Discriminative host sanctions in a fig-wasp mutualism.

In some mutualisms, cooperation in symbionts is promoted by hosts sanctioning "cheats," who obtain benefits but fail to reciprocate. In fig-wasp mutualisms, agaonid wasps pollinate the trees (Ficus spp.), but are also exploitative by using some flowers as larval food. Ficus can sanction cheats that fail to pollinate by aborting some un-pollinated figs. However, in those un-pollinated figs retained by trees, cheats successfully reproduce. When this occurs, wasp broods are reduced, suggesting sanctions increase offspring mortality within un-pollinated figs. We investigated sanction mechanisms of abortion and larval mortality against wasp cheats in the monoecious Ficus racemosa by introducing into figs 1, 3, 5, 7, or 9 female wasps (foundresses) that were either all pollen-laden (P+) or all pollen-free (P-). The abortion rates of P- figs were highest (-60%) when single foundresses were present. Abortion declined with increased foundresses and ceased with seven or more wasps present, irrespective of pollination. In un-aborted figs, wasp fitness (mean offspring per foundress) declined as foundress number increased, especially in P- figs. Reduced broods in P- figs resulted from increased larval mortality of female offspring as foundress number increased, resulting in more male-biased sex ratios. Overall sanctions estimated from both abortion rates and reduced offspring production strengthened as the number of cheats increased. In a second experiment, we decoupled pollination from wasp oviposition by introducing one pollen-free foundress, followed 24 h later by seven pollen-laden ovipositor-excised wasps. Compared with P+ and P- single-foundress figs, delayed pollination resulted in intermediate larval mortality and wasp fitness, which concurred with patterns of female offspring production. We conclude that fig abortion reflects both pollinator numbers and pollen presence. Sanctions within P- figs initiate soon after oviposition and discriminate against female offspring, thus reducing the benefits to cheats from adaptively biasing their offspring sex ratios. We suggest that costs to cheats via these discriminative sanctions are likely to promote stability in this mutualism.