Molecular tracing of white muscardine of the silkworm,Bombyx mori I: genetic structure analysis of Beauveria bassiana populations as the causal agents in China

Beauveria bassiana is a ubiquitous entomopathogenic fungus, which causes white muscardine in the silkworm, Bombyx mori, and is an important limiting factor in global silk production. Knowledge of the genetic structure of B. bassiana populations as the causal agents in China will aid in tracing the origins of muscardine and in developing effective control strategies. In the present study, a total of 489 B. bassiana isolates were obtained from silkworm cadavers in 12 different provinces of China. An analysis of the genetic structure of the causal agent populations using the internal simple sequence repeat technique to trace their origins revealed a high level (100%) of DNA polymorphism. An unweighted pair group method with arithmetic mean dendrogram revealed that the 489 B. bassiana isolates were heterogenic and polyphyletic and displayed typical regional distribution, although the genetic differentiations were not associated with geographic distance. The genetic differentiation (G_st) among populations reached 0.406, with the maximum G_st between populations reaching 0.647. The general gene flow (N_m) was low at 0.366, with the lowest at only 0.137, suggesting that the low gene flow maintained a high degree of differentiation. These data indicated that the white muscardine in each local silkworm population was predominantly caused by a native group of B. bassiana, rather than by any exotic strain as in the case of biological control based on B. bassiana insecticides.