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

Abstract

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.

Multiple lineages of Streptomyces produce antimicrobials within passalid beetle galleries across eastern North America

“…In total, we identified 25 compounds representing 12 distinct antimicrobial families plus 1 siderophore compound. We note that 14 of the compounds we identified here have been previously described to be produced by microbes associated with other insects ( Benndorf et al, 2018 ; Blodgett et al, 2010 ; Engl et al, 2018 ; Grubbs et al, 2020 ; Jiang et al, 2018 ; Kroiss et al, 2010 ; Mevers et al, 2017 ; Ortega et al, 2019 ; Poulsen et al, 2011 ; Schoenian et al, 2011 ; Seipke et al, 2011 ; Figure 3 ). Collectively, these results reinforce the findings above that O. disjunctus frass plays host to actinomycetes that produce a rich array of antimicrobial compounds.…”

Multiple lineages of Streptomyces produce antimicrobials within passalid beetle galleries across eastern North America

“…In total, we identified 25 compounds representing twelve distinct antimicrobial families plus one siderophore compound. We note that thirteen of the compounds we identified here have been previously described to be produced by microbes associated with other insects (Blodgett et al, 2010; Engl et al, 2018; Grubbs et al, 2019; Jiang et al, 2018; Kroiss et al, 2010; Mevers et al, 2017; Ortega et al, 2019; Poulsen et al, 2011; Schoenian et al, 2011; Seipke et al, 2011) ( Fig. 3 ).…”

First experimental evidence for active farming in ambrosia beetles and strong heredity of garden microbiomes

“…Here, insect farmers evolved a wide variety of strategies to selectively facilitate the growth of their cultivars [1,2,5]. These include the sequestration and compartmentalization of gardens [2], usage of antibiotic-producing bacteria [6][7][8] and the active monitoring and behavioural management of fungus-garden communities [9][10][11].…”