The genetic consequences of habitat specificity for fig trees in southern African fragmented forests

Abstract

Theory predicts that fragmentation will lead to reduced gene flow between populations, with loss of genetic diversity and increased population differentiation. However, these predictions may not always hold true, especially for long-lived woody plants and some fig trees (Ficus species) may not be affected by fragmentation because their fig wasps can transfer pollen for distances of over 100 km. Here we contrast the extent of genetic isolation caused by fragmentation among three southern African Ficus species with different habitat dependencies and distributional ranges. Two of the species are restricted to forest environments, which have been fragmented since at least the Pleistocene, and provide an indication of the long-term genetic effects of forest fragmentation. The third species is less forest-dependent, with a more general habitat association and more continuous populations. We found significant population differentiation in all three species. Populations of F. bizanae, a forest specialist with a highly restricted distribution, displayed the most genetic structure, followed by the second forest specialist, F. craterostoma. Populations of the habitat-generalist F. sur were the least genetically structured. Forest specialist Ficus species are clearly not immune to habitat fragmentation, despite extensive pollen flow, and other southern African forest trees are likely to have experienced similar or greater effects of habitat fragmentation. The strong genetic structure of F. bizanae suggests a limited seed dispersal range and local dispersal by the fig wasp pollinator, a possible adaptation to the limited range of its host fig tree.