Symbiosis and Dysbiosis in fungus - gardening ants: ants drive microbiome structure and homeostasis of fungal symbionts

One of the most important evolutionary transitions is the cooperation of vastly unrelated species (symbiosis). What stabilizes symbiotic associates or causes their collapse (dysbiosis) is a mystery. Recent advances due to high throughput sequencing techniques indicate that most symbioses (and eukaryotic organisms) are in fact communities of interacting bacteria, fungi in addition to macroscopic hosts and symbionts. Fungus-gardening (attine) ants have had a symbiotic relationship with specific fungal lineages for millions of years. These ants are excellent model systems for exploring the basis of specificity as the host and symbionts are macroscopic and can be easily experimentally manipulated. The most derived attine ants (the "higher attines") typically grow two broad clades of fungi. Most ants in the genus Trachymyrmex tend to grow Clade-B fungi, which are a group of undescribed Leucoagaricus species, whereas the leaf-cutting ants in the genera Acromyrmex and Atta tend to grow Clade-A fungi (Leucoagaricus gongylophorus). However, there are some species of Trachymyrmex that are known to grow Clade-A fungi and some leaf-cutting ants that grow Clade-B fungi. Previous work has shown switching the cultivar grown by Trachymyrmex septentrionalis ants, from Clade-B to Clade-A fungus, creates an unstable symbiosis between the ants and their grown cultivar, so that a sudden and catastrophic decrease in the size of their fungal garden invariably results. One hypothesis is that the stability of ant-fungal combinations is maintained by interactions among members of the microbiome of fungus-gardening ants and their fungus gardens. We explored whether changing fungal partners impacts the microbiomes of the host ants and their symbiotic fungus by performing cross fostering experiments that forced ants to grow novel fungi. Specifically, these experiments forced ants of two Trachymyrmex species (T. pomonae and T. septentrionalis ) that normally grow Clade-B fungi to grow Clade-A fungi and compared these to a species that is known to growth both Clade-A and Clade-B (T. arizonensis). The experiments revealed that Trachymyrmex ants altered their novel Clade-A garden microbiomes and that these were similar to that of the "control or sham switched" Clade-B fungus gardens. These results suggest that ants play a role in determining the structure of the microbiome of their fungus gardens. Since these combinations are not stable, it is possible that the "novel" microbiome structured by the Trachymyrmex ants is a factor in driving symbiotic collapse. Such findings suggest each fungal clade may have specificity with assorted bacterial communities.

演化过程中最重要的转变之一，便是亲缘关系极远的物种间的合作共生（symbiosis）。究竟是什么因素维持共生联合体的稳定，又是什么导致其崩溃（共生失调，dysbiosis），至今仍是未解之谜。近年来，高通量测序技术（high throughput sequencing）的发展揭示，绝大多数共生体系（以及真核生物，eukaryotic organisms）实际上是由宏观宿主、共生体与相互作用的细菌、真菌共同构成的群落。

培育真菌的蚂蚁（菌植蚁，attine ants）与特定真菌支系已经维持了数百万年的共生关系。由于宿主与共生体均为宏观生物且易于进行实验操作（experimental manipulation），这类蚂蚁是探究共生特异性机制的绝佳模型系统。

最为特化的菌植蚁——即「高等菌植蚁」（higher attines）——通常培育两类宽泛的真菌演化支（clade）。曲颊切叶蚁属（Trachymyrmex）的多数蚂蚁倾向于培育B演化支真菌（Clade-B fungi），这类真菌属于一群尚未被描述的白鬼伞属（Leucoagaricus）物种；而顶切叶蚁属（Acromyrmex）与切叶蚁属（Atta）的切叶蚁则多培育A演化支真菌（Clade-A fungi），即莲座白鬼伞（Leucoagaricus gongylophorus）。不过，部分曲颊切叶蚁属物种被证实可培育A演化支真菌，也有部分切叶蚁会培育B演化支真菌。

既往研究显示，将北方曲颊切叶蚁（Trachymyrmex septentrionalis）的栽培菌种从B演化支真菌更换为A演化支真菌，会使蚂蚁与其栽培菌种间形成不稳定的共生关系，最终必然导致其真菌花园（fungal garden）的规模出现突发性、灾难性的下降。

有假说提出，蚁菌组合的稳定性由培育真菌的蚂蚁及其真菌花园的微生物组（microbiome）成员间的相互作用所维持。本研究通过开展交叉抚育实验（cross fostering experiments），迫使蚂蚁培育新型真菌，以此探究更换真菌伙伴是否会影响宿主蚂蚁及其共生真菌的微生物组。具体而言，本实验迫使原本培育B演化支真菌的两种曲颊切叶蚁——苹果曲颊切叶蚁（T. pomonae）与北方曲颊切叶蚁（T. septentrionalis）——培育A演化支真菌，并以已知可同时培育A、B两类演化支真菌的亚利桑那曲颊切叶蚁（T. arizonensis）作为对照。

实验结果表明，曲颊切叶蚁会改变其新型A演化支真菌花园的微生物组结构，且该结构与「对照或假转移」（sham switched）的B演化支真菌花园的微生物组相似。上述结果提示，蚂蚁在其真菌花园的微生物组结构塑造中发挥了作用。由于这类新型组合并不稳定，曲颊切叶蚁所构建的「新型」微生物组可能是推动共生崩溃的因素之一。此类研究结果表明，每一类真菌演化支可能都与特定的细菌群落存在特异性关联。