Resource allocation patterns and strategies in the mutualism between figs and fig wasps

Abstract

Mutualism involves exchange of services and rewards between the partners, resulting in a net benefit to all those involved. In many mutualisms, hosts are larger partners that interact with several individuals of smaller mutualists that live on or within them and are termed symbionts. Partners have an incentive to cheat, leading to possible breakdown of the mutualism. Hosts may regulate interaction by selectively rewarding cooperative symbionts. However, this host-centric view, that has dominated mutualism studies, does not explain the role of symbionts in regulating trade in a mutualism. Mutualisms are mediated by resources and mechanisms of resource allocation in plants can be applied to study the exchange in plant-associated mutualisms. My thesis is an attempt to understand resource allocation patterns and the strategies employed by both partners in a prominent brood-site pollination mutualism between fig trees and their pollinator fig wasps. Fig trees are pollinated by pollinator wasps, which in turn get shelter and nutrition for their offspring developing inside galls within fig inflorescences called fig syconia. While seeds are attributed to increased allocation of resources, effect of galls on allocation has not been considered. In this thesis, we attempt to understand patterns of resource allocation in a syconium and the role of each partner in determining the allocation patterns.

In the first chapter, we determine the pattern of resource partitioning to different components of a fig syconium. We used dry mass as well as carbon and nitrogen stoichiometry as parameters to describe the allocation. We found that the syconial wall, which provides protection to the occupants, makes up the majority of the dry mass, although it is nutritionally less demanding. Our results also showed that a single pollinator wasp is costlier to produce compared to a single seed. In the second chapter, we measured the elementome of seeds, pollinators and the syconial wall tissue and expressed it as a biogeochemical niche (BN). We found that the BN of seeds and pollinators are significantly different allowing their co-existence within the syconium. We also measured trophic stoichiometric ratios (TSRs) values which were higher for nitrogen and sulphur in the case of male and female pollinators, indicating high resource mismatch for those elements. In the third chapter, we attempt to understand if individual differences in composition of seeds and pollinators result in differential allocation of resources to the syconium. We experimentally manipulated pollinators (foundresses) to produce syconia containing only seeds (S), only pollinators (G) and both seeds and pollinators (SG). We found that overall, the presence of both seeds and pollinators increased allocation to a syconium. Further, seeds are important determinants of allocation at low foundress density (numbers) and galls become important indicators of allocation at higher density. In the last chapter, we attempt to understand the role of plant growth hormones in the differential resource allocation. We measured the concentrations of two plant growth hormones: indole-3-acetic acid (IAA), an auxin and trans-Zeatin (tZ), a cytokinin, in S, G and SG syconia. IAA and tZ concentrations did not differ between S and G syconia indicating that galls might be mimicking seeds to garner resources. Further, SG syconia had higher hormone levels correlating with their increased size. Overall, my thesis provides a comprehensive view of resource allocation to a fig syconium and the role of both partners in determining patterns of allocation.