The ecology and evolution of endophytes

Abstract

The association between tall fescue and its endophyte is the best-known case of a symbiosis that is widespread in grasses, especially in cool-season, pooid grasses. Field and herbarium surveys have shown that many grasses are endophyte infected, often at 100% frequency. Ubiquitos endophyte infection with a host species is prima facie evidence that the association is mutualistic since there are mechanisms by which individuals could lose the infection (e.g. extended seed dormancy). Studies have shown that the frequency of endophyte infection tends to increase over time, suggesting an ecological advantage for infected (E+) plants. A 3 year demographic experiment with tall fescue in a grassland community in Louisiana revealed that E+ plants had significantly higher survival growth and flowering rates than E-plants. The mean fitness of E+ ball fescue was twice the mean fitness of E-plants. Greenhouse and common garden experiments have shown that E+ plants generally perform better than E-plants in mixtures. In order to separate the effects of endophyte-induced physiological changes in hosts versus protection against herbivory, a series of factorial competition experiments have been conducted where infection status, insect herbivory, density, and the identity of competing species were controlled. Preliminary results indicate that both infection and herbivory can affect competitive interactions among plants. The high frequency of seed-borne Acremonium endophytes within species of genera like Festuca, Lolium, Poa, and Stipa suggests that the association originated before speciation within these genera. The association of clavicipitaceous fungi with grasses spans a great range of interactions from localized ovarian parasites (e.g. Claviceps), systemic, choke-inducing fungi (e.g. Atkinsonella, Balansia, Epichloë), epiphytic fungi ( Atkinsonella, some species of Balansia, Myriogenospora), and completely seed-borne, asymptomatic endophytes such as in Lolium. The question of what type of associations are ancestral and which are derived remains unanswered. The potential of these fungi for contagious spread and the presence or absence of a sexual reproductive system may influence genetic variation within taxa, and therefore their evolution and coevolution with grasses. Seed-borne Acremonium endophytes do not spread contagiously or reproduce sexually. They are reproductively isolated from other endophytes and can become genetically differentiated by the accumulation of random mutations. Isozyme electrophoresis has revealed abundant genetic variation in many Acremonium endophytes from wild grasses but, in contrast, artificial selection of cultivated grasses apparently has eliminated much genetic variation of endophytes. Future research on endophytes will focus on quantifying genetic variation for important traits such as alkaloid production, stroma formation, and host compatability.