Consequences of pharmacophagous uptake from plants and conspecifics in a sawfly elucidated using chemical and molecular techniques

Abstract

Pharmacophagy involves the sequestration of specialised plant metabolites for non-nutritive purposes and commonly occurs in insects. Here we investigate pharmacophagy in the turnip sawfly, Athalia rosae, where adults not only collect specialised metabolites (clerodanoids) from a plant (Ajuga reptans), but also from the exterior of conspecifics via fighting. Using behavioural assays, chemical analytics, and RNAseq we show that when individuals nibble on conspecifics that have already acquired clerodanoids from A. reptans leaves, this nibbling results in the transfer of compounds between individuals. Furthermore, unlike other pharmacophagous insects, the acquisition of clerodanoids by A. rosae from the leaves of A. reptans does not induce the upregulation of known detoxification or sequestration genes and pathways. In contrast, pharmacophagous nibbling on conspecifics results in the upregulation of metabolic pathways associated with elevated metabolic rates and increased energy consumption. It therefore seems that individuals attack conspecifics to acquire clerodanoids despite the apparent metabolic costs of this form of pharmacophagy compared to clerodanoid uptake from a plant. Changes in the metabolic phenotype of A.rosae individuals consequently has profound consequences for social interactions with possible ramifications for their social niche.

Survival of the Sawfly Athalia rosae Upon Infection by an Entomopathogenic Fungus and in Relation to Clerodanoid Uptake

“…Even though we do not have a direct quantification of the amount of compounds taken up after nibbling on the leaves, we know that nibbling on A. reptans leaves for a short time leads to qualitative changes in the chemical profiles of adults. After such exposure, adults become attractive to their conspecifics, which exhibit a nibbling behavior only on exposed but not on non-exposed conspecifics (Paul et al, 2021). We observed this behavior toward insects exposed to clerodanoids in our experiment, but not toward insects of the non-exposed treatment.…”

“…This was confirmed by the characteristic nibbling behavior conspecifics exhibited toward insects of the clerodanoids-exposed treatment. Adult sawflies could have either incorporated plant compounds and/or spread some over the surface of their cuticle when contacting the leaves or while cleaning themselves (Paul et al, 2021). Further experiments are needed to determine the exact location of clerodanoids in and/or on the individual sawflies.…”

“…Plants used for this experiment were kept outside in pots and were flowering. Adults usually start within less than 20 min to voraciously "nibble" on the leaves (Müller, personal observation) and take up clerodanoid compounds in that way (Nishida et al, 2004;Paul et al, 2021). We refer to this experimental manipulation as "exposure treatment. "…”

Intergenerational Effects of Early Life Starvation on Life-History, Consumption, and Transcriptome of a Holometabolous Insect

“…For the F3 adults, pairs of non-sib females and males reared under the same larval starvation treatments (N or S) were assigned to one of two reproductive environment treatments (C- or C+). C+ individuals were exposed to a leaf section (1 cm 2 ) of A. reptans for 48 h prior to mating, giving individuals time to take up clerodanoids (preprint Paul et al 2021). Mated F3 females (2-9 days old) from each of these four treatments (NC-, NC+, SC-, SC+) and C-virgin females (NC-, SC-) were then placed in individual breeding boxes, as described above for the F2.…”

“…The leaf-consuming larvae can readily experience periods of starvation when their hosts are overexploited (Riggert 1939). The adults are nectar-feeding and in addition collect neo-clerodanoid-like compounds (hereafter called ‘clerodanoids’) from non-Brassicaceae plants, which improve their mating probability (Amano et al 1999) and affect the social interactions between adults of both sexes (preprint Paul et al 2021; preprint Paul and Müller 2021). We measured various key life-history traits to assess the combined and interactive effects of parental and offspring larval resource availability (starvation/non- starvation), and parental clerodanoid exposure on the offspring.…”