Fruit defence syndromes: the independent evolution of mechanical and chemical defences

Plants are prone to attack by a great diversity of antagonists against which they deploy various defence mechanisms, of which the two principle ones are mechanical and chemical defences. These defences are hypothesized to be negatively correlated due to either functional redundancy or a trade-off, i.e., plants which rely on increased mechanical defence should downregulate their degree of chemical defence and vice versa. A competing hypothesis is that different defences perform distinct functions and draw from different pools of resources, which should result in their independent evolution. We examine these competing hypotheses using two independent datasets of fleshy fruits we collected from Madagascar and Uganda. We sampled mechanical defences, indexed by fruit puncture resistance, and defensive defences, indexed by defensive volatile organic compounds, and examined their associations using phylogenetically-controlled models. In both systems, we found no correlation between mechanical and chemical defences, thus supporting the independent evolution hypothesis. This implies that fruit defence mechanisms reflect a more complex array of selection pressures and constraints than previously perceived.