evidence that native pathogens influence tree distributions, as predicted by the Janzen–Connell hypothesis, and suggest that similar ecological mechanisms operate in tropical and temperate forests.

Fungal Endophytes of Grasses: A Defensive Mutualism between Plants and Fungi

Many grasses are infected by systemic fungal endophytes (family Clavicipitaceae, Ascomycetes) that produce pysiologically active alkaloids in the tissues of their hosts. Infection makes grasses toxic to domestic mammals and increases resistance to insect herbivores. Some grasses are sterilized by endophyte infection while remaining vegetatively vigorous; other infected grasses remain completely fertile. Experiments demonstrate that plant growth and seed production can be increased by infection. This symbiotic association may be a defensive mutualism in which the fungi defend their hosts against herbivory, thereby defending their own resources. Recent studies suggest that defensive mutualism of endophytes with grasses may be widespread.

GRASSROOTS ECOLOGY: PLANT–MICROBE–SOIL INTERACTIONS AS DRIVERS OF PLANT COMMUNITY STRUCTURE AND DYNAMICS

A growing body of research on plant-microbe interactions in soil is con- tributing to the development of a new, microbially based perspective on plant community ecology. Soil-dwelling microorganisms are diverse, and interactions with plants vary with respect to specificity, environmental heterogeneity, and fitness impact. Two microbial pro- cesses that may exert key influences on plant community structure and dynamics are mi- crobial mediation of niche differentiation in resource use and feedback dynamics between the plant and soil community. The niche differentiation hypothesis is based on observations that soil nutrients occur in different chemical forms, that different enzymes are required for plant access to these nutrients, and that soil microorganisms are a major source of these enzymes. We predict that plant nutrient partitioning arises from differential associations of plant species with microbes able to access different nutrient pools. Feedback dynamics result from changes in the soil community generated by the specificity of response in plant- microbe interactions. We suggest that positive feedback between plants and soil microbes plays a central role in early successional communities, while negative feedback contributes both to species replacements and to diversification in later successional communities. We further suggest that plant-microbe interactions in the soil are an important organizing force for large-scale spatial gradients in species richness. The relative balance of positive feedback (a homogenizing force) and negative feedback (a diversifying force) may contribute to observed latitudinal (and altitudinal) diversity patterns. Empirical tests of these ideas are needed, but a microbially based perspective for plant ecology promises to contribute to our understanding of long-standing issues in ecology, and to reveal new areas of future research.

Hybridization and the colonization of novel habitats by annual sunflowers

Although invasive plant species often have a hybrid ancestry, unambiguous evidence that hybridization has stimulated the evolution of invasive behaviors has been difficult to come by. Here, we briefly review how hybridization might contribute to the colonization of novel habitats, range expansions, and invasiveness and then describe work on hybrid sunflowers that forges a direct link between hybridization and ecological divergence. We first discuss the invasion of Texas by the common sunflower and show that the introgression of chromosomal segments from a locally adapted species may have facilitated range expansion. We then present evidence that the colonization of sand dune, desert floor, and salt marsh habitats by three hybrid sunflower species was made possible by selection on extreme or “transgressive” phenotypes generated by hybridization. This body of work corroborates earlier claims regarding the role of hybridization in adaptive evolution and provides an experimental and conceptual framework for ongoing studies in this area.

Effects of Insect Herbivory and Fungal Endophyte Infection on Competitive Interactions among Grasses

Interactions among plants may be influenced by pests or parasites that dif- ferentially affect one competitor. The purpose of this study was to determine the effects of fungal parasitism and insect herbivory, alone and in concert, on plant competitive inter- actions. The effects of fungal endophyte (Acremonium spp.) infection and fall armyworm (Spodoptera frugiperda) herbivory on competitive interactions in one- and two-species mixtures of the grasses tall fescue (Festuca arundinacea), red fescue (F. rubra), and perennial ryegrass (Lolium perenne) were examined in greenhouse experiments. In general, herbivory reduced plant biomass whereas endophyte infection increased plant biomass. Endophyte- infected (E+) plants were less damaged by herbivory than uninfected (E-) plants of the same species. Studies on fall armyworm larval feeding and choice tests with the five grass species were generally consistent with the outcome of the competition experiments; E+ plants were less nutritious and less preferred than E- plants of the same species. There were significant interactions among factors so that the outcome of competition depended on the species identities and the presence or absence of endophytes and herbivores. In competition with Kentucky bluegrass (Poa pratensis), E+ and E- perennial ryegrass pro- duced similar biomass in the absence of herbivory, but E+ perennial ryegrass had nearly twice the biomass of E- plants when herbivores were present. E+ and E- tall fescue were poor competitors with orchard grass (Dactylis glomerata) when herbivores were absent, but E + tall fescue was a better competitor than E - plants and orchard grass when herbivores were present. This study indicates that competitive hierarchies among grasses are altered by interactions with insect herbivores and fungal endophytes, which have typically been ignored in past studies.

