How carton-nest fungi of the ant Lasius fuliginosus interact with each other and with the root-rot fungus Armillaria mellea

Lasius fuliginosus, a fungus-growing ant species distributed across Europe, hosts various fungi inside its carton nests in trees, including the nest fungus SP1 of the order Chaetothyriales, as well as the nest fungi SP5 and SP4 of the order Venturiales. The goal of this study was to gain a better understanding of the fungal interactions inside the L. fuliginosus nests as well as of potential interactions around the nests, including the effects of Armillaria mellea—a root-rot fungus infecting potential host trees. We performed two types of confrontation experiments on Petri dishes between the isolated nest fungi and A. mellea. Firstly, using de Wit experiments, we tested the fungal species in pairwise combinations at three different initial confrontation concentrations. Secondly, a linear confrontation setup focused on differences in directional growth of the fungal species in pairwise combinations as well as on the development of A. mellea rhizomorphs. For the fungi SP1, SP5, and SP4, we found positive influences on each other (SP1 on SP5, SP4 on SP1 and SP5, and SP5 on SP1) alongside no influence (SP1 on SP4, SP5 on SP4). SP1 had a significantly negative impact on the surface growth and directional growth of A. mellea, and SP5 triggered the strongest rhizomorph development of A. mellea, possibly a stress reaction of the root-rot fungus. Armillaria mellea did not negatively impact any of the nest fungi and even promoted the surface growth of SP1. The de Wit setup and the linear setup turned out to be complementary and together facilitated first insights into potential roles of the nest fungi in this association of ants and fungi in trees. Follow-up studies will need to assess how these findings under Petri dish conditions transfer to conditions in natural habitat, in the presence of both the ant and the tree host.

黑褐毛蚁（Lasius fuliginosus）是一种分布于欧洲的培菌蚁类物种，于其树上的纸质巢（carton nests）内寄居多种真菌，包括毛壳菌目（Chaetothyriales）的巢内真菌SP1，以及旋孢腔菌目（Venturiales）的巢内真菌SP5与SP4。本研究旨在更深入地解析黑褐毛蚁巢内的真菌互作关系，以及巢周潜在的互作过程，包括蜜环菌（Armillaria mellea）——一种侵染潜在宿主树木的根腐真菌——的影响。我们在培养皿（Petri dishes）上针对分离得到的巢内真菌与蜜环菌开展了两类对峙实验：首先采用德威特对峙实验（de Wit experiments），以三种不同的初始对峙浓度对真菌物种进行两两组合测试；其次采用线性对峙设置，聚焦于真菌物种两两组合时的定向生长差异，以及蜜环菌根状菌索（rhizomorphs）的发育情况。针对SP1、SP5与SP4这三种真菌，我们发现它们之间存在相互促进的关系（SP1对SP5、SP4对SP1与SP5，以及SP5对SP1），同时也存在无显著影响的组合（SP1对SP4、SP5对SP4）。SP1对蜜环菌的表面生长与定向生长均具有显著的抑制作用，而SP5则可诱导蜜环菌产生最强的根状菌索发育，这可能是该根腐真菌的应激反应。蜜环菌并未对任何一种巢内真菌产生负面影响，甚至可促进SP1的表面生长。德威特对峙实验设置与线性对峙设置互为补充，二者共同为解析该树栖蚁菌共生体系中巢内真菌的潜在功能提供了初步认识。后续研究需评估这些基于培养皿条件的研究结果，在同时存在蚂蚁与宿主树木的自然生境条件下的可转移性。