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

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 112; no. 28; pp. E3651 - E3660
• Main Authors: Pedrini, Nicolás, Almudena Ortiz-Urquiza, Carla Huarte-Bonnet, Yanhua Fan, M. Patricia Juáárez, Nemat O. Keyhani
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 14.07.2015; National Acad Sciences
• Series: PNAS Plus
• Subjects: Animals; Beauveria - enzymology; Beauveria - pathogenicity; Beauveria bassiana; benzoquinones; Benzoquinones - metabolism; Biological Sciences; Cells; E coli; entomopathogenic fungi; Escherichia coli; evolutionary arms race; Fungi; Galleria mellonella; Germination; host range; Host-Pathogen Interactions; hosts; insect cuticle; Insects; invertebrates; Mortality; Mutation; NAD(P)H dehydrogenase (quinone); Oxidoreductases - genetics; Oxidoreductases - metabolism; PNAS Plus; quinone reductase; Sitophilus oryzae; Tenebrionidae; Tribolium - metabolism; Tribolium - pathogenicity; Tribolium castaneum; Virulence; Tribolium castaneum; entomopathogenic fungi; insect cuticle; evolutionary arms race; quinone reductase
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1504552112

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 fungus Beauveria bassiana has a remarkably wide host range; however, some insects are resistant to fungal infection. Among resistant insects is the tenebrionid beetle Tribolium castaneum that produces benzoquinone-containing defensive secretions. Reduced fungal germination and growth was seen in media containing T. castaneum dichloromethane 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, whereas B. bassiana overexpressing BbbqrA ( Bb::BbbqrA ᴼ) 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 in Escherichia coli confirmed NAD(P)H-dependent benzoquinone reductase activity. The ΔBbbqrA strain showed decreased virulence toward T. castaneum , whereas overexpression of BbbqrA increased mortality versus T. castaneum . No change in virulence was seen for the ΔBbbqrA or Bb::BbbqrA ᴼ strains when tested against the greater wax moth Galleria mellonella or the beetle Sitophilus oryzae , neither of which produce significant amounts of cuticular quinones. The observation that artificial overexpression of BbbqrA results in increased virulence only toward quinone-secreting insects implies the lack of strong selection or current failure of B. bassiana to counteradapt to this particular host defense throughout evolution. Although entomopathogenic fungi and their invertebrate hosts share a >300 million year co-evolutionary history, little is known concerning the biochemical and genetic basis of insect defensive tactics and the countermeasures evolved and evolving by the pathogen to thwart these defenses. Our results provide a molecular mechanism to help explain why some insects are more resistant to broad host-range entomopathogenic fungi. We identify beetle cuticular secretions and a fungal detoxifying enzyme as components of an arms race between insects and the fungal pathogen, suggesting an evolving role for the quinone reductase enzyme as a specific virulence factor for host quinone detoxification. As races have winners and losers, this paper captures a snapshot where the host is leading the race.