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Interspecific mutualisms are regarded as having evolved from antagonistic or commensalistic interactions, with most mutualisms remaining facultative but some having coevolved into obligate reciprocal dependency. Underlying mutualism is an intrinsic conflict between the parties, in that each is under selection for increased exploitation of the other. Theoretical models suggest that this conflict is a source of evolutionary instability, and that evolution of 'cheating' by one party may lead to reciprocal extinction. Here we present phylogenic evidence for reversal of an obligate mutualism: within the yucca moth complex, distinct cheater species derived from obligate pollinators inflict a heavy cost on their yucca hosts by laying their eggs but not pollinating the yucca. Phylogenetic data show the cheaters to have existed for a long time. Coexisting pollinators and cheaters are not sister taxa, supporting predictions that evolution of cheating within a single pollinator is evolutionarily unstable. Several lines of evidence support a hypothesis that host shifts preceded the reversal of obligate mutualism. Host or partner shifts is a mechanism that can provide a route of evolutionary escape among obligate mutualists in general.

The genus Greya is closely related to the yucca moths, and Greya species interact with their host plants in many of the same ways as yucca moths. Females both pollinate and oviposit in the flowers oftheir host. Unlike yucca moths, however, pollinating Greya species share flowers with co-pollinators that do not oviposit on the host. We studied the interaction between Greya poli tel/a (Walsingham) (Lepidoptera: Prodoxidae) and Lithophragma parviflorum (Hook.) Torr. & Gray (Saxifragaceae) to evaluate the effect on seed output of a pollinating seed parasi te against a background of co-pollinators.Flowers were visited and pollinated mostly by bombyliid flies, solitary bees, and G. politella. Bombyliid flies alone composed 68-88% of the 5522 visits recorded over 2 yr. Although both male and female G. politella visited the flowers and probed for nectar, pollination by this species occurred only as females oviposited through the corolla tube, thereby passively transferring to the stigma pollen adhering to the abdomen. Visitation to flowers by ali pollinators averaged 0.3-1.9 visits/h during daylight hours. Consequently, most flowers were visited multiple times during the several days that stigmas were receptive, and 77% of the flowers tagged during the 4 yr had some developing seeds. Pollination did not depend upon visitation by G. politella. Flowers receiving G. politella eggs had the same probability of producing some seed and the same mean number of developing seeds as flowers visited only by other insects.Most flowers received the eggs of only one G. politella female, and the larvae ate 15-27% of the developing seeds. The final number of mature seeds remaining in attacked flowers did not differ from unattacked flowers except at one site in 1 yr, in which the values for attacked plants were marginally lower. Other sources ofvariation affecting seed output masked the effects of seed consumption by Greya.Overall, G. politella females have the potential to be mutualistic with L. parviflorum: they are effective pollinators, generally visit most plants and about halfthe flowers in the population, and impose a fairly small cost on seed output. Nonetheless, the abundant and effective co-pollinators, which do not eat the developing seeds, swamp Greya's mutualistic effects. Under the current conditions at Granite Point, the relationship between G. politella and L. parviflorum may be mostly commensalistic. The evolution of specialization to G. politella as an exclusive pollinator would seem to be possible only in L. parviflorum populations in which effective co-pollinators were either rare or unpredictable. That is, the potential for the evolution of specificity in this mutualism appears to depend upon the community context in which the interaction takes place rather than upon the simple outcome of the pairwise interaction between Greya and Lithophragma.

Pollination by seed parasites is unusual, and previously studied case involve plant species (figs, yuccas) that are pollinated mostly by one host-specific pollinator. Trollius europaeus (Ranunculaceae), however, is pollinated by four Chiastocheta species (Diptera: Anthomyiidae) that mate in the flowers, eat pollen and nectar, and complete larval development in the seeds. Strict resource partitioning is present between the fly species, in that different parts of the infructescence are used by larvae of each species, and fly species oviposit at different times in the life of a flower. The flies were shown to be the exclusive pollinators. Because the numbers of ovules fertilized per visit was a fixed proportion of remaining non-fertilized ovules, fly species differed in mean pollinator efficacy, with the last species contributing the fewest pollinations. Therefore, with larval consumption of seeds being fixed and seed initiation decreasing per visit, larvae from later ovipositions consumed more seeds than their mothers pollinated. The point at which costs and benefits were equal was found to be around 4-5 eggs; observed population means of eggs per flower varied between 2.3-7.25 during three years of study. The system is rather insensitive to variation in pollinator density, remaining mutualistic over a wide range. Small changes in pollinator efficacy, however, can tilt the equilibrium to a negative net effect for the plant. These data on Trollius/ Chiastocheta interactions provide the first extensive cost/ benefit analyses for mutualism based on seed parasites as pollinators.