Tenebrionid secretions and a fungal benzoquinone oxidoreductase form competing components of an arms race between a host and pathogen

Abstract

Entomopathogenic fungi and their insect hosts represent a model system for examining invertebrate-pathogen coevolutionary selection processes. Here we report the characterization of competing components of an arms race consisting of insect protective antimicrobial compounds and evolving fungal mechanisms of detoxification. The insect pathogenic fungusBeauveria bassianahas a remarkably wide host range; however, some insects are resistant to fungal infection. Among resistant insects is the tenebrionid beetleTribolium castaneumthat produces benzoquinone-containing defensive secretions. Reduced fungal germination and growth was seen in media containingT. castaneumdichloromethane extracts or synthetic benzoquinone. In response to benzoquinone exposure, the fungus expresses a 1,4-benzoquinone oxidoreductase,BbbqrA, induced >40-fold. Gene knockout mutants (ΔBbbqrA) showed increased growth inhibition, whereasB. bassianaoverexpressingBbbqrA(Bb::BbbqrAO) displayed increased resistance to benzoquinone compared with wild type. Increased benzoquinone reductase activity was detected in wild-type cells exposed to benzoquinone and in the overexpression strain. Heterologous expression and purification of BbBqrA inEscherichia coliconfirmed NAD(P)H-dependent benzoquinone reductase activity. TheΔBbbqrAstrain showed decreased virulence towardT. castaneum, whereas overexpression ofBbbqrAincreased mortality versusT. castaneum. No change in virulence was seen for theΔBbbqrAorBb::BbbqrAOstrains when tested against the greater wax mothGalleria mellonellaor the beetleSitophilus oryzae, neither of which produce significant amounts of cuticular quinones. The observation that artificial overexpression ofBbbqrAresults in increased virulence only toward quinone-secreting insects implies the lack of strong selection or current failure ofB. bassianato counteradapt to this particular host defense throughout evolution.