Pollinator sharing, copollination, and speciation by host shifting among six closely related dioecious fig species.

The obligate pollination mutualism between figs (Ficus, Moraceae) and pollinator wasps (Agaonidae, Hymenoptera) is a classic example of cospeciation. However, examples of phylogenetic incongruencies between figs and their pollinators suggest that pollinators may speciate by host shifting. To investigate the mechanism of speciation by host shifting, we examined the phylogenetic relationships and population genetic structures of six closely related fig species and their pollinators from southern China and Taiwan-Ryukyu islands using various molecular markers. The results revealed 1) an extraordinary case of pollinator sharing, in which five distinct fig species share a single pollinator species in southern China; 2) two types of copollination, namely, sympatric copollination by pollinator duplication or pollinator migration, and allopatric copollination by host migration and new pollinator acquisition; 3) fig species from southern China have colonized Taiwan repeatedly and one of these events has been followed by host shifting, reestablishment of host specificity, and pollinator speciation, in order. Based on our results, we propose a model for pollinator speciation by host shifting in which the reestablishment of host-specificity plays a central role in the speciation process. These findings provide important insights into understanding the mechanisms underlying pollinator speciation and host specificity in obligate pollination mutualism.