HEREDITARY SYMBIOSIS IN THE GRASS GENUS DANTHONIA

Hereditary symbiosis between the systemic, clavicipitaceous fungus Atkinsonella hypoxylon (Pk.) Diehl and five taxa of eastern North American Danthonia (Poaceae) was investigated by cultural and histological techniques. Many species of Danthonia, including D. alleni (or D. spicata var. alleni), D. compressa, D. epilis, D. sericea, and D. spicata produce both terminal panicles of wind-pollinated florets and obligately self-pollinated cleistogamous florets in the lower leaf axils, unlike most grasses. Terminal florets are aborted by infection while the basal florets produce viable, infected seeds (caryopses). The fungus grew out of surface-sterilized cleistogamous seeds and seedlings at high frequency in progenies from infected plants but not from uninfected plants. Infection of seeds from terminal panicles from apparently healthy plants was lacking in 11 of 13 population samples from all taxa and was infrequent in two other populations. Growth chamber tests indicated that germination rates of infected and uninfected seeds of D. epilis were similar. Microscopic observations of sections of seeds and seedlings revealed that the fungus occurred epiphytically on meristems, in leaf axils, and on leaf primordia. Because infected plants are partially fertile, the fungus may be repeatedly transmitted for many generations via self-fertilized cleistogamous seeds. The basal, and often subterranean, location of cleistogamous seeds may facilitate high infection frequencies in those Danthonia populations where cleistogamous seedling establishment predominates.