Cospeciation of figs and fig-wasps: a case study of endemic species pairs in the Ogasawara Islands

The interactions between figs (Ficus, Moraceae) and fig-wasps (Agaonidae, Chalcidoidea, Hymenoptera) are special plant-pollinator relationships that are highly species-specific, in that each fig species is pollinated by a single fig-wasp species that can breed only in that particular fig species. This study examined the degree of pre-mating isolation and genetic differentiation for three fig/fig-wasp pairs endemic to the Ogasawara Islands. Simple Y-tube tests revealed that fig-wasps from Ficus nishimurae and Higashidaira type chose their own host figs significantly more often, while fig-wasps from F. boninsimae did not. Based on RAPD markers, the genetic differentiation among the fig species was low, but F. boninsimae formed a cluster within the F. nishimurae cluster. The Higashidaira type has been derived from a subpopulation of F. nishimurae. Five mitochondrial DNA haplotypes were found in the fig-wasps and each haplotype correlated well with the fig species on their island of origin. These results led to a three-step hypothesis on the cospeciation process: (1) spatial separation of fig/fig-wasp populations arises; (2) gene flow is restricted and character differentiation of fig-wasps occurs; (3) there is further restriction of gene flow and genetic and character differentiation of figs.