Data from: Drechslerella stenobrocha genome ilustrates the mechanism of constricting rings and the origin of nematode predation in fungi

Background: Nematode-trapping fungi are a unique group of organisms that can capture nematodes using sophisticated trapping structures. The genome of Drechslerella stenobrocha, a constricting-ring-forming fungus, has been sequenced and reported, and provided new insights into the evolutionary origins of nematode predation in fungi, the trapping mechanisms, and the dual lifestyles of saprophagy and predation. Results: The genome of the fungus Drechslerella stenobrocha, which mechanically traps nematodes using a constricting ring, was sequenced. The genome was 29.02 Mb in size and was found rare instances of transposons and repeat induced point mutations, than that of Arthrobotrys oligospora. The functional proteins involved in nematode-infection, such as chitinases, subtilisins, and adhesive proteins, underwent a significant expansion in the A. oligospora genome, while there were fewer lectin genes that mediate fungus-nematode recognition in the D. stenobrocha genome. The carbohydrate-degrading enzyme catalogs in both species were similar to those of efficient cellulolytic fungi, suggesting a saprophytic origin of nematode-trapping fungi. In D. stenobrocha, the down-regulation of saprophytic enzyme genes and the up-regulation of infection-related genes during the capture of nematodes indicated a transition between dual life strategies of saprophagy and predation. The transcriptional profiles also indicated that trap formation was related to the protein kinase C (PKC) signal pathway and regulated by Zn(2)-C6 type transcription factors. Conclusions: The genome of D. stenobrocha provides support for the hypothesis that nematode trapping fungi evolved from saprophytic fungi in a high carbon and low nitrogen environment. It reveals the transition between saprophagy and carnivory of these fungi and also proves new insights into the mechanisms of mechanical trapping.背景：捕线虫真菌（Nematode-trapping fungi）是一类独特的生物类群，可借助精密的捕获结构捕获线虫。本研究已完成缩环形成真菌细钻孢霉（Drechslerella stenobrocha）的基因组测序及解析，为阐释真菌捕食线虫的进化起源、捕获机制及腐生-捕食双重生活方式提供了全新视角。 结果：本研究对通过缩环结构机械捕捉线虫的细钻孢霉基因组进行了测序。该基因组大小为29.02 Mb，相较于少孢节丛孢（Arthrobotrys oligospora）的基因组，其转座子（transposons）与重复诱导点突变（repeat induced point mutations）的出现频率极低。在少孢节丛孢基因组中，参与线虫侵染的功能蛋白（如几丁质酶、枯草杆菌蛋白酶及黏附蛋白）发生了显著扩增；而细钻孢霉基因组中介导真菌-线虫识别的凝集素基因（lectin genes）则相对偏少。两个物种的碳水化合物降解酶（carbohydrate-degrading enzyme）谱均与高效纤维素降解真菌的酶谱特征相似，这表明捕线虫真菌起源于腐生真菌。在细钻孢霉中，线虫捕获阶段，腐生酶基因的下调与侵染相关基因的上调，表明其完成了腐生与捕食双重生活策略的转换。转录组分析结果还显示，捕器形成与蛋白激酶C（Protein Kinase C, PKC）信号通路密切相关，并受Zn(2)-C6型转录因子（Zn(2)-C6 type transcription factors）调控。 结论：细钻孢霉的基因组数据为"捕线虫真菌起源于高碳低氮环境中的腐生真菌"这一假说提供了有力支撑。该研究揭示了这类真菌腐生与捕食的生活策略转换机制，同时为机械捕获的分子机制提供了全新见解。