Mutualism between fly and fungus depends on fungal reproductive modes

Background/Question/Methods Botanophila flies act as vectors of spermatial spores of sexually reproducing Epichloë fungi, and were thought to be the primary means by which cross fertilization of the fungi occurred; yet, recent studies on this system have called this assertion into question. We hypothesized that the discrepancy between earlier and more recent studies is due to different types of reproductive modes used by different species of Epichloë. Specifically, we predict that type I fungi (which only reproduce sexually) should depend less on flies for spore transfer, as they risk not reproducing if the fly fails as a vector. In contrast, type II fungi (which reproduce both sexually and asexually) should depend more upon flies given that they have a “backup” mode of reproduction (asexual). We carefully observed the interaction between Botanophila and Epichloë  typhina (a type I fungus) at our field site in the forest of Phalempin, France to determine the level of dependence between fly and fungus.

Results/Conclusions Feeding by both the larvae and other fungivores (primarily slugs) was quite high on both conidial and perithecial tissue of the fungus and increased over the study period. Unlike past studies from our lab (on type II Epichloë spp.) that showed Botanophila flies were the main vector effecting cross fertilization, we found no net benefit to the fungus of having flies visit. This is consistent with more recent studies (on type I Epichloë) by other labs and with our hypothesis that mutualism with Botanophila is based on reproductive mode of Epichloë.