POLLEN-MEDIATED SELECTIVE ABORTION IN YUCCAS AND ITS CONSEQUENCES FOR THE PLANT–POLLINATOR MUTUALISM

Abstract

Mutualistic interactions contain an underlying evolutionary conflict, in that natural selection favors individuals better at exploiting their mutualistic partner, thus potentially destabilizing the mutualism. In the obligate mutualism between yuccas and yucca moths, selective abortion of flowers that receive many moth eggs or receive poor pollen load has been suggested as a mechanism that can prevent evolution of excessive exploitation by moths. Using controlled pollinations, varying pollen amount and source, we asked whether variation in pollination quality of the magnitude seen among moths affects patterns of selective abortion. Pollination experiments in two Yucca filamentosa populations revealed that both pollen quantity (large/small) and quality (self/one-outcross/two-outcross donors) affected fruit retention, seed set, and seedling growth rate. A pollen load larger than that typically provided by moths increased fruit retention. Outcrossed flowers were selectively retained to fruit, and flowers receiving a mixed outcross load were selectively retained over single-outcross flowers in one population. Seed set per fruit was significantly lower with a small pollen load, and lower when a single outcross source was used than in self or mixed outcross treatments. Outcross seeds germinated at a greater frequency and produced plants that grew faster than those that resulted from self-pollination, yielding a cumulative inbreeding depression of 0.73–0.81. Early fruit abortion is a major mortality factor to the yucca moth's progeny, and the female can increase retention by providing high-quality pollinations. Nonetheless, individual pollen loads are usually below levels that provide higher probability of retention, and commonly observed geitonogamous pollinations also appear to conflict with expected strong selection on moths to provide pollinations that give high retention probability. Other factors, such as combined visits by multiple females, floral position effects on retention, and possible costs of moth movement among plants, may interact with the pollination-mediated effects in molding female behavior. These factors do not diminish the potential of selective abortion to promote evolutionary stability of the interaction as they do not provide the moth with a mechanism to circumvent selective abortion.