Plant defences against ants provide a pathway to social parasitism in butterflies

Abstract

Understanding the chemical cues and gene expressions that mediate herbivore-host-plant and parasite-host interactions can elucidate the ecological costs and benefits accruing to different partners in tight-knit community modules, and may reveal unexpected complexities. We investigated the exploitation of sequential hosts by the phytophagous-predaceous butterfly Maculinea arion, whose larvae initially feed on Origanum vulgare flowerheads before switching to parasitize Myrmica ant colonies for their main period of growth. Gravid female butterflies were attracted to Origanum plants that emitted high levels of the monoterpenoid volatile carvacrol, a condition that occurred when ants disturbed their roots: we also found that Origanum expressed four genes involved in monoterpene formation when ants were present, accompanied by a significant induction of jasmonates. When exposed to carvacrol, Myrmica workers upregulated five genes whose products bind and detoxify this biocide, and their colonies were more tolerant of it than other common ant genera, consistent with an observed ability to occupy the competitor-free spaces surrounding Origanum. A cost is potential colony destruction by Ma. arion, which in turn may benefit infested Origanum plants by relieving their roots of further damage. Our results suggest a new pathway, whereby social parasites can detect successive resources by employing plant volatiles to simultaneously select their initial plant food and a suitable sequential host.

Keywords: Maculinea; ant; gene expression; host detection; myrmecophily; plant volatiles.

Figure 1.

Life cycle of Ma. arion and its interaction with Myrmica ants and O. vulgare, indicated by this and previous [,–14] studies. (1) Female butterfly is attracted to flowers then, when close, by volatiles promoting oviposition (2) on flowerbuds of O. vulgare plants that co-occur with a nest of any Myrmica species; (3) final-instar larvae abandon O. vulgare and are adopted by Myrmica ants, in whose nests they live for 11 months, feeding on ant brood (4, 5) and acquiring more than 98% of their final biomass (5). Larval survival in the study race of Ma. arion is highest with Myrmica scabrinodis and Myrmica sabuleti and zero with ants of other genera [15].

Figure 2.

Effects of Myrmica ants on O. vulgare defence hormone production and gene induction. (a) Myrmica presence induces increases in the defence hormone JA, its conjugate JA-Ile and JA precursor OPDA; f.wt = fresh weight; *p < 0.001 between plants with and without Myrmica. (b) Myrmica presence causes a 10-fold induction for a monoterpene synthase gene (OvTPS2) that encodes a protein catalysing the formation of γ-terpinene, a direct precursor for thymol and carvacrol. Large increases also occurred in transcript levels of three cytochrome P450 genes involved in carvacrol and thymol biosynthesis: CYP71D180, CYP71D179/182 and CYP71D178; **p < 0.01, ***p < 0.001.

Figure 3.

Responses of Ma. arion to ant-induced plant volatiles. (a) Olfactometry choice bioassays of gravid female Ma. arion show a strong preference for carvacrol and its isomer thymol over clean air, as well as for the odour of O. vulgare exposed to ants (=treated plant) or of controls dosed with carvacrol rather than that of isolated control plants. Females also chose a blend of carvacrol and thymol rather than thymol alone; *p < 0.05, **p < 0.01. (b) EAG of Ma. arion antennal responses to the two main aromatic monoterpenes emitted by O. vulgare. (c) Antennal responses in EAGs to odours from plants grown with Myrmica among their roots (=treated) and to the odour of My. scabrinodis ant colonies.

Figure 4.

Tolerance of different ant genera to carvacrol. (a) Worker survival times when entrapped in cells containing 20 ppm pure carvacrol. Boxplots show median, quartile, maximum and minimum survival; outliers are open circles; Kruskal–Wallis test_{all ants} H₄ = 37.71, n = 63, d.f. = 4_, p < 0.001; pairwise: My. scabrinodis ≠ L. alienus (U = 41.577, p < 0.001), ≠ T. caespitum (U = 33.154, p < 0.001), ≠ F. cinerea (U = 27.077, p = 0.002), ≠ T. erraticum (U = 22.864, p = 0.023). (b) Gene expression of odourant receptor 1 and odourant-binding protein (OBP6) in My. scabrinodis ants after exposure to carvacrol. (c) Expression of detoxification-related genes in the five ant genera tested after exposure to carvacrol: acetylcholinesterase (AChE), GST and a cytochrome P450 (CYP4509E2). Error bars indicate s.e.m. for each gene, different letters indicate significant (p < 0.05) differences.