AUTHOR=Grubbs Kirk J. , Surup Frank , Biedermann Peter H. W. , McDonald Bradon R. , Klassen Jonathan L. , Carlson Caitlin M. , Clardy Jon , Currie Cameron R. 

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

JOURNAL=Frontiers in Microbiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2020.562140

DOI=10.3389/fmicb.2020.562140

ISSN=1664-302X

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