Abstract:

The fig-fig-pollinating wasp symbiosis provides a model system for investigating the mutualistic interaction between plants and animals. A simple one-to-one rule was proposed to describe the highly specialized reciprocal relationships between figs and their species-specific pollinating fig wasps based on the initial studies: each fig tree species is obligatorily pollinated by one fig wasp species, and each wasp species can only reproduce in one fig species. With the deepening of research, however, increasing cases of breakdown in species-specificity have been reported, especially the reveal of cryptic pollinating fig wasp species progressively weaken the universality of one-to-one rule in fig-fig-pollinating wasp symbiosis. The documented cases of breakdown in the one-to-one rule were divided into two types, including copollinator and pollinator sharing, which have their own different mechanisms. Pollinator sharing is pollination of different species of Ficus by the same pollinator, while copollinator is the co-occurrence of more than one species of pollinators in one host fig. Here, the research progress on the breakdown of one-to-one rule is presented in three stages, i.e. morphological description, multidisciplinary evidence and discovery of cryptic pollinating wasp species. Some future challenges and research prospects resulted from the breakdown of one-to-one rule are outlined. Copollinators may lead to the interspecific hybridization between pollinator species, while pollinator sharing may result in the hybridization of host figs. Nonetheless, the hybridization in fig-fig-pollinating wasp symbiosis is very rare and suggests a strong level of reproductive isolation. Furthermore, the incidence and pattern of pollinator sharing and copollinator are differentiated in dioecious and monoecious figs, which suggests a differentiated host-specificity. The identification of cryptic species triggers the research of its speciation and co-occurrence, as well as the reassessment of pollinator biodiversity. Failing to recognize cryptic species also will limits the effectiveness of the controlled experiment in fig-fig-pollinating wasp symbosis. Each pair of fig and pollinating wasp possesses its distinctive coevolutionary history, and there are more than 750 pairs across the globe, which provide rich materials to explore the coevolutionary process and mechanism between plants and animals. The integrated study of formation, maintenance and breakdown mechanisms of the host-specificity will contribute to the understanding of fig-fig-pollinating wasp mutualism, and the coevolutionary process and mechanism between plants and animals.

Key words: fig, fig pollinating wasp, one-to-one rule, cryptic species, copollinator, pollinator sharing