The cost of mutualism in a fly-fungus interaction.

Botanophila flies act as "pollinating" parasites of the ascomycetous fungus, Epichloë elymi. Flies transfer fungal spermatia (gametes) among fungi as they visit their hosts for oviposition. Fly larvae consume the products of cross-fertilization (ascospores). We tested whether the cost to the fungus of engaging in the obligate mutualism rises as fly visitation increases and whether mechanisms operate to prevent excessive exploitation of the fungus. Fungi and flies were monitored over 3 years. We recorded the reproductive output of fungi, the amount of feeding by fly larvae on host reproductive tissues, and the mortality of fly eggs and larvae. In two of three years, fly eggs were randomly dispersed; eggs were clumped in the remaining year. The reproductive output of fungi did not decrease with increasing egg load; rather, fungal reproductive output tended to increase as more eggs were laid on fungi. Larval feeding was only weakly associated with the number of eggs on fungi. The lack of over-exploitation of the fungus by the fly shows that the interaction was stable during the period of time we conducted our study. Our data suggest the stability was primarily due to high fly egg/larval mortal-ity that increased as egg load increased.