Origanum vulgare terpenoids modulate Myrmica scabrinodis brain biogenic amines and ant behaviour

Abstract

Coordinated social behaviour is fundamental for ant ecological success. However, even distantly-related organisms, such as plants, have evolved the ability to manipulate ant collective performances to their own advantage. In the parasitic system encompassing Maculinea butterflies, Myrmica ants, and Origanum vulgare plants, the ant-plant interaction elicits the release of a volatile terpenoid compound (carvacrol) which is used by the gravid butterfly to locate the ideal oviposition site. Here we show that this ant-plant association is maintained by the effect of O. vulgare terpenoids on ant behaviour and that food plants might gain protection by Myrmica ants by chemically manipulating workers to forage in their surroundings. The variation in the locomotor ability of three ant species (Formica cinerea, Tetramorium caespitum, and Myrmica scabrinodis) was studied after treatment with the two major O. vulgare terpenoid volatile compounds (i.e., carvacrol and thymol). The brain levels of three biogenic amines (dopamine, tyramine and serotonin) were analysed in ants exposed to the O. vulgare terpenoids by HPLC-ESI-MS/MS. Carvacrol and thymol increased the locomotor activity of all ant species tested, but if blended reduced the movement propensity of Myrmica scabrinodis. Dopamine and tyramine production was positively correlated with the worker locomotor activity. In Myrmica ants, both brain biogenic ammines were negatively correlated with the aggressive behaviour. Blends of O. vulgare volatiles affected the locomotor ability while increased the aggressiveness of Myrmica workers by altering the aminergic regulation in the ant brains. This behavioural manipulation, might enhance partner fidelity and plant protection. Our findings provide new insights supporting a direct role of plant volatiles in driving behavioural changes in social insects through biogenic amine modulation.

Fig 1. Ant locomotor activity.

Data are expressed as the frequency of crossing the bisector traced on the Petri lid, taken as a proxy for worker locomotor activity affected by treatments (C: carvacrol; T: thymol; Ct: 3:1 (v/v) carvacrol/thymol; Tc: 1:3 (v/v) carvacrol/thymol) and controls (CTRL and DMSO) on Formica cinerea (A), Tetramorium caespitum (B) and Myrmica scabrinodis (C). Boxplots show median, quartile, maximum and minimum values; different letters indicate significant differences (Tukey’s HSD post hoc, P < 0.05; S2 Table).

Fig 2. Dopamine content.

Effects of treatments (C: carvacrol; T: thymol; Ct: 3:1 (v/v) carvacrol/thymol; Tc: 1:3 (v/v) carvacrol/thymol) and controls (CTRL and DMSO) on dopamine contents in ant brains of Formica cinerea (A), Tetramorium caespitum (B) and Myrmica scabrinodis (C). Values are expressed as pmol dopamine per ant brain. Boxplots show median, quartile, maximum and minimum values; different letters indicate significant differences (Tukey’s HSD post hoc, p < 0.05; S3 Table).

Fig 3. Tyramine content.

Effects of treatments (C: carvacrol; T: thymol; Ct: 3:1 (v/v) carvacrol/thymol; Tc: 1:3 (v/v) carvacrol/thymol) and controls (CTRL and DMSO) on tyramine contents in ant brains of Formica cinerea (A), Tetramorium caespitum (B) and Myrmica scabrinodis (C). Values are expressed as pmol tyramine per ant brain. Boxplots show median, quartile, maximum and minimum values; different letters indicate significant differences (Tukey’s HSD post hoc, p < 0.05; S3 Table).

Fig 4. Aggression index.

Aggression index between heterocolonial M. scabrinodis workers treated with different compounds or pure air (CTRL: non-treated; CC: carvacrol vs. carvacrol; TT: thymol vs. thymol; Ct-Ct: 3:1 (v/v) carvacrol/thymol vs. 3:1 (v/v) carvacrol/thymol, and Tc-Tc 1:3 (v/v) carvacrol/thymol vs. 1:3 (v/v) carvacrol/thymol). Data are expressed as number of aggressive behaviours on the total number of interaction (see methods for details). Boxplots show median, quartile, maximum and minimum values; different letters indicate significant differences (Tukey’s HSD post hoc, p < 0.05; S4 Table).