Evolutionary biomechanics: host specificity and the role of “wax barriers” in Macaranga-ant mutualisms
Supervisor: Walter Federle
Interactions between insects and plants such as herbivory, pollination and seed dispersal play a key role for almost all land
ecosystems. The insects' extraordinary diversity may be based to a large extent on their ability to interact with plants, which has led to reciprocal adaptations and coevolution over hundreds of millions of years. While chemical mechanisms of insect-plant interactions such as feeding inhibitors, toxins and signals have been studied in detail, the role of physical factors remains poorly characterized. The aim of this project is to study the complex ecological effects and evolutionary implications of physical factors in a model system, the mutualism between Macaranga trees and ants in SE Asia. This is a system of approximately 30 obligate ant-plants, which are inhabited by at least 12 morphospecies of specialised ants of the genera Camponotus and Crematogaster, which protect their hosts against herbivory and overgrowing vines.
15 species of Macaranga trees have waxy stems, which are very slippery for most insects but not for their ant partners which are specialised “wax-runners”. Thus, the plant’s “wax barriers” protect the ant partners against competitors and predators (Federle et al. 1997; Federle & Rheindt 2005). This project will analyse the detailed mechanisms of wax-running, clarify the consequences of wax barriers for host specificity in the Macaranga-ant mutualism, and trace the evolutionary development of the ants’ wax-running ability in the Crematogaster (Decacrema) clade, which comprises at least eight species of wax-runners and non-wax-runners (Feldhaar et al. 2003).
Field experiments will be conducted in Borneo on selected Macaranga-ant associations to clarify the mechanisms underlying host plant specificity. Colony transplantation and preference experiments will test the extent to which the ants’ host plant specificity is based on the presence of wax barriers, chemical cues or on imprinting. To study the mechanisms of wax-running, 3D high-speed analysis of the ants’ climbing locomotion, ablation experiments, manipulations of the wax layer, whole-body / single-leg force measurements under different conditions will be performed in the laboratory, using host plants and ant colonies kept in the greenhouse. The evolutionary development of identified
traits and adaptations will be studied by incorporating available phylogenetic information of both ants and plants. The student will be given training in field work, as well as a variety of biomechanical laboratory techniques, including force measurements, 3D high-speed motion analysis, light and electron microscopy, as well as computer-based analysis. The project will suit a biologist with field work experience and quantitative skills for biomechanical work in the laboratory.
Federle, W., Maschwitz, U., Fiala, B., Riederer, M. & Hölldobler, B. 1997. Slippery ant-plants and skilful climbers: Selection and protection of specific ant partners by epicuticular wax blooms in Macaranga (Euphorbiaceae). Oecologia 112, 217-224.
Federle, W. & Rheindt, F. 2005. Macaranga ant-plants hide food from intruders: correlation of food presentation and presence of wax barriers analysed using phylogenetically independent contrasts. Biol. J. Linn. Soc. 84, 177-193.
Feldhaar, H., Fiala, B., Gadau, J., Mohamed, M. & Maschwitz, U. 2003. Molecular phylogeny of Crematogaster subgenus Decacrema ants (Hymenoptera: Formicidae) and the colonization of Macaranga (Euphorbiaceae) trees. Mol. Phyl. Evol. 27, 441-452.