Fungal metabolic plasticity and sexual development mediate induced resistance to arthropod fungivory

Abstract

Prey organisms do not tolerate predator attack passively but react with a multitude of inducible defensive strategies. Although inducible defence strategies are well known in plants attacked by herbivorous insects, induced resistance of fungi against fungivorous animals is largely unknown. Resistance to fungivory is thought to be mediated by chemical properties of fungal tissue, i.e. by production of toxic secondary metabolites. However, whether fungi change their secondary metabolite composition to increase resistance against arthropod fungivory is unknown. We demonstrate that grazing by a soil arthropod, Folsomia candida, on the filamentous fungus Aspergillus nidulans induces a phenotype that repels future fungivores and retards fungivore growth. Arthropod-exposed colonies produced significantly higher amounts of toxic secondary metabolites and invested more in sexual reproduction relative to unchallenged fungi. Compared with vegetative tissue and asexual conidiospores, sexual fruiting bodies turned out to be highly resistant against fungivory in facultative sexual A. nidulans. This indicates that fungivore grazing triggers co-regulated allocation of resources to sexual reproduction and chemical defence in A. nidulans. Plastic investment in facultative sex and chemical defence may have evolved as a fungal strategy to escape from predation.

Keywords: chemical defence; facultative sexuality; induced resistance; insect–fungus ecology; mycotoxins.

Figure 1.

Folsomia candida food choice. Food choice (+PF, fungivore-challenged versus –PF, unchallenged) of F. candida as a function of increasing post-fungivory period (PFP) and observation period (1.5, 20 and 44 h). Fungivore preference was measured by subtracting the proportion of individuals on −PF colonies from the proportion of individuals on +PF colonies resulting in Δ-values ranging from +1 (full preference for PF) to −1 (full preference for −PF). Δ-values not significantly different from zero indicate no preference (n = 20 per treatment, see electronic supplementary material, table S1 for statistical details). (*p < 0.05, ***p < 0.001, n.s. not significantly different from zero, error bars are s.e.m.).

Figure 2.

Folsomia candida feeding and fitness responses. (a) Mean body length of F. candida after feeding for 7 days (on −PF or +PF colonies (n = 20 per treatment). (b) Mean number of faecal pellets produced by experimental F. candida after feeding for 7 days on −PF or +PF colonies (n = 20 per treatment, error bars are s.e.m.).

Figure 3.

Induction of sexual fruiting bodies. (a) Box-plots depicting the number of cleistothecia produced in response to untouched control colonies (−PF), arthropod feeding (+PF) or artificial wounding (WO) (n = 20 per treatment). (b) Food choice of F. candida when offered asexual conidia and sexual cleistothecia. Positive Δ-values indicate preference for the conidia (n = 8; error bars are s.e.m.). (c) Representative image of the effect of long-term (three weeks) F. candida grazing on A. nidulans. Both the entire vegetative and asexual reproductive tissue has been consumed and the whole arena is covered with faecal pellets. Only the sexual fruiting bodies remain intact. (d) Box-plots depicting the number of cleistothecia found in A. nidulans exposed to long-term fungivory (+PF) compared with the number of cleistothecia formed by undisturbed control colonies (−PF). (n = 10 per treatment).

Figure 4.

Quantification of A. nidulans secondary metabolites by HPLC-MS/MS. (a) Peak area of ion chromatograms of secondary metabolites that were identified by using published mass spectra (see electronic supplementary material, figure S4). m/z values represent the transition of mass-to-charge ratios at which the HPLC peak areas were quantified. (n = 6 per treatment, error bars are s.e.m.) (b) Chemical structures of secondary metabolites enhanced in response to F. candida grazing.