Table2.DOCX

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），学名Heterodera glycines Ichinohe，隶属于线虫动物门（Nematoda），是大豆的主要病原物。该线虫可在全球范围内造成严重的大豆产量损失，且防控难度较高——因其胞囊能够保护卵粒，而卵可存活近十年之久。采用非寄主作物轮作以及利用真菌、细菌等生防生物是颇具前景的防控策略，但由于目前对该线虫寄生物的生物学特性了解不足，这些手段的应用仍受到限制。
本研究采用高通量元条形码测序（high-throughput metabarcoding）技术，对与SCN胞囊相关的真菌群落进行了表征：SCN胞囊是土壤中的一类特殊微环境，可能同时携带线虫寄生物与植物病原菌。研究团队从明尼苏达州东南部的长期作物轮作试验地块中采集SCN胞囊，在两个生长季内设置三个采样时间点，以此表征胞囊内定殖真菌的多样性，并探究作物轮作制度与季节变化对真菌群落的影响。
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类群，包括被认为主要与植物和枯落物相关的被孢霉亚门（Mortierellomycotina）与球囊菌门（Glomeromycotina，如丛枝菌根真菌），这提示这些类群可能具备线虫卵寄生物的功能。
物种丰富度同时受作物轮作制度与季节的影响：在轮作初期以及生长季末期的秋季样本中，物种丰富度更高。作物轮作与季节同样会显著影响真菌群落组成，研究发现多个真菌纲类群在大豆早期轮作样点中相对丰度更高，包括散囊菌纲（Eurotiomycetes）、粪壳菌纲（Sordariomycetes）以及圆盘菌纲（Orbiliomycetes，如捕食线虫真菌）。多个真菌属的相对丰度与大豆种植年限的增加呈显著正相关。
真菌群落同样随季节发生显著变化，在生长季中期群落分化程度最高。被归类为被孢霉亚门未定类群（Mortierellomycotina_cls_Incertae_sedis）与粪壳菌纲的操作分类单元（operational taxonomic unit, OTU）占比在生长季中期上升，而圆盘菌纲的OTU占比在中期下降，球囊菌纲（Glomeromycetes）的OTU占比在秋季升高。具有动物病原生活型的真菌生态类群，以及此前从被寄生的SCN卵中分离得到的潜在卵寄生物类群，在生长季中期的相对丰度均有所提升。其中动物病原类群包含已知的生防生物（如厚垣孢普可尼亚菌Pochonia chlamydosporia）与新的候选生防生物。
本研究增进了学界对农业生态系统中食线虫真菌生态学的认识，同时为其作为SCN生防制剂的应用提供了理论支撑。