Ancient plants with ancient fungi: liverworts associate with early-diverging arbuscular mycorrhizal fungi

Arbuscular mycorrhizas are widespread in land plants including liverworts, some of the closest living relatives of the first plants to colonise land 500 MYA. Previous investigations reported near-exclusive colonisation of liverworts by the most recently evolved arbuscular mycorrhizal fungi, the Glomeraceae, indicating a recent acquisition from flowering plants at odds with the widely-held notion that arbuscular mycorrhizal-like associations in liverworts represent the ancestral symbiotic condition in land plants. We performed an analysis of symbiotic fungi in 674 globally-collected liverworts using molecular phylogenetics and electron microscopy. Here we show every order of arbuscular mycorrhizal fungi colonises early-diverging liverworts, with non-Glomeraceae being at least ten times more common than in flowering plants. Arbuscular mycorrhizal fungi in liverworts and other ancient plant lineages (hornworts, lycopods and ferns) were delimited into 58 taxa and 36 singletons, of which at least 43 are novel and specific to liverworts. The discovery that early plant lineages are colonised by early-diverging fungi supports the hypothesis that arbuscular mycorrhizas are an ancestral symbiosis for all land plants.

丛枝菌根（arbuscular mycorrhizas）广泛分布于陆生植物中，包括苔类植物（liverworts）——后者是5亿年前首批登陆陆地的植物现存的近缘类群之一。既往研究曾报道，苔类植物几乎仅被演化最晚的丛枝菌根真菌——球囊霉科（Glomeraceae）所定殖，这提示该共生关系是近期从被子植物中获得的，这与广为接受的“苔类植物中的类丛枝菌根共生代表了陆生植物的祖先共生状态”的观点相悖。本研究采用分子系统发育学与电子显微镜技术，对全球采集的674份苔类植物样本中的共生真菌展开了分析。研究结果显示，所有目级分类阶元的丛枝菌根真菌均可定殖早期分化的苔类植物，且非球囊霉科真菌的占比至少比被子植物中高出10倍。研究人员将苔类植物及其他古老植物类群，包括角苔类（hornworts）、石松类（lycopods）和蕨类（ferns），中的丛枝菌根真菌划分为58个分类单元与36个单型类群，其中至少43个为全新的、仅分布于苔类植物的特有类群。这一“早期分化的真菌定殖早期植物类群”的发现，支持了“丛枝菌根是所有陆生植物的祖先共生关系”这一假说。