Asexual Fungal Symbionts Alter Reproductive Allocation and Herbivory over Time in Their Native Perennial Grass Hosts

Although most plants harbor microbial symbionts, the temporal effects of symbionts on resource allocation and herbivore resistance of perennial hosts are unknown. Neotyphodium endophytes are asexual, vertically transmitted fungal symbionts of grasses that are thought to interact mutualistically with their hosts, mainly by deterring herbivores. To test age-specific effects of Neotyphodium infection and herbivory on resource allocation, I conducted a 4-year field experiment with four genotypes of an infected perennial native grass from which the endophyte was removed and for which herbivory and water availability were controlled. In the absence of herbivory, infection increased allocation to reproductive effort in the first two growing seasons. Infected plants also flowered earlier. Herbivory increased allocation to reproduction in the first year but much more so in infected than in uninfected plants. Infected plants also had greater herbivore loads in early stages, suggesting that infected plants are more tolerant to herbivory. Asexual fungal symbionts thus fundamentally alter host resource allocation and resistance and tolerance to herbivores over time. Increased reproductive effort in early host ontogeny should benefit the symbiont by increasing transmission but perhaps at the expense of lifetime host fitness. If so, then the conventional notion of asexual endophytes as mutualistic hostages of their hosts is incorrect.