Prevalence of Beauveria pseudobassiana among entomopathogenic fungi isolated from the hard tick, Ixodes ricinus.

Human and animal disease-transmitting hard ticks (Acari: Ixodidae) are of great concern for public health and animal farming. Alternatives to tick control by chemical acaricides are urgently needed, and one intensively evaluated biocontrol strategy is based on the use of tick-pathogenic filamentous fungi. An indispensable prerequisite for the development of tick-derived fungal isolates into registered myco-acaricides is their sound taxonomic characterisation. A set of fungal strains isolated from ixodid ticks in the Republic of Moldova was genetically characterised at the genus and species level together with further tick-derived fungal isolates from different geographic locations in Europe and North America. In a previous study, the same isolates had been assigned to the species Beauveria bassiana. Using a recent molecular taxonomic approach based on phylogenetic reconstruction from both internal transcribed spacer (ITS) and protein-encoding gene sequences, all fungi investigated were conclusively assigned to one of the two "hyphomycete" genera, Beauveria or Isaria (Ascomycota; Hypocreales; Cordycipitaceae). Within the genus Isaria, two species, Isaria farinosa and Isaria fumosorosea, were equally represented. Within the genus Beauveria, the comparatively rare species Beauveria pseudobassiana was found to strongly prevail among the isolates from Moldova, and one of the two tick-derived Beauveria strains from North America could be assigned to this species as well. In particular, the previous classification as B. bassiana could not be confirmed for any of the characterised tick pathogens from Europe and North America. The data presented here lend support to the hypothesis that within the genus Beauveria specific adaptation to ticks might have occurred within the species B. pseudobassiana. To test this hypothesis, a more extensive molecular taxonomic survey carefully reconsidering previous taxonomic assignments of tick-derived fungal isolates is needed.