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

Abstract

The fungal endophyte Epichloë sylvatica (Clavicipitaceae, Ascomycota) may obligatorily 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 disease), whereas in the asexual cycle the fungus remains asymptomatic and transmits vertically by seeds. Variation in the reproductive system thus determines whether the symbiosis is mutualistic or parasitic. In order to assess the relative effects of each genotype on fungal reproduction, we used naturally infected seed families of B. sylvaticum and experimentally infected plants with different combinations of plant and fungal genotypes. The results of one experiment suggested a maternal effect of the host association on 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 also be responsible for whether an endophyte association is beneficial or pathogenic. We discuss the results in the light of current theories about the evolution of mutualism and the maintenance of sex.