EVIDENCE FOR GENETIC DIFFERENTIATION BETWEEN CHOKE-INDUCING AND ASYMPTOMATIC STRAINS OF THE EPICHLOE GRASS ENDOPHYTE FROM BRACHYPODIUM-SYLVATICUM

Life cycle and breeding system variation in Epichloe grass endophytes (choke disease) is tightly linked to the degree of stroma formation. I t is not known whether this variation results from differences in host resistance, fungal virulence, or environmental conditions. We found g enetic differentiation between 173 asymptomatic (NS) and 93 stromata-f orming (S) Epichloe strains isolated from one grass species, Brachypod ium sylvaticum, based on 13 presumed allozyme loci, of which six were variable. The fungal strains originated from 10 sites in Switzerland, three sites of which were represented by both NS and S subpopulations. In total, 19 allozyme genotypes, that were nonrandomly distributed am ong S and NS were detected. Genetic variation measured as G(ST) betwee n S and NS strains isolated from the same site ranged from 0.73 to 0.9 8. Clonality, measured as linkage disequilibrium at one site, was sign ificant in the NS subpopulation (P much less than 0.001), but not in t he S subpopulation (P = 0.21), implying asexual reproduction by NS str ains as well as successful horizontal transmission of S strains. Since all seeds are usually infected vegetatively, horizontal transmission implies the occurrence of multiple host infections. Altogether, these results provide indirect evidence that NS and S strains do not belong to one panmictic population and that differentiation patterns of strom a formation found in nature are due to genetic differences among fungi in associations with their host plants. We discuss the direction of e volution of disease expression in this system. The distribution of gen etic variability suggests that the asymptomatic strains were derived f rom stromata-forming populations.