Manipulating Hosts Within Mutualisms: Role of Plant Hormones in Selective Resource Allocation

Abstract In some mutualisms involving plants, photoassimilates are provided as rewards to symbionts. Endophagous organisms often manipulate host plants to increase access to photoassimilates. Host manipulations by endophagous organisms that are also mutualists are generally less understood. We show host plant manipulations by symbionts and the role of phytohormones, i.e. the auxin indole-3-acetic acid (IAA), and the cytokinin trans-zeatin (tZ), in the brood-site pollination mutualism between fig trees and pollinator fig wasps. In this interaction, pollinator wasps pollinate Ficus flowers within a closed inflorescence called a syconium, in exchange for flowers that develop into galls nourishing pollinator offspring. To examine host manipulation by pollinator galls, we compared growth hormones released by syconial occupants within three experimentally produced treatment groups of syconia: S (containing only seeds), G (containing only pollinator galls) and SG (containing seeds and pollinator galls). We harvested syconia from each treatment in the early and mid-phases of syconial maturation when maximal growth occurs and measured hormone levels Hormone levels were reduced by mid-phase in general; however, they were mostly sustained in G syconia in the mid-phase, suggesting that galls manipulate the host to continuously access resources. We found no difference in IAA and tZ levels of S and G syconia. IAA concentrations were higher in SG syconia. From the perspective of the maintenance of the mutualism, syconium volume and hormone concentrations were highest when both seeds and galls were present (SG treatment), indicating that joint control by both partners over allocation of resources to syconia.