Mutualistic transfer of Epichloë spermatia by Phorbia flies

Abstract

We studied the interactions between insects and the spermatial stremata of Epichloë spp. they visit in North America and Great Britain. We found that Phorbia flies formed a close association with Epichloë at all study sites. Adult flies visited Epichloë stromata for feeding and egg laying. Stromata upon which Phorbia deposited eggs produced perithecia more frequently than those that lacked eggs. Furthermore, when we accounted for feeding on perithecia, stromata possessing eggs produced 5–7 times more perithecia as those lacking Phorbia eggs. Excluding insects from spermatial stromata with mesh bags prevented perithecia from being produced. A laboratory test gave no support for the hypothesis that spermatia are transferred by wind. Field observations of natural stromata revealed little visitation by non- Phorbia insects. Laboratory and field observations of Phorbia on stromata showed that flies display a stereotypical behavior in which they drag their abdomen immediately following deposition of an egg. Microscopic inspection of fly feces revealed that spermatia pass through the gut of the fly intact. Our results show that Phorbia is the primary vector of Epichloë spermatia and that the mode of transfer is likely through the gut of the fly.

Key Words:

• Epichloë
• Great Britain
• insect-fungus interactions
• North America
• Phorbia
• spermatia