Host manipulations within mutualisms: Role of plant hormones in selective resource allocation

Abstract

In some mutualisms involving host 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 poorly 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 an enclosed inflorescence called a syconium, in exchange for flowers that develop into galls nourishing pollinator offspring. To examine host manipulation by pollinator galls by affecting host fitness through seed reduction, 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 early and mid-phases of syconial maturation when maximal growth occurs and measured hormone levels. Hormones were reduced by mid-phase in general; however, their levels 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 mutualism, syconium volume and hormone concentrations were highest when both seeds and galls were present (SG treatment), indicating that both partners control allocation of resources to syconia.