Plants inhabited by ants (myrmecophytes) have evolved in a diversity of tropical plant lineages. The Paleotropical tree genus Macaranga (Euphorbiaceae) includes ~300 species. Twenty-six west Malesian species of Macaranga are myrmecophytic, and they vary in their morphological specializations for ant association. In order to ascertain the origins and diversification of myrmecophytism in Macaranga, phylogenetic analyses of selected Malesian species were conducted using morphological and nrITS DNA characters, followed by the mapping of ant-plant associations and allied traits onto the resulting phylogenetic trees. Combined analysis of morphological and ITS data resulted in a well-supported hypothesis of relationships. Mapping 'myrmecophytism' on all most parsimonious trees resulting from the combined analysis indicated that myrmecophytism evolved independently in Macaranga between two and four times and was lost between one and three times (five changes). Mapping morphological traits on the phylogeny suggested that myrmecophytism was not homologous among lineages, and that each independent origin involved a suite of different specializations for the ant-plant association. A number of different morphological, ecological and biogeographic factors appear to have facilitated and constrained this radiation of ant-plants.

Key words: Macaranga, myrmecophytism, west Malesia