Dissertation Research: Evolution of Host Affiliation in an Ant-Plant Mutualism: Phylogeny of the Crematogaster Inhabitants of Macaranga (Euphorbiaceae).

Harvard University, Cambridge MA

Investigators

Abstract

In the aseasonal rain forests of Southeast Asia, ants of the genus Crematogaster live exclusively inside stems of trees in the genus Macaranga (Euphorbiaceae), forming an obligate partnership from which both parties benefit. The ants remove vines and herbivores from their hosts and subsist exclusively on food bodies produced by the plants and honey-dew from scale-insect which they tend inside the stems. The interaction involves ca. 8 ant lineages and ca. 26 host species. Based on a molecular phylogeny of the Crematogaster, we will investigate evolutionary trends in their association with Macaranga hosts. In particular we will examine correlations between host traits and ant traits that influence patterns of association. Preliminary results indicate that the presence or absence of copious epicuticular wax blooms on the stems of Macaranga have played an important role in determining the identity of the ant tenants on Macaranga. We also aim to address some of the mechanisms that underlie the phylogenetic patterns in host association, for example, what are the relative contributions of contemporary and historical processes in explaining patterns of association? Additionally, using the molecular phylogeny and Scanning Electron Microscopy, we seek to resolve species limits in the ants as suggested by morphological data. Because the integrity of our conclusions are contingent upon the assumption that the Macaranga-inhabiting ants form a natural group, we will verify the monophyly of the ants by including in the molecular phylogeny other members of their subgenus not found with Macaranga. The ecology, systematics and biogeography of Macaranga have been thoroughly investigated, affording a more holistic understanding of the system beyond an investigation based solely on the ants. The phylogenetically comprehensive scope of our study will afford an evolutionary perspective to the growing body of research on the ecological aspects of this interaction. This study will provide a comparison for other phylogenetically-based ant-plant studies and help illuminate ecological and evolutionary themes in ant-plant mutualisms. Contrasts with mutualistic systems where symbionts are directly inherited from the previous generation (such as yucca and yucca moths, and figs and fig wasps) may reveal why cospeciation more often explains the evolution of pollinating mutualisms than it does ant-plant mutualisms.