Table4.XLSX

The soybean cyst nematode (SCN), Heterodera glycines Ichinohe (Phylum Nematoda), is a major pathogen of soybean. It causes substantial yield losses worldwide and is difficult to control because the cyst protects the eggs which can remain viable for nearly a decade. Crop rotation with non-host crops and use of biocontrol organisms such as fungi and bacteria offer promising approaches, but remain hampered by lack of knowledge of the biology of nematode parasitic organisms. We used a high-throughput metabarcoding approach to characterize fungal communities associated with the SCN cyst, a microenvironment in soil that may harbor both nematode parasites and plant pathogens. SCN cysts were collected from a long-term crop rotation experiment in Southeastern Minnesota at three time points over two growing seasons to characterize diversity of fungi inhabiting cysts and to examine how crop rotation and seasonal variation affects fungal communities. A majority of fungi in cysts belonged to Ascomycota and Basidiomycota, but the presence of several early diverging fungal subphyla thought to be primarily plant and litter associated, including Mortierellomycotina and Glomeromycotina (e.g., arbuscular mycorrhizal fungi), suggests a possible role as nematode egg parasites. Species richness varied by both crop rotation and season and was higher in early years of crop rotation and in fall at the end of the growing season. Crop rotation and season also impacted fungal community composition and identified several classes of fungi, including Eurotiomycetes, Sordariomycetes, and Orbiliomycetes (e.g., nematode trapping fungi), with higher relative abundance in early soybean rotations. The relative abundance of several genera was correlated with increasing years of soybean. Fungal communities also varied by season and were most divergent at midseason. The percentage of OTUs assigned to Mortierellomycotina_cls_Incertae_sedis and Sordariomycetes increased at midseason, while Orbiliomycetes decreased at midseason, and Glomeromycetes increased in fall. Ecological guilds of fungi containing an animal-pathogen lifestyle, as well as potential egg-parasitic taxa previously isolated from parasitized SCN eggs, increased at midseason. The animal pathogen guilds included known (e.g., Pochonia chlamydosporia) and new candidate biocontrol organisms. This research advances knowledge of the ecology of nematophagous fungi in agroecosystems and their use as biocontrol agents of the SCN.

大豆孢囊线虫（soybean cyst nematode, SCN），即大豆异皮线虫（Heterodera glycines Ichinohe，隶属线虫动物门（Phylum Nematoda）），是大豆的主要病原物。该线虫在全球范围内造成严重的产量损失，且防控难度极高——因其胞囊可保护卵粒，而这些卵可存活近十年之久。采用非寄主作物轮作以及施用真菌、细菌等生防生物的策略虽颇具应用前景，但目前对寄生线虫类生物的了解不足，限制了该类防控手段的推广应用。本研究采用高通量元条形码（metabarcoding）技术，对与大豆孢囊线虫胞囊相关的真菌群落进行表征；线虫胞囊是土壤中一类兼具线虫寄生菌与植物病原菌定植潜力的特殊微环境。本研究在明尼苏达州东南部的长期作物轮作定位试验田中，于两个生长季内的三个时间点采集大豆孢囊线虫胞囊，旨在解析胞囊内定殖真菌的多样性，并探究作物轮作与季节变化如何影响真菌群落结构。胞囊内的真菌类群绝大多数隶属于子囊菌门（Ascomycota）与担子菌门（Basidiomycota），但同时存在多个早期分化的真菌亚门——这类亚门通常被认为主要与植物及枯落物相关，包括被孢霉亚门（Mortierellomycotina）与球囊菌亚门（Glomeromycotina，如丛枝菌根真菌），这提示上述类群可能具备线虫卵寄生能力。物种丰富度同时受作物轮作与季节的影响，在轮作初期以及生长季末的秋季样本中更为丰富。作物轮作与季节变化同样会影响真菌群落组成，研究鉴定出散囊菌纲（Eurotiomycetes）、粪壳菌纲（Sordariomycetes）与圆孢霉纲（Orbiliomycetes，如捕食线虫真菌）等多个真菌类群，在早期大豆轮作处理中相对丰度更高。多个真菌属的相对丰度与大豆连作年限的增加呈显著正相关。真菌群落同样随季节更替发生显著变化，且在生长季中期的群落分化程度最高。在生长季中期，隶属于被孢霉亚门地位未定类群（Mortierellomycotina_cls_Incertae_sedis）与粪壳菌纲（Sordariomycetes）的操作分类单元（operational taxonomic unit, OTU）占比上升，而圆孢霉纲（Orbiliomycetes）的相对丰度下降，而球囊菌纲（Glomeromycetes）在秋季的占比有所提升。具有动物病原生活方式的真菌生态类群，以及此前从被寄生的SCN卵中分离得到的潜在卵寄生类群，在生长季中期的占比均有所上升。这类动物病原生态类群包含已知的生防生物（如淡紫拟青霉（Pochonia chlamydosporia））与新的潜在生防生物候选菌株。本研究增进了人们对农业生态系统中食线虫真菌生态学的认知，同时为其作为大豆孢囊线虫生防制剂的应用提供了理论支撑。