Regulation of Seed and Pollinator Production in the Fig-Fig Wasp Mutualism

Abstract

Female agaonine fig wasps enter Ficus fruits (figs), where they pollinate the flowers and oviposit into the ovaries of a proportion of the flowers via their styles. The fig trees are totally dependent on these fig wasps for sexual reproduction, as the wasps fertilize the figs' seeds and transfer pollen between trees, while the wasps need the fig trees' ovaries as feeding sites (galled seeds) for their progeny. Since the wasp progeny destroy seeds, the question arises as to why selection has not led to increasingly higher fig wasp fecundities and the eventual collapse of the mutualism. The stability of the mutualism could be maintained by pollinators having short ovipositors, restricting oviposition to short-styled flowers so that long-styled flowers produce seeds. However, in most of the monoecious Ficus species where measurements were taken, pollinators generally possessed ovipositors of sufficient length to reach the ovaries of a large majority of the flowers. This was confirmed by the presence of pollinator progeny in flowers with long styles. Complete seed destruction may also be avoided by a proportion of fig ovaries being inviolate to oviposition by fig wasps, independent of their style lengths. However, experimental increases in pollinator foundresses per fig in F. burtt-davyi showed that all the ovaries that could be reached by the wasps' ovipositors were potentially exploitable by agaonine fig wasps. Relative production of wasps and seeds was found to be largely dependent on the interplay between the number of eggs inside each wasp, the number of accessible ovaries in individual figs, and the average number of foundresses entering each fig. On average, there were not enough foundresses entering each fig to utilize all accessible ovaries. Possible factors affecting the total entry numbers of wasps into figs include ostiolar closure rates and wasp population densities in the areas surrounding fig trees. As many ovaries remain unused because of a frequent shortage of wasp eggs per fig, fig trees can continue to produce enough seeds for the continued evolutionary stability of the fig-fig wasp mutualism.