Effects of fungal endophytes on the seed and seedling biology of Lolium perenne and Festuca arundinacea

SummaryMany grasses are infected by endophytic fungi that grow intercellularly in leaves, stems, and flowers and are transmitted maternally by hyphal growth into ovules and seeds. The seed biology and seedling growth of endophyte-infected and uninfected perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) were investigated under controlled environmental conditions. The percentage of filled seeds produced by infected tall fescue was over twice of uninfected tall fescue; infected and uninfected perennial reegrass had similar percentages. Weights of seeds from infected and uninfected plants were similar in both species. Seeds from infected plants of both species exhibited a higher rate of germination than seeds from uninfected plants. Shoot growth in the greenhouse was compared by making three sequential harvests of above-ground plant parts from infected and uninfected plants of both species. Infected perennial ryegrass plants produced significantly more biomass and tillers than uninfected plants after 6 and 10 weeks of growth and significantly more biomass after 14 weeks of growth. Infected tall fescue plants produced significantly more biomass and tillers than uninfected plants after 10 and 14 weeks of growth. The physiological mechanism of enhancement of growth is not known. The results of this study suggest that infected plants may have a selective advantage in populations with uninfected members.

Conspecific Negative Density Dependence and Forest Diversity

Keep Your Distance Conspecific negative density dependence (CNDD), whereby the abundance of a species is limited by negative interactions between individuals of the same species, is thought to have an important influence on the composition and dynamics of forest communities, but studies have generally been limited to few species and small areas. Johnson et al. (p. 904) analyzed CNDD in over 200,000 plots from a database of more than 3 million individuals of 151 species spanning 4 million square kilometers across forests in the eastern United States and found that the strength of CNDD strongly predicted the relative abundance of tree species. Because tree seedlings are unlikely to become established where conspecific adults are common, CNDD may provide a general mechanism maintaining diversity in forests. Tree seedlings have a harder time establishing themselves in forests containing many adults of the same species. Conspecific negative density-dependent establishment, in which local abundance negatively affects establishment of conspecific seedlings through host-specific enemies, can influence species diversity of plant communities, but the generality of this process is not well understood. We tested the strength of density dependence using the United States Forest Service’s Forest Inventory and Analysis database containing 151 species from more than 200,000 forest plots spanning 4,000,000 square kilometers. We found that most species experienced conspecific negative density dependence (CNDD), but there was little effect of heterospecific density. Additionally, abundant species exhibited weaker CNDD than rarer species, and species-rich regions exhibited stronger CNDD than species-poor regions. Collectively, our results provide evidence that CNDD is a pervasive mechanism driving diversity across a gradient from boreal to subtropical forests.

Clavicipitaceous endophytes of grasses: Their potential as biocontrol agents

Many grasses harbour systemic clavicipitaceous endophytes in their leaves, stems, and seeds. Laboratory experiments and field observations have shown that infected plants are often toxic to livestock and more resistant to insect herbivores than uninfected conspecifics. Recently developed inoculation techniques allow artificial infection of grasses and the development of new varieties with high levels of endophyte infection. The fungi have applied potential as biocontrol agents against insect pests of grasses although their utilization may be limited to non-pasture situations. Exploitation of this naturally occurring symbiosis may obviate the need for chemical pesticides in managed grasslands.

Fungal endophytes of grasses and their effects on an insect herbivore

SummaryThe effects of endophytic fungi (Tribe Balansiae, Clavicipitaceae, Ascomycetes) of grasses on an insect herbivore were studied by feeding paired groups of larvae of the fall armyworm (Spodoptera frugiperda, Noctuidae, Lepidoptera) leaves from either infected or uninfected individuals. Perennial ryegrass infected by “the Lolium endophyte”, tall fescue infected by Epichloe typhina, dallisgrass infected by Myriogenospora atramentosa, Texas wintergrass infected by Atkinsonella hypoxylon, and sandbur infected by Balansia obtecta were utilized. The endophytes of ryegrass and fescue previously have been shown to be toxic to mammalian herbivores and to deter feeding of some insect herbivores. In this study we extend the antiherbivore properties of those endophytes to the fall armyworm and demonstrate that fungal endophytes in three other genera have similar antiherbivore properties. For most grasses, survival and weights of fall armyworm larvae fed infected leaves were significantly lower and larval duration was significantly longer compared to larvae fed uninfected leaves. Resistance to herbivores may provide a selective advantage to endophyte-infected grasses in natural populations.