Defensive Symbiosis of the Wood Tiger Moth (Arctia plantaginis)

Abstract

Microbial contributions to the protection of insects can impact on a host’s fitness but the dynamics of these symbioses can vary more than many nutritional-symbiont associations seen in the literature. The wood tiger moth (Arctia plantaginis) secretes defensive fluids when attacked by avian predators, and they are home to bacterial communities including genera known to synthesise compounds similar to those found in the secretions. I studied the role bacteria play in the efficacy of the defensive secretions against avian predators and their contributions to the pyrazine chemical components of the secretions. I characterised the spatial and temporal variability of the secretion’s bacterial taxa, and the impact on the life histories of A. plantaginis following their depletion. The former was done by manipulating the microbiome with antibiotics and testing the subsequent defensive secretions with predator assays and GC/MS. The latter used sequencing of the 16s rRNA gene to identify the bacteria in defensive secretions from wild moths, while life history traits of A. plantaginis were recorded with gene expression data following antibiotic treatment of larvae. Bacteria-depleted secretions did not illicit hesitation from birds in the predator assays, but the birds’ perception of the secretion’s taste remained unchanged. Chemical analysis showed no changes in the secretion’s methoxypyrazine concentrations. Analysis of the microbiome revealed that bacterial taxa remained similar across a wide geographic area and multiple genetic populations of A. plantaginis, but there were significant changes in the microbiome composition over time. Following depletion of bacteria, A. plantaginis up-regulated their growth related genes and down-regulated immune system genes. They reached adulthood sooner, while adult females were significantly lighter without any loss in fecundity. The bacteria are contributing to olfactory cues directed towards avian predators, but it is not one of the prominent methoxypyrazines meaning further relevant compounds are present in the defensive secretions that have not been identified yet. The loosely associated bacterial taxa may form a functional core in which multiple taxa can contribute to the secretion’s efficacy. The need for the immune system in controlling bacteria in the moth’s body is costly for the host. Keywords: Bacteria; chemical defences; microbiome.