Colony Foundation and Symbiont Transmission in An Ant-Plant Farming Mutualism

The Fijian farming ant Philidris nagasau and its six epiphytic plant associates in the genus Squamellaria have a unique farming mutualism. Models and predictions for coevolutionary processes differ between interactions that are transmitted vertically, from parent to offspring, and horizontally, between unrelated individuals. It is already known that these ants exclusively nest in specialist structures (domatia) of associated plants and cultivate, disperse and protect these hosts and their offspring over the lifespan of a colony. What is unclear is how this mutualism is maintained and transmitted between generations of colonies. In this project, I employed population genetics and phylogenetics approaches to test hypothesised Dependent Colony Foundation (DCF) and vertical transmission of symbionts to daughter colonies. These mechanisms are expected to result in extremely limited female dispersal in ants and a high degree of congruence between plant and ant genealogies. Population and individual-level genetic structure analyses indicated a high degree of male bias in dispersal and some support for fine-scale spatial structuring within a short (<1.5 km) distance. Phylogenetic analyses recovered clades of symbionts within colonies, but colonies were never entirely monophyletic. Maximum parsimony reconciliations of maximum-likelihood phylogenies were conducted in eMPRess. These revealed significant congruence between ant and plant trees and support a degree of vertical transmission. However, high numbers of events in which plant lineages transferred to a different ant colony lineage were also observed. These “transfer” events are consistent with high frequencies of horizontal transmission and suggest that transmission of the Philidris nagasau-Squamellaria system may be more mixed than anticipated.