Centre d’Ecologie Fonctionnelle et Evolutive (CEFE-CNRS), 1919 Route de Mende, 34293 Montpellier Cedex 5, France.
Ants and flowering plants are each characterised by a rich panoply of chemically mediated intra- and interspecific interactions. What happens when these two worlds come together? I will examine this question for one of the three main classes of mutualistic interactions between the two groups, those involving protection of plants by ants. In these interactions, ants are incorporated to various degrees into the defence systems of plants against herbivores and pathogens, adding a new dimension to classical questions about optimal defence. How are biotic and chemical defences linked to each other, from functional and evolutionary points of view? Biotic defence also brings a completely new set of questions into plant defence theory. Because ant partners are acquired by the plant, not inherited, biotic defence can play only a limited role in early stages of plant ontogeny, before ants are acquired. In ant-plant symbioses, plants differ in the timing of onset in ontogeny of ant-related traits, so that relative roles of biotic and chemical defence over ontogeny vary among systems. Chemical signals must mediate many phases of these interactions, but little is known about this aspect of their functioning. How do founding queens of host-specific plant-ants locate individual hosts? How are ant patrolling activities oriented to parts of the plant where they most increase plant fitness? How are evolutionary conflicts between ants and plants resolved? In these horizontally transmitted symbioses, neither partner has a genetic interest in the reproduction of the other, and investment in reproduction instead of growth can reduce benefits to the partner. How does the plant protect its flowers against castrating ants? Chemical ecology can contribute not only to understanding how protection mutualisms have evolved and how they work, but also how they remain evolutionarily stable.