Biology of the Epichloë–Botanophila interaction: An intriguing association between fungi and insects

Epichloë fungi (Ascomycota: Clavicipitaceae) are endophytes of grasses that can produce epiphytic stromata on the culms of their hosts. The fungal stromata are visited by Botanophila flies for feeding and egg laying. We review research that has documented the heterothallic mating system of Epichloë, the mutualistic service of spermatization flies provide for the fungus, and host selection by flies. Flies display an active, stereotypical behavior immediately following oviposition by which spermatia are transferred endozoochorously to stromata. After eggs hatch larvae feed on developing perithecia. Several studies have focused on the cost (consumption of ascospores) to the fungus of engaging in the mutualism with its insect visitors. Generally, researchers have found benefits outweigh costs suggesting the mutualism is stable, however exceptions have been reported for Epichloë infecting some commercially cultivated grasses. Because Epichloë fungi are obligate outcrossers and flies are the major (perhaps only) vector of spermatia, the possibility exists that flies could promote reproductive isolation among Epichloë species through specific behavior. This idea has been tested using different approaches including observations of flies within screened cages containing Epichloë species and an analysis of fly gut content, which revealed no or very moderate selectivity by flies. Volatile fungal compounds are responsible for fly attraction and differ among Epichloë species. However, in a field bioassay using species-specific blends of the two predominant volatiles – a sesquiterpene alcohol (chokol K) and a methyl ester – flies showed no preference for specific blends. That is, flies do not appear able to distinguish between different fungal species based on their odor profiles. Thus, it appears that the flies' role in maintaining reproductive isolation among Epichloë species may be minor and that mechanisms of post-zygotic reproductive isolation are more important in keeping compatible species distinct. However, ethological mechanisms such as ‘stroma constancy’ favoring intraspecific mating may operate at a local scale and need to be investigated further.