Shifting Costs and Benefits of Systemic Endophyte Interactions With Native Grass Hosts

Asexual, seed born endophytes in the genus Neotyphodium are well known for conferring a wide range of benefits to agronomic grass hosts, like tall fescue and perennial ryegrass. These benefits include increased resistance to drought, vertebrate and invertebrate herbivores and seed predators, pathogens and root feeders as well as increasing germination rates, nutrient uptake, and allelopathic interactions with competitors. The role of systemic endophytic infections in native pooid grasses, however, is largely unknown. Many native pooid grasses are infected, but infection frequencies are much more variable within and between populations and between species than in agronomic grasses. From past NSF support, we have tested multiple conventional hypotheses regarding the maintenance and cost and benefits of Neotyphodium infections in Arizona fescue (Festuca arizonica), a widespread native grass. Our results indicate that the endophyte does not generally provide the aforementioned benefits to the host under typical, ambient environments. However, the endophyte does increase growth, reproduction and competitive abilities, but only at the extreme ranges of water availability. We propose here to continue this research with Arizona fescue and another highly infected native grass, sleepy grass (Achnatherum robustum). Our overarching hypothesis is that the costs and benefits of Neotyphodium infection depend on plant and endophyte genotypes, water availability, and herbivory. Genotype and environmental factors, in turn, influence alkaloid production, which is the major avenue for costs and benefits of infection. We will implement two major field experiments that control plant genotype and endophyte infection, soil moisture levels and herbivory by invertebrates and vertebrates. We will also test three other novel and unconventional hypotheses to explain variability in infections in natural populations. These include tests of transmission mode of the endophyte, how infection alters sex ratio of the host plant, and whether Neotyphodium infections allow more pathogenic microbes to invade the host. This research is among the first to comprehensively test the cost and benefits of systemic and symbiotic endophytes in non-agronomic, native grass species. The research addresses basic questions of how species interactions, especially mutualisms, vary with genotype and environments. Although basic in nature, the research has important implications for restoration and maintenance of native semi-arid grasslands, especially those altered by anthropogenic factors.