Multistrain infections of the grass Brachypodium sylvaticum by its fungal endophyte Epichloe sylvatica

Endophytes of the genus Epichloe (Clavicipitaceae, Ascomycota) are systemic symbionts of cool-season grasses. Their interactions with grass hosts may vary between mutualistic and pathogenic depending on the mode of endophyte reproduction. Sexual strains prevent flowering and seed set (choke disease) of the host and can be horizontally transmitted by ascospores, while asexu al strains remain asymptomatic and are vertically transmitted through seeds . In Switzerland nearly all plants of Brachypodium sylvaticum (Huds.) P.B. are infected by Epichloe sylvatica Leuchtmann & Schardl, but choke symptoms are formed very rarely, and are restricted to particular locations and to a minority of plants at those locations. Earlier research has revealed that E, sylvatica is genetically differentiated into sexual and asexual subpopu lations. Given the high level of infection and assuming horizontal transmis sion of sexual strains, multiple host infections have been predicted. In th is study, 25 plants out of 63 examined by isozyme analysis were found to be infected by two Or three different endophyte genotypes. In most cases endo phyte genotypes appeared to be correlated with the symptom type of a partic ular tiller, suggesting that the fungal genome controls choke formation and that the sexual and asexual subpopulations are separated at the ramet (til ler) level rather than at the genet (plant) level. These conclusions were f urther supported by analyses with log-linear models of the population struc ture of E. sylvatica at four locations where choke symptoms were present. T hese analyses also revealed a geographic structure in the asexual subpopula tion but not in the sexual subpopulation which could be caused by the diffe rent dispersal ranges of their propagules. The rare occurrence of sexually reproducing strains and the dominance of a single genotype in asymptomatic plant populations may be explained by the colonization history of B. sylvat icum and its endophyte in Switzerland.