ENDOPHYTE-HOST ASSOCIATIONS IN FORAGE GRASSES. VIII. HETEROTHALLISM IN EPICHLOË TYPHINA

Experiments conducted indicate the mating system in Epichloe typhina to involve simple bipolar heterothallism. Perithecia were formed when condia were transferred to a compatible stroma. Perithecial development followed the growth of mycelium over the surface of the stroma. The perithecial stroma consisted of two layers: an inner layer composed of the original mycelium of the conidial stroma and an outer layer that appears to be composed of mycelium growing from the inner stromal layer. Perithecia were initiated in the outer layer, just above the interface of the two layers. Natural onset of perithecial formation was associated with evidence of visitation by a prasitic fly (Phorbia phrenione). CONSIDERABLE ATTENTION has been focused recently on a group of clavicipitaceous fungi occurring endophytically in certain grass species of the subfamily Festucoidae (Latch, Christensen, and Samuels, 1984; White and Cole, 1985; White, 1987). These fungi have been implicated as the source of alkaloids which cause toxicosis in grazing cattle (Lyons, Plattner, and Bacon, 1986; White and Morgan-Jones, 1987). They have also been demonstrated to impart increased resistance to certain insect pests of grasses (Siegel, Latch, and Johnson, 1985; Clay, Hardy, and Hammond, 1985). Endophytic mycelium is present intercellularly in ground tissue of leaves and culms and is observable within carpels and caryopses. The endophytes are propagated vegetatively by growth of mycelium from the parental grass plant into the ovary, followed by penetration of the developing embryo within the caryopsis (Freeman, 1904; White and Cole, 1986). Endophytic mycelium of the ascomycete Epichloe typhina (Fr.) Tul. is widespread in certain grasses. Herbarium studies (White, 19 8 7) suggest that E. typhina is present in about half the individuals of Elymus canadensis L. in North America. The fungus is expressed externally (Fig. 1-10) as an ectophytic stroma, which is produced on the uppermost leaf sheath I Received for publication 24 October 1986; revision accepted 27 February 1987. The authors are grateful to Dr. Jerry Brand for use of materials and to the University of Texas for laboratory space at the Brackenridge Field laboratory where experiments were conducted. We also wish to acknowledge Drs. Philip M. Halisky and Gareth Morgan-Jones for their constructive reviews of the manuscript. 2 Current address: Dept. of Botany, Ohio State University, Columbus, OH 43210. (Fig. 1) of the culms of some of the endophytecontaining plants of E. canadensis (White, 1987). Occurrence of the stroma on the grass culm prevents emergence of the inflorescence and is commonly known as "choke disease" (Halisky, Saha, and Funk, 1985). Each spring a small percentage (1-10%) of the endophytecontaining plants of E. canadensis bear the ectophytic phase (Bultman and White, unpublished). In contrast, when Agrostis tenuis Sibth. and Dactylis glomerata L. are infected with E. typhina, a stroma always forms, resulting in the complete suppression of caryopsis formation by infected plants (Western and Cavet, 1959). In these grasses, spread of the endophyte appears to be limited to clonal growth of the host grass, with infection by ascospores an apparently rare event. Consequently, infected populations of these species are small and infrequently encountered (Bradshaw, 1959). While the endophyte reduces the fitness of infected individuals of A. tenuis and D. glomerata, it does not appear to substantially reduce fitness of infected individuals of E. canadensis. Similarly, the related endophytes of other grasses, such as Festuca arundinacea Schreb. and Lolium perenne L., have not been demonstrated to produce an ectophytic phase and thus apparently do not reduce fitness of those grasses (Latch et al., 1984; Siegel et al., 1985; White et al., 1987; White and Morgan-Jones, 1987). These exclusively vegetatively-reproducing forms have been referred to as nonchoke-inducing endophytes (Halisky et al., 1985). The close phylogenetic relationship of the nonchoke-inducing endophytes and the choke-inducing E. typhina has been inferred from the morphological similarities of the conidial states in vitro and on the common en-