Longevity, early emergence and body size in a pollinating fig wasp – implications for stability in a fig–pollinator mutualism

Abstract

Summary Fig trees (Ficus ) are pollinated only by agaonid wasps, whose larvae also gall fig ovules. Each ovule develops into either a seed (when pollinated) or a wasp (when an egg is also laid inside) but not both. 2. Ovipositing wasps (foundresses) favour ovules near the centre of the enclosed inflorescence (syconium or ' fig'), leaving ovules near the outer wall to develop into seeds. This spatial stratification of wasps and seeds ensures reproduction in both partners, and thereby enables mutualism persistence. However, the mechanism(s) responsible remain(s) unknown. 3. Theory shows that foundresses will search for increasingly rare inner ovules and ignore outer ovules, as long as ovipositing in outer ovules is sufficiently slow and/or if inner ovules confer greater fitness to wasps. The fig-pollinator mutualism can therefore be stabilized by strong time constraints on foundresses and by offspring fitness gradients over variation in ovule position. 4. Female fig wasps cannot leave their galls without male assistance. We found that females in outer ovules were unlikely to be released. Inner ovules thus have added value to foundresses, because their female offspring are more likely to mate and disperse. 5. For those offspring that did emerge, gall position (inner/outer) and body size did not influence the order in which female pollinators exited syconia, nor did early emerging wasps enjoy increased life spans. 6. We also found that the life spans of female wasps nearly doubled when given access to moisture. We suggest that conflict resolution in the fig-pollinator mutualism may thus be influenced by tropical seasonality, because wasps may be less able to over-exploit ovules in dry periods due to time constraints.

Spatial heterogeneity and host repression in fig-fig wasp mutualism

“…The pollination and oviposition efficiency of foundresses might greatly affect the trade-off between the figs and pollinators wasps [22,28,29]. We also examined whether the pollination and oviposition efficiency of foundresses varies seasonally.…”

Spatial Stratification of Internally and Externally Non-Pollinating Fig Wasps and Their Effects on Pollinator and Seed Abundance in Ficus burkei

“…Ovule length was determined as pedicel length plus the length of the seed or wasp gall, excluding what remained of the style. Pedicel length was not used for two reasons: (1) some outer ovules have no pedicels, which would result in many zeros in the dataset, (2) there is often no distinct landmark where the pedicel joins the seed or gall, making accurate, repeatable measurements difficult [13,15]. The numbers of female pollinators (foundresses) and INPFWs that had been present in each syconium when it had been receptive could not be accurately counted.…”

“…For all models, we initially included the independent (explanatory) variables "site" and "tree" as factors to account for unmeasured variance between localities and trees [13][14][15]33], and fig volume as a covariate. The inclusion of fig volume in this manner removed the need to use "fig" as an additional nested factor within "tree."…”

“…In three morphologically and phylogenetically distinct Australasian Ficus, this results in additional spatial stratification of pollinator and parasitic NPFW galls, showing that inner ovules provide pollinator offspring with "enemy-free space" [13]. Additionally, female pollinators in outer ovules are unlikely to be released from their galls by males [15]. Clearly, biased oviposition and biased survival are not mutually independent mechanisms, because biased survival is likely to contribute to selection for biased oviposition [13].…”

“…[12]) or biased offspring survival (e.g. [13][14][15]). Ovipositing foundress wasps may favour inner ovules because of shorter handling times offered by the short styles [10,12].…”