Inflorescence size predicts host-symbiont conflict in monoecious fig-wasp mutualisms

In monoecious fig-wasp mutualisms (Ficus; ~350 spp.), tiny wasps obligately pollinate fig-tree inflorescences (‘figs’). Although pollination enables seed production wasp symbionts also oviposit into flowers, replacing potential seeds with wasp offspring. Consistently across Ficus, ~40-60% of developed flowers produce seeds. Although several processes are likely involved, a general explanation for why wasps do not exploit more flowers has been elusive. However, inter-specific scaling between host-symbiont reproductive traits suggests that as figs become larger across Ficus, the increase in wasp-eggs-per pollen-receptive fig will fail to match the increase in flower numbers. The potential for wasps to exploit hosts should thus decline due to an increasing excess of flowers. We tested these predictions, which were upheld, using data from 23 fig-wasp mutualisms from four continents. As fig size increases across Ficus, wasp egg-to-flower ratios, a measure of host-symbiont conflict-of-interest, declines, but the likelihood of a wasp egg successfully becoming an adult offspring increases. Host-symbiont conflict-of-interest thus varies systematically across Ficus due to variable relationships between key mutualist reproductive traits and fig size. We suggest that mutualism stability is more dependent upon mechanisms curtailing wasp flower exploitation in systems with small figs, and on preventing high foundress numbers in those with large figs.

Methods
Methods are detailed in the published paper.