Who holds the reins? Context-dependent resource allocation in the mutualism between fig trees and their fig wasp pollinators.

Abstract

Mutualisms are consumer-resource interactions, in which goods and services are exchanged. Biological market theory states that exchanges should be regulated by both partners. However, most studies on mutualisms are one-sided, focusing on the control exercised by host organisms on their symbionts. In the brood-site pollination mutualism between fig trees and their symbiont wasp pollinators, galled flowers are development sites for pollinator larvae and are exchanged for pollination services. We determined if pollinator galls influenced resource allocation to fig inflorescences called syconia and considered feedbacks from the host tree. We experimentally produced syconia containing only seeds (S), only pollinator galls (G) or seeds and galls (SG) with varying number of introduced female pollinator wasps, i.e., foundress wasps. Biomass allocation to syconia was affected by foundress numbers and treatment groups; SG treatments received highest biomass allocation at low foundress numbers, and both G and SG treatments at high foundress numbers. Seeds are important determinants of allocation at low foundress numbers; galls are likely more influential at high foundress numbers. Most allocation in the G and SG treatment was to the syconium wall, likely as protection from parasitoids and temperature/humidity fluctuations. Dry mass of individual seeds and wasps (except at low foundress numbers) was unchanged between treatment groups, indicating seeds and wasps regulate resource flow into them, with lower flow into galls containing the smaller males compared to females commensurate with sexual dimorphism. We demonstrate the importance of considering the direct role of symbionts in accessing resources and controlling exchanges within mutualisms.