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

Abstract

Fungal cultivation is a defining feature for advanced agriculture in fungus-farming ants and termites. In a third supposedly fungus-farming group, wood-colonizing ambrosia beetles, an experimental proof for the effectiveness of beetle activity for selective promotion of their food fungi over others is lacking and farming has only been assumed based on observations of social and hygienic behaviours. Here, we experimentally removed mothers and their offspring from young nests of the fruit-tree pinhole borer, Xyleborinus saxesenii . By amplicon sequencing of bacterial and fungal communities of nests with and without beetles we could show that beetles are indeed able to actively shift symbiont communities. Although being consumed, the Raffaelea food fungi were more abundant when beetles were present while a weed fungus ( Chaetomium sp.) as well as overall bacterial diversity were reduced in comparison to nests without beetles. Core symbiont communities were generally of low diversity and there were strong signs for vertical transmission not only for the cultivars, but also for secondary symbionts. Our findings verify the existence of active farming, even though the exact mechanisms underlying the selective promotion and/or suppression of symbionts need further investigation.

Comparing the succession of microbial communities throughout development in field and laboratory nests of the ambrosia beetle Xyleborinus saxesenii

“…The ASVs identifying Wolbachia were excluded from further analyses, since insect related infection was not in the focus of our research on fungus garden material. It is worth mentioning, however, that Wolbachia has been present in several analyses of laboratory nests by now (this study and Nuotclà et al, 2021;Diehl et al, 2022), whereas material of field nests never contained Wolbachia.…”

Volatiles of fungal cultivars act as cues for host-selection in the fungus-farming ambrosia beetle Xylosandrus germanus

“…Bacteria and yeasts have been also isolated from X. germanus individuals or infested galleries, but their functions have not been determined (Hulcr et al, 2012;Agnello et al, 2017;Rassati et al, 2019). In some cases, bacteria found in association with ambrosia beetles have been considered as beneficial (Haanstad and Norris, 1985;Grubbs et al, 2011Grubbs et al, , 2020Barcoto et al, 2020;Diehl et al, 2022). Further studies are needed to characterize the specific ecological role of all these microorganisms for X. germanus.…”

“…Ambrosia beetles in the tribe Xyleborini (Scolytinae), which are an ecological clade within the bark beetles, tunnel into host trees for the purposes of cultivating their nutritional fungal mutualists (Hulcr and Stelinski, 2017;Diehl et al, 2022). The role(s) that MVOCs play within the fungus-farming ambrosia beetles is emerging, but could function as short-or long-range cues to indicate the suitability of host substrates for brood production (Hulcr et al, 2011;Kuhns et al, 2014;Egonyu and Torto, 2018;Kandasamy et al, 2019;Biedermann and Vega, 2020;Ranger et al, 2021a;Nones et al, 2022).…”

Friend or Foe: Ambrosia Beetle response to volatiles of common threats in their fungus gardens

“…Whether all bark and ambrosia beetles are able to detect and react to fungal weeds is doubtful, but recent experiments on the ambrosia beetle, Xyleborinus saxesenii (Ratzeburg), revealed that this species can detect and selectively remove an Aspergillus fungal pathogen (Nuotclá et al 2019). Furthermore, beetles can actively promote their nutritional mutualists over fungal weeds (Diehl et al 2022). So far, it is unknown, however, how X. saxesenii and other farming beetles detect the presence of pathogens within nests.…”