Cycloheximide-Producing Associated With and Fungus-Farming Ambrosia Beetles.

Symbiotic microbes help a myriad of insects acquire nutrients. Recent work suggests that insects also frequently associate with actinobacterial symbionts that produce molecules to help defend against parasites and predators. Here we explore a potential association between Actinobacteria and two species of fungus-farming ambrosia beetles, and . We isolated and identified actinobacterial and fungal symbionts from laboratory reared nests, and characterized small molecules produced by the putative actinobacterial symbionts. One 16S rRNA phylotype of (XylebKG-1) was abundantly and consistently isolated from the galleries and adults of and nests. In addition to , the symbiont that cultivates, we also repeatedly isolated a strain of sp. that is an antagonist of this mutualism. Inhibition bioassays between XylebKG-1 and the fungal symbionts from revealed strong inhibitory activity of the actinobacterium toward the fungal antagonist sp. but not the fungal mutualist Bioassay guided HPLC fractionation of XylebKG-1 culture extracts, followed by NMR and mass spectrometry, identified cycloheximide as the compound responsible for the observed growth inhibition. A biosynthetic gene cluster putatively encoding cycloheximide was also identified in XylebKG-1. The consistent isolation of a single 16S phylotype of from two species of ambrosia beetles, and our finding that a representative isolate of this phylotype produces cycloheximide, which inhibits a parasite of the system but not the cultivated fungus, suggests that these actinobacteria may play defensive roles within these systems.