Imperfect vertical transmission of fungal endophytes in native grass species

The fixation of vertically-transmitted endosymbionts has led to major evolutionary transitions such as the evolution of mitochondria and chloroplast.  We use the facultative symbiosis between native grasses and vertically-transmitted fungal endophytes to explore ecological factors influencing the fixation of vertical transmission.  In grasses, endophyte are vertically transmitted from maternal plants to seeds, and imperfect transmission can occur at three life history transitions: within individual tillers of the maternal adult, at the maternal adult-seed transition, and/or at the seed-seedling transition.  We collected individual plants from two or more populations of eight native grass species and followed endophyte infection in the seeds and seedling each plant produced.  Imperfect vertical transmission as well as significant differences among infection frequencies at the adult, seed, and seedling stages were found in at least one population of all species.  Although most of the imperfect vertical transmission occurred between the adult tiller and seed stages, the mode of imperfect vertical transmission varied among grass species and among populations within the same species.  For example, we observed perfect transmission in one population of Festuca subverticillata and two different types of imperfect vertical transmission in two other populations.  We are currently exploring how imperfect vertical transmission in F. subverticillata responds to biotic and abiotic stress with factorial design greenhouse experiments manipulating endophyte infection, drought, and herbivory.  We expect that the endophyte will confer drought tolerance and resistance to herbivory and will be transmitted at a greater frequency when plants are exposed to these stresses.