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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,  Xyleborinus saxesenii  and  Xyleborus affinis . 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  Streptomyces  (XylebKG-1) was abundantly and consistently isolated from the galleries and adults of  X. saxesenii  and  X. affinis  nests. In addition to  Raffaelea sulphurea , the symbiont that  X. saxesenii  cultivates, we also repeatedly isolated a strain of  Nectria  sp. that is an antagonist of this mutualism. Inhibition bioassays between  Streptomyces griseus  XylebKG-1 and the fungal symbionts from  X. saxesenii  revealed strong inhibitory activity of the actinobacterium toward the fungal antagonist  Nectria  sp. but not the fungal mutualist  R. sulphurea.  Bioassay guided HPLC fractionation of  S. griseus  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  S. griseus  XylebKG-1. The consistent isolation of a single 16S phylotype of  Streptomyces  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.

Cycloheximide-Producing Streptomyces Associated With Xyleborinus saxesenii and Xyleborus affinis Fungus-Farming Ambrosia Beetles