Yuccas, yucca moths, and coevolution: A review

The obligate pollination mutualisin between yuccas (Agavaceae) and yucca moths (Lepidoptera, Prodoxidae), in which the adult moth pollinates yucca flowers and her progeny feed on developing seeds, is one of the classically cited examples of coevolution. While known since 1872, our understanding of the ecology and evolution of this association has increased dramatically in the past decade. Here I review current information on organismal diversity and phylogenetic relationships, ecological relationships, origin and reversal of the mutualism, and the potential for analyzing patterns of co-speciation and the historical role of coevolution on specific traits in driving diversification in the interaction. Major novel developments in recent years include the recognition of a large species complex of pollinators, previously thought to be one polyphagous species; a majority of all moth species are monophagous. Considerable life history diversity has been unveiled, and mechanisms that maintain a mutualistic equilibrium by preventing over-exploitation documented. Phylogenctic and ecological information, including data from other, newly discovered facultative pollinators in the Prodoxidae, have been used to erect a hypothesis for the evolution of obligate mutualism. Application of a molecular clock to phylogenetic data suggests that the plant-moth association arose at least 40 Mya, and that the obligate mutualisin evolved very quickly after this event. Two separate events of reversal of mutualism have been identified, involving derived "cheater" moth species that oviposit into fruits resulting from pollination by other pollinator species. This appears to have happened not through selection for cheating, but rather as a byproduct of a phenological shift to an unexploited seed resource, in which case pollination behavior became redundant. Analyses of parallel diversification and character coevolution are hampered by incomplete phylogenetic information at the species level, especially for the plants, but also for the pollinators. Available data indicate considerable deviation from strict co-speciation, and no evident examples of this process. Analyses of the role of coevolutionary processes in driving the diversification of yuccas and yucca moths will be possible once fully resolved phylogenies become available.