Fungal genotype controls mutualism and sex in Brachypodium sylvaticum infected by Epichloe sylvatica

The fungal endophyte Epichole sylvatica (Clavicipitaceae, Ascomycota) may o bligatorily infect the woodland grass Brachypodium sylvaticum, on which it can display two alternative modes of reproduction. During the sexual cycle, external stromata suppress host flowering and production of seed (choke di sease), whereas in the asexual cycle the fungus remains asymptomatic and tr ansmits vertically try seeds. Variation in the reproductive system thus det ermines whether the symbiosis is mutualistic or parasitic. In order to asse ss the relative effects of each genotype on fungal reproduction, we used na turally infected seed families of B. sylvaticum and experimentally infected plants with different combinations of plant and fungal genotypes. The resu lts of one experiment suggested a maternal effect of the host association o n the choke rate in the offspring, while the results of a second experiment clearly indicated that the fungal genotype determines stroma formation and thus the mode of reproduction, Since sexual reproduction of the fungus is closely tied with disease expression on the host, the fungal genotype may a lso be responsible for whether an endophyte association is beneficial or pa thogenic. We discuss the results in the light of current theories about the evolution of mutualism and the maintenance of sex.