EVOLUTIONARY STABILITY OF MUTUALISM BETWEEN YUCCAS AND YUCCA MOTHS

INTERSPECIFIC mutualisms inherently possess a conflict of interests be tween the interacting species in that fitness increases of one species occur at the expense of the other. This holds for mutualisms as diver se as plant associations with mycorrhizal fungi or nitrogen-fixing bac teria, animals and endosymbionts, and obligate plant-pollinator associ ations(1-6). Prevailing models of interspecific cooperation predict th at mutualistic interactions are evolutionarily stable only when both i nteracting species possess mechanisms to prevent excessive exploitatio n(3-6). In light of this, it is paradoxical that some of the classical examples of coevolved obligate mutualism seemingly do not meet this c riterion. In mutualisms involving seed parasites that actively pollina te their hosts, such as yucca/ yucca moth and fig/fig wasp interaction s, there is no apparent means of retaliation on behalf of the plant. P redictions from theory suggest that a cryptic mechanism, such as selec tive abortion of flowers with heavy egg loads, could stabilize these i nteractions(4,6-9). Here we present the first empirical evidence that such a mechanism in fact exists in the yucca/yucca moth interaction. A strong negative effect exists between moth egg number and probability of flower retention. Furthermore, we show a strong positive effect be tween the number of pollinations received and the probability of flowe r retention. Selective maturation of fruit with low egg loads and high pollen loads provides a mechanism to increase the quantity and possib ly quality of seeds produced, and simultaneously select against moths that lay many eggs per flower or provide low-quality pollinations(4,6, 8,10). Not only can these results explain the stability of this type o f interaction, but selection for high-quality pollination also provide s a mechanism to help explain the evolution of active pollination amon g yucca moths.