Genetic diversity and population structure of the Chinese Fungus Metarhizium rileyi causing green muscardine in silkworm

• Green muscardine caused by Metarhizium rileyi is a threat to silkworm sericulture.
• Green muscardine in each local silkworm population was predominantly caused by a native group of M. rileyi.
• Population genetic structure of 156 M. rileyi isolates obtained in China was analyzed.
• UPGMA dendrogram clustered strains from Qianshan County into a single clade.
• No obvious host specificity (between silkworm and other Lepidoptera) in M. rileyi isolates.

Green muscardine caused by Metarhizium rileyi affects sericulture, and is typically enzootic and occurs frequently at low incidence. We collected 152 M. rileyi isolates from silkworm cadavers in eight sericulture areas in seven provinces of China, and four strains from other Lepidoptera larvae in Qianshan(QS) County, Anhui province. Nine microsatellite inter-simple sequence repeat (ISSR) primers produced 91 distinct and reproducible bands, revealing a high level (90.11%) of DNA polymorphism. Unweighted Pair Group Method with Arithmetic Mean (UPGMA) cluster analysis divided the populations into four groups, with isolates from Qianshan County forming a single branch. All the 156 M. rileyi isolates were heterogenic and polyphyletic and did not displayed typical regional distribution except strains from Qianshan country. PCA analysis of the nine populations of M. rileyi revealed similar phylogenies among accessions. Genetic differentiation index (Gst) among eight enzootic populations was 0.3789 and gene flow (Nm) was 0.4098, suggesting the low gene flow maintained a high degree of differentiation. Gst between the enzootic population of Qianshan County and other seven populations exceeded the threshold of severe differentiation, with moderate differentiation between the remaining seven enzootic populations. Analysis of molecular variance (AMOVA) showed most ISSR variation (61%) among isolates occurred within populations. No significant correlation was observed between geographical and genetic distance. According to cluster analysis based on single enzootic population, every enzootic population showed dominance, namely mainly constituted of strains with high genetic similarity. These data indicated that the green muscardine in each local silkworm population was predominantly caused by a native group of M. rileyi. Furthermore, Gst and Nm of M. rileyi from silkworm and other Lepidoptera larvae in Qianshan County were 0.1174 and 1.8791, respectively, suggesting strains isolated from different hosts in Qianshan County do not show obvious host specificity. This demonstrated that host transfer may take place in silkworm and other insects.

(a) Silkworm cadavers infected with green muscardine. (b) Dendrogram derived from unweighted pair group method analysis (UPGMA) clustering based on ISSR markers for the 156 Metarhizium rileyi isolates.Figure optionsDownload as PowerPoint slide