Plants inhabited by ants (myrmecophytes) have evolved in a diversity of tropical plant lineages. Macaranga
includes approximately 300 paleotropical tree species; in western Malesia there are 26 myrmecophytic species that
vary in morphological specializations for ant association. The origin and diversification of myrmecophytism in Macaranga
was investigated using phylogenetic analyses of morphological and nuclear ITS DNA characters and studies
of character evolution. Despite low ITS variation, the combined analysis resulted in a well-supported hypothesis of
relationships. Mapping myrmecophytism on all most parsimonious trees resulting from the combined analysis indicated
that the trait evolved independently between two and four times and was lost between one and three times (five
changes). This hypothesis was robust when tested against trees constrained to have three or fewer evolutionary
transformations, although increased taxon sampling for the ITS analysis is required to confirm this. Mapping morphological
traits on the phylogeny indicated that myrmecophytism was not homologous among lineages; each independent
origin involved a suite of different specializations for ant-plant association. There was no evidence that
myrmecophytic traits underwent sequential change through evolution; self-hollowing domatia evolved independently
from ant-excavated domatia, and different food-body production types evolved in different lineages. The multiple
origins of myrmecophytism in Macaranga were restricted to one small, exclusively western Malesian lineage of an
otherwise large and nonmyrmecophytic genus. Although the evolution of aggregated food-body production and the
formation of domatia coincided with the evolution of myrmecophytism in all cases, several morphological, ecological,
and biogeographic factors appear to have facilitated and constrained this radiation of ant-plants