Transfection of entomopathogenic Metarhizium species with a novel mycovirus confers hypervirulence against two lepidopteran pests

Although most known mycoviruses are asymptomatic or reduce the virulence of their host fungi, those that confer hypervirulence to entomopathogenic fungus still need to be explored. Here, we discovered and studied a novel mycovirus in Metarhizium flavoviride, isolated from Laodelphax striatellus. Based on molecular analysis, we tentatively designated the mycovirus as Metarhizium flavoviride partitivirus 1 (MfPV1), a novel species in genus Gammapartitivirus, family Partitiviridae. MfPV1 has two double-stranded (ds) RNAs as its genome, 1,775 and 1,575 bp in size respectively, encapsidated in isometric particles. When we transfected commercial strains of M. anisopliae and M. pingshaense with MfPV1, conidiation was significantly enhanced (t-test; P-value < 0. 01), and the significantly higher mortality rates of the larvae of Plutella xylostella and Spodoptera frugiperda, two important lepidopteran pests were found in virus-transfected strains (ANOVA; P-value < 0.05). Transcriptomic analysis showed that transcript levels of pathogenesis-related genes in MfPV1-infected M. anisopliae were obviously altered, suggesting increased production of metarhizium adhesin-like protein, hydrolyzed protein and destruxin synthetase. Further studies are required to elucidate the mechanism whereby MfPV1 enhances the expression of pathogenesis-related genes and virulence of Metarhizium to lepidopteran pests. This study presents experimental evidence that the transfection of other entomopathogenic fungal species with a mycovirus can confer significant hypervirulence and provides a good example that mycoviruses could be used as synergistic agent to enhance the biocontrol activity of entomopathogenic fungi. To investigate a novel mycovirus induced hypervirulence in the host fungus, we analyzed the RNA-seq data from hyphae of three virus-infected and three virus-free strains of M. anisopliae cultured in cicada slough induced PBD medium.

尽管目前已报道的多数真菌病毒（mycovirus）无明显症状或会减弱宿主真菌的毒力，但能够增强昆虫病原真菌毒力的真菌病毒仍有待探索。本研究从灰飞虱（Laodelphax striatellus）分离得到的黄绿绿僵菌（Metarhizium flavoviride）中，发现并研究了一株新型真菌病毒。经分子生物学分析，我们将该真菌病毒暂定名为黄绿绿僵菌分病毒1号（Metarhizium flavoviride partitivirus 1, MfPV1），隶属于分病毒科（Partitiviridae）γ分病毒属（Gammapartitivirus），为该属的一个新物种。MfPV1的基因组包含两条双链RNA（double-stranded RNA, dsRNA），长度分别为1775 bp和1575 bp，病毒粒子呈等轴对称形态。我们将MfPV1转染至商业用绿僵菌（Metarhizium anisopliae）和平沙绿僵菌（Metarhizium pingshaense）菌株后，发现产孢量显著提升（t检验，P<0.01）；转染病毒的菌株对两种重要鳞翅目害虫——小菜蛾（Plutella xylostella）和草地贪夜蛾（Spodoptera frugiperda）幼虫的致死率也显著升高（方差分析，P<0.05）。转录组分析显示，感染MfPV1的绿僵菌中致病相关基因的转录水平发生了显著变化，提示其黏附素样蛋白、水解蛋白以及绿僵菌素合成酶的表达量有所提升。后续仍需开展进一步研究，以阐明MfPV1增强绿僵菌致病相关基因表达及其对鳞翅目害虫毒力的具体分子机制。本研究提供了实验证据，证明通过转染真菌病毒可使其他昆虫病原真菌获得显著的强毒力表型，同时也为利用真菌病毒作为协同因子提升昆虫病原真菌的生防活性提供了良好范例。为探究真菌病毒诱导宿主真菌产生强毒力的分子机制，我们分析了三组感染MfPV1和三组未感染MfPV1的绿僵菌菌丝的RNA测序数据，这些菌株均在蝉蜕诱导的PBD培养基中培养。