Mechanisms of speciation in Southeast Asian ant-plants of the genus Macaranga (Euphorbiaceae) associated with Crematogaster ants

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Biotic interactions between organisms can be a crucial factor driving diversification and speciation. We are investigating myrmecophytic ant-plant associations as a model system for the evolution of mutualistic interactions in general. In myrmecophytes, the ants benefit from food and preformed nest sites supplied by their host whereas the host plants benefit from protection against herbivores and vines. The most prominent ant-plant system of the palaeotropics consists of the pioneer tree genus Macaranga and its manifold associations with Crematogaster-ants. Our previous work has shown that genetic distances between myrmecophytic Macaranga species are small, suggesting a recent radiation. We envisage two hypotheses that may account for this observation. First, the obligatory symbiosis with ants reduces gene flow among plant populations, resulting in population differentiation, and eventually allopatric speciation. Alternatively, the acquisition of mutualistic ant partners served as a key innovation which enhanced the fitness of the plant partners in certain habitats, thereby promoting ecological differentiation. To test the first of these hypotheses, we will initiate a comparative study on two pairs of myrmecophytic vs. non-myrmecophytic Macaranga species to analyse (1) population structure and the extent of gene flow using nuclear microsatellite markers, and (2) historic population divergence (phylogeography) based on chloroplast haplotypes.