PARENT SESSION
Organized Oral Session 3: Mutualists as parasites: How mutualistic are mutualists, and why?
Organizer(s): JR Powell, BE Wolfe, and JN Klironomos
Monday, August 8, 8:00 AM - 11:30 AM, Meeting Room 511a, Level 5, Palais des congrès de Montréal

Coevolutionary adaptations stabilizing obligate ant-plant mutualism.

Heil, Martin^*,1, ¹ Dept. of General Botany - FB 9, Essen, Germany

ABSTRACT- Several species of the ant genus Pseudomyrmex in Central America are highly specialized to live only in hollow thorns of obligate Acacia ant-plants (myrmecophytes). The plants produce food resources to nourish their ant inhabitants, which play a crucial role in plant defense to herbivores. Besides cellular food bodies, the plants produce extrafloral nectar (EFN). Feeding experiments and analyses of stable isotopes revealed no hint on the use of any external food sources by the ants. Myrmecophytic Acacia species secrete EFN constitutively. This represents a derived trait since EFN in Acacia and related genera usually is secreted in response to leaf damage, thus representing an induced defense trait. Producing a highly attractive food source such as EFN constitutively increases the risk of exploitation by other, non-defending insects such as wasps and generalist ant species. Feeding experiments demonstrated, however, that EFN of Acacia myrmecophytes is less attractive to generalist ants than EFN of related, yet non-myrmecophytic species. One difference is the complete absence of sucrose (a disaccharide highly attractive to generalist ants) from the EFN of myrmecophytes. The absence of sucrose appears to result from the presence of invertase, i.e. a sucrose-cleaving enzyme, in the EFN of myrmecophytes. Correspondingly, Pseudomyrmex ants specialized to live on Acacia myrmecophytes show only low invertase activities in their digestive tract and have lost the capacity to induce this enzyme after ingestion of its substrate. Valuable resources are produced for exchange between the two partners involved in the mutualism. The behavioral and physiological characters as described here have obviously evolved in order to act as filters preventing these resources from being exploited by generalist ants and other potential parasites.

Key words: ant-plan mutualism, indirect defense, nectar, coevolution