The distribution of the Epiphytic fungus Atkinsonella texensis and its effects on the performance of its plant host, Nassella leucotricha
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I investigated the distribution of the epiphytic fungus Atkinsonella texensis and its effects on the performance of its plant host, Nassella leucotricha. I conducted surveys to determine the effects of small-scale heterogeneity and host plant density on the rate of disease incidence in the host plant population. In all three sites surveyed, the incidence of disease was highest beneath woody canopies and lowest in the open. Where Nassella was growing beneath woody plants, there was a positive relationship between the incidence of infection and Nassella density. In contrast, no relationship was detected between Nassella density and the incidence of infection in the open grassy areas. The effects of A. texensis on plant size and resistance to insect herbivores were studied in a natural population of Nassella. Infection had no effect upon the proportion of leaves damaged by grasshoppers, suggesting that the alkaloids produced by A. texensis do not spread throughout the plant. Infection significantly increased host plant vegetative size (leaf number), perhaps by diverting resources normally used for plant reproduction to vegetative growth. Relative amounts of herbivory, however, were not higher on these larger, infected plants. The effects of A. texensis on its host’s reproduction, size, resource allocation patterns, competitive abilities, and tolerance of herbivory were studied in a greenhouse experiment. Atkinsonella texensis sterilized Nassella; infected plants produced fungal stromata in place of inflorescences. Infection was found to have no effect on the total above-ground biomass produced by Nassella. Instead, infection altered resource allocation: infected pairs allocated less to fungal reproduction than uninfected plants did to plant reproduction. As a result, infected plants produced more vegetative biomass than uninfected plants. The effect of simulated herbivory was independent of the effects of infection and competition on Nassella. Because infection also did not reduce the amount of herbivore damage in the field, infection appears to have no beneficial effects on Nassella. Therefore, A. texensis is a parasite, unlike many of its close relatives. The relationship between Nassella and A. texensis may represent the earliest stage in the evolution of the mutualisms that now exist between similar fungi and their plant hosts.