Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/1/2004
Publication Date: 7/1/2004
Citation: Castrillo, L.A., Vandenberg, J.D., Griggs, M. 2004. Vegetative compatibility groups in indigenous and mass-released strains of the entomopathogenic fungus beauveria bassiana: likelihood of recombination in the field. Journal of Invertebrate Pathology. 86:26-37. Interpretive Summary: Control of insect pests of crops costs farmers billions of dollars every year. However, many pests have become resistant to conventional, as well as some biological, insecticides. Alternative control measures being investigated include the use of insect-pathogenic fungi, and some strains have been mass-released in the field for control of certain pests. The success of this approach may depend on the ultimate establishment of these strains in the field. There has been no investigation to date that has shown whether these inundative releases have an impact on indigenous strains of the same fungus already present in the field. We evaluated 34 strains of the insect-pathogenic fungus Beauveria bassiana for their genetic relatedness and for their ability to exchange genetic information through a process called vegetative compatibility. We identified a group of closely related strains most of which were originally isolated in northeastern North America from the Colorado potato beetle. We showed, for the first time, that strains of this fungus can exchange genetic information not only in petri dish cultures but also within insect hosts. We also found that one commercial strain is compatible with this group but the other is not. While the frequency of recombination inside an insect host was low, the exchange of genetic information did occur in our laboratory tests and could have an impact on field populations of fungi. Consequently, when developing fungi for insect control, attention must be paid to possible genetic changes among introduced and indigenous strains. These findings improve our understanding of the impacts of fungal insecticides and will help us design effective strategies for their use as safe biological control agents.
Technical Abstract: Using nitrate non-utilizing (nit) mutants, we determined vegetative compatibility groups (VCG) among strains of Beauveria bassiana representing strains indigenous to North America, isolated from diverse insect hosts, and strains that have been mass released for insect control. Genetic similarity among these strains was analyzed using random amplified polymorphic DNA (RAPD) markers. Our data revealed 23 VCGs among the 34 strains tested, with most of these groups comprised of only a single strain. We also observed a VCG comprised of eight genetically similar strains isolated from Colorado potato beetles (CPB). Co-inoculation studies of CPB larvae with complementary nit mutants from the same or from different VCGs revealed heterokaryosis in four out of five same-VCG pairs, with only 5 to 17 % of the sporulating cadavers generating few parasexual recombinants. In contrast, none of the infected beetles treated with non-compatible pairs generated recombinants. The large number of VCGs observed and the low frequency of in vivo recombination limited to vegetatively compatible strains indicate that this self/non-self recognition system may be an effective barrier preventing genetic exchange between dissimilar strains in the field.