Research progress of interspecific hybridization in genus Ficus

Abstract

Hybridization plays a vitally important role in biological evolution and speciation. Although occurring frequently in nature, the prevalence of hybridization events is unevenly distributed across the plants. It is generally considered unlikely for the obligate insect-pollinated plants, due to the much stronger prezygotic barriers which were developed during their long co-evolutionary with the host-specific pollinators, such as the fig–fig-pollinating wasp mutualism. Species-specificity in fig pollination appears to be extreme compared with most other insect pollination mutualisms. Most fig species are pollinated by only one wasp species and most wasps are associated with just a single fig species. However, increasing cases of host shift have been reported with the deepening of research, which raises the research and discussion on hybridization between host fig species. Here, the research progress on the interspecific hybridization in Ficus is presented from the following five aspects, artificial hybridization, hybridization between exotic and native species, natural hybridization of island species, natural hybridization of sympatric allied species, and hypothesized hybridization by phylogenetic incongruence. Some further research prospects on the Ficus interbreeding are also outlined. Artificial F1 hybrids between the closely related species of the same subgenus are usually fertile. However, intensity difference of cross-fertilize compatibility between species of different subgenera exists between the different breeding systems. There is unidirectional or bidirectional but asymmetrical interspecific gene flow. Introgression may be the most prominently and prevalently hybridization process in Ficus because of the extreme dependence on pollinating fig wasps for sexual reproduction. A series of prezygotic isolation mechanisms maintain the genetic and morphological identity of fig species, including chemical, mechanical, geographic, temporal and ecological isolation. However, the postzygotic isolation is weak effective. The reproductive fitness of pollinating fig wasp is reduced obviously in atypical host species. Future research work needs to be carried out in investigating the effect of hybridization on fig biodiversity and the stability of the fig–fig-pollinating wasp mutualism, and exploring hybridization pattern and consequence of the hybrid zone, and the factors that affect the host shift and hybridization, and so on.