Fig trees (Ficus, Moraceae) and their pollinating wasps (Chalcidoidea, Agaonidae) are
involved in an obligate mutualism where each partner relies on the other in order to
reproduce: the pollinating fig wasps are a fig tree’s only pollen disperser whilst the fig
trees provide the wasps with places in which to lay their eggs. Mutualistic interactions
are, however, ultimately genetically selfish and as such, are often rife with conflict. Fig
trees are either monoecious, where wasps and seeds develop together within fig fruit
(syconia), or dioecious, where wasps and seeds develop separately. In interactions
between monoecious fig trees and their pollinating wasps, there are conflicts of interest
over the relative allocation of fig flowers to wasp and seed development. Although fig
trees reap the rewards associated with wasp and seed production (through pollen and
seed dispersal respectively), pollinators only benefit directly from flowers that nurture
the development of wasp larvae, and increase their fitness by attempting to oviposit in
as many ovules as possible. If successful, this oviposition strategy would eventually
destroy the mutualism; however, the interaction has lasted for over 60 million years
suggesting that mechanisms must be in place to limit wasp oviposition.
This thesis addresses a number of factors to elucidate how stability may be
achieved in monoecious fig systems. Possible mechanisms include: 1) a parasitoidcentred
short ovipositor hypothesis in Ficus rubiginosa, which suggests that a subset of
flowers are out of reach to parasitoid ovipositors making these ovules the preferred
choice for ovipositing pollinators and allowing seeds to develop in less preferred ovules;
2) the presence of third-party mutualists such as non-pollinating fig wasps (F. burkei)
and patrolling green tree ants on the fig surface (F. racemosa) that limit pollinator and
parasitoid oviposition respectively; and 3) selection on fig morphology which constrains
the size (and therefore fecundity) of the associated pollinators. I discuss the lack of
evidence for a single unifying theory for mutualism stability and suggest that a more
likely scenario is the presence of separate, and perhaps multiple, stabilising strategies in
different fig/ fig-wasp partnerships.