Table_2_Volatiles of fungal cultivars act as cues for host-selection in the fungus-farming ambrosia beetle Xylosandrus germanus.xlsx

Many wood-boring insects use aggregation pheromones during mass colonization of host trees. Bark beetles (Curculionidae: Scolytinae) are a model system, but much less is known about the role of semiochemicals during host selection by ambrosia beetles. As an ecological clade within the bark beetles, ambrosia beetles are obligately dependent on fungal mutualists for their sole source of nutrition. Mass colonization of trees growing in horticultural settings by exotic ambrosia beetles can occur, but aggregation cues have remained enigmatic. To elucidate this mechanism, we first characterized the fungal associates of the exotic, mass-aggregating ambrosia beetle Xylosandrus germanus in Southern Germany. Still-air olfactometer bioassays documented the attraction of X. germanus to its primary nutritional mutualist Ambrosiella grosmanniae and to a lesser extent another common fungal isolate (Acremonium sp.). During two-choice bioassays, X. germanus was preferentially attracted to branch sections (i.e., bolts) that were either pre-colonized by conspecifics or pre-inoculated with A. grosmanniae. Subsequent analyses identified microbial volatile organic compounds (MVOCs) that could potentially function as aggregation pheromones for X. germanus. To our knowledge, this is the first evidence for fungal volatiles as attractive cues during host selection by X. germanus. Adaptive benefits of responding to fungal cues associated with an infestation of conspecifics could be a function of locating a suitable substrate for cultivating fungal symbionts and/or increasing the likelihood of mating opportunities with the flightless males. However, this requires solutions for evolutionary conflict arising due to potential mixing of vertically transmitted and horizontally acquired symbiont strains, which are discussed.

众多木质甲虫在大量入侵宿主树木的过程中会使用聚集信息素。鞘翅目（Curculionidae: Scolytinae）中的蠹虫被选为模型系统，但对于聚集甲虫在宿主选择过程中半化学物质的作用，了解甚少。聚集甲虫作为鞘翅目内的一个生态类群，其营养来源完全依赖于真菌共生体。外来聚集甲虫在园艺环境中对树木的大规模入侵是可能的，但其聚集线索一直扑朔迷离。为了阐明这一机制，我们首先对德国南部的外来大规模聚集性聚集甲虫Xylosandrus germanus的真菌共生体进行了表征。静气嗅觉计生物测定实验记录了X. germanus对其主要营养共生体Ambrosiella grosmanniae的吸引，以及在一定程度上对另一种常见真菌分离株（Acremonium sp.）的吸引。在双选择生物测定实验中，X. germanus更倾向于被分支部分（即螺栓）所吸引，这些分支部分要么已被同种个体先期殖民，要么预先接种了A. grosmanniae。后续分析鉴定出了可能作为X. germanus聚集信息素的微生物挥发性有机化合物（MVOCs）。据我们所知，这是首次有真菌挥发性物质作为X. germanus宿主选择吸引线索的证据。对与同种昆虫入侵相关的真菌线索作出反应的适应性益处可能在于寻找适宜的培养基育真菌共生体以及增加与不能飞翔的雄性交配机会的可能性。然而，这需要解决由于垂直传递和水平获得共生体菌株的潜在混合所引起的进化冲突，相关讨论如下。