Genetic differentiation between Ficus pumila var. pumila and Ficus pumila var. awkeotsang and their pollinators

The obligate mutualism between pollinating fig wasps in the family Agaonidae (Hymenoptera: Chalcidoidea) and Ficus species (Moraceae) is often regarded as a classic example of co-evolution. Each fig species is generally pollinated by an unique species of fig wasps. Figs depend on its own specific species of wasp for pollination and the larvae of these wasps develop in their specific host. This relationship between figs and their pollinators is considered to be species-specific and may lead to co-speciation. The endemic jelly fig (Ficus pumila L. var. awkeotsang (Makino) Corner; awkeotsang) is a variety of creeping fig (F. pumila L. var. pumila; pumila) and both of them are thought to share a common pollinator wasp, Wiebesia pumilae (Hill). Since jelly and creeping figs are different in their habitats, forms of figs, and biochemical component of figs, it is reasonable to suggest the existence of genetic differentiation in between. Moreover, the differences between host figs may also have promoted genetic differentiation between their pollinators. According to our hypothesis, three DNA segments from figs, including chloroplast trnT-trnL intergenic spacer (IGS1), third intron of nitrate reductase (NIA-i3), and ribosomal intergenic transcript spacer II (ITS II), and one gene from wasps, mitochondrial cytochrome oxidase I (mtCOI), were sequenced and analyzed. Between two figs, no fixed difference was found in NIA-i3 and ITS II, and genetic distance recovered from IGS1 was only 0.3%. The nucleus gene flow between two fig populations is evident. In contrast, genetic distance derived from mtCOI gene was 11.9% between two pollinators. These results insinuate that the pollinator fig wasps may have been differentiated before the divergence of their host, and different varieties of F. pumila may be in the early stage of speciation. Within different pollinators, the long-term female effective population size of awkeotsang pollinator is estimated to be 2.3 times larger than that of pumila pollinator. Results derived from Bayesian skyline plot and mismatch distribution suggest the former have underwent population expansion during the last glacial epoch. Phylogeographic analysis indicates that populations of pumila pollinator can be divided into two groups, samples from Kaohsiung and off-shore islands are distinct from samples from the rest of Taiwan Island. Comparing with the outgroups, the indistinguishable relationships between pumila pollinator and the pollinator of F. sarmentosa, Wiebesia callida, was observed, implying the possibility of recent host transfer. The detailed mechanism may need further investigation.