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 attine ants and fungus-farming termites. In a third supposedly fungus-farming group, wood-colonizing ambrosia beetles (Curculionidae: Scolytinae and Platypodinae), 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 behaviors. Here, we experimentally removed mothers and their offspring from young nests of the fruit-tree pinhole borer, Xyleborinus saxesenii (Scolytinae). 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 low diverse 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.

Effects of selection for early dispersal on the ambrosia beetleXyleborinus saxeseniiand its fungal symbionts

“…In the initial stage, a LSU sequences database of stock culture of fungi isolated from X. saxesenii was employed. The database comprised eighteen reference sequences of twelve fungal species (Diehl et al, 2022). The identity threshold was set at 97% due to the fact that the database comprises sequences of known fungal symbionts of X. saxesenii.…”

“…The unclassified ASVs were employed as the input for the second stage, which utilised a custom reference database constructed from NCBI data using BCdatabaser (Keller et al, 2020). The second database comprised 82 250 sequences (Diehl et al, 2022). In order to ensure accurate classification, the identity threshold was set at 99%.…”

“…Ambrosia beetles are typically associated with mutualistic fungi, which they cooperatively cultivate in monoculture within tunnel systems and which serve as food for both larvae and adults (Biedermann & Vega 2020). Xyleborinus saxesenii cultivates two nutritional fungi that develop in succession, Dryadomyces sulphureus and Raffaelea canadensis (Biedermann et al, 2013;Diehl et al, 2022;Francke-Grosmann, 1975). Other fungal species are also present within the nests of X. saxesenii, although they are not as closely associated with the beetle (Biedermann et al, 2013).…”

“…Other fungal species are also present within the nests of X. saxesenii, although they are not as closely associated with the beetle (Biedermann et al, 2013). However, the accumulation of less beneficial fungi within nests over time also affects the fitness of late-dispersing offspring, which vertically transmit all these fungi to newly founded nests (Diehl et al, 2022). A strong correlation has been identified between highly productive nests, their durability, long philopatric periods and alloparental care by adult daughters , Nuotclá et al 2021).…”

“…Furthermore, we sought to ascertain the impact of offspring philopatry has on fungal communities. It was hypothesized that these communities would be less beneficial in nests of females with long philopatric periods and that they would contain higher relative abundances of R. canadensis than D. sulphureus fungal cultivars (Diehl et al, 2022).…”