Bacterial and fungal microbiome study on active farming in ambrosia beetles

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.

真菌种植是切叶蚁（attine ants）与真菌种植白蚁所具备的先进农业的标志性特征。在第三个被推测具备真菌种植行为的类群——蛀木食菌甲虫（鞘翅目象甲科：小蠹亚科与长小蠹亚科）中，目前仍缺乏实验证据证明此类甲虫可选择性促进其食用真菌的生长并抑制其他真菌类群，此前仅通过观察其社会与卫生行为推测该类群存在农业种植行为。本研究以小蠹亚科的果树小蠹（Xyleborinus saxesenii）的幼龄巢穴为研究对象，通过实验移除其中的母代甲虫及其子代；通过对有甲虫与无甲虫巢穴的细菌和真菌群落进行扩增子测序（amplicon sequencing），我们证实甲虫确实能够主动调控共生群落结构。尽管莱弗菌属（Raffaelea）的食用真菌会被甲虫取食，但在有甲虫存在的巢穴中，该类真菌的丰度反而更高；而与无甲虫巢穴相比，杂菌毛壳菌属（Chaetomium sp.）的丰度以及整体细菌多样性均显著降低。核心共生群落的多样性普遍偏低，且不仅栽培菌株，次级共生菌也存在显著的垂直传播迹象。本研究证实了此类甲虫主动农业种植行为的存在，尽管其选择性促进和/或抑制共生菌群的确切机制仍有待进一步探究。